Visibility-reducing species in the denver “brown cloud”—I. Relationships between extinction and chemical composition

Armspheric Envmvmmr Vol. 15. No. 12, P&I 2473-2484. 1981 0@3&6981/81/122473-12 102.00/O Printed m Great Britam

0 1981 Pergamon Press Lid

VISIBILITY-REDUCING SPECIES IN THE DENVER “BROWN CLOUD”-4 RELATIONSHIPS BETWEEN EXTINCTION

AND CHEMICAL COMPOSITION

PETER J. GROBLICKI, GEORGE T. WOLFF and RICHARD J. COUNTESS*

Environmental Science Department, General Motors Research Laboratories, Warren, MI 48090, U.S.A.

(First receiued 23 October 1980 and infinalform 13 Jdy 1981)

Abstract-Simultaneous measurements of the extinction coefficient due to particle absorption and particle scattering were made for 41 consecutive days during November and December, 1978 in Denver, Colorado. The scattering coefficient was measured with integrating nephelometers operating at both ambient conditions and in a heated mode. In this manner, the scattering due to absorbed water could be distinguished from the scattering due to dry particles. The absorption coefficient was determined by the integrating plate method. In addition, 4-h measurements of sulfate, nitrate, ammonium, organic carbon, elemental carbon and fine particulate mass, as well as continuous measurements of nitrogen dioxide were made.

In Denver, on the average, the constant Rayleigh scattering is 7 “i, of the total extinction. The contributions to the variable part of the extinction are 7 % by NO2 absorption, 29 7; by particle absorption and 64% by scattering due to particulate matter plus associated water. If the water was removed from the aerosol, the light scattering by the aerosol would decline by 38 %.

Using multivariate statistical analyses, a model was developed which relates the concentrations of these species to the various components of the extinction coefficient. The model predicts extinction coefficients which are in excellent agreement with the observed values with a correlation coefficient of 0.97. According to the model, ammonium sulfate accounts for 20 % of the variable part of the visibility reduction, ammonium nitrate 17 %, organic compounds 13 %, elemental carbon 38 %, and other particulate matter 7 %. Absorption by NO* is responsible for 6 % of the visibility reduction. Thirty one per cent of the extinction is due to absorption of light by elemental carbon. Most of the visibility reduction is due to particles smaller than 2.5 pm, although a minor contribution is due to elemental carbon larger than 2.5 pm.

INTRODUCTION

The beautiful view of the mountains west of Denver is often obscured by the greyish-brown cloud of pollutants trapped by atmospheric temperature inversions. Denver’s location in the Platte River Valley is especially conducive to the formation of these inversions (Crow, 1976) which trap all pollutants including the small particles responsible for the visibility reduction. The 1978 Denver Haze Study (Wolff et al., 1980; Heisler et ai., 1980) acquired data on meteorological conditions, gaseous pollutants, aerosol chemical composition, aerosol physical properties and aerosol optical properties. This report describes instrumental measurements and statistical analyses used to characterize the aerosol’s contribution to scattering and absorption and the effect of humidity on light scattering.

The 1978 Denver Brown Cloud Study

In November and December of 1978, General Motors Research Laboratories (GMR) and the Motor Vehicle Manufacturers Association organized a study aimed at understanding the causes of the Denver brown cloud (Wolff et al., 1980; Heisler et al., 1980).

l Current a~Iiation: Environmen~l Research & Tech- nology, Westlake Village, California 91361, U.S.A.

All pollutant measurements in this paper and in part II (Wolff et ai., 1981) were made from 13 November to 23 December, 1978, at the GMR site located in a large, open parking lot of the Mile High Kennel Club in Commerce City, Colorado. The site (see Fig. 1) was located in the Platte River Valley about 6 km northeast of downtown Denver, between a heavily industrialized area and a residential area, and did not appear to be dominated by any single source. The site was typically downwind of most significant sources, including the

Fig. 1. Map of the Denver, Colorado area, showing location of GMR sampling site relative to major emission

sources.

2473

2474 PETER J. GROBLICKI er al.

city itself and all four power plants (Crow, 1976). Consequently, the pollutant samples collected at the GMR site are thought to be representative of both the primary and secondary pollutants which make up the Denver haze.

Details of the sampling program and the chemical analyses have been previously reported (Wolff et al., 1980, Fern-ran er aZ., 1981, Countess et al., 1980, Cadle et al., 1980 and Countess et al., 1981). Consequently, only a brief synopsis will be presented here.

Particulate samples were collected every 4-h for chemical speciation, with two pairs of Environmental Research & Technology (ERT) sequential filter sam- plers operating at a flow rate of approx. 70 P min - ’ (Heisler et al., 1978). One pair employed Ghia Teflon filters while the other employed pre-fired Gelman Micro-Quartz filters. Each pair consisted of one sampler designed to collect total suspended particulates (upper cut size about 3O~m) and one fitted with a cyclone to eliminate particles larger than 2.5pm (Heisler et al., 1978).

Sulfate and nitrate were determined from the samples on Teflon filters by ion chromatography while the ammonium ion content was determined by a spectrophotometric technique. Elemental analyses on the Teflon filters were done by X-ray fluorescence and neutron activation by Columbia Scientific and NEA Laboratories, respectively. Aerosol mass was also determined on the Teflon filters.

Both organic and elemental carbon were measured on the Micro-Quartz filters using the GMR carbon

analyzer described by Cadle et al. (1980). Appar- ent organic carbon, C,,, is the carbon that can be volatilized from a filter in flowing helium at 650” C and apparent elemental carbon, C,,, is that removed by flowing oxygen at 650” C. It is possible that elemental carbon may be overestimated if some organic carbon chars and produces elemental carbon (Cadle and Groblicki, 1981).

In addition to particulate sampling, a complete array of gaseous pollutants and meteorological parameters was measured continuously by GMR’s mobile Atmos- pheric Research Laboratory (ARL) and this is described by Ferman et al. (1981). Samples for gaseous ammonia determination were collected on two stacked oxalic acid-impregnated glass fiber filters which were preceded by a third filter to remove particles.

A complete listing of measured pollutant parameters and nomenclature is presented in Table 1.

In addition, particulate composition and light scattering and absorption were measured at four other sites by Environmental Research and Technology, Inc. (ERT) (Heisler et al., 1980). The ground based data were supplemented by aircraft data and photographic data to delineate the extent and appearance of the cloud. The data gathered at the GMR site is the basis for the analysis in this report.

Ch~~a~terizatio~ of’ aisibility

The light coming from a distant object is affected by four atmospheric processes: absorption by particles and gases and scattering by particles and gases. In this

Table 1. Summary of certain key parameters measured at the GMR site

Frequency of Instrumental or Parameter Symbol* measurement analytical method

Total particulate mass TSP 12h High-volume sampler Total particulate mass TPM 4h Sequential filter (G30pm) sampler (SFS) Coarse particulate mass CPM 4h SFS ( > 2.5, < 30j6m) Fine particulate mass FPM 4h SFS (4 2.5 Ini) Sulfates so:- 4h SFS, ion chromatography Nitrates NO; 4h SFS, ion chromatography Total carbon C, 4h SFS, carbon analyzer Apparent organic carbon C a0 4h SFS, carbon analyzer Apparent elemental carbon C

Cher%al 4h SFS, carbon analyzer

Elements? 4h SFS, X-ray fluorescence (XRF), symbol neutron activation (NAA)

Ammonium NH: 4h SFS, s~trophotometry Ammonia NH, 4h SFS, s~trophotometry Sulfur dioxide SO2 continuous (C) Flame photometry Nitrogen oxides NO, NO,, NO, C Chemiluminescence Ozone 0, C Ultraviolet absorption Carbon monoxide co 15min Flame ionization/gas

chromatography (FID/GC) Methane CH, 15min FID/GC Nonmethane hydrocarbons NMHC 15min FID Individual hydrocarbons _ 90 min FID/GC

- * The addition of a “c” or “I” subscript denotes the coarse or fine aerosol fraction. t Elements include: by XRF (Al, Si, S, K, Ca, Ti, V, Mn, Fe, Zn, Br and Pb), by NAA (V and As). $ Samples were collected for the ~~, 08Wt200 and 1200-1600-h periods only.

Visibility-reducing species in the Denver “brown cloud”-1 2415

study, the contribution of each process to the total atmospheric extinction was estimated. Absorption by particles (b,,) was measured using the Integrating Plate Method of Lin et al. (1973). Absorption of visible light by gases (b,,) would be essentially all due to nitrogen dioxide which was measured with a chemiluminescent analyzer (Ferman et al., 1981). The scattering from dry particulate matter (b,,) was measured with a heated integrating nephelometer. The difference between b,, and the scattering measured by a nephelometer main- tained at ambient temperature is attributed to scattering by water associated with the particles (b,,).

The contribution from gaseous Rayleigh scattering (b,.J is considered to be a constant at 0.2 x 10m4 m- r. The atmospheric extinction coefficient b,,,, is the sum of the individual contributions:

be,, = b,, + b,, + b,, + b,, + b,, (1)

The extinction coefficient is usually (Middleton, 1952) related to the meteorological visual range (u,) by:

3.912

v =K-- m (2) ex,

To use this equation with our point measurements, it is assumed that the aerosol is homogeneous over the entire sight path and that the instrumental measurements parallel the complex psycho-physical processes occurring in the human eye. When the atmosphere was homogeneous, Hall (Heisler et al., 1980) obtained good agreement between b,,, measured directly by a long path method and the sum of the individually measured components.

The remainder of this report considers the contributions to the non-Rayleigh scattering portion of the extinction, i.e. the portion of the extinction that is affected by varying atmospheric conditions, desig- nated by b;,,.

EXPERIMENTAL

Particle scattering measuremenfs

Integrating nephelometers. Two Meteorology Research, Inc. Model 1550 “flashlamp” integrating nephelometers were used in this study. One was located outdoors in a ventilated cabinet at close to ambient temperature, the other was preceded by a MRI model 461 Air Sample Heater. In addition, a 6-day comparison was made of the Model 1550 nephelometers with a modified Model 1561 photon-counting narrow wavelength range nephelometer provided by the University of Washington (U of W). This latter model was similar to the nephelometers in use at the other sampling sites of the study.

Calibration of nephelometers. The nephelometers were calibrated by filling them with filtered ambient air or with filtered dichlorodifluoromethane (FC-12). The scale of the flashlamp nephelometers was established by assigning a value of zero to the scattering from particle free air and a value of 3.54 x 10m4 m- 1 to the scattering from 101.3 kPa(760 torr) of FC-12 at 298 K. The FC-12 calibration value used for the photon counting nephelometer was 2.16 x 10-4m-‘. The barometric pressure and nephelometer temperature at the time of calibration were used to determine the calibration factor corresponding to the actual number density of FC-12

molecules present at the time of calibration. Since the completion of the data analysis reported in this

and the subsequent paper (Wolff et al., 1981), Ruby and Waggoner (1981) have suggested that the model 1550 nephelometer be calibrated using a value of 3.154 x 10-4m-’ for the difference between the scattering of 1 atmosphere of particle free air and FC-12 at 298 K. However, to maintain continuity and compatibility with other reports ofthe Denver Brown Cloud Study (Heisler et a/., 1980; Van Vahn er al., 1979) we have not made this adjustment to the scattering values. The effect of the new calibration would be to reduce the values of scattering to 0.89 of those reported.

Data acyuisition and processing. The output voltages from the nephelometers were recorded on strip chart recorders. These voltages as well as the temperatures of the cases of the instruments were recorded by a Hewlett-Packard 3052A data acquisition system.

The voltages were converted to scattering coefficients using the zero and span-calibration data to generate a time-varying zero and sensitivity factor, thus providing a first order correction for instrument drift. A problem arose with the ambient temperature nephelometer-its sensitivity and zero varied with theambient temperature. Linear regression of the zero and span-calibration data against temperature showed a decrease in zero reading of 0.13 S, of full scale per degree K temperature rise and a decrease m FC-12 reading of 0.41 I><, per degree K temperature rise. These temperature coefficients are in addition to the pressure and temperature effects on the number density of the calibrating gas. These temperature coefficients were used to obtain the zero and sensitivity for all measured nephelometer temperatures.

Particle absorption measurements

A modification of the Integrating Plate Method (IPM) of Lin et al. (1973) was used for the measurement of the absorption coefficient (b,,) of the Denver aerosol. In this method, aerosol collected on a filter is supported on an opal glass plate and placed in a light beam. The opal glass serves to direct light scattered by the particles back to the detector. The amount of light transmitted through the filter and particles is compared with that transmitted through the blank filter. The difference in transmission is ascribed entirely to light absorption by the particles. We modified the method of Lin et a/.

(1973) by reducing the optical beam size and scanning the filter across the beam, thus allowing the use of the unexposed filter edge as the reference. Measurements were made using 550 nm light. More details of the measurement method and a comparison of results to the original method are given in Appendix A.

Gaseous light absorption determination

A modified Monitor Labs Model 8440/E chemiluminescent nitrogen oxides analyzer with an in-line Teflon particulate filter was used to measure nitrogen dioxide. The analyzer was equipped with a low-noise package and had a sensitivity of less than 1 ppb. The zero point was established using hydrocarbon free air while calibrations were performed using NO2 permeation tubes and diluted NO cylinder gas. All channels were scanned every minute and the voltages were averaged over S-min periods. Additional details on our NO, measurements are contained in Kelly et al. (1981).

To determine the effect of NO, on visibility, we used the extinction coefficient of NO, at a wavelength of 550 nm which is in the center of the optically sensitive region of the human eye (Fleagle and Businger, 1963). The relationship between bag and NO, concentration was developed using the data of Nixon (1940) which was presented in a more usable form by Hodkinson (1966). The resulting equation at 550 nm is:

b,, ( x 104) = 3.3 (NO,) (3)

where NO2 is in units of ppm and b,, is in units of 10m4 m- ‘.

2476 PETER J. GROBLICKI et al.

RESULTS AND DISCUSSION

Comparison of nephelometers

During the six days that the three nephelometers were operated simultaneously there was extremely good agreement between them. Both the heated and the U of W nephelometers had a correlation coefficient of 0.982 with the ambient nephelometer. The correlation coefficient between the heated and U of W nephelometers was 0.9995. Linear regressions of the data gave the following relationships:

U of W = 0.9397 Heated + 0.0208 x 10e4 m-r (4)

U of W = 0.5360 Ambient +0.1425 x 10-4m-1.

(5) Both the correlation coefficients and the regression

analyses indicate that the U of W nephelometer and the heated nephelometer produce nearly identical readings in spite of the physical differences in the two instruments. Although no heater was used with the U of W nephelometer, the connecting tubing and heat from the internal lamp appear to be sufficient to heat the aerosol enough to dry it before it is measured. Thus we conclude that the GMR heated nephelometer measurements are comparable to the nephelometer measurements made at the other sites during this study. The readings from both these “heated” nephelometers were highly correlated with the ambient temperature nephelometer. This correlation is explored further in a later section dealing with humidity effects.

Our value of 1.06 for the ratio of b,, as measured by the two models of integrating nephelometers is some- what less than the ratios near 1.2 measured by Ruby and Waggoner (1981). The difference cannot be ascribed to the new PC-12 calibration values, since both values in the ratio would be multiplied by 0.89. The available data do not allow a choice between this difference being a consequence of the aerosol proper-

14 12

$ 10

“Ezs

14f

hap q

$6 bsp n

“4 bswn

2 bag 11/13 11/15 11117 H/l9 11121

ties in Denver compared to Seattle or of poor humidity control in the nephelometer. At other sitesand times in Denver, Heisler et al. (1980) observed that the scattering varied with wavelength as the 0.7 power. Such wavelength dependence of the scattering would be compatible with our ratio of 1.06, but no data on the wavelength dependence of the scattering is available for the General Motors Research site. Ahernatively, if the photon~ounting nephelometer did not heat the aerosol sufficiently to fully dry it out, then lower values of the ratio would be observed.

Summary of measurements

Figures 2 and 3 illustrate the 4-h variation in the contributions to extinction from absorption by particulates (b,,), scattering by dry particles (b,,), scattering by water associated with the particles (b,,), and absorption by NOZ (b,e). Average and maximum values are presented in Table 2. Some of the important features of Figs 2 and 3 and Table 2 are listed below.

(1) There is a large day to day variation in b,,, and sometimes from one sampling period to the next. The large variation in pollutant concentrations is primarily due to variations in dispersion (Ferman et al., 1981; Wolff et al., 1981). From Fig. 2, the elevated pollution episodes are readily identified.

(2) In general, b,, is the most important contributor to the extinction.

(3) The second most important parameter is b!P The mean value of b,d(b,, + bsp) is consistent with previously reported ratios in Denver and an industrialized area in St. Louis (Waggoner and Charlson, 1977).

(4) The b,, is a significant part of the bexp The variability in this term is thought to be a function of relative humidity (r.h.) and this will be discussed in detail later.

(5) At a wavelength of 550nm, b, from NOz is

2 2 0 12/l 12/3 12/5 12/7 12/9 12/11 12113 12/15 12/17 12/19 12/21

Day Day

Fig. 2. Temporal distribution of b ~~~~~~~~.~, 4,. q . and bag, &j; units


hap bsp n

bsw

bag

Fig. 3. Percentage contribution to extinction from bat,, 69, b,,, q , b,,, 0 , and b,,, q .

Table 2. Summary of extinction measurements

Mean Maximum

Parameter Value* %t Value* %t

b 0.66 28.9 4.38 61.1 b aP

SP 0.92 40.4 6.01 96.2 b SW 0.55 24.1 4.95 63.1 b i”= b&J

0.15 6.6 0.83 36.8 2.28 100 14.43

* All extinction coefficients in units of 10m4 m - I. t “/, of total extinction excluding Rayleigh scattering. $ Does not include Rayleigh scattering of 0.2 x lo- 4 m - I.

ture nephelometer. No effect on the nephelometer reading was observed, although the instrument was

capable of detecting a reduction of a few per cent. Several statistical analyses were performed to de-

termine the relationship between b,, and particulate mass. The first were simple linear regressions of b,, vs FPM, and b,, vs CPM. The results are plotted in Figs 4(a) and (b). The rz values for the FPM and CPM are 0.95 and 0.06 respectively. A significant contribution of CPM to b,, is not evident. Next, a multiple-regression analysis on 208 data points was used to obtain the relationship between scattering and mass:

small compared to the extinction contribution from

particles but it is still significant during most sampling

periods.

bsp( x lo4 m- ‘) = 0.033 FPM -0.003 CPM (6)

The following sections combine the optical

measurements with the determinations of the chemical species present in the aerosol. The major objectives in this report are to identify the parameters and chemical species influencing each of the four extinction terms.

Extinction by particulate scattering (b,,)

Coarse andJine mass contributions. In Denver, an

average of 42 7” of the total mass less than 30-pm dia. was fine particulate matter, FPM, with a diameter < 2.5 pm, and SSo/, was coarse particulate matter, CPM, with a diameter > 2.5 pm to < 30 pm (Countess et al., 1980). For similar conditions, Heintzenberg and Quenzel (1973) have calculated that removing all particles greater than 2.5~pm dia. would cause less than a 2 7; reduction in scattering. Thus, the FPM collected by our filter samples should account for 98 ‘x of the light scattering material. This theoretical prediction was verified in the following experiment. During a period with a high scattering coefficient, a 1” HASL cyclone which removed particles larger than 2.5 pm was placed on the inlet of the ambient temnera-

0 50 100 150 200 250 0 50 loo 150 2w 250 FIN Part1culateMassl~g/m3t CoarsePart~culates Mas~(~glm3)

Fig. 4. Correlation of b,, with fine and coarse particulate mass.

with an rz value of 0.96. The negative contribution of the CPM cannot be theoretically justified but is probably an artifact due to a frequent inverse relationship between FPM and CPM. On certain re- latively clean days with windy conditions, wind blown crustal material is aerosolized in the CPM. These conditions, however, are not conducive for the accu- mulation of FPM and this inverse relationship prob-


Table 3. Linear correlation coefficients (r) between b, and the principal FPM species

Species Y: of FPM’ Number of

r observations

Ammonium (NH:) 7.4 0.74 232 Sulfate (SO:-) 8.9 0.78 232 Nitrate (NO;) 12.2 0.60 232 Organic carbon (C& 21.6 0.82 204 Elemental carbon (C,) 15.3 0.83 204 Remainder 31.0 0.77 204

* From Countess et al. (1980).

ably overwhelms any contribution from CPM to b,, Waggoner and Weiss (1980) have summarized data

from other locations on the ratio of FPM to b,,. Their values of this ratio ranged from 0.31 to 0.34 g m- 2. Linear regression of our data yields

FPM = 29 x IO4 b,,+6.2 (7)

where FPM is in pg m -3 and b,nisinunitsofm-‘.The coefficient corresponds to a ratio of FPM to b,, of 0.29 g m- 2, which is in reasonable agreement with the results of Waggoner and Weiss. Their value of 0.31 g m- 2 for two industrial areas are even closer to our results for this industrial area of Denver. A further correction for the 6”;, difference in response between the two nephelometers (U of W and heated model 1.550-see equation 6). brings our value into agreement with theirs. Thus. theory, ex~riment and statistical analyses of the data all confirm that the FPM de- termines the scattering properties of the Denver winter aerosol.

Contributions of chemical species to b,,. Essentially all of the principal chemical species in the FPM and CPM have been identified (Countess er al., 1980). Linear correlation coefficients between each of the CPM species and b,, were examined and. again, no relationships were found. For the FPM, however, several strong correlations were found. The correlation coefficients presented in Table 3 indicate that all the major FPM species have significant correlation coefficients with b,,. Such an analysis, however, can be misleading if the independent variables are themselves highly correlated. According to Wolff et al. (1981), this is indeed the case for Denver, as most pollutants vary together as a function of dispersion. Proof of this high degree of correlation is shown in Table 4 where all values are significant at the 99.99”;, level.

Table 4. Linear correlation coefficients for the major FPM species

so: -- NO; NH: C,,

NO; 0.55 NH: 0.85 0.84 c

C:: 0.52 0.62 0.57 0.43 0.59 0.62 0.96

--

In order to determine the individual contribution of each species to the b,,, multivariate analysis is necessary. An appropriate procedure that has been used for similar applications in the past (White and Roberts, 1975; Cass, 1979; Trijonis and Yuan, 1978a; 1978b) is multiple regression analysis. In previous analyses, investigators considered only SO:- and the remainder of the total suspended particulate (TSP) or SO:-, NO; and the remainder.

The greater detail of the present study’s chemical analyses allows specific inclusion in the model of the scattering from C,, and C,,, which, in Denver, are equal to or greater in concentration than SOi- and NO;. All the SOi- and NO; are assumed to be present as the ammonium salts and the measured value of C,, is multiplied by I .2 to account for the associated oxygen and hydrogen (Countess et ui., 1980). The ‘*remainder” (R) is the FPM minus the concentrations of (NH& SO.+, NH,NO,, C, and C,. The remainder consists chiefly of crustal material. water, lead salts and other trace elements.

The 4-h averaging period of the filters often included times when atmospheric concentrations were changing rapidly due to the breaking or the formation of inversions. To eliminate biased 4-h partial averages of b,,, we required that at least 20min of b,, data be available from each of the 4 h in the averaging period. This resulted in 157 4-h sampling periods with valid concurrent data for each of the 6 parameters included in the model. Multiple regression yielded the coefficients given in (8) with the high value of r2 of0.94. All particulate species are in kig m _ 3,

(bsp x 104) = 0.066 [(NH& SO,]

+ 0.028 [NH,NO,]

+ 0.044 [ 1.2 C,,] -t 0.032 [C,,]

+ 0.017 [Remainder] - 0.17. (8)

Values for thecoefficients for sulfate, nitrate and the remainder from previous investigations have been summarized by Trijonis et al. (1978b). Our values for the coefficients are all within the range of these previous results. Our correlation coefficient, however, is much better than any previously reported. We feel that this is due to three factors: (1) only fine particulate data were used whereas previous investigators used total particulate data which included the less efficient

Visibility-refusing species in the Denver “brown cloud”--1 2479

(in terms of light scattering) coarse particles, (2) our analysis included C,, and C, which comprised 40.5 % of the FPM and (3) the use of the heated nephelometer eliminated the effects of relative humidity.

Normalizing the coefficients to the coefficient of the remainder term, the relative efficiency of light scattering per unit mass for the principal species can be calcuiated. These relative efficiencies in order of importance are: SOi- = 3.9, C, = 2.6, C,, = 1.9, NO; = 1.6 and remainder, R = 1.0. For a dry particulate

then, SO: has a significantly higher specific scattering coefficient than the other species.

Eflecfs of relative humidity

In previous visibility models (White and Roberts, 1977; Cass, 1979) b,, was strongly dependent on relative humidity (r.h.). Cass converted visibility measurements into estimates of the atmospheric extinction coefhcients, but the nephelometer used to obtain the data analysed by White and Roberts did not have a heater. Since White and Roberts’ nephelometer was located in an air-conditioned trailer, the relative humidity at which the particles were actually measured could be quite different from the ambient relative humidity and the nephelometer’s relative humidity could be less than or greater than the ambient humidity. In the Denver study, by using two nephelometers--one heated and one at ambient conditions- we are able to isoiate the humidity term, b,,, which is the difference between the ambient and heated b,, values.

The dependence of b,, on relative humidity (r.h.) is shown in Figs 5 and 6. In Fig. 5, the diurnal pattern of b,, closely follows that of r.h. In Fig. 6, the temporal variations are again similar except for the three indicated periods characterized by moderate to heavy rates of snowfall. Consequently, we have not included these three periods in our statistical analyses. We assume that b,, is affected by the relative humidity (where p = xr.h./lOO) and the concentrations of the fine particulate hygroscopic species (NH,), SO0 and NH4N03. Cass (1979) and White and Roberts (1977) have suggested the following possibilities for the dependence of b,, on r.h.:

- % RH .-. bs,.,

z @ 1.2: 1 01

“ e

.5 40 0.8 ,a“

j/ 2 z g 20

_L_____ __,. ,..~ d . .._._ ,,’ .___-.._-, 0.4

010 0 2 4 6 81012141616202224

Timeof Day

Fig. 5. Average diurnal variation of b,, and relative humidity.

~~~n~~y Days

-.- bsw

'0°Y5

11111 11119 11127 1215 12113 12121 Day

Fig. 6. Temporal comparison of b,, and relative humidity.

Multiple regression analysis was used to obtain the coefficients a and b for each model. As shown in Table 5 all three models result in excellent correlations with (9) producing the best correlation (r* = 0.87). Consequently, (9) will be used in subsequent calculations.

It is interesting to note that all three models suggest

that rh. has a greater effect on sulfate than nitrate. This

a(NH,),SO, bNH,N03 b,, = ~

1-p + _ .---.

1-p (Cass, 1979)

b SW

= a (NH&SC4 + b.NH,NO, (I - /~)0.6’ (1 -p)“.67

(Cass, 1979)

6,, = a(NH~)*SO~~* + bNH~N0~~’ (White and Roberts, 1977).

Tabie 5. Summary of multiple regression analysis with b, as the dependent variable

Equation No. Model* a b r2

(9) aS/(I--p)+bN/(l--p) 0.0173 0.0147 0.87 (IO) as/(1 - JI)‘.~’ + bN/( 1 - I,c)n.6’ 0.031 0.021 0.84 (II) aSp2 f bN$ & 0.129 0.120 0.83

* S = fine (NH&SO,; N = fine NH,NO,; p = Xr.h./lOO.

2480 PETER J. GROBLICKI et 01.

is consistent with the results of Cass (1979) but not with those of White and Roberts (1977). In the latter case, all of the r.h. effect was incorporated into the nitrate term. Our results and Cass’ (1979) results are more reasonable since both sulfate and nitrate are hygroscopic.

sources whose concentrations are determined by atmospheric dispersion conditions.

As shown in Fig. 7, there is a good fit of the data to a simple linear model:

bapf x lo4 = 0.118 Caef.

Extinction by particulate absorption (b,,) 5.0 1 I In the 1973 Denver study, Waggoner and Charlson

(1977) reported that the mean b,P/b,x, ratio was about 0.45 and speculated that the principal absorbing species was “graphitic” or elemental carbon, though no carbon measurements had been made. Since then, many researchers (Rosen et al., 1979; Hansen et al., 1979; Patterson, 1979; Weiss et al., 1979; Nolan, 1979) have accumulated evidence that carbon is the principal particulate light absorbing species in the ambient air. All of these investigators reported good correlations between total carbon and b,, but they still had to speculate that elemental carbon (C,J, not organic carbon (Ca,) was the principal absorber because they never made specific C,, measurements. Theoretical analyses (Heintzenberg, 1979; Faxvog and Roessler, 1978) and laboratory results (Roessler and Faxvog; 1979 a, b; Truex and Anderson, 1979) support a major contribution of C,, and a negligible contribution of C,, to b,, in an urban atmosphere. For example, the specific b,, for C,, (i.e. b,, per unit mass of C,) generated from acetylene and propane flames ranged from 8 (Roessler and Faxvog, 1979a) to 17mZg-’ (Truex and Anderson, 1979) while for C,, generated from cigarette smoke, the specific b,, was about 0.06 mz g- 1 (Roessler and Faxvog, 1979b).

0 5 10 15 20 25 30 35 Fine Elemental Carbon (pg/m3)

Fig. 7. Relationship between bat,rand elemental carbon.

There is no significant improvement in the fit when terms including Caof or crustal elements are included. The slope corresponds to a specific absorption coefficient of 11.8 mz g- r, which is well within the range of 8-17mZg-’ measured for soot by Roessler and Faxvog (1979a) and Truex and Anderson (1979). Thus, our data support the suggestion that most of the light absorption in Denver is due to fine elemental carbon in the particulate matter.

The Denver data set represents the first ambient data base containing extensive simultaneous measurements of C,,, C,, and b,, in both the fine and coarse fractions. As in the case with scattering, the FPM was responsible for most, 88.4 :,, of the b,,. Consequently, we will first focus on the fine particle absorption, bapf, before we address the coarse absorption.

Absorption by fine carbon. The linear correlation coefficients between b apf and the principal species in the FPM presented in Table 6 show the highest correlation of bapf with C,r. Since the correlation coefficient between Caor and Caer is 0.96, the high correlation between Caor and bapf is not surprising. There is a weak correlation of bapf with SOi- and NO; but this is expected for materials from different

Absorption by coarse carbon. The contribution of the CPM to the absorption was investigated in two ways. First, the absorption coefficient was determined for the FPM and for the total particulate. The difference between these two measurements is the absorption due to the CPM, although the accuracy is reduced due to the nature of difference measurements. For the 4-h Nuclepore filter samples, 11.6 % of the total absorption was due to CPM, on the average. A more direct measurement compared the absorption by the fine and coarse fractions separated by the dichotomous sampler. After correcting for the fine aerosol that “contaminates” the coarse fraction, the CPM was found to contribute 12% of the absorption. These findings are consistent with the observation that an average of 18 % of the elemental carbon is in the CPM (Countess et al., 1980).

Table 6. Linear correlation coefficients (r) between b,, and the principal fine particle species

Multivariate analysis was used to relate the b,, from the total particulate to both fine and coarse elemental carbon:

Fine species r

C,, 0.93 C s$;

0.86 0.48

NO, 0.52

Number of observations

206 206 229 229

b,, ( x 104) = 0.125 C&r+ 0.038 C,, (13)

with r equal to 0.96. The correlation coefficient did not increase when

crustal species were incorporated. This result agrees with the observation that the filters were white after being heated in air to burn off the carbon. (Colored crustal species would be expected to affect b,,.) This is

(12)


also in agreement with the findings that absorbing material could not be extracted from the filters by either organic solvents or acids (Heisler et al., 1980). Thus, the total b,, can be accounted for by fine and coarse elemental carbon.

Extinction by NO1 absorption

The absorption by gaseous NOz as inferred from the measured NOz con~ntrations was presented earlier in Figs 2 and 3 and Table 2. The average 6, was less than the contribution from Rayleigh scattering of the ambient air.

Observed us calculated extinction

0 2 4 6 8 10 12 14 b’axt (calculated) x lo4

Fig. 8. Comparison of observed b,,, with model prediction.

The models for each contribution to the total extinction, equations (3), (8) (9) and (13) can be Extinction budget by chemical species

combined into one equation which will describe the The individual terms from (14) can be rearranged to total extinction coefficient. The resulting equation is: group together the contributions from each chemical

b’,t ( x 104) =

0.066s + 0.028N + 0.044 x 1.2 x Caof -I- 0.032 Caef i 0.017R - 0.17

+ 0.0173s + 0.0147N

J-P) ___ + 0.125 Caef + 0.038 C,, (l-Pj \ v ,

(14)

b SW

+ 3.3 NO, / bV ’ ag

b ap

The observed chemical composition and equation species involved, allowing the fraction of the extinction (14) were used to caiculate the total extinction coef- due to each chemical species to be calculated. Rather ficient for each 4-h period. The calculated extinction than neglect the intercept term (-0.17) in the sub- coefficient is plotted against the observed coefficient in Fig. 8. The resulting relationship is in good agreement

model for b,, it was apportioned to each species, based

for all ranges of bext and the correlation coefficient is upon the fraction of b,, due to that species. The fraction of the extinction due to each of the measured

0.975. species is:

fraction of extinction

due to fine (NH&SO4

=f = S(O.O66+0.0173 (1 -/A)-’ -0.011 bsp’) s

bkxt (15)


due to fine NH4N03

~(0.028+0.0147 (1 -j~-‘-5xlO-~~$ G&Z--.

bkxt


due to fine C,,


due to C,,

=fc,, = 1.2.C& (0.044 - 7 x 10-sbs;‘)

b’ext

C&f (0.157-5 X 10-3b,~1)+0.037 C,, =fc, =1- b;xt

fraction of extinction due to "fR z.z R (0.017-2.9 x 10-3ba

remainder of fine particles bkxt

(16)

(17)

(18)

(19)


due to NO,

3.3 NO* =-kc,, = b (20)

cxt


where all symbols have the same definition as previously given. Using the site average concentrations of each species (Table VIII, Countess et al., 1980) and the mean r.h. of 55 “/, the resulting budget is presented in Table 7. These b,, and b,, values differ slightly from those in Table I, because Table 2 is based on 196 concurrent measurements of bspr b,, hap and b,, while Table 7 is based on 113 concurrent samples which were also analysed for the various chemical species. Consequently, the total number of observations is lower due to some missing samples.

The results in Table 7 indicate that C,, is the most important visibility reducing specie because it absorbs as well as scatturs light. Sulfate is next in importance, followed closely by C,, and nitrate.

Table 7. Contribution of the chemical species to the extinction coetTicient, b;,,

Species* Mean “/, contribution

(NH,)zSG, 20.2 NH4N03 17.2 C CE-scattering

12.5 6.5

C&-absorption C,-total Remainder FPM NO,

31.2

Total

37.7 6.6 5.7

100.

* Fine particulate species. t Includes fine and coarse particulate C,,.

CONCLUSIONS

The most significant finding in this study of the Denver wintertime aerosol is the role of b,, in visibility reduction. On the average hap accounts for 3 17; of the total extinction. Based on theory, laboratory studies by other researchers, and our own statistical results, all of the particle light absorption (b,& appears to be due to elemental carbon (Cd. Since fine C,, also scatters light, the total contribution of C,, to extinction is 38 “i,. This means that, per unit mass, C, is the most effective visibility reducing particulate species in the atmosphere and, during the winter in Denver, it accounts for the largest part of the visibility reduction.

Another significant result is the large effect of water on bext. On the average, the b,, for dry particles, measured with a heated integrating nephelometer, accounted for 40 “/ of the bext, while water associated with the aerosol, as inferred from the nephelometer operating at ambient temperature, accounted for an additional 24%. We assume that the water was associated only with sulfate and nitrate, and the statistical analyses indicate that this is a reasonable assumption.

Particulate scattering is essentially all caused by fine particles, i.e. those less than 2.5~Frn diameter. Of these

fine particles, sulfate is the largest contributor in Denver to total extinction, at 20 %, followed by nitrate, 17 %, and organic carbon, 13 %. These three species plus elemental carbon account for nearly all of the b,,

Scattering by the remaining constituents of the fine particulate fraction contributes only 6.6 y; to the total extinction. For average relative humidities, on a unit mass basis, sulfates are also the most effective particulates in scattering light followed by nitrate, organic carbon, elemental carbon, and the remaining fine mass. However the relative efficiency of the sulfate and nitrate changes with the relative humidity.

The final contributor to the extinction is gaseous NO, which, on the average, accounts for 6 % of the bext

in Denver.

Acknowiedgements-The assistance of Carolina Ang, Carrie Masters and Patricia Mulawa in making the measurements is appreciated. The authors are especially grateful to Martin A. Ferman for assisting in the implementation of many aspects of the project. The statistical anaiysesand data reduction were performed by Carolina Ang, Richard Herrmann and Denise Pierson. Ray Weiss, Alan Waggoner and Bob Charlson of the University of Washington provided comparisons for the nephelometers and b,, method as well as many helpful discussions.

REFERENCES

Cadle S. H. and Groblicki P. J. (1981) An evaluation of methods for the determination of organic and elemental carbon in particulate samples. In Particuhzte Carbon: Atmospheric Lifi Cycle, Wolff G. T. and Khmisch R. L. (eds), Plenum Press, NY (in press).

Cadle S. H., Groblicki P. J. and Stroup D. P. (1980) An automated carbon analyzer for particulate samples. Analyt. Chem. 52, 2201-2206.

Cass G. R. (1979) On the relationship between sulfate air quality and visibility with examples in Los Angeles. Atmospheric Enviro~ent 13, 1069-1084.

Countess R. J., Cadge S. H., Groblicki P. J. and Wolff G. T. (1981) Chemical analysis of size-segregated samples of Denver’s ambient particulate. J. Air Pollur. Control Ass. 31, 247-252.

Countess R. J., Wolff G. T. and Cadle S. H. (1980) The Denver winter aerosol: a comprehensive chemical characterization. J. Air Pollur. Control Ass. 30, 1194- 1200.

Crow L. W. (1976) AirtIow study related to EPA field monitoring program, Denver Metropolitan Area. in Denver Air Pollution Study, 1973, Vol. I, EPA 6OQ/9-76 007a, Research Triangle Park, NC, pp. 3-29.

Faxvog F. R. and Roessler D. M. (1978) Carbon aerosol visibility vs particle size distribution. Appl. Opt. 17, 2612- 2616.

Ferman M. A., Wolff G. T. and Kelly N. A. (1981) An assessment of the gaseous pollutants and meteorological conditions associated with Denver’s Brown Cloud. J. Envir. Sci. Hlth. A. 16, 315-339.

Fleagle R. G. and Businger J. A. (1963) Atmospheric Physics, Academic Press, NY, p. 276.

Hansen A. D. A., Rosen H., Dod R. L. and Novakov T. (1979) Optical characterization of ambient and source particulates. In Proc. Carbonaceous Particulate in the Atmosphere, LBL-9037, Lawrence-Berkeley Laboratories, Berkeley, CA, pp, 116-121.

Visibility-reducing species in the Denver “brown cloud’-1 2483

Heintzenberg J. (1979) Light-scattering parameters of internal and external mixtures of soot and nonabsorbing material in atmospheric aerosols. In Proc. C~r~~ceo~ Parricuiate in the Atmosphere, LBL-9037, Lawrence- Berkelev Laboratories. Berkeley, CA, pp. 278-281.

Heintzenderg J. and Quknzel H. (1973) dn the effect of the loss of large particles on the determination of scattering coefficients with integrating nephelometers. Atmospheric ~nvironmenf 7,503-507. - _

Heisler S. L., Henry R. C., Watson J. G. and Hidy G. M. (1980) The 1978 Denver winter haze study. Motor Vehicle Manufacturers Association, Detroit, ML

Hodkinson R. J. (1966) Calculations of color and visibility in urban atmospheres polluted by gaseous NO*. Int. J. Air War. Poilut. 10, 137-144.

Kelly N. A., Wolff G. T and Ferman M. A. (1981) Background pollutant measurements in air masses affect- ing the eastern United States-I. Air masses arriving from the northwest. Atmospheric Environment (in press).

Lin C. I., Baker M. and Charlson R. J. (1973) Absorption coefficient of atmospheric aerosol: a method for measurement. Appl. Opt. 12, 13561363.

Middleton W. E. K. (19.52) Vision Through the Atmosphere. University of Toronto Press, Toronto.

Nixon J. K. (1940) Absorption coefficient of NO, in the visible spectrum. J. them. Phys. 8, 1.57-160.

Nolan J. L. (1979) Measurements of lift-a~orbing aerosols from combustion sources. In Proc. Carbonaceo~ Par- ticulate in the Atmosphere, LBL-9037, Lawrence-Berkeiey Laboratories, Berkeley, CA, pp. 265-269.

Patterson E. M. (1979) Optical properties of urban aerosols containing carbonaceous material. In Proc. Carbonaceous Particulate in the Atmosphere, LBL-9037, Lawrence- Berkeley Laboratories, Berkeley, CA, pp. 247-251.

Roessler D. M. and Faxvog F. R. (1979a) Optoacoustic measurement of optical absorption in acetylene smoke. J. opt. Sot. Am. 69, 169991704.

Roessler D. M. and Faxvog F. R. (1979b) Photoacoustic determination of optical absorption to extinction ratio in aerosols. Appl. 0~;. 19, 5788581.

Rosen H.. Hansen A. D. A.. Gundel L. and Novakov T. (1979) Identification of the graphitic carbon component of source and ambient particulates by Raman spectroscopy and an optical attenuation technique. In Proc. Conference on Carbonaceous Particles in the Atmosphere, LBL-9037, Lawrence-Berkeley Laboratories, Berkeley, CA, pp. 49- 55.

Ruby M. G. and Waggoner A. P. (1981) Intercomparison of integrating nephelometer measurements. Envir. Sci. Technol. 15, 109-l 13.

Trijonis J. and Yuan K. (1978a) Visibility in the Southwest. EPA-6~~3-78-039, IJS. EPA, Research Triangle Park, NC.

Trijonis J. and Yuan K. (1978b) Visibility in the Northeast. EPA 600!3-78-075, U.S. EPA, Research Triangle Park, NC.

Truex T. J. and Anderson J. E. (1979) Mass monitoring of carbonaceous aerosols with a spectrophone. Atmospheric Environment 13, 507-509.

Van Valin C. C., Wellman P. L. and Pueschel R. F. (1979) Aerosol formation/transformation in Denver’s emission plume. Presented at Fourth International Conference of the Commission on Atmospheric Chemistry and Global Pollution, Boulder, Colorado, 12-18 Aug. 1979.

Waggoner A. P. and Charlson R. J. (1977) Measurement of aerosol optical properties. In Denver Air Pollution Study, 1973, Vol. II, EPA-@O/9-77-001, Research Triangle Park, NC, pp. 35-56.

Wannoner A. P. and Weiss R. E. (1980) Comnarison of fine p&cle mass concentration and’ligh~~atte~ing extinction in ambient aerosols. Atmospheric Environment 14, 623- 626.

Weiss R. E., Waggoner A. P., Charlson R. J., Thorsell D. L.,

Hall J. S. and Riley L. A. (1979) Studies of the optical, physical and chemical properties of light-absorbing aerosols. in Proc. Car~~ceo~ Part~cuIate in the Atmosnhere. LBL-9037. Lawrence-Berkelev Laboratories, Berkeley, CA, pp. 257-262.

White W. H. and Roberts P. T. (1977) On the nature and origins of visibility reducing species in the Los Angeles Basin. Atmospheric Environment 11. 803-812.

Wolff G. T., Groblicki P. J., Countess R. J. and Ferman M. A. (1980) The design of the Denver “Brown Cloud” Study, Paper No. 80-58.4, Presented at the Annual Meeting of the Air Pollution Control Association, Montreal, Canada, June, 1980. (Also available from General Motors Research Laboratories, Warren, MI, 48090 as GMR-3050).

Wolff G. T., Countess R. J., Groblicki P. J., Ferman M. A., Cadle S. H. and Muhlbaier J. L. (1981) Visibility-reducing species in the Denver “brown cloud”. Part II. Sources and temporal patterns. Atmospheric Environment. 15,

2485-2502.

APPENDIX A: MEASUREMENT OF ABSORPTION COEFFICIENT

Optical measurements

To simplify field operations, the transmission of the biank filters was not determined before the filters were used. In contrast to the work of Lin et al. (1973) a smaller optical beam size was used, allowing determination of the un- absorbed reference intensity using the unexposed rim of the filter and determination of the particulate absorption using the central portion of the same filter. A system was con- structed which scanned the exposed filter across the beam (550nm) ofa spectrophotometer. Opaque regions of the filter holder allow the zero of the system to be determined. Then, the intensity of the light beam passing through only the opal glass is obtained as a check on the stability of the spectrophotometer light source. Next, the intensities through the edge, and center of the filter are measured. The filter was oriented such that the particles were nearest the light source (Weiss et al., 1979).

Experimental apparatus

The system used for these measurements is based upon a Bausch and Lomb Spectronic 70 grating spectrophotometer. The door of the sample compartment was removed and a mounting plate fastened over the opening. The opal glass supporting the filter was moved in and out of the beam using a linear guide mechanism based upon a Portalign. The filter was pushed through the beam at a constant rate by a Sage Instruments Model-355 syringe pump. By synchronizing the drive speed and the recorder chart speed both to 1 inch min- I, the length of the transmission profile obtained on the recorder was the same size as the filter diameter.

Problems and precautions

Although a high quality, white, flashed opal glass diffuser (Ealing No. 26-6528) was used, visual inspection showed ripples to be present in the glass. By careful selection, an orientation was found which provided a uniform intensity profile across the central portion of the glass. The usable width of the flat, central portion was increased by blackening the brass holder for the opal glass, as well as the 3 mm thick cylindrical edges of the opal glass plate. Striations were usually observed in the Nuclepore filters themselves. When a filter was scanned perpendicular to these striations, variations in transmission were observed. Scanning parallel to the


striations showed the filters to be uniform in this direction. In Comparison with University of Washington measurements

order to minimize the effect of filter blank inhomogeneities on the results, intensities were used from the exposed section of

At the time of the field experiments, Ray Weiss of the

the filter adjacent to the rim. University of Washington measured b,, on I1 filters, using

Transmission through a blank filter depends upon the the same equipment used to determme b,, at the other

optical contact between the filter and the opal glass. Repro- locations in the study. The correlation coefficient for the two

ducibility was enhanced by laying a microscope slide on top measurement techniques was 0.985 and linear regression

of the filter, causing it to lie flat and in good optical contact showed the values were related by:

with the opal glass. b,, (GMR) = -0.036 x lo-“ + 1.093 b,, (U of W). (Al)

Documents

Visibility-reducing species in the denver “brown cloud”—I. Relationships between extinction and chemical composition