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2.5. PM2.5 Composition, Sources, and Air Concentrations

Composition of PM

Industrial emissions Major sources

Composition of ultrafines Motor vehicles and elemental

carbon Secondary vs primary aerosol

Local vs distant sources Exceedances in SE

Diurnal and seasonal variability

Radiative and optical properties

Internal mixing of constituents

Urban plumes under stagnation conditions

Urban plumes under advective conditions

Ozone transport from urban to rural areas

2.5. PM2.5 COMPOSITION, SOURCES, AND AIR CONCENTRATIONS Paul Solomon and David Allen

SOS was selected by EPA’s Office of Air Quality

Planning and Standards (EPA/OAQPS) to establish the

first of two initial Supersites. The other initial Supersite

was in Fresno-Bakerfield, CA. Both sites were part of

EPA's effort under the leadership of Paul Solomon and

Richard Scheffe to increase the nation's capacity to

monitor both coarse (PM10) and fine (PM2.5) fractions in

a reliable way in various parts of the United States.

The Atlanta Supersite Experiment was established

under the leadership of SOS' Chief Scientist, Bill

Chameides. SOS brought together in Atlanta during

August 1999, the most comprehensive array of

particulate-matter measurement instruments ever

assembled in the US and about 150 of the nation’s most

competent aerosol research scientists and a few from

abroad.

David Allen of the University of Texas in Austin, TX and Matt Fraser of Rice University in

Houston, TX also were selected by EPA/OAQPS to lead the Houston Supersite, one of eight

EPA Supersites around the United States that are part of EPA’s PM Supersite Program – a

cooperative endeavor with other carefully selected public-health and regional-haze investigators.

Here very detailed, chemical and physical characterization measurements of ambient coarse and

fine aerosols were made in all months of the year (2001-2002) at multiple, carefully selected

urban and rural locations in southeast Texas.

The objectives of both the Atlanta and Houston Supersite programs were to “advance

scientific understanding of atmospheric processes regarding formation and accumulation of PM.”

The Atlanta Supersite Experiment was located on Jefferson Street in Atlanta, GA at a mixed

commercial and industrial area about 8 km west of the city center. Here detailed PM

characterization measurements had been made routinely as a part of EPRI’s and the Southern

Company’s SEARCH and ARIES programs for more than a year. An extraordinarily diverse

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variety of instruments were assembled at this site for comparison purposes and to determine the

mass, particle size distribution, chemical composition of individual particles, and chemical

composition of hourly-collected filter samples collected on all days of the week, and

simultaneous gas and particle measurements.

Scientific findings are summarized below – mainly from the Atlanta Supersite Experiment in

August 1999, year-round, rural and urban measurements at the Houston Supersite in southeast

Texas, and both January and July measurements at a rural site near Anderson, SC.

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2.5.1. Composition of PM in the Southeastern US The major components of total PM mass on average for the urban Atlanta Supersite study

[and both rural and urban locations within the Houston Supersite program] were: organic

material (~35%) [25-30%], sulfate (~34%) [30-40%]; ammonium (~12%) [7-10%], elemental

carbon (~3%) [2-5%], nitrate (~2%) [1-4%], and crustal material (~3%) [

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• Sulfate, ammonium ion (which neutralizes the sulfate ion), organic carbon, and elemental carbon are the major constituents of PM2.5; the annual average concentrations of these major components were spatially homogeneous across southeast Texas.

• Nevertheless, localized events with high mass fractions of sulfate or carbon occurred frequently at many monitors in this region.

• Concentrations of sulfate were slightly higher in the spring and late fall than in the summer; carbon concentrations were highest in the late fall.

• High organic-carbon to elemental-carbon ratios suggest that much of the carbonaceous material in PM2.5 in southeast Texas is not emitted directly, but is formed in the air through reactions involving both gaseous biogenic and anthropogenic VOC emissions.

KEY CITATIONS: Husain, H. and C. Christoforou. 2003. Concentration and Chemical Composition of PM2.5 Particles at a Rural Site

in South Carolina. SOS Final Report. Clemson University. 35 pp. Modey, W.K., E.J. Eatough, Y. Pang, and N.L. Eatough. 2004. Performance and evaluation of the PC-BOSS for

fine PM2.5 sampling during the summer EPA Supersite Program in Atlanta. J. Air Waste Manage. Assoc. (in press).

Russell, M.M. and D.T. Allen. 2004. Seasonal and spatial trends in primary and secondary organic carbon concentrations in southeast Texas. Atmos Environ. 38:3225-3239.

Russell, M.M., D.T. Allen, D.R. Collins, and M.P. Fraser. 2004. Daily, seasonal and spatial trends in PM2.5 mass and composition in southeast Texas. Aerosol. Sci. Technol. 38(S1):14-26, doi:10.1080/02786820390229138.

Solomon, P.A., W. Chameides, R. Weber, A. Middlebrook, C.S. Kiang, A.G. Russell, A. Butler, B. Turpin, D. Mikel, R. Scheffe, E. Cowling, E. Edgerton, J. St. John, J. Jansen, P. McMurry, S. Hering, and T. Bahadori. 2003b. Overview of the 1999 Atlanta Supersite project. J. Geophys. Res. 108(D7), 8413, doi:10.1029/2001JD001458.

Weber, R., D. Orsini, A. Sullivan, M. Bergin, C.S. Kiang, M. Chang, Y.N. Lee, P. Dasgupta, J. Slanina, B. Turpin, E. Edgerton, S. Hering, G. Allen, P. Solomon, and W. Chameides. 2003. Transient PM2.5 aerosol events in metro Atlanta: Implications for air quality and health. J. Air Waste Manage. Assoc. 53:84-91.

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2.5.2. Sources of Fine Particles in Houston, TX Fresh hydrophobic ultrafine particles are emitted by industrial sources in the Ship Channel

area of Houston; they grow in size and become more hydrophilic as they grow.

Particle growth within the VOC-rich ship channel plume exceeded that expected solely from

SO2 oxidation. But particle growth within the plume of the Parish power plant was generally

consistent with condensation of the oxidation products of SO2 when the plume did not pass over

substantial sources of VOCs.

There are five major sources of primary PM2.5 emissions in southeast Texas. They include:

1) mobile sources, 2) cooking of foods, 3) point sources, 4) geological sources, and 5) wild fires

and open burning.

• Primary mobile-source emissions are significant; evidence suggests that these emissions account for about 25-35% of PM2.5 mass in SE Texas. In fact, diesel engines in heavy duty trucks, trains, and farm or construction equipment, and gasoline engines in cars, trucks, boats, and hand tools, as well as jet-fueled aircraft, account for most primary emissions of PM2.5 in southeast Texas.

• Primary emissions from cooking of foods are significant in all urban areas; evidence suggests that these emissions account for about 10-15% of PM2.5 mass in urban areas.

• Point sources of primary PM10 particles are significant, but point-sources of primary PM2.5 particles have not yet been quantified. Thus, additional research is needed to determine the importance, size distributions, and chemical compositions of these PM2.5 primary emissions.

• Geological sources (wind-blown dust) are a relatively minor contributor to the total mass of PM2.5.

• Fires are a sporadic, but significant source of primary PM2.5 emissions in Texas. On an annual average basis, they contribute about 1-2% of the total mass of fine particles in the Houston-Galveston area; but these emissions tend to be concentrated on specific days with fire events.

KEY CITATIONS: Allen, D.T. 2002. Particulate Matter Concentrations, Compositions, and Sources in Southeast Texas: State of the

Science and Critical Research Needs. Report to the Texas Environmental Research Consortium. 93 pp. http://www.harc.edu/harc/Projects/AirQuality/Projects/Status/Reports.aspx

Brock, C.A., M. Trainer, T.B. Ryerson, J.A. Neuman, D.D. Parrish, J.S. Holloway, D.K. Nicks, Jr., G.J. Frost, G. Hübler, F.C. Fehsenfeld, J.C. Wilson, J.M. Reeves, B.G. Lafleur, H. Hilbert, E.L. Atlas, S.G. Donnelly, S.M. Schauffler, V.R. Stroud, and C. Wiedinmyer. 2003. Particle growth in urban and industrial plumes in Texas. J. Geophys. Res. 108(D3), 4111, doi:10.1029/2002JD002746.

NOAA Aeronomy Laboratory. 2003. Texas 2000 Air Quality Study - Phase II Analysis of NOAA Data. Final Report to Texas Commission on Environmental Quality Houston/Galveston Air Quality Science Evaluation. 158 pp.

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ftp://ftp.tnrcc.state.tx.us/pub/OEPAA/TAD/Modeling/HGAQSE/Contract_Reports/Data_Analysis/TexAQS2000_NOAA_Data_Analysis.pdf 2.5.3. Emission Sources of PM2.5 and PM10 in Atlanta, GA A source identification technique called positive matrix factorization was used with daily

integrated particulate matter data on mass concentration and composition collected in Atlanta

between August 1998 and August 2000. For PM2.5 particles, eight major types of emission

sources were identified: 1) SO42--rich secondary aerosol sources (56%), 2) motor vehicle sources

(22%), 3) wood smoke sources (11%), 4) NO3--rich secondary aerosol sources (7%), 5) mixed

cement kiln and organic carbon sources (2%), 6) airborne soil sources (1%), 7) metal recycling

facilities (0.5%), and 8) a miscellaneous source that includes bus stations and metal processing

facilities (0.3%). Invariably, NH4+ [presumably mainly from agricultural sources] was

associated with both the SO42--rich and NO3--rich secondary aerosols.

For PM10 particles, five major types of sources were identified: 1) airborne soil sources

(60%), 2) NO3--rich secondary aerosol sources (16%), 3) SO42--rich secondary aerosol sources

(12%), 4) cement kiln facilities (11%), and 5) metal recycling facilities (1%).

Summary finding from this work can be stated:

• Sulfate-rich secondary aerosol was the primary contributor to Atlanta PM2.5 mass; airborne soil was the largest primary source of PM10 particle mass in Atlanta.

KEY CITATION: Kim, E., P.K. Hopke, and E.S. Edgerton. 2003. Source identification of Atlanta aerosol by positive matrix

factorization. J. Air Waste Manage. Assoc. 53:731-739.

2.5.4. Composition of Ultrafine Particles in Atlanta, GA • The composition of the ultrafine (less than 100 nm) particles was dominated by carbon

compounds. The major composition classes (expressed as percentage of particle mass) were: organic carbon (~74%), potassium (~8%), iron (~3%), calcium (~2%), nitrate (~2%), elemental carbon (~1.5%), and sodium (~1%).

• The total mass of ultrafine particles (

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Bahadori. 2003b. Overview of the 1999 Atlanta Supersite project. J. Geophys. Res. 108(D7), 8413, doi:10.1029/2001JD001458.

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2.5.5. Elemental Carbon in PM2.5 Time-resolved ambient particulate organic and elemental carbon were measured during the

Atlanta Supersite Study using five different instruments, in order to investigate temporal trends

of carbon-containing aerosols and to determine the contributions of primary and secondary

organic carbon to particulate organic carbon. Major findings of this work are summarized

below.

• In spite of the large uncertainties inherent in measuring carbon-containing particulate matter, which is very complex in composition, and in utilizing different operational techniques for measurement, there was generally good agreement between measurement systems.

• Organic matter and elemental carbon comprised ~40% and ~8%, respectively, of PM2.5 mass on average during the August 1999 Atlanta Supersite Study.

• Motor vehicles were indicated as a primary emission source of elemental carbon in Atlanta, using carbon monoxide as a tracer. Elemental carbon concentrations tended to peak at 0600-0900 EST, which is probably indicative of motor vehicle emissions, and had a much smaller peak in the evening.

KEY CITATIONS: Lim, H.-J. and B.J. Turpin. 2002. Origins of primary and secondary organic aerosol in Atlanta: Results of time-resolved

measurements during the Atlanta Supersite Experiment. Environ. Sci. Technol. 36:4489-4496. Lim, H.-J., B.J. Turpin, E. Edgerton, S. Hering, G. Allen, H. Maring, and P. Solomon. 2003. Semicontinuous aerosol

carbon measurements: Comparison of Atlanta Supersite measurements. J. Geophys. Res. 108(D7), 8419, doi:1029/2001JD001214.

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2.5.6. Secondary Formation of Organic Aerosols Contributions of primary emissions and secondary aerosol formation to measured organic

carbon in Atlanta in August 1999 were estimated to be roughly equal. Secondary organic carbon

estimates are estimated to have a + 10% uncertainty.

Poor correlation between elemental carbon and organic carbon in Atlanta in summer is

indicative of secondary organic aerosol formation. The ratio of organic carbon to elemental

carbon closely tracked daytime variations in ozone concentrations. Thus, the ratio increased

during the afternoon in correspondence to photochemical activity.

Ratios of organic carbon to elemental carbon suggest that much of the carbonaceous material

in southeast Texas is secondary organic aerosol, that is, it is formed in the atmosphere as the

result of the reactions of gas phase VOC emissions. Point sources and area/non-road emissions

are the dominant contributors to formation of anthropogenic secondary organic aerosol in the

Houston urban area, but on a regional basis, biogenic emissions of secondary organic aerosol

precursors overwhelm anthropogenic sources.

Radiocarbon dating (14C/13C ratios) indicates that at some suburban and rural locations in

southeast Texas, formation of secondary organic aerosol is dominated by reactions involving

biogenic emissions. These locations are primarily north and southwest of Houston’s urban core.

• Secondary formation of organic aerosols tended to be large compared to primary emissions. This was true at both the Atlanta and Houston Supersites

KEY CITATIONS: Dechapanya, W., M.M. Russell, and D.T. Allen. 2002. Estimates of anthropogenic secondary organic aerosol

formation in Houston, Texas. Aerosol Sci. Technol. 38(1):156-166, doi:10.1080/02786820390229462. Lemire, K.R., D.T. Allen, G.A. Klouda, and C.W. Lewis. 2002. Fine particulate matter source attribution for

Southeast Texas using 14C/13C ratios. J. Geophys. Res. 107(D22), 4613, doi:10.1029/2002JD002339. Lim, H.-J. and B.J. Turpin. 2002. Origins of primary and secondary organic aerosol in Atlanta: Results of time-

resolved measurements during the Atlanta Supersite Experiment. Environ. Sci. Technol. 36:4489-4496. Lim, H.-J., B.J. Turpin, E. Edgerton, S. Hering, G. Allen, H. Maring, and P. Solomon. 2003. Semicontinuous

aerosol carbon measurements: Comparison of Atlanta Supersite measurements. J. Geophys. Res. 108(D7), 8419, doi:1029/2001JD001214.

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2.5.7. Local and Regional Sources of PM2.5 in Southeast Texas

Sulfate makes up about 30-40% and organic carbon and elemental carbon make up about 25-

30% of the total, annual average, PM2.5 mass in southeast Texas. Point sources of SO2 emissions

are the dominant source of locally generated sulfate in PM2.5. There is also evidence for

significant local sources of the carbonaceous material found in PM2.5. The sulfate in PM2.5 often

is not completely neutralized; thus ammonia emissions influence the total mass of PM2.5. The

dominant source of ammonia in Texas is cattle and other livestock; in most of the state’s urban

areas, on-road sources and domestic activities (use of cleaning products, human perspiration and

respiration, human wastes and pet wastes) dominate ammonia emissions.

But it appears that not all of the sulfate or carbonaceous material observed in PM2.5 in

southeast Texas is emitted or formed locally. Examination of the spatial distributions of PM2.5

and composition and analysis of air parcel back trajectories indicate that:

• When high concentrations of fine particulate matter mass, sulfate and organic carbon are observed throughout southeast Texas, back-trajectory analyses of these air parcels often indicate high concentrations of background sulfate and organic carbon in PM2.5 that extend far upwind. This suggests that much that much sulfate and carbonaceous aerosol is transported into southeast Texas from the eastern half of North America.

• But, high concentrations of fine particulate matter mass and organic carbon are observed at isolated monitors in southeast Texas, suggesting local source contributions are important on some days.

KEY CITATION: Russell, M.M., D.T. Allen, D.R. Collins, and M.P. Fraser. 2004. Daily, seasonal and spatial trends in PM2.5 mass

and composition in southeast Texas. Aerosol. Sci. Technol. 38(S1):14-26, doi:10.1080/02786820390229138.

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2.5.8. Concentrations of PM2.5 Mass in the Southeastern US Data collected and analyzed in connection with the Atlanta Supersite Experiment lead to the

following conclusions:

• The average air concentration of PM2.5 mass during the August 1999 Atlanta Supersite Experiment was 31.3 µg m-3, with a peak value of 47.2 µg m-3. Thus, the 24-hour PM2.5 standard was not exceeded. Interestingly, the 1-hr ozone standard was exceeded on several days, for multiple hours, during the study. Sulfate and ammonium ion concentrations were well correlated with PM2.5 mass; but organic carbon and elemental carbon concentrations were not very well correlated.

• Monitoring data from January 2001 to February 2002 at the Atlanta Speciation Trends Network site showed an average total PM2.5 concentration in the winter of 12 µg m-3, and 20 µg m-3 in the summer, with the highest 24-hour average totaling µg m-3.

• Annual PM2.5 mass concentrations measured from March 1999 to February 2000 in the ASACA study in Atlanta exceeded the annual NAAQS of 15 µg m-3 at all four monitoring sites, with annual averages ranging from 19.3 to 21.2 µg m-3. One site violated the daily NAAQS of 65 µg m-3.

Analyses of PM measurements at a rural site during summer and winter indicate:

• At a rural site near Anderson, SC, the average PM2.5 mass during July 2001 was 20.9 µg m-3, with a high of 41.2 µg m-3 on July 18 and a low of 4.4 µg m-3 on July 25. The overall average in January 2002 was 9.4 µg m-3, with a high of 18.2 µg m-3 on January 18 and a low of 3.7 µg m-3 on January 25. Across all sampling events, the average annual mass concentration was 15.1 µg m-3, just above the new NAAQS annual standard of 15 µg m-3.

Data on air concentrations of PM2.5 mass in southeast Texas indicate that:

• Annual average mass concentrations of PM2.5, over wide regions of eastern and southeastern Texas, range from approximately 10 µg m-3 to 15 µg m-3, which is close to the annual average NAAQS of 15 µg m-3.

• When averaged over long time periods, PM2.5 mass concentrations are spatially homogeneous throughout southeast Texas.

• Local events with moderately high air concentrations of PM2.5 mass occur at many monitors in this region but only rarely exceed the annual average NAAQS.

KEY CITATIONS: Butler, A.J., M.S. Andrew, and A.G. Russell. 2003. Daily sampling of PM2.5 in Atlanta: Results of the first year

of the Assessment of Spatial Aerosol Composition in Atlanta study. J. Geophys. Res. 108(D7), 8415, doi:10.1029/2002JD002234.

Husain, H. and C. Christoforou. 2003. Concentration and Chemical Composition of PM2.5 Particles at a Rural Site in South Carolina. SOS Final Report. Clemson University. 35 pp.

Russell, M.M., D.T. Allen, D.R. Collins, and M.P. Fraser. 2004. Daily, seasonal and spatial trends in PM2.5 mass and composition in southeast Texas. Aerosol. Sci. Technol. 38(S1):14-26, doi:10.1080/02786820390229138.

Solomon, P.A., W. Chameides, R. Weber, A. Middlebrook, C.S. Kiang, A.G. Russell, A. Butler, B. Turpin, D. Mikel, R. Scheffe, E. Cowling, E. Edgerton, J. St. John, J. Jansen, P. McMurry, S. Hering, and T. Bahadori. 2003b. Overview of the 1999 Atlanta Supersite project. J. Geophys. Res. 108(D7), 8413, doi:10.1029/2001JD001458.

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2.5.9. Variability in Concentrations of PM2.5 in the Southeastern US Measurements of the air concentrations of PM2.5 mass in Atlanta and surrounding rural areas

indicate that:

• Winter and summer data from a rural site near Anderson, SC showed higher mass concentrations in summer than in winter. Sulfate ion and ammonium ion concentrations increased in summer, but nitrate ion concentrations decreased in summer at this site. Comparison of these SC air concentration data with those for similar rural sites in GA and NC showed that the NC sites generally had higher and the GA sites generally had lower air concentrations of PM2.5 mass during late 2001 and early 2002.

• ASACA data in Atlanta showed that most PM2.5 constituents peaked during summer months; but nitrate, metals, and elemental carbon usually showed some enhancement during winter due mainly to lower inversion heights. Diurnally, there were discernible early morning and late night peaks that corresponded to rush-hour traffic patterns and inversion heights, respectively.

• Comparison of data from Atlanta, GA and Fresno, CA showed that seasonal differences in meteorology and amounts of emissions have significant influences on seasonal variability in the composition of PM2.5 at both locations.

Data on the air concentrations of PM2.5 mass in southeast Texas indicate that:

• Throughout the region, concentrations are slightly higher in the spring and late fall than in summer.

• A consistent and strong morning peak in PM2.5 mass concentrations is observed throughout the region and a weaker and slightly less consistent peak in mass concentration is observed in the late afternoon to early evening.

KEY CITATIONS: Butler, A.J., M.S. Andrew, and A.G. Russell. 2003. Daily sampling of PM2.5 in Atlanta: Results of the first year

of the Assessment of Spatial Aerosol Composition in Atlanta study. J. Geophys. Res. 108(D7), 8415, doi:10.1029/2002JD002234.

Chu, S.-H., J.W. Paisie, and B.W.-L. Jang. 2003. PM data analysis – A comparison of two urban areas: Fresno and Atlanta. J. Geophys. Res. (in review)

Husain, H. and C. Christoforou. 2003. Concentration and Chemical Composition of PM2.5 Particles at a Rural Site in South Carolina. SOS Final Report. Clemson University. 35 pp.

Russell, M.M., D.T. Allen, D.R. Collins, and M.P. Fraser. 2004. Daily, seasonal and spatial trends in PM2.5 mass and composition in southeast Texas. Aerosol. Sci. Technol. 38(S1):14-26, doi:10.1080/02786820390229138.

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2.5.10. Radiative Forcing by Aerosols in Atlanta During the Atlanta Supersite Experiment, studies were made of both the optical and radiative

forcing properties of the aerosols observed in Atlanta. These studies showed that:

• Light scattering was dependent on a wide range of chemical components of the aerosols. • Light absorption is most strongly linked to the elemental carbon component. • The average direct aerosol radiative forcing properties of the Atlanta was about minus 11

+ 6 watts m-2; this value is about 10 times larger than global mean estimates for aerosols.

KEY CITATION: Carrico, C.M., M.H. Bergin, J. Xu., K. Baumann, and H. Maring. 2003. Urban aerosol radiative properties:

Measurements during the 1999 Atlanta Supersite experiment. J. Geophys. Res. 108(D7), 8422, doi:10.1029/2001JD001222.

2.5.11. Composition of Aerosols in Atlanta Chemical composition of single particles was measured using the Particle Analysis by Laser

Mass Spectrometry (PALMS) instrument during the Atlanta Supersite Study. Particle

composition was generally internally mixed. The predominant particle consisted of organic

species and sulfate, and often contained other components such as nitrate, ammonium, halogens,

metals, soot/hydrocarbon, and aluminosilicates. More than 20% of the negative ion spectra of

single particles contained nitrate ion peaks, which showed a clear maximum during the morning

at high relative humidity and a smaller maximum in the afternoon. About 45% of the negative

spectra contained ions indicative of oxidized organics, with similar morning and afternoon

maxima. At high relative humidity, the nitrate peaks were often mixed internally with sulfate.

The single particle data also indicated the presence of organic sulfur containing compounds,

which might account for 10 — 15% of sulfur observed in Atlanta PM2.5.

• The composition of particles measured during the Atlanta Supersite Study was generally internally mixed, with components of organic matter, sulfate, nitrate, ammonium and other constituents.

KEY CITATIONS: Lee, S.-H., D.M. Murphy, D.S. Thomson, and A.M. Middlebrook. 2002. Chemical components of single

particles measured with particle analysis by laser mass spectrometry (PALMS) during the Atlanta Supersite Experiment: Focus on organic/sulfate, lead, soot, and mineral particles. J. Geophys. Res. 107(D1), 4003, doi:10.1029/2000JD000011.

Solomon, P.A., W. Chameides, R. Weber, A. Middlebrook, C.S. Kiang, A.G. Russell, A. Butler, B. Turpin, D. Mikel, R. Scheffe, E. Cowling, E. Edgerton, J. St. John, J. Jansen, P. McMurry, S. Hering, and T. Bahadori. 2003b. Overview of the 1999 Atlanta Supersite project. J. Geophys. Res. 108(D7), 8413, doi:10.1029/2001JD001458.

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2.6. Particle Phase Measurement Technologies - Development and Intercomparison of Techniques

Integrated Filter Methods for Fine Particle Composition

Integrated Filter Measurements of Semi-Volatile Fine Particulates

Semi-Continuous Methods for Measuring Fine Particle Chemical Composition–Atlanta Supersite Intercomparison

Mass Spectroscopic Methods for Measuring Fine Particle Chemical Composition

Methods for Measuring Fine Particle Density

2.6. PARTICLE PHASE MEASUREMENT TECHNOLOGIES - DEVELOPMENT AND INTERCOMPARISON OF TECHNIQUES

Rodney Weber

Measurements of the physical and chemical properties

of aerosols are needed to gain insight into sources, atmospheric transformations, and impacts of ambient

aerosol particles on human health and the environment. Aerosol particle properties of interest include size

distributions, density, optical properties, total mass,

chemical composition, and the aerosol particle mixing state. Methods capable of fast sampling rates and

automated operation are especially useful since they provide large data sets that are better integrated with

meteorological and gas phase measurements and thus

have the potential to provide greater insights than highly time-integrated measurements. Although improvements

are still being made, automated instrumentation to measure aerosol physical properties, such as number

concentrations, size distributions, and optical properties

have existed for some time. It is only recently that significant progress has been achieved in developing online methods for measuring particle chemical composition, much of it with SOS

support. This section focuses on recent developments in aerosol measurement instrumentation

that has been aided by the SOS research program. Following the approach of the Atlanta Supersite Study, instrumentation is divided into three categories, integrated methods, semi-

continuous methods, and methods based on mass spectrometry.

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2.6.1.1 Assessment of Integrated Filter Methods for Measuring Fine Particle Chemical Composition

Collection of particles onto filter substrates for extended integration periods, followed by off-line

extraction and analysis, is the standard method for measuring fine particle total mass and chemically speciated mass concentrations. One of the findings from the Atlanta Supersite Study

was gained from a unique experiment in which side-by-side comparisons were made between 12 different integrated filter methods for measuring PM2.5 mass and chemical composition. The

results indicate:

• Integrated filter methods showed good agreement for PM2.5 mass (most samplers within ±20%), and sulfate and ammonium (most samplers within 10%).

• There were larger discrepancies between methods (±3 0 to 35%) for nitrate, possibly due to the low ambient concentrations. Higher variability also was found for the organic (OC) and elemental (EC) carbonaceous fractions of PM2.5. For all OC samplers the variability ranged between 35 and 45%. EC variability was high, especially between the different analytical methods.

KEY CITATION: Solomon, P., K. Baumann, E. Edgerton, R. Tanner, D. Eatough, W. Modey, H. Maring, D. Savoie, S.

Natarajan, M. B. Meyer, and G. Norris. 2003. Comparison of integrated samplers for mass and composition during the 1999 Atlanta Supersites project. J. Geophys. Res. 108(D7), 8423, doi:10.1029/2001JD001218.

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2.6.1.2 Integrated Filter Measurements of Semi-Volatile Fine Particulates by the PC-BOSS

Semi-volatile compounds associated with aerosol particles include ammonium nitrate and

semi-volatile organics that are mainly from secondary organic aerosol formation. Although these compounds can comprise a significant fraction of the PM2.5 mass, they are difficult to

measure accurately due to evaporation loss during sampling. Standard single filter methods, such as the methods currently accepted by the US EPA, including the PM2.5 Federal Reference

Method (FRM), will underestimate semi-volatile compounds. The PC-BOSS sampler has been

developed to quantify the fine particle composition, including the semi-volatile compounds. The instrument has been deployed in a number of studies, including the Atlanta Supersite Study, to

quantify the semi-volatile aerosol compounds. A key finding from the range of studies with the PC-BOSS is that:

• An estimated 10% to 50% of the fine particulate mass was not measured with the PM2.5 FRM sampler due to the loss of semi-volatile organic material and ammonium nitrate during sampling.

These types of methods are necessary to accurately characterize PM2.5 chemical components and

to aid in setting relevant policy standards. KEY CITATIONS: Eatough, D.J., R.W. Long, W.K. Modey, and N.L. Eatough. 2003. Semi-volatile secondary organic aerosol in urban

atmospheres: Meeting a measurement challenge. Atmos. Environ. 37:1277-1292. Modey, W.K., Y. Pang, N.L. Eatough, and D.J. Eatough. 2001. Fine particulate (PM2.5) composition in Atlanta, USA:

Assessment of the particle concentrator-Brigham Young University organic sampling system, PC-BOSS, during the EPA supersite study. Atmos. Environ. 35(36):6493-6502.

Modey, W.K., E.J. Eatough, Y. Pang, and N.L. Eatough. 2004. Performance and evaluation of the PC-BOSS for fine PM2.5 sampling during the summer EPA Supersite Program in Atlanta. J. Air Waste Manage. Assoc. (in press).

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2.6.2 Advances in Semi-Continuous Methods for Measuring Fine Particle Chemical Composition–Atlanta Supersite Intercomparison

A significant outcome of SOS efforts in instrument development was the assistance given to

a range of methods for automated measurements of particle chemical composition. Five relatively new methods for measurement of bulk fine particle ionic composition, and five for

measurement of carbonaceous compounds were intercompared as part of the Atlanta Supersite Study. These methods included:

Inorganic composition: • Aerosol Dynamics, Inc Integrated Collection and Vaporization Cell (ICVC) for nitrate

and sulfate, now commercially available through R&P (e.g., Rupprecht and Patashnick Sulfate and Nitrate Monitors, Albany NY). (Hering and Stolzenburg, 1997; Stolzenburg and Hering, 2000)

• Netherlands Energy Research Foundation (ECN): nitrate and sulfate. (Slanina et al., 2001) • Georgia Tech/Brookhaven National Lab Particle Into Liquid Sampler (PILS). (Orsini et al.,

2003; Weber et al., 2001). • Atmospheric Research and Analysis (ARA) nitrate. • Texas Tech University (TT): nitrate and sulfate. (Boring et al., 2002)

Carbonaceous Composition • Rutgers University/Oregon Graduate Institute Thermal optical carbon analyzer (OC, EC,

TC). A similar instrument is commercially available (Sunset Labs) • Rupprecht and Patashnick (R&P) 5400 ambient carbon particulate monitor (OC, EC, TC)

(Turpin et al., 1990). • Radiance Research particle soot absorption photometer (PSAP, Seattle WA) (EC). • Aerosol Dynamics Inc integrated collection and vaporization cell (ICVC) for carbon

(OC). • Magee Scientific AE-16 aethalometer (EC).

Two separate publications detailed the performance of the semi-continuous methods based on an unprecedented side-by-side ambient study.

1) Ionic Compounds: Five semi-continuous PM2.5 instruments for measurements of fine particle (PM2.5) nitrate and sulfate were compared. It was found that most instruments were in close agreement with r-squared values between instrument pairs typically ranging from 0.7 to 0.94. Based on comparison between individual semi-continuous devices and 24-hour integrated filter measurements, most instruments were within 20 to 30% for nitrate (~0.1-0.2 µg m-3) and 10 to 15% for sulfate (1-2 µg m-3). Within 95% confidence intervals, linear regression fits suggest no biases existed between the semi-continuous techniques and the 24-hour integrated filter measurements of nitrate and sulfate, however, for nitrate, the semi-continuous intercomparisons showed significantly less variability than intercomparisons amongst the 24-hour integrated filters. (Weber et al., 2003b, JGR)

2) Carbonaceous Compounds: As observed in the integrated filter inter-comparisons, the measurement of the aerosol carbonaceous component is subject to larger uncertainties than the inorganic compounds. Semicontinuous aerosol carbon measurements made during

30 June 2004 | State of SOS-3: 1995 - 2003 | 95

Atlanta using five different samplers showed moderate correlations between pairs of organic carbon measurements and high correlations between pairs of elemental carbon measurements. The two semi-continuous samplers capable of a total carbon (TC) measurement were in good agreement, within 5% based on the slope of a Deming least squares fit, and r-squared of 0.83. Regression slopes between pairs of the OC measurements were within 22%, but in some cases intercepts as large as 2.3 ugC/m3 suggest there were significant systematic differences. Large differences were found between pairs of EC measurements. Regression slopes indicate differences ranged from 4 to 45%. The study demonstrates the successful operation of automated semi-continuous carbon analyzers and illustrates the need for standards to decrease uncertainties in current OC-EC measurements. (Lim et al., 2003, JGR) Many of the semi-continuous methods developed with SOS assistance have had a significant

impact on other federally funded projects, and are likely to impact regulation of PM2.5. Key findings are summarized below.

Data from new semi-continuous instrumentation deployed at the Atlanta Supersite have

enabled the following new insights into aerosol sources and atmospheric processing: • Secondary organic aerosol, which comprised about 46% of the measured organic

carbon, was from a combination of in situ photochemical production and transport of more aged secondary organic aerosol. This is based on diurnal patterns and correlations with ozone and carbon monoxide and estimates of the fraction of OC from secondary organic aerosol formation processes from mean 1-hour fine particle OC and EC data.

• Transient PM2.5 episodes in which particle mass rapidly rises and falls over a period of a few hours but which go undetected with traditional time-integrated measurements are ubiquitous. Continuous highly time resolved measurements of fine particle mass and chemical composition at the EPA Atlanta Supersite Study, August 1999, revealed the transient episodes. Speciated composition data show that these events are driven by sudden increases of two specific aerosol chemical components that dominate at different times, carbonaceous events in the early morning and sulfate events in late afternoon. Apart from providing insights into sources, the unique chemical nature of these transient events may have specific health effects that previous epidemiologic studies based on highly averaged aerosol data could not readily resolve.

• Partitioning between the gaseous and condensed phases was in reasonable agreement with predictions of a thermodynamic equilibrium model. Application of the thermodynamic equilibrium model ISORROPIA to near real-time measurements of fine particle sulfate, nitrate, and ammonium, and gas phase ammonia and nitric acid showed good agreement, with an indication of potential bias in estimates of acidity/alkalinity.

KEY CITATIONS: Lim, H.-J. and B.J. Turpin. 2002. Origins of primary and secondary organic aerosol in Atlanta: Results of time-resolved

measurements during the Atlanta Supersite Experiment. Environ. Sci. Technol. 36:4489-4496.

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Lim, H.-J., B.J. Turpin, E. Edgerton, S. Hering, G. Allen, H. Maring, and P. Solomon. 2003. Semicontinuous aerosol carbon measurements: Comparison of Atlanta Supersite measurements. J. Geophys. Res. 108(D7), 8419, doi:1029/2001JD001214.

Weber, R., D. Orsini, Y. Duan, K. Baumann, C.S. Kiang, W. Chameides, Y.-N. Lee, F. Brechtel, P. Klotz, P. Jongejan, H. ten Brink, J. Slanina, C.B. Boring, Z. Genfa, P. Dasgupta, S. Hering, M. Stolzenburg, D.D. Dutcher, E. Edgerton, B. Hartsell, P. Solomon, and R. Tanner. 2003a. Intercomparison of near real time monitors of PM2.5 nitrate and sulfate at the U.S. Environmental Protection Agency Atlanta Supersite. J. Geophys. Res. 108(D7), 8421, doi:10.1029/2001JD001220.

Weber, R., D. Orsini, A. Sullivan, M. Bergin, C.S. Kiang, M. Chang, Y.N. Lee, P. Dasgupta, J. Slanina, B. Turpin, E. Edgerton, S. Hering, G. Allen, P. Solomon, and W. Chameides. 2003b. Transient PM2.5 aerosol events in metro Atlanta: Implications for air quality and health. J. Air Waste Manage. Assoc. 53:84-91.

Zhang, J., W.L. Chameides, R. Weber, G. Cass, D. Orsini, E. Edgerton, P. Jongejan, and J. Slanina. 2002. Validity of thermodynamic equilibrium assumption for fine particulate composition: Nitrate and ammonium during the 1999 Atlanta Supersite Experiment. J. Geophys. Res. 108(D7), 8414, 10.1029/2001JD001592.

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2.6.3 Advances in Mass Spectroscopic Methods for Measuring Fine Particle Chemical Composition

Four particle mass spectrometers operated side-by-side during the Atlanta Supersite Study:

1) Particle Analysis by Laser Mass Spectrometer (PALMS) of the National Oceanic and Atmospheric Administration, 2) University of California at Riverside’s Aerosol Time-of-Flight

Mass Spectrometer (ATOFMS), 3) University of Delaware’s Rapid Single-Particle Mass Spectrometer II (RSMS-II), and 4) Aerodyne’s Aerosol Mass Spectrometer (AMS). Of these,

the PALMS and ATOFMS were fairly well established techniques, whereas the RSMS-II and

AMS were in earlier stages of development and thus the Atlanta Supersite Study provided a valuable opportunity to assess these instrument’s performance. Key findings are summarized

below.

• Particle sizes were measured most accurately with ATOFMS, RSMS-II, and AMS. The RSMS-II system can obtain composition information on individual particles as small as 15 nm. The three systems that utilize laser desorption/ionization, (PALMS, ATOFMS, and RSMS-II), produce mass spectra that are qualitative and representative of individual particles. The AMS instrument, which uses a two-step volatilization on a heated surface and ionization by electron impact, can produce quantitative results representative of the ensemble of particles measured.

• Single-particle positive ion classifications from the Atlanta data by the laser-based instruments are broadly consistent and revealed similar trends as a function of size for organic, sulfate, and mineral particles. The AMS, which is the most quantitative of the mass spectrometers compared, had nitrate to sulfate molar ratios that were highly correlated with those of the semi-continuous instruments discussed above. Based on insights from the Atlanta study, subsequent studies, such as those undertaken at the New York EPA Supersite in the summer of 2002 (PEMTACS), demonstrated the quantitative measurement capabilities of the AMS. Overall, the strength and primary focus of the laser-based instruments are their ability to find associations between chemical species in individual particles with high time resolution.

KEY CITATIONS: Drewnick, F., J.J. Schwab, O. Hogrefe, S. Peters, L. Husain, D. Diamond, R. Weber, and K. Demerjian. 2003.

Intercomparison and evaluation of four semi-continuous PM2.5 sulfate instruments. Atmos. Environ. 37(2003):3335-3350.

Middlebrook, A.M., D.M. Murphy, S.-H. Lee, D.S. Thomson, K.A. Prather, R.J. Wenzel, D.-Y. Liu, D.J. Phares, K.P. Rhoads, A.S. Wexler, M.V. Johnston, J.L. Jimenez, J.T. Jayne, D.R. Worsnop, I. Yourshaw, J.H. Seinfeld, and R.C. Flagan. 2003. A comparison of particle mass spectrometers during the 1999 Atlanta Supersite Project. J. Geophys. Res. 108(D7), 8424, doi:10.1029/2001JD000660.

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2.6.4 Advances in Methods for Measuring Fine Particle Density A new technique that measures the density of aerosol particles in the diameter range 0.1 to 0.3

um was developed and used during the Atlanta Supersite Study. The technique offers an

alternative to calculating density from measured aerosol composition. Particles are selected based on known electrical mobility and then measurements of their mass are made with an

aerosol particle mass analyzer; density is determined based on geometric diameter, which is

equal to electrical mobility equivalent diameter for spherical particles, and mass. • Particles measured in August 1999 in urban Atlanta typically included a major

mass peak with a density in the ~1.5 to 1.7 g cm-3 range at 3-6% relative humidity. These data agreed with calculated densities based on measured size-resolved composition within about 5%.

KEY CITATION: McMurry, P.H., X. Wang, K. Park, and K. Ehara. 2002. The relationship between mass and mobility for atmospheric

particles: A new technique for measuring particle density. Aerosol Sci. Techol. 36:227-238.

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2.7. Gas Phase Measurement Technologies - Development and Intercomparison of Techniques

Chemical ionization & proton transfer mass spectrometry

Odd hydrogen radical measurements

NMHC measurements Formaldehyde measurements

Carbonyl measurements NO2 and NOy measurements

Organic nitrate measurements CO measurements

2.7. GAS PHASE MEASUREMENT TECHNOLOGIES - DEVELOPMENT AND INTERCOMPARISON OF TECHNIQUES

Eric Apel and David Parrish

Full understanding of the photochemistry that

produces ozone and PM2.5 requires ambient

measurements of the precursors, intermediates (radicals

as well as more stable species), and products. These

measurements must be made with high and well-defined

sensitivity, accuracy, precision, and specificity, and with

fast time response (particularly ~ 1 Hz for aircraft

measurements). The achievement of these goals is an

evolutionary process requiring careful instrument

development and operation in the field. This section

summarizes the major progress of the SOS research

program in this area.

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2.7.1 Major Advance: Chemical Ionization and Proton Transfer Mass Spectrometry The combination of mass spectrometry with chemical ionization (CIMS) potentially provides

a sensitive and specific measurement technique for many atmospheric species. SOS-sponsored

work includes the development, testing, and deployment of CIMS techniques for the

measurement of HNO3, isoprene, and ammonia as well as the deployment and testing of proton

transfer (PTR-MS) instruments. The PTR-MS instruments can detect most gas-phase organic

species with excellent time response, but their sensitivity and specificity are limited for many

compounds by the manifold of organic species and fragment ions with similar masses.

Highlights of this work are summarized below.

• Specific HNO3 Measurement Developed. A CIMS instrument to measure gas-phase HNO3 was developed and demonstrated to be sensitive with fast response (detection limit of approximately 10 pptv at 1 Hz), accurate, precise, and interference-free. It has been tested in a ground-based intercomparison and deployed on aircraft during SOS 1999 and 2000 field intensives.

• PTR-MS Deployed and Tested. PTR-MS instruments have been deployed at ground sites and on aircraft along with other instruments for measurement of several organic species. Intercomparison of the coincident measurements will help to define the capabilities of the PTR-MS instruments.

• CIMS Isoprene and Ammonia Measurements Developed. CIMS techniques for measurement of isoprene and ammonia have been developed and tested in ground-based intercomparisons. These techniques promise to provide sensitive and fast aircraft measurements of those species. The selectivity of the isoprene technique must be tested by comparison to other techniques.

KEY CITATIONS: Huey, L.G., E.J. Dunlea, E.R. Lovejoy, D.R. Hanson, R.B. Norton, F.C. Fehsenfeld, and C.J. Howard. 1998.

Fast time response measurement of HNO3 in air with a chemical ionization mass spectrometer. J. Geophys. Res. 103:3355-3360.

Fehsenfeld, F.C., L.G. Huey, D.T. Sueper, R.B. Norton, E.J. Williams, F.L. Eisele, R.L Mauldin, III, and D.J. Tanner. 1998. Ground-based intercomparison of nitric acid measurement techniques. J. Geophys. Res. 103:3343-3353.

Leibrock, E., and L.G. Huey. 2000. Ion chemistry for the detection of isoprene and other volatile organic compounds in ambient air. Geophys. Res. Letters 19:1763-1766.

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2.7.2 Major Advances: Odd Hydrogen Radical Techniques The hydroxyl radical (OH), the main oxidant in atmospheric chemistry, cycles rapidly with

the hydroperoxyl radical (HO2), initiating the production of ozone and other pollutants. A now

widely accepted measurement technique for OH and HO2, together called HOX, is laser-induced

fluorescence in detection chambers at low pressure. This technique was applied for the first time

to continuous, 24-hour measurements on 10-meter tall towers in Nashville in 1999 and at

TEXAQS 2000 in Houston. In addition, a new, unique instrument was developed to measure the

OH loss rate due to reactions with other atmospheric chemicals. This new instrument, the Total

OH Loss-rate Measurement (TOHLM), was successfully deployed for the first time in Nashville

in 1999. The combination of HOX measurements and TOHLM provides powerful new

diagnostics for understanding and testing the oxidation chemistry of any environment.

• First continuous OH and HO2 measurements in urban environments. The measurements, when compared to models, test the fundamental atmospheric chemistry that underpins chemical transport models. For Nashville SOS, OH and HO2 measurements agree to within a factor of two with model calculations near midday, but tend to be larger than models in the evening, at night, and for periods when nitrogen oxides are especially abundant. These observations indicate unidentified HOx sources and questions about HOx-NOx chemistry.

• Total OH Loss-rate Measurement tests the completeness of measured VOC inventories. The presence of unmeasured VOC is indicated if TOHLM-measured OH loss rates are greater than those calculated from the sum of VOC measurements and OH reaction rate coefficients. Preliminary Nashville observations indicate that OH loss rates are about twice those calculated, suggesting unmeasured VOC.

KEY CITATION: Kovacs, T.A. and W.H. Brune. 2000. Total OH loss rate measurement. J. Atmos. Chem. 39(2):105-122,

doi:10.1023/A:1010614113786.

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2.7.3 Non-Methane Hydrocarbons (NMHCs): Intercomparisons of Techniques NMHCs, emitted from a variety of sources, are precursors to ozone and PM2.5, and some

species are considered toxins. NMHCs are the primary photochemical fuel in urban and many

rural areas whereas carbon monoxide and methane play this role in regions remote from sources.

Biogenic NMHCs dominate the VOC chemistry in many highly forested regions and even in

urban areas situated in regions surrounded by forests, as is the case in most cities located in the

SOS domain (southeastern U.S.) [e.g., Chameides et al., 1997]. Anthropogenic NMHCs

dominate in most other urban areas.

• Accuracy of NMHC Measurements Tested. Because of the large role of NMHCs in ozone formation, it is imperative that measurements accurately reflect the true atmospheric composition. SOS has taken a leadership role in the atmospheric science community and has partnered with the NOAA Climate and Global Change Program in conducting “The Non Methane Hydrocarbon InterComparison Experiment” (NOMHICE). This experiment was designed to assess the accuracy of analytical methods used to determine mixing ratios of atmospheric non-methane hydrocarbons (NMHCs).

• NMHC Measurements Intercompared in the SOS Network. To help ensure quality measurements and to understand where there are problems, intercomparisons of NMHC measurements are conducted in the SOS network before and during every field study. Prior to the Nashville 99 Field Study nine measurements were intercompared. For each measurement, a set of canisters were simultaneously collected at the Cornelia Ft. site in Nashville, TN and distributed to 4 groups for analysis. Figure 2.6.1 shows the mean ratio of each laboratory’s analysis of a given compound to that of the reference laboratory (EPA-Lonneman). It is apparent that for some species, the accuracy of the analyses is good (agreement to within 11% for all groups), but for some species there were errors up to a factor of 5. Some of the problems could be corrected during the experiment. For example, the discrepancy of the Argonne data for i-pentane was determined to be due to an overlap with another peak and corrected. The precision of the measurements of the individual species by the different laboratories is indicated by the standard deviations of the ratios. These standard deviations ranged from very good (±3%) to quite poor (factor of 3).

• High Quality Multi-Component Standards Developed. Eight identical standards containing 100 NMHC were developed and tested for the TexasAQS 2000 study. These standards were used as a common calibration source for all participants in the study.

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KEY CITATIONS: Chameides, W.L., R.D. Saylor, and E.B. Cowling. 1997. Ozone pollution in the rural United States and the new

NAAQS. Policy Forum. Science 276:916. Apel, E.C., J.G. Calvert, T.M. Gilpin, F.C. Fehsenfeld, D.D. Parrish, and W.A. Lonneman. 1999. The Non-

Methane Hydrocarbon Intercomparison Experiment (NOMHICE): Task 3. J. Geophys. Res. 104:26069-26086.

30 June 2004 | State of SOS-3: 1995 - 2003 | 104

2.7.4 Formaldehyde Technique Development and Intercomparisons Formaldehyde (CH2O) is a primary emission product from internal combustion engines and

is produced in the atmosphere by the photochemical oxidation of methane and non-methane

hydrocarbons (NMHCs). It is the most abundant gas-phase carbonyl compound in both urban

areas and the remote troposphere. Formaldehyde is extensively connected with the odd

hydrogen (HOx = H+OH+HO2) and odd nitrogen (NOx =NO + NO2) chemical cycles. It is also a

major source for HO2 and for CO in air not strongly perturbed by anthropogenic sources.

Consequently, accurate measurements of formaldehyde are critical to understanding the overall

tropospheric chemistry leading to hydrocarbon oxidation, the processes controlling the odd

hydrogen cycles and nitrogen cycles, and the global budgets of OH and CO.

• Two Measurement Techniques Developed. SOS has encouraged the development of fast and sensitive techniques to measure formaldehyde. Two techniques, tunable diode laser absorption spectroscopy (TDLAS) and coil/2,4-dinitrophenylhydrazine (CDNPH), have emerged which satisfy the necessary criteria for ground-based and aircraft-based measurements. These techniques have been used extensively in SOS field studies. The TDLAS technique can provide 1-second averages, while the CDNPH technique has been limited to averages of at least 1 minute.

• Two Field Intercomparisons Completed. A blind intercomparison of six ambient formaldehyde measurement techniques was conducted at the National Center for Atmospheric Research (NCAR) in Boulder, CO, from May 29 to June 3, 1995. It was concluded that gas phase standards should be employed with any of the measurement techniques, and the cartridge measurement methods are limited by long collection periods, generally lower precision, and the incomplete understanding of potential interferences from ozone and possibly other compounds. Airborne CH2O measurements by TDLAS and CDNPH techniques indicated that, on average, both instruments measured identical ambient CH2O concentrations to better than 0.1-ppbv over the 0 to 0.8-ppbv-concentration range. However, significant differences, larger than the combined 2σ total uncertainty estimates, were observed in 29% of data set. It is clear that careful attention must be paid to the behavior of CH2O in the inlet for accurate airborne measurements.

• CH2O Gas Phase Standards Developed. Through SOS support, formaldehyde standards have been produced in high-pressure cylinders at the ppmv level. Techniques have been developed, including FTIR and GC-FID, to verify the concentration of the standard. Long-term stability tests are presently being conducted.

KEY CITATION: Gilpin, T., E. Apel, A. Fried, B. Wert, J. Calvert, Z. Genfa, P. Dasgupta, J.W. Harder, B. Heikes, B. Hopkins,

H. Westberg, T. Kleindienst, Y.-N. Lee, X. Zhou, W. Lonneman, and S. Sewell. 1997. Intercomparison of six ambient [HCHO] measurement techniques. J. Geophys. Res. 102:21,161-21,188.

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2.7.5 Carbonyl Technique Development and Intercomparisons Aldehydes and ketones and other oxygenates are produced by the oxidation of hydrocarbons,

and some are emitted directly. Until recently, this compound class has received much less

attention than its VOC counterpart, the NMHCs. This is largely due to difficulties encountered

in measuring these compounds. SOS has taken a leading role in developing carbonyl standards,

carbonyl measurement techniques, and evaluating the techniques with intercomparisons.

Through these studies, it is becoming apparent that the carbonyls and other oxygenates may be

more ubiquitous than previously thought, and hence contribute significantly to photochemical

processes in the troposphere.

• The first quantified and verified carbonyl standards. Carbonyl standards have been prepared gravimetrically with both calibrated permeation sources and in specially treated aluminum cylinders. Techniques such as atomic emission detection (AED) (Apel et al., 1998a) and FTIR have been applied to verify the accuracy of the standards. Table 2.6.1. demonstrates the verification of a prepared standard.

Table 2.6.1. Quantification of Standards Results Compound High Pressure Cylinder

gravimetric (ppm) High Pressure Cylinder

analyzed (ppm)* FTIR (ppm)

Methanol 3.00 ± 0.03 3.0 ± 0.1 2.9 ± 0.2 Ethanol 3.02 ± 0.03 n.d. 2.9 ± 0.3 Acetone 3.03 ± 0.03 3.03 ± 0.08 3.2 ± 0.2 MEK 3.04 ± 0.03 n.d 2.5 ± 0.3 Acetaldehyde 3.03 ± 0.03 n.d 3.2 ± 0.4 Propanal 3.03 ± 0.03 n.d n.d. Butanal 3.02 ± 0.03 n.d n.d.

*analysis based on calibration factors from permeation tubes n.d. - not determined • Intercomparison completed. Cartridge–based (Si-Gel and C18) and GC-MS

measurements have been intercompared through SOS. Serious discrepancies were found and more work is needed to resolve these differences.

• New techniques developed. A new relatively fast-response (15 minute cycle) GC-FID technique has been developed to measure carbonyls and other oxygenates aboard aircraft. A new GC-MS technique is currently being developed to measure carbonyls with a 5-minute time response.

KEY CITATIONS: Apel, E.C., J.G. Calvert, J.P. Greenberg, D. Riemer, R. Zika, T.E. Kleindienst, W.A. Lonneman, K. Fung,

and E. Fujita. 1998a. Generation and validation of oxygenated volatile organic carbon standards for the 1995 Southern Oxidants Study Nashville Intensive. J. Geophys. Res. 103:22,281-22,294.

Apel, E.C., J.G. Calvert, D. Riemer, W. Pos, R. Zika, T.E. Kleindienst, W.A. Lonneman, K. Fung, E. Fujita, P.B. Shepson, T.K. Starn, and P.T. Roberts. 1998b. Measurements comparison of oxygenated volatile organic compounds at a rural site during the 1995 SOS Nashville Intensive. J. Geophys. Res. 103:22,295-22,316.

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2.7.6 NO2 and NOy Technique Development and Intercomparisons. The oxides of nitrogen (NO and NO2) are precursors of ozone and PM2.5, and the total

oxidized nitrogen family (NOy) in an air parcel represents the total emissions of these precursors

that remain in the atmosphere. Improvements in the measurement of these species are

summarized below.

• Improved NO2 measurements by photolysis-chemiluminescence. A new photolytic converter utilizing the focused UV output from a high-pressure mercury (Hg) arc lamp was developed and tested. The new configuration permits simple and accurate retrieval of ambient NO2 data at very high time resolution, is more specific, provides increased sensitivity, and is less expensive to operate than previous photolytic converter designs.

• Validation of an aircraft inlet for HNO3 and NOy. Rapid and quantitative sampling of NOy species, including HNO3, has been demonstrated using a short, heated Teflon inlet. In flight, standard addition calibrations of HNO3 at the aircraft inlet demonstrate freedom from significant surface adsorption of HNO3, which has significantly compromised measurements through other aircraft inlets.

• Intercomparisons of ground-based NO2 and NOY measurements. Intercomparisons during SOS field intensives have demonstrated that laser-induced fluorescence, differential optical absorption spectroscopy, and photolysis-chemiluminescence techniques are all capable of accurately quantifying atmospheric NO2 above 1 ppbv. Further, MoO and Au converters were shown to be capable of accurately measuring NOy above 2 ppbv in the urban and suburban environments typical of the SOS region. These studies further concluded that generation of reliable NO2 or NOy data still demands skilled operators and dedicated, critical oversight during the measurement process.

KEY CITATIONS: Ryerson, T. B., E.J. Williams, and F.C. Fehsenfeld. 2000. An efficient photolysis system for fast-response NO2

measurements,. J. Geophys. Res. 105:26,447-26,461. Ryerson, T.B., L.G. Huey, K. Knapp, J.A. Neuman, D.D. Parrish, D.T. Sueper, and F.C. Fehsenfeld. 1999.

Design and initial characterization of an inlet for gas-phase NOy measurements from aircraft, J. Geophys. Res. 104:5483-5492.

Williams, E.J., K. Baumann, J.M. Roberts, S.B. Bertman, R.B. Norton, F.C. Fehsenfeld, S.R. Springston, L.J. Nunnermacker, L. Newman, K. Olszyna, J. Meagher, B. Hartsell, E. Edgerton, J.R. Pearson, and M.O. Rodgers. 1998. Intercomparison of ground-based NOy measurement techniques. J. Geophys. Res. 103:22,261-22,280.

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2.7.7 Organic Nitrate Technique Development Several advances have been made in the measurements of organic nitrates through the course

of SOS. Emphasis has been put on improving separation and quantitation of compounds that are

of biogenic origin, and on developing rapid methods for aircraft measurements.

• Measurements of peroxyacyl nitrates (PANs). Gas chromatographic methods for PANs have been refined to provide rapid and sensitive measurement. Aircraft-based instrumentation was developed to measure four of the major compounds of interest, PAN, PPN, PiBN, and MPAN, every 3.5 min. The measurement of PANs by proton-transfer reaction mass spectrometry (PTR-MS) was deployed during the Nashville 99 intensive (Hansel and Wisthaler, 2000). While still in the development stage, this method has the potential to provide rapid (10 sec) measurements of PAN aboard aircraft.

• Development of PAN calibration systems. Two different calibration methods for PAN have been developed: a diffusion source and photochemical production of PAN in acetone/air/CO/NO mixtures. The diffusion system relies on an NOy measurement for calibration, while the photochemical source relies on a known, efficient conversion of an NO standard to PAN. The two were compared during the TEXAQS 2000 study and were found to agree within 5%.

• Development of organic nitrate measurements. An automated system for the measurement of the organic nitrates produced from OH radical attack on isoprene was developed and deployed at the Dickson site during SOS 99. These compounds result when the peroxy radicals derived from OH reaction with isoprene, react with NO to produce a set of 8 isomeric RONO2 species. The maximum concentrations of the sum of these species were in the 100-200 pptv range, much higher than observed in a previous study. Comparison of these two data sets provides a good opportunity to examine the NOx-dependence of this aspect of isoprene photochemistry.

KEY CITATIONS: Williams, J., J.M. Roberts, S.B. Bertman, C.A. Stroud, F.C Fehsenfeld, K. Baumann, M.P. Buhr, K. Knapp,

P.C. Murphy, M. Nowick, and E.J. Williams. 2000. A method for the airborne measurement of PAN, PPN and MPAN. J. Geophys. Res. 105(D23):28,943–28,960.

Hansel, A. and A. Wisthaler. 2000. A method for real-time detection of PAN, PPN, and MPAN in ambient air. Geophys. Res. Lett. 27:895-898.

Grossenbacher, J.W., P.B. Shepson, T. Thornberry, M. Witmer-Rich, M.A., Carroll, I. Faloona, D. Tan, W. Brune, E. Apel, D. Riemer, and H.A. Westberg. 2000. Measurements of isoprene nitrates above a forest canopy. J. Geophys. Res. 109, D11311, doi:10.1029/2003JD003966.

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2.7.8 Carbon Monoxide Technique Development and Intercomparisons Carbon monoxide is a long-lived gas primarily emitted in automobile exhaust, which makes

it a useful tracer for urban pollution plumes. The result of an SOS effort to develop an

instrument for aircraft measurements of this species is summarized below.

• Development of an instrument based upon vacuum UV resonance fluorescence of CO. An instrument was developed that is capable of fast (~1 Hz), accurate (~5%), and precise (~1 ppbv) measurement of CO from an aircraft platform. Intercomparisons with other techniques demonstrate that it is highly specific with no identified interferences (see Figure 2.7.2).

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Figure 2.7.2. Time series of coincident 5-second-average measurements of CO. The solid lines give tunable diode laser absorption (darker) and the vacuum ultraviolet fluorescence (lighter) results and the dotted line indicates the aircraft altitude. The features labeled 1 through 4 are intercepted urban plumes. KEY CITATION: Holloway, J. S., R.O. Jacoubek, D.D. Parrish, C. Gerbig, A. Volz-Thomas, S. Schmitgen, A. Fried, B. Wert, B.

Henry, and J.R. Drummond. 2000. Airborne intercomparison of vacuum ultraviolet fluorescence and tunable diode laser absorption measurements of tropospheric carbon monoxide. J. Geophys. Res. 105:24,251-24,261.

30 June 2004 | State of SOS-3: 1995 - 2003 | 109

2.8. Atmospheric Dynamics and Mixing on Urban and Regional Scales

Daytime transport processes Nighttime transport processes

Variations in mixing height Vertical distribution of

ozone, precursors, and aerosols

Off- to on-shore flow reversal Heterogeneity of industrial

plumes

2.8. ATMOSPHERIC DYNAMICS AND MIXING ON URBAN AND REGIONAL SCALES

Allen White and Christoph Senff

While a thorough understanding of the atmospheric

chemistry associated with the formation and destruction

of atmospheric pollutants is critical in air quality

research, knowledge of atmospheric dynamics and

mixing processes is equally important to gain a full

understanding of the problem. Vertical and horizontal

transport mechanisms as well as the temporal and spatial

evolution of mixed layer height often play a critical role

in the distribution of pollutants on urban and regional

scales. In this section, we summarize key scientific

findings from the SOS’ Nashville ’95 and Nashville ‘99

studies, and TexAQS 2000 that pertain to atmospheric dynamics and mixing processes.

2.8.1. Daytime Transport Processes During SOS field studies, a number of meteorological processes contributed to horizontal

and vertical transport of pollutants during the daytime. A schematic summarizing many of these

processes is shown in Fig. 2.8.1. SOS used a variety of ground-based and airborne observing

systems to study transport mechanisms. Key findings are summarized below.

• Horizontal Advection. We found that synoptically driven winds were the dominant daytime horizontal transport mechanism. Mesoscale circulations caused by topography or land use differences also contribute to daytime transport. During TexAQS 2000, synoptic flow exported the Houston/Ship Channel and Texas City pollution plumes to rural, source-free areas, resulting in ozone concentrations well above the ozone standard far downwind of the Houston metropolitan area. Many of these ozone exceedances were missed by the surface monitoring network due to sparse network coverage in rural areas.

• Boundary-layer Venting. Under light wind conditions, we observed substantial (~40%) horizontal variations in daytime mixing height due to the urban-rural contrast in the surface energy balance (the “urban dome”). The dome allowed venting of urban emissions aloft, making them available for horizontal transport but unavailable for vertical mixing downwind of the dome during the day (refer to D in Fig. 2.8.1).

• Convection. Cumulus clouds vented pollutants from the boundary layer and reduced the sunlight available for photochemistry. Because direct measurements of cumulus venting are difficult to obtain experimentally, we were unable to quantify this process during SOS’ Nashville ‘95 and Nashville ’99 studies or during TexAQS 2000. Deep vertical

30 June 2004 | State of SOS-3: 1995 - 2003 | 110

mixing associated with convective storms may have resulted in stratosphere/troposphere exchange.

Figure 2.8.1. Transport between the surface and the atmospheric boundary layer (ABL) and between the ABL and free troposphere: A. fluxes from nearly homogeneous surfaces to ABL, B. fluxes from inhomogeneous surfaces to ABL, C. transport across capping inversion (Zi) by entrainment/detrainment and cumulus venting, and D. direct injection to troposphere caused by horizontal variations in boundary-layer depth.

• Subsidence. Synoptic scale subsidence associated with high pressure strengthened the boundary-layer capping inversion, thereby inhibiting vertical transport of momentum and pollutants and cumulus convection. This behavior, combined with the stagnant conditions resulting from relaxation of the synoptic-scale pressure gradient, allowed pollutants to accumulate locally during the day (Banta et al., 1998).

• Morning Transition. The morning transition caused photochemically aged pollutants from the residual layer to interact with pollutants emitted at night into the shallow nocturnal boundary layer. During the 1999 SOS Nashville Intensive, the breakup of the nocturnal inversion occurred at an urban site 1-2 h earlier than at three rural sites. In the humid environment in the SOS field studies, surface water vapor mixing ratio was often an excellent meteorological tracer for the timing of the morning transition (Fig. 2.8.2).

Figure 2.8.2. 1-min time series of CO and NOy concentrations, the concentration ratio of NOx to NOy, and water vapor mixing ratio (rmix) measured near the surface on 15 July 1999 at the Dickson, Tennessee site. The early morning increase in rmix is due to surface evaporation. The sharp transition in rmix near 0920 CDT occurs after the nocturnal inversion is fully eroded and as the mixed layer grows rapidly through the residual layer and entrains drier air from aloft. The CO and NOy concentrations decrease because nocturnal surface emissions of these gases were confined to the shallow nocturnal boundary layer. The NOx/NOy ratio decreases because the residual layer contains photochemically aged air from the previous day. KEY CITATIONS: Banta, R.M., C.J. Senff, A.B. White, M. Trainer, R.T. McNider, R.J. Valente, S.D. Mayor, R.J. Alvarez, R.M.

Hardesty, D. Parrish, and F.C. Fehsenfeld. 1998. Daytime buildup and nighttime transport of urban ozone in the boundary layer during a stagnation episode. J. Geophys. Res. 103:22,519–22,544.

White, A.B., B.D. Templeman, W.M. Angevine, R.J. Zamora, W.W. King, C.A. Russell, R.M. Banta, W.A. Brewer, and K.J. Olszyna. 2002. Regional contrast in morning transitions observed during the 1999 Southern Oxidants Study Nashville/Middle Tennessee Intensive. J. Geophys. Res. 107(D23), 4726, doi:10.1029/2001JD002036.

30 June 2004 | State of SOS-3: 1995 - 2003 | 111

2.8.2. Nighttime Transport Processes During the night, the effect of surface friction is confined to a shallow layer (10s of meters)

near the surface. In general, vertical motions not associated with convective storms are

considerably weaker at night than during the day. Thus, during SOS, nocturnal transport tends to

redistribute pollutants horizontally rather than vertically. Key features are summarized below.

• Low-Level Jet. At night, the winds above a shallow (10s of meters) layer at the surface accelerated as the atmosphere decouples from the surface.

• Inertial Oscillation. The nocturnal winds rotated in time in accordance with the principles of the inertial oscillation. McNider et al. (1998) demonstrated the persistent nature of this phenomenon using wind spectra obtained from wind profilers deployed during the SOS95 Nashville Intensive. Under sufficiently weak synoptic forcing, the low-level jet and inertial oscillation dominated nocturnal transport. Trajectories derived from the wind profiler network deployed during SOS95 demonstrated the combined effect of these important features (see Fig. 2.8.3).

• Vertical transport is suppressed at night. In the absence of convective storms, the atmosphere stabilized at night, which suppressed any significant vertical transport. In many cases, intermittent turbulence has been observed in the nocturnal boundary layer, which may be linked to wind shear associated with the low-level jet. The effect of intermittent turbulence on pollution levels at the surface is an important topic of current SOS research.

Figure 2.8.3. Overnight forward trajectories calculated from the network of 915-MHz boundary-layer wind profilers deployed during SOS95. Winds were averaged over the 400-m vertical intervals shown in the key. Trajectories were calculated from an origin centered on Nashville, at 36.2Ε N, 86.8Ε W (after Banta et al., 1998).

KEY CITATIONS: Banta, R.M., C.J. Senff, A.B. White, M. Trainer, R.T. McNider, R.J. Valente, S.D. Mayor, R.J. Alvarez, R.M.

Hardesty, D. Parrish, and F.C. Fehsenfeld. 1998. Daytime buildup and nighttime transport of urban ozone in the boundary layer during a stagnation episode. J. Geophys. Res. 103:22,519–22,544.

McNider, R.T., W.B. Norris, R.I. Clymer, S. Gupta, R.M. Banta, R.J. Zamora, A.B. White, and M. Trainer. 1998. Meteorological conditions during the 1995 Southern Oxidants Study Nashville/Middle Tennessee Field Intensive. J. Geophys. Res. 103:22,225–22,243.

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30 June 2004 | State of SOS-3: 1995 - 2003 | 112

2.8.3. Variations in Mixing Height During the SOS95 and SOS99 studies, it was found that the daytime mixing height can vary

considerably in the Nashville, TN area, especially when comparing the urban and adjacent rural

areas. As mixing height is an important parameter affecting air pollution concentrations, this

finding has significant implications for the regional distribution of ozone and particulates. Key

results are summarized below.

• Remote sensors provide reliable measurements of mixing height. A comparison of mixed layer depth estimates deduced from wind profiler and airborne lidar data showed very good agreement under clear or partly cloudy conditions (White et al., 1999). This result confirmed that radar wind profilers and lidars are well suited to measure the depth of the mixed layer and its variability.

• Mixing height variability is tied to land use differences. Variations in mixing height are related to differences in surface characteristics, such as soil and vegetation type as well as surface moisture (see Fig. 2.8.4). During SOS99 Nashville Intensive, these different surface characteristics were reflected in varying energy, ozone, and carbon fluxes at the surface.

Figure 2.8.4. Time height cross section of aerosol backscatter gradient indicating the top of the mixed layer measured with the NOAA/ETL airborne lidar during midday on 12 July 1995 during a northwest to southeast transect over Nashville, TN (city of Nashville is in center of panel). The urban heat island over Nashville is clearly visible at flight times near 11:30 LST. Over forested areas to the northwest of Nashville (left side of panel) the mixed layer is strongly suppressed, while over suburban and agricultural terrain to the southeast (right side of panel) the mixed layer is only slightly shallower than over the city.

• Differences in mixing height are most pronounced under light wind conditions. Under stagnant conditions, air parcels tend to dwell over regions of one surface type, which allows surface heating differences to express themselves as variations in mixing height. Stronger flow moves air parcels over many surface types, thus producing a more uniform mixing height

• Urban heat island. The strong differences in surface heating between the Nashville urban area and the surrounding agricultural and forested areas resulted in significantly deeper mixed layers over the city, especially under stagnant conditions. We frequently observed urban mixing heights of 2 km or more, which was as much as 800 m higher than the mixing heights over adjacent rural areas (see Figs. 2.8.4 and 2.8.5).

30 June 2004 | State of SOS-3: 1995 - 2003 | 113

Figure 2.8.5. Hourly measurements of mixed layer depth on 4 July 1999 from the wind profiler network deployed in and around Nashville, TN during SOS 99. The wind profiler at Cornelia Fort airport shows a maximum mixed layer depth of about 2.1 km while all other profilers deployed in rural areas around Nashville detect peak mixed layer heights of only about 1.5 km.

Time (LST) • Ozone concentrations are anti-correlated with mixing height. Peak ozone

concentrations in the Houston/Ship Channel pollution plume downwind of the sources were found to be anti-correlated with mixing height. In the Houston area, mixing depth typically increases with distance away from the coast. Thus, transport of the Houston/Ship Channel pollution plume to coastal areas tends to produce higher ozone peak values than transport to inland areas.

• Model prediction of mixing depth. We found that the Pennsylvania State National Center for Atmospheric Research Mesoscale Model 5 (PSU/NCAR MM5) had difficulty accurately predicting mixing depths under both stable and unstable atmospheric conditions. Deficiencies in the atmospheric radiation parameterizations caused excess energy in the model’s surface layer (see Fig. 2.8.6). MM5’s response to this additional input resulted in daytime mixed layers that were too deep.

Figure 2.8.6. Solar radiative flux predicted by MM5 (line) and measured with a spectral pyranometer (asterisks) at New Hendersonville, Tennessee during SOS95. The model bias leads to model errors in the surface energy budget which impact the depth and strength of vertical mixing and thermally driven circulations such as the land-sea breeze. KEY CITATIONS: Angevine, W.M., A.B. White, C.J. Senff, M. Trainer, R.M. Banta, and M.A. Ayoub. 2003. Urban-rural contrasts

in mixing height and cloudiness over Nashville in 1999. J. Geophys. Res. 108(D3), 4092, doi:1029/2001DJ0001061.

White, A.B., C.J. Senff, and R.M. Banta. 1999. A comparison of mixing depths observed by ground-based wind profilers and an airborne lidar. J. Atmos. Oceanic Technol. 16, No. 5:584-590.

Zamora, R.J., S. Solomon, E.G. Dutton, J.W. Bao, M. Trainer, R.W. Portmann, A.B. White, D.W. Nelson, and R.T. McNider. 2003. Comparing MM5 radiative fluxes with observations gathered during the 1995 and 1999 Nashville Southern Oxidants Studies. J. Geophys. Res. 108(D2), 4050, doi:10.1029/2002JD002122.

30 June 2004 | State of SOS-3: 1995 - 2003 | 114

2.8.4. Vertical Distribution of Ozone, Precursors, and Aerosols The vertical distribution of ozone, precursors, and aerosols is influenced to a large extent by

the dynamical processes described above. The key findings are:

• The vertical distribution of pollutants in the daytime mixed layer. Mixing processes due to convective or mechanical turbulence acted to smooth out vertical inhomogeneities in the daytime boundary layer. Figure 2.8.7 shows a high-ozone layer near the surface mixing vertically as convective turbulence increased over the course of the morning.

Figure 2.8.7. Series of vertical profiles of ozone concentration measured with the NOAA/ETL airborne ozone lidar on the morning of 12 July 1995. The ozone profiles are each spaced about 50 min apart, starting at 7:30 CDT and show the vertical redistribution of ozone as the mixed layer grows.

• The vertical distribution of pollutants at night. Due to a lack of vertical mixing (see

Section 2.8.2) pollutants tended to form horizontal layers or patches that persisted throughout the night until they were mixed out by the growing boundary layer the next morning. Fig. 2.8.8 depicts the cross section of a power plant plume that had been injected into the stabilizing evening atmosphere. In the absence of any significant vertical mixing the power plant plume stayed confined to a thin vertical layer.

Figure 2.8.8. Cross section of Cumberland power plant plume measured with the NOAA/ETL airborne ozone lidar on the evening of 4 July 1999 at about 10 km downwind of the power plant. The plume is easily identified by its low-ozone signature. Note that the plume is confined to the layer between 800 and 1200 m ASL.

30 June 2004 | State of SOS-3: 1995 - 2003 | 115

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• Pollutant concentrations in the free troposphere are affected by long-range transport or stratosphere-troposphere exchange processes. Ozone sonde and aircraft measurements from the Nashville ‘95 and ‘99 campaigns showed that concentrations of ozone and other pollutants in the free troposphere were highly variable and were primarily affected by regional to continental-scale advection of clean or polluted air masses. Another significant process contributing to high tropospheric ozone concentrations is the intrusion of stratospheric air. Through entrainment processes (see Section 2.8.1) pollutant concentrations in the lower free troposphere can impact the air quality in the atmospheric boundary layer and at the surface.

• The improper treatment of aerosols in models contributed to forecast errors in solar radiation reaching the surface. Aerosol absorption and scattering reduce the amount of sunlight that reaches the Earth’s surface. During the 2002 New England Air Quality Study, the correlation between observed aerosol optical depth and incoming solar radiation measured at Durham, NH was greater than the correlation between observed aerosol optical depth and predicted incoming solar radiation from the ETA model (see Fig. 2.8.9). The model also was unable to produce the observed slope for the line of regression between these two variables. This improper treatment of aerosols in the ETA model contributed to a large radiation bias (see also Fig. 2.8.6).

Figure 2.8.9. Correlation between observed solar irradiance and aerosol optical depth measured at Thompson Farm in Durham, NH during the 2002 New England Air Quality Study along with correlation between the observed optical depth and solar irradiance predicted by the ETA model.

KEY CITATIONS: Banta, R.M., C.J. Senff, A.B. White, M. Trainer, R.T. McNider, R.J. Valente, S.D. Mayor, R.J. Alvarez, R.M.

Hardesty, D. Parrish, and F.C. Fehsenfeld. 1998. Daytime buildup and nighttime transport of urban ozone in the boundary layer during a stagnation episode. J. Geophys. Res. 103:22,519–22,544.

Senff, C.J., R.M. Hardesty, R.J. Alvarez II, and S.D. Mayor. 1998. Airborne lidar characterization of power plant plumes during the 1995 Southern Oxidants Study. J. Geophys. Res. 103(D23):31,173-31,189.

Wotawa, G. and M. Trainer. 2000. The influence of Canadian forest fires on pollutant concentrations in the United States. Science 288:324-328.

Zamora, R.J., E.G. Dutton, M. Trainer, S.A. McKeen, J.M. Wilczak, and Y.-T. Hou. 2004. The accuracy of solar irradiance calculations used in medium range forecast models. Mon. Wea. Rev. (accepted for publication).

30 June 2004 | State of SOS-3: 1995 - 2003 | 116

2.8.5. Off- to On-Shore Flow Reversal In coastal areas, the land - sea breeze circulation can act to focus large concentrations of

ozone and other pollutants. Very high concentrations of pollutants can be expected when the

morning offshore flow is followed by a period of stagnant winds and the sea breeze recirculates

the aged pollutants released in the morning back over the sources areas. Key features are

summarized below.

• Off- to on-shore flow reversal was observed in Houston during TexAQS 2000 in conjunction with very large accumulations of ozone. Severe ozone exceedances on flow-reversal days were linked to a combination of two meteorological factors: (1) Light-wind conditions that allow buildup of ozone plumes over source areas during the middle of the day, and (2) Afternoon sea breeze that transports aged pollutants back over source areas, thus reinforcing the already high ozone concentrations (see Fig. 2.8.10). The distribution of pollutants and the severity of the ozone event depend on the morning offshore flow regime, the timing of the sea breeze onset, and the strength of the sea breeze.

Figure 2.8.10. Cross section of ozone measured with the NOAA/ETL airborne ozone lidar during TexAQS 2000 in the late afternoon on 30 August. Very high ozone concentrati