Winter-spring evolution and variability of HO x reservoir species, hydrogen peroxide, and methyl hydroperoxide, in the northern middle to high latitudes

Winter-spring evolution and variability of HOx reservoir species,

hydrogen peroxide, and methyl hydroperoxide,

in the northern middle to high latitudes

Julie A. Snow,1 Brian G. Heikes, and John T. MerrillCenter for Atmospheric Chemistry Studies, Graduate School of Oceanography, University of Rhode Island, Narragansett,Rhode Ialand, USA

Anthony J. Wimmers and Jennie L. MoodyDepartment of Environmental Sciences, University of Virginia, Charlottesville, Virginia, USA

Christopher A. CantrellNational Center for Atmospheric Research, Boulder, Colorado, USA

Received 5 February 2001; revised 20 May 2002; accepted 23 May 2002; published 8 February 2003.

[1] The Tropospheric Ozone Production about the Spring Equinox (TOPSE) experimentexamined the evolution of tropospheric chemical compositions from February to May2000 over North America, 40 to 85 N. Hydrogen peroxide (H2O2) and methylhydroperoxide (CH3OOH) were investigated using instrumental observations aboard theNCAR C-130 research aircraft. Primary TOPSE results indicate both photochemistry andatmospheric dynamics are critical factors controlling the variability of peroxides in thisregion. From February to May, H2O2 and CH3OOH mixing ratios increased with thegreatest relative changes at mid-altitudes. H2O2 ranged from below the detection limit(BDL = 25 pptv) to 380 pptv in winter and from BDL to 1330 pptv during spring. Wintermeasurements of CH3OOH were from BDL (35 pptv) to 740 pptv with higher levels ofBDL to 1400 pptv measured during spring. Peroxides also decreased with latitude at allaltitudes. These findings are consistent with those expected from photochemical theory.Evidence also supports a source of CH3OOH to the Arctic from the transport ofsubtropical air masses. Air mass back trajectories and GOES-derived specific humidityproducts indicate transport of moist tropical air to the study region coincides with elevatedlevels of CH3OOH up to 940 pptv. The concurrence of this transport regime withepisodic elevated CH3OOH events suggests a source of HOx to the Arctic. However,evidence from this study shows CH3OOH does not greatly contribute to total HOx

production which is dominated primarily by reactions of O(1D) and H2O at low latitudesand CH2O at high latitudes. INDEX TERMS: 0365 Atmospheric Composition and Structure:

Troposphere—composition and chemistry; 0368 Atmospheric Composition and Structure: Troposphere—

constituent transport and chemistry; KEYWORDS: TOPSE, hydrogen peroxide, methyl hydroperoxide, HOx

reservoirs, arctic photochemistry, GOES

Citation: Snow, J. A., B. G. Heikes, J. T. Merrill, A. J. Wimmers, J. L. Moody, and C. A. Cantrell, Winter-spring evolution and

variability of HOx reservoir species, hydrogen peroxide, and methyl hydroperoxide, in the northern middle to high latitudes,

J. Geophys. Res., 108(D4), 8362, doi:10.1029/2002JD002172, 2003.

1. Introduction

[2] The Tropospheric Ozone Production about the SpringEquinox (TOPSE) program was designed to investigate theevolution of atmospheric trace gases and aerosols during theNorthern Hemisphere’s winter to spring transition in photo-chemistry and meteorology. TOPSE consisted of 38 science

flights, from February to May 2000, over North Americafrom 40 to 85 N (Figure 1). With an extensive payload,TOPSE documented this seasonal transition over a largevertical and geographic range [Atlas et al., 2002]. Ofparticular interest was recording the springtime ozone (O3)maximum observed over North America in an attempt toevaluate photochemical and dynamic factors. Ozone pro-duction in this region is dependent on the amount ofhydrocarbons (HCs), nitrogen oxides (NO and NO2), water(H2O), and O3 and can be limited by either HCs or NOx. Atransition from a HC limited atmosphere in winter to a NOx

limited atmosphere in summer has been suggested by

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. D4, 8362, doi:10.1029/2002JD002172, 2003

1Now at the University of Washington, Bothell, USA.

Copyright 2003 by the American Geophysical Union.0148-0227/03/2002JD002172

TOP 10 - 1

several studies [Jacob et al., 1995; Kleinman, 1991]. HOx

(HO and HO2) is also a critical component of O3 produc-tion. Therefore, determining the photochemical develop-ment and distribution of NOx, HCs and radical speciesand reservoirs over the Northern Hemisphere mid-tropo-sphere as a function of latitude, altitude, and season is ofgreat interest.[3] Gas phase hydrogen peroxide (H2O2) and methyl

hydroperoxide (CH3OOH) are important HOx reservoirspecies [Logan et al., 1981]. H2O2 and CH3OOH providea source of HOx through primary photochemical reactions:

H2O2 þ hv ! 2 HO; ð1Þ

CH3OOHþ hv ! CH3Oþ HO; ð2Þ

CH3Oþ O2 ! CH2Oþ HO2: ð3Þ

When the photolysis of formaldehyde (CH2O) is alsoconsidered, CH3OOH has the potential to produce twoadditional HO2 molecules. CH2O photolysis proceedsthrough two channels, one of which, the radical channel,produces two HO2 molecules through the oxidation ofatomic hydrogen and the formyl radical,

Hþ CHOþ 2O2 ! COþ 2 HO2: ð4Þ

As a sink, H2O2 sequesters two perhydroxyl radicals (HO2),and CH3OOH sequesters an organic peroxy radical (RO2)and HO2. The peroxy radicals, in addition to nitric oxide(NO), are necessary for net ozone production.[4] Previous studies have shown H2O2, CH3OOH, and

CH2O contribute about 30% of the gross odd-hydrogenproduction in the troposphere, while the remainder of odd-

hydrogen production comes from the photolysis of ozone inthe presence of water vapor [Lee et al., 1997]. Hydro-peroxides become most important as HOx sources in regionsof low water vapor, such as high latitudes and high altitudes[Jaegle et al., 1997; Finlayson-Pitts and Pitts, 1986]. Uppertroposphere (UT) in-situ measurements of hydroperoxides,specifically CH3OOH, have shown an enhancement in airmasses down wind of convective events [O’Sullivan et al.,1999; Newell et al., 1996; Lee et al., 1995; Pickering et al.,1995]. These authors suggest the vertical transport ofboundary layer air elevated in CH3OOH is the source ofthe observed enhancements.[5] Enhanced UT peroxides are noteworthy because

model studies have shown peroxides do indeed increaseHOx concentrations when transported to the UT [Crawfordet al., 2000; Cohan et al., 1999; Folkins et al., 1998;Prather and Jacob, 1998; Jaegle et al., 1997; Lee et al.,1995]. Jaegle et al. [1997] found the injection of CH3OOHto the UT increased HOx levels for a week downwind of theconvective event. Most of these studies, however, tookplace in tropical regions where these trace gases are elevatedin the boundary layer. In the high latitudes where there arelower light conditions, such as found during TOPSE, it isexpected that the lifetime of these HOx reservoir specieswould increase (C. A. Cantrell et al., Peroxy radicalbehavior during the winter-to-spring seasonal transition atmiddle to high latitudes during TOPSE, manuscript sub-mitted to Journal of Geophysical Research, 2002), makingthe transport of CH3OOH to midlatitude and high altituderegions a source of HOx that may be important in springtimeO3 production. The transport of H2O2 to Arctic regions islimited by its shorter lifetime and removal by heterogeneousreactions and precipitation [O’Sullivan et al., 1999; Law-rence and Crutzen, 1998].

Figure 1. Map of the TOPSE study area showing the flight tracks of the NCAR C-130. Highlighted arenotable locations including towns where the aircraft landed: JEFFCO airport in Broomfield, CO,Winnipeg, MB, Canada, Churchill, MB, Canada, and Thule, Greenland.

TOP 10 - 2 SNOW ET AL.: DISTRIBUTION OF MIDDLE TO HIGH LATITUDE PEROXIDES

[6] Aircraft studies of peroxides in mid to high-latituderegions are limited [Heikes et al., 1996a; Schnell et al., 1987;Schnell, 1984]. These studies have shown, however, thatmaximum peroxide mixing ratios are found around 2 km andthat mixing ratios generally decrease with latitude. Manysurface studies have also shown seasonal cycles in H2O2 andCH3OOH [Riedel et al., 2000; McConnell et al., 1997; deServes, 1994]. The peroxides are generally lower in darkconditions (winter) and increase with solar irradiation (sum-mer), suggesting the importance of photochemical reactionsas a source of hydroperoxides in high latitudes. Additionally,de Serves [1994] has observed the dominance of organicperoxides in the winter and H2O2 in the late spring, indicat-ing photochemistry may be responsible for altering theperoxide speciation as well. The limited amount of high-latitude data previously recorded makes TOPSE a uniqueand important example of peroxide distributions in thisregion over an extended temporal period.

2. Methods

2.1. H2O2 and CH3OOH

[7] Using a technique described by Lee et al. [1995],H2O2 and CH3OOH were quantified aboard the NCAR C-130. Ambient air was collected through an inlet in the bellyof the aircraft and mixed with collection solution (pH 6) in acontinuous flow glass scrubbing coil. Based on Henry’s Lawequilibrium partitioning, gas-phase H2O2 and CH3OOHwere collected in the aqueous phase. The collection solutionwas then analyzed real-time using a high performance liquidchromatography (HPLC) instrument with a C-18 packedcolumn. This allows for the separation of H2O2 and organicperoxides, specifically CH3OOH. After separation, the per-oxides were reacted with peroxidase and p-hydroxyphenyl-acetic acid enzymes to produce a fluorescent hydroperoxidederivative. The resulting derivative is proportional to thequantity of peroxides in the ambient sample. Running twoHPLC systems simultaneously, ambient samples were takenevery two minutes. Aqueous standards from diluted stockswere used to calibrate the instrument before each flight,while gas phase standard addition and zero air blanks wereintermittently run through the inlet during flight, preferablywhen ambient peroxide levels were low and stable. Gasphase detection limits were 25 pptv and 35 pptv for H2O2

and CH3OOH respectively. Uncertainty in the measurementsof H2O2 and CH3OOH is ± (detection limit + 30% ofambient mixing ratio) for a 2-minute averaged sample. Thistechnique has successfully quantified hydroperoxides duringother field programs aboard the NASA DC-8 and P3-B[O’Sullivan et al., 1999; Heikes et al., 1996a; Heikes etal., 1996b; Heikes, 1992].

2.2. Air Mass Back Trajectories

[8] The University of Virginia used gridded data fromNOAA’s Air Resources Laboratory FNL archive to calcu-late 3-dimensional air mass back trajectories based on themodel wind fields. Air Resources Laboratory archives theNational Center for Environmental Prediction’s final modelrun, which includes meteorological fields at 00, 06, 12, and18 UTC data. Air Resources Laboratory then converts the 1-degree latitude-longitude grid to hemispheric polar stereo-graphic grids for the FNL archive. TOPSE trajectories were

calculated for each flight track such that the trajectory endpoints correspond to the altitude and location of the aircraftevery 5 min.

2.3. Altered Water Vapor

[9] Altered water vapor (AWV) imagery was producedfrom the GOES-water vapor channel 6.7 mm brightnesstemperature [Wimmers et al., 2002; Wimmers and Moody,2001; Moody et al., 1999]. Images were processed using ananalytical expression that calculates the logarithm of thevertically weighted average of UT specific humidity scaledas ‘‘altered brightness temperature’’ in units of degreesKelvin. Because specific humidity acts as a quasi-conservedtracer, the AWV makes air mass type immediately apparentand is useful in tracing air mass origin. AWV generallyresolves water in the UT at midlatitudes between 250 and400 hPa with the pole-ward limit set by the viewing domainof the satellite. The transition occurring at an altered bright-ness temperature of 250 K, indicated by the change in colorfrom blue to purple, generally coincides with the intersec-tion of the 2 Potential Vorticity Unit (PVU) surface with the300 hPa layer. This delineates the descent of the tropopausebelow 300 hPa [Wimmers et al., 2002].

2.4. Steady State Model

[10] A steady state calculation was made for the radicalsHO2 and CH3O2. All species related to the production, loss,or inter-conversion of the radicals were constrained in thecalculation by the measured species when available. Thisincludes such variables as temperature, pressure, O3, jO

1D,H2O, H2O2, CH3OOH, CH2O, CH4, CO, NMHCs, andNOx. The calculation was made with a steady state equationfor each measured HC reacting with OH and producing thespecific RO2 radical. A complete description of this modelcan be found in Cantrell et al. (submitted manuscript, 2002)and Cantrell et al. [1996].

3. Results and Discussion

[11] TOPSE examined the midlatitude to Arctic tropo-sphere over a large latitudinal, altitude, and temporal range.Table 1 summarizes the mission, date, and locale of eachflight during the entire TOPSE program.

3.1. H2O2 and CH3OOH Distributions

[12] H2O2 and CH3OOH mixing ratios increased fromwinter to spring during TOPSE (Figures 2 and 3). Thisresult is expected based on the increase in photochemicalproduction due to increased solar radiation throughout theseason and has been observed in previous studies [e.g., Rayet al., 1992]. Between 0 and 2 km, H2O2 exhibited adecrease in median mixing ratios from S to N. The highestmedian values were observed between 60 and 70 N (Figure2a) during missions 6 (M6) and 7 (M7) when H2O2 levelsreached 660 pptv and 590 pptv respectively. CH3OOHshowed a similar latitudinal trend in surface measurementswith peaks during M6 and M7 of 510 pptv and 460 pptv,respectively. Near surface measurements of CH3OOH werealso lower in the winter than in the spring. Winter measure-ments ranged from BDL to 390 pptv, and spring levels werebetween BDL and 510 pptv.[13] These results compare well with surface studies. For

example, de Serves [1994] observed total peroxides of less

SNOW ET AL.: DISTRIBUTION OF MIDDLE TO HIGH LATITUDE PEROXIDES TOP 10 - 3

than 40 pptv during the winter and between 100 and 400pptv during the spring at the surface at Alert (82.5 N). Inaddition, a decrease in total peroxides, which includes bothH2O2 and CH3OOH, of �40 to 50 pptv per degree oflatitude has also been observed moving S to N [Van Valin,1987]. Similar results have been observed over the highsouthern latitudes. Annual surface measurements of hydro-peroxides in Antarctica show maximum mixing ratios dur-ing sunlight periods (H2O2 � 200 and CH3OOH � 190pptv) and a minimum during dark periods (H2O2 � 50 andCH3OOH � 90 pptv) [Riedel et al., 2000]. Furthermore,McConnell et al. [1997], using surface snow in Antarcticaas an archive for atmospheric H2O2, also documented anannual cycle with a peak of 280 pptv during the summersolstice and a minimum of 1 pptv during the winter solstice.[14] Both H2O2 and CH3OOH exhibit a general decrease

with latitude in the middle and UT (2–5 km and >5 km)(Figures 2b–2c). This result has been previously observedduring the Subsonic Assessment (SASS) Ozone and Nitro-gen Oxide Experiment (SONEX) in late fall, 1997, over theNorth Atlantic (B. G. Heikes, unpublished results, 1997).Poleward of 60 N and below 8 km, H2O2 levels fell below200 pptv, and CH3OOH levels fell below 100 pptv; how-ever, lower latitude measurements reached a maximum ofonly 320 pptv throughout the entire SONEX mission.

Table 1. Flight Numbers and Dates for Each Mission During

TOPSE

Mission Number Flight Number Date (2000) Locations

M1 5 February 4 Jeffco-Winnipeg6 February 4 Winnipeg-Churchill7 February 7 Churchill-Churchill8 February 9 Churchill-Jeffco

M2 9 February 21 Jeffco-Winnipeg10 February 21 Winnipeg-Churchill11 February 23 Churchill-Thule12 February 25 Thule-Churchill13 February 27 Churchill-Jeffco

M3 14 March 5 Jeffco-Winnipeg15 March 5 Winnipeg-Churchill16 March 7 Churchill-Churchill17 March 8 Churchill-Jeffco

M4 18 March 19 Jeffco-Winnipeg19 March 19 Winnipeg-Churchill20 March 21 Churchill-Thule21 March 23 Thule-Thule22 March 24 Thule-Churchill23 March 26 Churchill-Jeffco

M5 24 April 2 Jeffco-Winnipeg25 April 2 Winnipeg-Churchill26 April 4 Churchill-Thule27 April 6 Thule-Thule28 April 7 Thule-Churchill29 April 10 Churchill-Churchill30 April 11 Churchill-Jeffco

M6 31 April 23 Jeffco-Winnipeg32 April 23 Winnipeg-Churchill33 April 25 Churchill-Thule34 April 27 Thule-Thule35 April 28 Thule-Churchill36 April 30 Churchill-Jeffco

M7 37 May 15 Jeffco-Winnipeg38 May 15 Winnipeg-Churchill39 May 18 Churchill-Thule40 May 19 Thule-Thule41 May 22 Thule-Winnipeg42 May 23 Winnipeg-Jeffco

Figure 2a. Latitudinal trends of H2O2, CH3OOH, andH2O below 2 km. The data were binned in 5� latitudegroups by mission and the median value was plotted.

Figure 2b. Latitudinal trends of H2O2, CH3OOH, and H2Obetween 2 km and 5 km. The data were binned in 5� latitudegroups by mission and the median value was plotted.


During the winter to spring transition documented duringTOPSE, mid-altitude H2O2 mixing ratios increased, as didtheir variability. CH3OOH was more variable than H2O2

with elevated levels measured during M3, between 50 and55 N, and M4, between 60 and 65 N. During M3 maximumlevels of 920 pptv from 2–5 km and 760 pptv above 5 kmwere measured. Maximum values reached 680 pptv from2–5 km and 690 pptv above 5 km during M4. These highvalues were associated with transport from the sub-tropicsand are discussed in more detail in section 2.4. Similarresults were found during PEM-West A on several flightsnorth of 45 N, but only a limited portion of each flight wasbelow 8 km. During this program Heikes et al. [1996a]found that H2O2 and CH3OOH exhibited a latitudinal trendat all altitudes.[15] Altitude profiles also indicate significant variability

in H2O2 and CH3OOH throughout the winter-spring tran-sition at all latitudes. At latitudes below 50 N, H2O2 wasfairly constant with altitude during the winter and earlyspring (M1 �100 pptv and M4 �150 pptv) (Figure 3a).Later, in March and April (M5–M7), a maximum wasobserved between 2 and 4 km. During M5 mid-troposphereH2O2 levels reached 750 pptv and during M7 they reached930 pptv. These results are slightly higher than measure-ments made over the western Atlantic (below 3 km) duringWATOX; mean H2O2 was 120 pptv and below 50 pptv inthe boundary layer [Heikes et al., 1988]. This type ofaltitude profile, with a maximum around 2 km, has beenobserved in the tropics [O’Sullivan et al., 1999], the NorthPacific [Heikes et al., 1996a], and over continental regions[Ray et al., 1992; Heikes, 1992]. Lower mixing ratios athigher altitudes are due to the decrease of water with

Figure 2c. Latitudinal trends of H2O2, CH3OOH, andH2O above 5 km. The data were binned in 5� latitudegroups by mission and the median value was plotted.

Figure 3a. Altitude distributions of H2O2, CH3OOH, and H2O below 50 N. Data were binned every 1km and the median value was plotted for each mission.


altitude. Water vapor enhances the production of H2O2

[Finlayson-Pitts and Pitts, 1986] both directly and indi-rectly through the production of HOx (O

1D + H2O); there-fore, a decrease in the concentration of H2O in theatmosphere corresponds to a decrease in H2O2. H2O2 isalso at a minimum in the lower troposphere (below 1 km)due to precipitation removal [O’Sullivan et al., 1999],increased heterogeneous reactions with ice, clouds, andparticles [Audiffren et al., 1999; Das and Husain, 1999;Lawrence and Crutzen, 1998; Snider and Murphy, 1995],and surface deposition [Hall et al., 1999; Heikes et al.,1996a; Watkins et al., 1995]. Studies have shown thatagreement between modeled and measured H2O2 improveswhen models include heterogeneous processes [Heikes etal., 1996a; Jonson and Isaksen, 1992], suggesting much ofthe H2O2 variability observed in the lower troposphere canbe explained by these reactions. The greatest relative changein H2O2 from winter to spring was observed around 2 km,where the additive impact of sunlight and water peaked toproduce a maximum in H2O2 mixing ratios in the spring(Figures 3a–3c). CH3OOH was more variable with altitudeand season than H2O2. However, during M7, CH3OOHlevels were between 2 and 6 times larger than during M1(Figure 3a). As with H2O2, springtime maximum developedbetween 2 and 6 km reaching 1170 pptv during M7.[16] At northern latitudes (50 to 75 N), H2O2 ranged from

BDL to 180 pptv during M1 and BDL to 270 pptv duringM3 (Figure 3b). Later in the spring, higher levels wereobserved between 1 and 3 km, similar to observations inlower latitudes. During M4 a peak of 820 pptv wasmeasured, and in M7 the peak was 640 pptv. CH3OOHincreased from winter to spring at all altitudes with highermixing ratios at high altitudes during the winter and at lower

altitudes during the spring. A maximum of 760 pptv wasobserved during M3 between 6 and 7 km. CH3OOH mixingratios are typically greater in the lower troposphere due toincreased sources and thus photochemical production[O’Sullivan et al., 1999; Tremmel et al., 1994]. Therefore,the elevated UT measurements suggest a possible sourcefrom the sub-tropical boundary layer transported over longdistances.[17] High latitude observations of H2O2 and CH3OOH

were similar in structure to those made at latitudes below 75N; however, they were lower in magnitude (Figure 3c).H2O2 was almost constant with altitude until M4, afterwhich time it increased through M7. A maximum value of580 pptv was observed later in spring between 2 to 4 km,whereas below 1 km and above 5 km mixing ratios neverexceeded 500 pptv. CH3OOH did not show a mid-altitudemaximum as observed in lower latitudes; CH3OOH wasgenerally higher in the boundary layer (BDL to 460 pptv)than in the UT (BDL to 270 pptv).[18] An interesting observation during TOPSE was the

general dominance of CH3OOH during the winter and thedominance of H2O2 during the spring at high latitudes.Figure 4 shows the ratio of H2O2/CH3OOH during M3(March 5–8) and M7 (May 15–23) poleward of 50 N. Inearly March the median ratio falls below 1 for all altitudesshowing larger CH3OOH mixing ratios were observed. Thetransition from an organically dominated peroxide regime toan H2O2 dominated system occurred in late March and byMay (M7) the median ratio was greater than 1 at all altitudes.de Serves [1994] found a similar result during the PolarSunrise Experiment at Alert. He found organic peroxideswere higher than H2O2 until day 95 after which time H2O2

was higher than the organic peroxides. Although the experi-

Figure 3b. Altitude distributions of H2O2, CH3OOH, and H2O between 50 and 75 N. Data were binnedevery 1 km and the median value was plotted for each mission.


ment ran for only 7 days, it suggests two regimes, also notedin the TOPSE data set. It appears organic peroxides, specif-ically CH3OOH, may be the dominant peroxide in latewinter/early spring when lower sunlight, lower tempera-tures, and lower water vapor exist, while H2O2 may domi-nate in late spring and summer when abundant sunlight,higher temperatures and more water are present. A transitionfrom modeled HO2 to RO2 was also noted (Cantrell et al.,submitted manuscript, 2002) suggesting a shift from RO2 toHO2 drives the transition in peroxides. Furthermore, hydro-carbon mixing ratios decreased by 6.2 ppbC between Feb-ruary and March north of 58 N [Blake et al., 2002],contributing to a decrease in the source of CH3OOH prima-rily in the lower altitudes. However, the transition in per-oxide speciation is less striking at lower altitudes whereremoval processes reduce the H2O2 concentrations.[19] The primary factors controlling peroxides in the north-

ern mid to high-latitudes are their photochemical lifetimes.During the dark winter months, the lifetimes of H2O2 andCH3OOH are several weeks in comparison to their lifetimesof hours to days in warm, sunlit conditions [Cantrell et al,submitted manuscript, 2002;Wang et al., 2002; Jaegle et al.,2000]. These differing lifetimes have a large impact on thevariability of peroxides over the TOPSE study area. The longlifetimes, at high latitudes and during the winter months,mean the impact of air masses originating outside of the localregime becomes greater. And even in these vertically strati-fied winter conditions, peroxide variability is reducedthrough quasi-horizontal mixing. At lower latitudes and inlate spring and summer, the lifetimes of the peroxides drop

dramatically, increasing their variability and making localphotochemistry more important. This was observed duringTOPSE where the variability detected during the winter tospring transition was less at high latitudes than at lowlatitudes. Evidence of long-range subtropical transport is alsonoted during late winter (Figures 2b and 3b), contrasting thestrong photochemically produced mid-altitude maximummeasured during the late spring (Figures 3a–3c).

3.2. H2O Distributions

[20] Surface H2O concentrations increased during thetransition from winter to spring across all latitudes (Figure2a). In addition, a strong latitudinal trend with lowerconcentrations at higher latitudes was found. Between 40and 45 N concentrations ranged from 0.8 to 5.2 g/kg, whilebetween 80 and 85 N concentrations were between 0.4 and1.4 g/kg. In the mid-troposphere, H2O concentrations werevariable, however concentrations still decreased withincreasing latitude and time (Figure 3b). The peak presentin every mission between 55 and 60 N is an artifact of low-elevation sampling owing to the aircraft’s landings inChurchill, Manitoba Canada. Above 5 km there was still aclear latitudinal trend with concentrations ranging between

Figure 3c. Altitude distributions of H2O2, CH3OOH, andH2O above 75 N. The median values of each 1 km bin wereplotted for each mission.

Figure 4. A comparison of the H2O2/CH3OOH ratioduring M3 in March and M7 in May. CH3OOH dominatedduring the cold, dark winter while H2O2 was greater thanCH3OOH during the end of spring. The box has lines at thelower quartile, median, and upper quartile values. Thewhiskers are lines extending to 1.5 times the interquartilerange and the plus indicates any outliers with values beyondthe ends of the whiskers.


0.02 and 1.0 g/kg at the lowest latitude and between 0.01and 0.2 g/kg at the highest latitude (Figure 2c). H2Oexhibited a sharp decrease in concentration and variabilitywith altitude (Figures 3a–3c). Although the general trendwas for concentrations to increase from February to March,a strong temporal trend was not observed due to the largevariability in H2O concentrations during the program. Lowaltitude H2O concentrations decreased with latitude whilehigh altitude concentrations remained low at all latitudes.[21] A correlation between H2O and H2O2 is expected [e.g.

Van Valin et al., 1990] but was not always observed. The lackof correlation could be a result of the typically dark and dryconditions observed during TOPSE. Excess NOx levels alsosuppress H2O2 production, typically in the winter-time whenhydrocarbon-limited conditions [Jacob et al., 1995; Klein-man, 1991] and a ratio of H2O2/(NOy-NOx) < 0.2 [Sillman,1995] exists. However, TOPSE conditions were NOx limitedwith H2O2/(NOy-NOx) ratios between 0.8–0.3 and thereforeshould not be limiting H2O2 production.[22] Another suggested cause comes from a surface study

at Niwot Ridge, Colorado. Watkins et al. [1995] found gasphase H2O2 had a negative correlation with relative humid-ity, suggesting the importance of wet removal processes onH2O2 concentrations. The removal of H2O2 in convectiveevents at lower latitudes has been observed in recent studies[Cohan et al., 1999; O’Sullivan et al., 1999] and may play arole in controlling the variability of H2O and H2O2 throughlong-range transport. However, both positive and negativerelationships between relative humidity and H2O2 wereobserved during TOPSE. For example, on May 23 theaircraft transected a front and there was a sharp transitionfrom dry air to moist air at 46 N. Relative humidityincreased from less than 10% to 60% during this transectwhile H2O2 mixing ratios increased from below the detec-tion limit to greater than 400 pptv. During this flight, apositive relationship between relative humidity and H2O2

was observed. In contrast results from April 11 show therewas a negative relationship between H2O2 and relativehumidity; H2O2 mixing ratios drop to below 200 pptv whilerelative humidity increased to 50% as the aircraft movednorth of 50 N. This flight, flown through primarily moist airoriginating at lower latitudes, suggests wet removal pro-cesses are important mechanisms controlling the H2O2 andH2O relationship.

3.3. Measurement and Model Comparison

[23] H2O2 measurements were generally lower thansteady state model calculations by factors of 2 to 4 through-out the TOPSE program. Previous studies have invokedheterogeneous reactions [Heikes et al., 1996a; Jonson andIsaksen, 1992] that improved measurement to model com-parisons. Heterogeneous reactions on snow and ice as wellas the oxidation of SO2 in cloud droplets are importantmechanism controlling the variability of H2O2. Cantrell etal. (submitted manuscript, 2002) applied a first order lossmechanism rate of 0.1 hr�1 in order to match the measuredH2O2. Wang et al. [2002] found a much larger discrepancybetween measured and modeled H2O2 using a diel steadystate box model; measured H2O2 fell below simulatedvalues by as much as a factor of 10 at lower altitudes.Furthermore, the addition of wet scavenging in the modelreconciled HNO3 measurement/model differences but did

not provide agreement between the measured and modeledH2O2. Other removal processes may therefore be critical inunderstanding the low measured to modeled H2O2 ratiosduring TOPSE.[24] Measurement and modeled CH3OOH were in closer

agreement; steady state modeled CH3OOH was no greaterthan 2.5 times the measured CH3OOH. Due to the betteragreement, Cantrell et al. (submitted manuscript, 2002) useda much smaller first order loss mechanism rate of 0.02 hr�1,consistent with the lower solubility of CH3OOH. Wang etal. [2002] also found better agreement between the simu-lated and measured CH3OOH except during the month ofMarch, when median measured CH3OOH increased withaltitude suggesting a possible source from convective out-flow. However, it does not appear that convective transportfrom the subtropics was more prevalent in March thanduring the other months of the TOPSE program.

3.4. Evidence of Subtropical Transport: A Case Studyof Flights 14 and 15

[25] During flights 14 and 15, CH3OOH mixing ratiosreached 920 pptv around 3 km (Figure 5). These relativelyhigh mixing ratios coincided with the sampling of an airmass with recent subtropical origin. From the findings of theair mass back trajectories and the AWV images (Figure 6), itbecomes clear that the sampled air parcels originated in themidlatitudes and subtropics. Three types of transport pat-terns are indicated by the trajectories, all of which arecommon airflow patterns around cyclones [Carlson, 1998;Cooper et al., 1998; Merrill et al., 1986]. We suggest thevertical transport of boundary layer air elevated in CH3OOHoccurred in convective storms in regions off the westerncoast of the United States. The advection of that airmassalong the storm track, and in some cases ascending aroundthe cyclone further southeast, transported the convectivelyinfluenced air mass to the sampling site.[26] The location of the air mass 70, 46, and 22 hours prior

to flights 14 and 15 is highlighted in Box A in Figure 6. 70hours before the flight the air mass was very moist, withbrightness temperatures below 230 K and scattered withconvective activity. Trajectories suggest as the air massmoved east, it split taking two distinct pathways. Three ofthe air parcels suggest transport around the cyclone to thesouth (the three air parcels moving toward the bottom of BoxA in the image 70 hours prior to sampling), while the other 3trajectories indicate continued movement with the stormtrack directly to the east, northeast (the remaining threeparcels). Within the 2 days before the flight, AWV imagesshowed sporadic convective activity throughout the entireregion, including west of the Baja peninsula where the 3southern bound parcels possibly traveled. Meteorologicalanalysis indicates that the tongue of dry air near 130W in the70 hour image was wrapped into the subtropical disturbancenear the center of Box A in the 46 hour image. Then one daybefore sampling, the trajectories were tightly clustered overthe Northwest United States in a relatively moist (�230 K)air mass. Taking the lifetime of CH3OOH to be several daysin the UT [Jaegle et al., 2000; Cohan et al., 1999], elevatedlevels from convection 1–3 days prior to sampling would bedetected even if further vertical mixing had occurred. H2O2

was not elevated during these flights (<300 pptv) consistentwith precipitation removal in the days preceding the flight.


Ratios of H2O2/CH3OOH <1 are common in convectivelyinfluenced air masses due to the wash out of H2O2 inconvective storms [O’Sullivan et al., 1999].[27] Other in-situ gases indicate the transport of clean

near surface air during this event (Box A in Figure 5).Higher levels of water vapor, for example, correlate wellwith the elevated CH3OOH. In addition, lower levels of O3

(25 ppbv) and CO (100 ppbv) were measured during thesame time period. These low pollution indicators make itunlikely that local pollution from Winnipeg, Canada isresponsible for the elevated CH3OOH. No correlation of

CH3OOH with marine convective tracers such as methyliodide (CH3I), bromoform (CHBr3), or dimethyl sulfide(DMS) was observed. However, CH3I and DMS haverelatively short lifetimes [Sharma et al., 1999; Blake etal., 1997], and all have unique and localized sources[Sharma et al., 1999; Berg et al., 1983; Lovelock, 1975].[28] Previous studies have shown peroxide concentrations

are controlled by air mass origin [Aneja and Das, 1995; Rayet al., 1992; Van Valin, 1987]. Jaegle et al. [1997] showedsynoptic storms at midlatitudes play an important role in thesupply of peroxides to the UT. Although the frequency of

Figure 5. H2O2, CH3OOH, H2O, O3, and CO levels along the flight tracks. Warmer colors correspondto higher mixing ratios. Elevated CH3OOH and H2O can be seen between 2 and 5 km around 50 N andcorrespond to decreases in O3 and CO. The box labeled A corresponds to the same air mass in the AWVimages.


storms is lower at higher latitudes, the lifetimes of peroxidesare longer, possibly making peroxides an important sourceof HOx to the Arctic. Several studies have previouslyobserved long-range transport from midlatitudes coincidentwith elevated peroxides and water vapor [Riedel et al.,2000; Van Valin et al., 1990].[29] Furthermore, several studies have focused on the

existence of O3 mini holes. An O3 minihole is a synopticscale area of strongly reduced O3 (<20 ppbv) directlyrelated to weather systems. They are typically associatedwith breaking Rossby waves in the UT that allow for thepoleward injections of O3 poor air originating in subtropicallatitudes [McCormack and Hood, 1997; Peters et al., 1995].Their occurrence typically peaks in late winter [McCormackand Hood, 1997] however they have been observed in thefall over the North Atlantic [Grant et al., 2000] and Europe,where they are most frequently found [James, 1998]. Mini-hole activity exhibits a close spatial correlation to storm-track regions in the Northern Hemisphere [James, 1998].We suggest events of these types, similar to the eventdiscussed here, are also associated with elevated CH3OOH.[30] Grant et al. [2000] reported an O3 minimum (<20

ppbv) event stretching from 38 N to 47 N over the easternUnited States. The event was produced by a synoptic-scaledisturbance where tropical marine boundary layer air fromthe south was transported along a ‘‘warm conveyor belt’’[Grant et al., 2000; Carlson, 1998; Cooper et al., 1998].The warm conveyor belt is generally associated with well-developed midlatitude cyclones or synoptic-scale disturban-

ces and in some cases may be accompanied by a strato-spheric intrusion. Grant et al. [2000] measured H2O2 levelsof �162 pptv and CH3OOH levels of 96 pptv, significantlylower than observed during TOPSE. CO values were <100ppbv, also lower than measured during TOPSE, suggestingthe TOPSE air mass may have mixed with preexistingmidlatitude air containing a higher level of CO.

3.5. HOx Production in Subtropical Air Masses

[31] Although we show the transport of CH3OOH in sub-tropical air masses does occur during TOPSE, a closeexamination of the relative sources of HOx shows this typeof transport may not contribute greatly to total HOx pro-duction. Figure 7 shows the flight track and CH3OOH andH2O concentrations measured during three flights on Feb-ruary 9 and 27, and May 23. These flights moved souththrough dry air masses poleward of 46 N into moist airmasses south of 46 N. AWV images and trajectories revealthese air masses have midlatitude and subtropical origins.South of 46 N CH3OOH mixing ratios increase from below250 pptv to as large as a 1 ppbv on May 23. In addition H2Oconcentrations increased across this transect from less than0.5 to 2 g/kg. A calculation of the relative contribution toHOx from H2O2, CH3OOH, CH2O, and O(1D) and H2O isshown in Figure 8. Almost no increase in the contributionfrom CH3OOH is noted in the subtropical air compared tothe dry air to the north. The contribution to HOx fromCH3OOH was primarily between 0 and 30% across alltransects, higher than the contribution from H2O2 which

Figure 6. AWV images for March 2, 3, 4, and 5 (flights 14 and 15). The black and white dots, withinthe black box labeled A, on the March 2, 3 and 4 images denote the location of the sampled air massindicated by air mass back trajectories. The white streamers show the path and direction of the air mass12 hours before and after the dot. The white line on the image for March 5 is the aircraft flight track, andthe black segment (A) indicates the location of the aircraft at the time the AWV image was collected.


remained around 10%. CH2O dominated HOx production inthe dry air masses to the north, while reactions betweenO(1D) and H2O became more important in the moist airmasses south of 46 N.[32] The steady state model was used to examine the

relative importance of peroxides, CH2O, and O(1D) andH2O to the HOx budget (Cantrell et al., submitted manu-script, 2002). Results were similar to those from the threeflights discussed above; CH2O provided a significant sourceof HOx at high latitudes due to the decrease in temperatureand H2O vapor concentrations combined with large solarzenith angles. Furthermore, at lower latitudes and loweraltitudes where H2O vapor concentrations were higher, thephotolysis of O3 and subsequent reactions of O(1D) andH2O were the dominant source of HOx. Similar results werefound by Wang et al. [2002] using a diel steady state boxmodel. Therefore it can be concluded that CH3OOH andH2O2 were not primary sources of HOx during TOPSE,contributing less than 40% even within subtropical air

masses containing relatively large concentrations ofCH3OOH. Results from the SONEX campaign [Tan et al.,2000], which took place in October-November over theNorth Atlantic, also show the dominant HOx sources to beO(1D) + H2O and CH2O. However, H2O2 was also asignificant source of HOx during that study, contrastingthe small contribution H2O2 makes to HOx productionduring TOPSE. The TOPSE results also contrasts a studyin the tropics, which found the convective transport of bothCH3OOH and H2O2 was an important source of HOx to theUT [Cohan et al., 1999; Jaegle et al., 1997].

3.6. Stratospheric Intrusion: A Case Study of Flight 23

[33] Flight 23 occurred in the same geographical regionas flights 14 and 15, however the meteorological conditionson March 26 (Flight 23) were very different from March 5(Flights 14 and 15). Air mass trajectories show the airoriginated in the NW Pacific, traveling to the central UnitedStates in approximately 5 days. The air mass was moving

Figure 7. The altitude profiles and CH3OOH and H2O mixing ratios during flights on February 9(square), February 27 (circle), and May 23 (plus). These flights transect from dry air (poleward of 46 N)to air with subtropical origins (south of 46 N).


Figure 8. The percent contribution of H2O2, CH3OOH, CH2O, and O1D + H2O to total HOx onFebruary 9 and 27, and May 23. These flights crossed from dry air into moist subtropical air around 46 N.

Figure 9. The transition from blue to purple, occurring at 250 K in the AWV image, indicates the areawhere stratospheric air is below 300 hPa. The black segment (B) along the flight track shows where theaircraft sampled stratospheric air during this flight section. AWV images from March 23, 24, and 25show the air mass was relatively dry with stratospheric characteristic up to 3 days prior to sampling. Thesampled air mass was tracked with air mass back trajectories. The black and white dots (within box B)indicate the location of the air mass at the time the image was collected while the white streamersrepresent the direction the air mass traveled 12 hours before and after the image was taken.


rapidly and descending slightly over time. The AWVimages (Box B in Figure 9) show a very dry air mass wassampled during flight 23, in stark contrast to the situationduring flights 14 and 15. Where brightness temperatures>250 K, the transition from blue to purple in this image,indicate the intrusion of the tropopause below approxi-mately 300 hPa. In several locations the flight track passedthrough regions with brightness temperatures >250 K,indicating a stratospheric air mass was most likely sampled.73 hours before the flight, trajectories indicate the air masswas located north and west of the sampling site. 3 trajecto-ries place the air mass in very dry air above 60 N, while the

other 3 trajectories are located on the fringes of moister airmasses to the south. The trajectories are more tightlyclustered throughout the finger-like intrusions of dry Arcticair 49 hours before the flight. 25 hours before the flight thetrajectories are even more tightly clustered in a very dry(>255 K) air mass just poleward of 60 N and moving south.[34] During Flight 23, H2O2 and CH3OOH are low,

poleward of 51 N and above 6 km, with maximum levelsreaching 100 and 150 pptv respectively (Box B in Figure10) compared to 280 and 920 pptv for flights 14 and 15(Box A in Figure 5). High altitude water vapor was low(<1 g/kg) but increased at altitudes below 4 km. Water

Figure 10. The altitude profiles show the mixing ratios of H2O2, CH3OOH, H2O, O3, and CO duringFlight 23. O3 > 100 ppbv (red) can be seen at altitudes >4 km and is coincident with low H2O, peroxides,and CO, (Box B) suggesting stratospheric air was sampled.


vapor and O3 were highly anti-correlated, another indicationof stratospheric air. O3 increased from 27 ppbv below 1 kmto values >100 ppbv between 4 and 8 km during differentportions of the flight. During the particular time perioddiscussed here (Box B in Figure 10), O3 is >70 ppbv.Associated with the O3 increase was a decrease in CO downto 95 ppbv, suggesting the high O3 was not directly relatedto pollution. Decreases in peroxide levels are expected instratospheric air and have been observed in other fieldprograms [Riedel et al., 2000; Heikes et al., 1996a]. Furtherdiscussion of stratospheric intrusions during TOPSE can befound in Browell et al. [2002] and Wimmers et al. [2002].

4. Summary and Conclusion

[35] TOPSE documented temporal, latitudinal, and alti-tude patterns in H2O2 and CH3OOH. As winter progressedto spring, the mixing ratios of peroxides generallyincreased. A decrease in peroxides with increasing latitudeat all altitudes was also observed. H2O2 developed a strongmaximum between 1 and 4 km during the transition fromwinter to spring at all latitudes. A mid-altitude maximumwas also observed in CH3OOH; however, it was stronger atlower latitudes. Much of the variability in the peroxides canbe explained by air mass origin. The concurrence of trans-port from the midlatitudes and sub-tropics with episodicelevated CH3OOH contrasts with the very low peroxidelevels observed during stratospheric intrusions. Althoughhigh CH3OOH mixing ratios were found in sub-tropicaltransport, they coincided with increased water vapor, thedominant source of HOx in those air masses. These resultsimply mid to high-latitude peroxide levels are controlled byboth photochemical processes, which are most important atlower latitudes and higher insolation levels, and meteoro-logical and dynamical factors, such as air mass origin andtransport pattern, which are important at high latitudes andin dark conditions.

[36] Acknowledgments. The authors would like to thank E. Atlas,and the pilots and crew of the NCAR C-130 for safely and successfullyguiding us through the TOPSE project, and the National Science Founda-tion for supporting this research through grant number OPP9907808 toURI. The authors would also like to thank D. Blake and N. Blake for thenon-methane hydrocarbon and CO data, A. Fried for the CH2O data, B.Ridley for the O3 and NOx data, and E. Scheuer, B. Lefer, and F. Flocke fortheir help throughout the entire field program.

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��C. A. Cantrell, National Center for Atmospheric Chemistry Studies,

Boulder, CO 80305, USA. ([email protected])B. G. Heikes and J. T. Merrill, CACS, Graduate School of Oceanography,

University of Rhode Island, South Ferry Rd., Narragansett, RI 02882-1197,USA. ([email protected]; [email protected])J. L. Moody and A. J. Wimmers, Department of Environmental Sciences,

University of Virginia, Charlottesville, VA 22903, USA. ([email protected]; [email protected])J. A. Snow, Interdisciplinary Arts and Sciences, University of

Washington, Bothell, 18115 Campus Way NE, Bothell, WA 98011-8246,USA. ( [email protected])


Documents

Winter-spring evolution and variability of HO x reservoir species, hydrogen peroxide, and methyl hydroperoxide, in the northern middle to high latitudes