Ozone and aerosol distributions in the ... - Wiley Online Library

JOURNAL OF GEOPHYSICAL

RESEARCH, VOL. 99, NO. D1, PAGES 1739-1755, JANUARY

20, 1994

Ozone and aerosol distributions in the summertime troposphere over Canada E. V. Browell, 1 M. A. Fenn, 2 C. F. Butler, 2 W. B. Grant, 1 R. C. Harriss, 3 and M. C. Shipham1 Abstract. Measurementsof ozone (03) and aerosoldistributionswere made with an airborne lidar system in the lowland and boreal forest regions of eastern Canada during July-August 1990 as part of the NASA Global TroposphericExperiment/Arctic BoundaryLayer Expedition (ABLE) 3B. Aerosol and 03 profileswere measured simultaneouslyabove and below the Electra aircraft from near the surface to above the tropopauseon long-rangeflights over these important ecosystems.A broad range of atmosphericconditions were encounteredduring repeated flights over intensive study sites in the Hudson Bay lowlands near Moosonee, Ontario, and over the boreal forest near Schefferville, Quebec. The troposphericcompositionin this high-latitude region was found to be strongly influencedby stratosphericintrusions. Regions of low aerosol scatteringand enhanced03 mixing ratios were correlatedwith descendingair from the lower stratosphere.Over 33% of the troposphere(0-12 km) along our flight track at latitudesfrom about45ø to 55øNhad significantlyenhanced03 due to stratospheric intrusions,and in the middleto upper tropospherethe extent of the enhanced03 generally exceeded 40%. Ozone mixing ratios of 80 parts per billion by volume (ppbv) near 6 km were common in strong intrusions. In the boundary layer over the lowlands,

03 wasin the20-30ppbvrangewitha vertical 03 gradient of 6.7ppbvkm-• to about 45 ppbv at 3 km. Above 6 km the backgroundtropospheric03 profile was nearly constantwith an average value of 53 ppbv. Due to forest fires in Canada and Alaska, plumesfrom biomass-burningsourceswere observedon many flights. Biomass-burning plumesinfluencedabout 25% of the free tropospherebelow 4 km, and in some of the plumes, 03 was enhancedby 10-20 ppbv over ambientlevels of 30-45 ppbv. Several air massestransported from the tropical Pacific were observed over Canada in the middle to upper tropospherewith 03 levels 10-20 ppbv below backgroundvaluesof 50-55 ppbv. Introduction

A primary objective of the NASA Arctic Boundary Layer Expedition (ABLE) 3 program was to investigate the sum-

mertime sourcesand/or sinks for tropospheric03 in the Arctic and subarctic regions of North America [Harriss et al., 1992, this issue (a)]. The 1988 expedition (ABLE 3A) focused primarily on the Arctic and subarctic regions near Barrow and Bethel, Alaska, respectively (see ABLE 3A, Journal of Geophysical Research, 97(D15), 1992), and the 1990expedition (ABLE 3B) examinedthe lowland regionsof the Hudson Bay and the boreal forest areas of eastern Canada near Schefferville, Quebec. A primary objective of the ABLE 3 series of expeditions was to examine the

and variability of 03 duringthe summermonths,(2) vertical transport of 03 associatedwith stratosphere-troposphere exchange, (3) photochemicalproduction and/or destruction of 03 at highlatitudesandlong-rangetransportof 03 and 03 precursorsfrom urban/industrial areas, and (4) the role of surface deposition and photochemical processesin determining03 concentrationsin the atmosphericboundarylayer (BL).

This paper reports the results of both mesoscale and large-scale studies of the distribution of aerosols and 03 using an airborne lidar system. These results, together with airbornein situ measurementsof 03 [Andersonet al., this issue], aerosol size distributions[Gregory et al., this issue], and aerosol chemical composition [Gorzelska et al., this following componentsof the 03 budget: (1) survey of issue]and the meteorologicalanalysesof atmospherictranstropospheric03 across the North American subarcticto port [Shipham et al., this issue;Bachmeier et al., this issue], establisha better perspective on the large-scaledistribution provide insights into factors determining the summertime troposphericdistribution of 03 and aerosolsin the highI Atmospheric Sciences Division, NASALangley Research Cen- latitude regionsof North America. ter, Hampton, Virginia.

2Science Applications International Corporation, Hampton, Virginia.

3Institute fortheStudyof Earth,Oceans, andSpace,University Experimental Techniques

of New Hampshire, Durham.

Copyright 1994 by the American GeophysicalUnion.

An airborne differential absorption lidar (DIAL) system [Browell et al., 1983, 1992; Browell, 1989] was used to

Paper number 93JD01832.

provide vertical profiles of 03 and aerosolsfrom near the surfaceto above the tropopausealong the flight track of the

0148-0227/94/93 JD-01832505.00

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NASA Wallops Electra aircraft. Simultaneouszenith and nadir lidar measurementswere made from a range of about 750 m above and below the aircraft to slightly above the tropopausein the zenith case and to about 300 m above the surfacein the nadir case. An 03 measurementaccuracyof better than 10% or 3 parts per billion by volume (ppbv), whichever is larger, with a vertical resolutionof 210 m and a horizontal resolution of about 6 km (assumingan aircraft

speedof 100m s-1) wasobtained [Browell,1983;Browell et al., 1983, 1985a, b]. In addition, comparisonsbetween airborne DIAL and in situ 03 measurementswere made throughout ABLE 3B to constantly verify that their agreement was consistent with the above stated accuracy. In the airborne DIAL system, two frequency-doubled Nd:YAG lasers are used to pump two high-conversionefficiency, frequency-doubled, tunable dye lasers. The four lasers are mounted on a structure that supports all of the laser power supplies,the laser beam transmittingoptics, and the dual telescope and detector packagesfor simultaneous nadir and zenith 03 and aerosolmeasurements.One of the frequency-doubleddye lasersis operatednear 286 nm for the DIAL on-line wavelength of 03, and the other one is operated near 300 nm for the off-line wavelength. The DIAL wavelengthsare produced in sequentiallaser pulses with a time separation of-•300 txs to ensure that the same atmospheric scattering volume is sampled at both wavelengths during the DIAL measurement. Half of each ultraviolet laser

DISTRIBUTIONS

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sectionspresentedhere was 300 m for the nadir data and 500 m for the zenith data with 1-min horizontal averagingin both cases. Detailed characteristics

of the current airborne DIAL

systemand the 03 DIAL techniqueare given by Browell [1989]. In addition

to the DIAL

measurements

the Electra

had

instrumentation for in situ measurementsof 03, aerosolsize distribution, aerosol number density, aerosol composition, carbonmonoxide(CO), methane (CH4), nitric oxide (NO), nitrogendioxide (NO2), peroxyacetylnitrate (PAN), nitric

acid, total nitrogen-containing compounds(NOy), nonmethanehydrocarbons,meteorologicalparameters, such as temperature, dew point, and winds, and other trace gas and aerosolspecies.A general descriptionof these systemsand their measurementsis given by Harriss et al. [this issue(a)], and more details are provided in companionpapers in this issue.

Data Results and Discussion

Twenty-two missions were conducted as part of the ABLE 3B field experiment between July 6 and August 15, 1990. A list of the missionsis given by Harriss et al. [this issue(a)]. During this field experiment, a total of 19 missions were flown north of about 45øN latitude with nine flights in the region between North Bay, Ontario, and Churchill, Manitoba, and 10 flights from Goose Bay, Labrador. The flight tracks for the missions flown during ABLE 3B are beam is transmitted in the zenith and nadir directions. The shown in Figure 1. dye laser output at 600 and 572 nm that remains after the Ozone and aerosol distributions were measured remotely frequency-doublingprocessand the residual 1064-nmoutput with the DIAL system on almost all of the flights during from the frequency-doubled Nd:YAG laser are also transABLE 3B (aircraft power problems interrupted measuremitted for aerosol profile measurements.The output beams ments on two flights). The DIAL system provided nearly are transmitted out of the aircraft coaxially with the receiver completealtitude coverageof 03 and aerosoldistributions telescopesthrough 40-cm-diameter fused silica windows. from near the surface to the tropopause region along the The backscattered laser energy at the four wavelengths is aircraft flight track. These data provided information on the collected by two back-to-back 36-cm-diameter telescopes. large-scalevariationsof 03 and aerosolsin the types of air The lidar returns in the ultraviolet, visible, and infrared are massesencountered during the ABLE 3B field experiment, separated with dichroic optics and directed onto different and these measurementswere compared with the measuredetectors. The analog signalsfrom the detectorsare digitized ments made in the Arctic and subarctic during ABLE 3A at 10 MHz to 12 bits, and the average digitized signals are [Browell et al., 1992]. It should be noted that because nearly stored every 3 s (average of 15 lidar returns) on a 1600-bpi all of the flights during the ABLE field experiments were nine-track magnetic tape. Ozone concentrationsand aerosol conducted during relatively clear days, there will be some distributions are calculated in real time, and the output can biases in the data sampling that cannot be avoided. Also, be displayed on a video screen or can be continuously while attemptswere made to samplemany differenttypes of plotted in color with two ink-jet plotters for in-flight and air masses,the atmospherewas not truly randomly sampled, postmission analysis. and this may result in differencesin describingthe average The

vertical

and

horizontal

resolution

of the

aerosol

measurementscan be as small as that defined by the digitization rate of the lidar return (15 m) and the distance traveled by the aircraft in 3 s (-300 m), respectively. The relative aerosol distribution is derived from the range-correctedlidar signal at the 1064-nm laser wavelength. This wavelength is used because of its enhanced sensitivity to aerosol scattering and low sensitivity to molecular scattering. To provide adequate signal statisticsfor an accurate 03 DIAL calculation, the UV lidar signals require vertical smoothingof at least 210 m and horizontal averaging over at least 1 minute (-•6 km). The actual resolution used in the DIAL calculation dependedupon many factors during each mission, including the transmitted laser energy and solar background conditions, which greatly affected the zenith measurements,in particular. Unless otherwisespecified,the vertical resolution used for the DIAL-derived 03 cross

air mass characteristics.

The air masses observed during ABLE 3B above 45øN were broadly divided into six main categories: background Arctic air in the free troposphere, BL air, stratospheric intrusions, convective outflows, biomass-burning plumes,

and low 03 air. Examples are presented below for the variabilitythat has been observedin 03 and aerosoldistributions under different meteorological conditions for these

different types of air masses. Average 03 and aerosol characteristicsare discussedfor selectedair masstypes, and an analysisis given of the extent to which major air masses were observed in various altitude regions during ABLE 3B. Boundary Layer Air and Background Arctic Air in Free Troposphere

Lowertroposphere. Typical distributionsof aerosolsand 03 in the lower troposphereover the JamesBay lowlands

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Bay

Figure 1.

Map of 1990 Arctic Boundary Layer Expedition (ABLE) 3B flight tracks.

are shown in Plates 1 and 2, respectively. Aerosol concentrations are naturally higher in the BL since the source of aerosols is at the surface, and aerosol loading generally decreaseswith increasingaltitude. As a result, aerosolsare an excellent tracer of BL dynamics. The depth of the boundary layer can be seen in Plate 1 to vary between 1.5 and 2.5 km along the flight track with much deeper vertical transport associatedwith cloudsthat rise to above 3.5 km in some regions. The higher aerosol scattering to the south of Fraserdale (left-hand side of Plate 1) is thoughtto result from the high relative humidity and resulting larger aerosol size associated

with

a line of clouds

and showers.

Over

the

lowlands (Plate 2), the BL depth remained quite stable at about 1.7 km with only slightly more scattering in the BL than above it. A thin plume above the BL is evident in the aerosol data with

some indication

that it has a source at a

gradient of 6.7ppbvkm-1 fromabout34ppbvat 1.5kmto 48ppbvat 3.6km.Thisagrees wellwiththe7.0ppbvkm-1 gradient derived from in situ measurementsbelow 5 km by Anderson et al. [this issue] during ABLE 3B and with the 7.4

ppbvkm-1gradient foundforcontinental polarairmasses in the lower troposphere during ABLE 3A [Browell et al., 1992].

Within the BL the 03 decreasesrapidly with decreasing altitude to about 25 ppbv at 0.7 km (see Figure 2). At this altitude within the BL, 03 was consistentlybetween 20 and 25 ppbv. This is less than the 29 ppbv found at the same altitude

in the BL over the tundra

of southwestern

Alaska

[Browellet al., 1992].The lower 03 valuesfound in the BL during ABLE 3B are consistent with the factor of 2-4 lower

surfaceresistancevalues for the uptake of 03 found over Canadacomparedto that observedover the tundra of Alaska

point in the B L near point 2 shownon the right-hand side of [Ritter et al., this issue]. the plate. The 03 distributionindicatesthat 03 gradually Upper troposphere. The average 03 profile in the upper decreases with decreasing altitude to the lowest values tropospherealongthe sameflight track as in Plate 2 is shown closest to the surface. in Figure 3. The 03 values rangebetween 51 and 56 ppbv up The flow of air in the troposphereon July 9, 1990(flight 2), to about 7.5 km before the 03 increasesrapidly into the was being controlled by a strong polar vortex and an stratosphere.The tropopauselevel is at an altitude of about associated surface low located over extreme eastern Hudson 7.8 km, as defined by the altitude where the upward linear Bay and western Quebec. The polar jet was situated along extension of the average upper tropospheric 03 profile the southernedge of the polar vortex from northern Minne- intersects with the downward linear extension of the stratosota to the Canadian maritime provinces. An isentropic spheric 03 profile (03 > 100 ppbv). A similar case with a trajectory analysis for flight 2 indicates that the air near the higher tropopause(--•11 km) is shown in Figure 4. The 03 surface(potential temperature of {9 = 290-295 K) came from profileis againnearly constantwith altitudewith 03 between Manitoba and Saskatchewanand higher in the troposphere about 50 and 55 ppbv up to the tropopause. This trend is (O -> 300-305 K) the air came southward across central distinctlydifferent than the 0 3 profile observedin the upper Canada from the Arctic Ocean [Shipham et al., this issue, troposphereduringABLE 3A, where the 03 increasedwith witha verticalgradient of 5.2ppbvkm-1 to a value Figures3 and 4]. Figure 2 showsthe average03 profile for altitude the combinationof the atmosphericcross sectionsshownin of >68 ppbv at 8 km [Browell et al., 1992].The differencesin Plates 1 and 2. The 03 has a nearly linear increasewith a these two cases is thought to be due to the general increase

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Plate 1. Aerosol (top) and 0 3 (bottom)distributionsin boundarylayer and backgroundfree troposphere obtained from nadir airborne DIAL measurements on July 9, 1990, over the lowland region near Fraserdale,Ontario. The relative amountof atmosphericbackscatteringand 03 mixing ratios in parts per billion by volume (ppbv) are defined by the color scalesat the top of each display. Black representsvalues greater than the maximum level given on the color scale. Geometric altitudes are given in kilometers above sea level (asl), and local time (LT) is shown at the top of each display. Aircraft latitude and longitude information in degreesis given at the bottom of each display at each reference time.

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RELATIVE AEROSOL SCATTERING X 1000 (IR) 0

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20

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Plate 2. Extensionof aerosoland 0 3 data presentedin Plate I over the JamesBay lowlandswest of Moosonee, Ontario, on July 9, 1990.

in the influence of stratosphericintrusions and large-scale subsidence at higher latitudes on determining the background03 distributionin the upper troposphere. The abovetropospheric03 distributions(Figures2-4) are thought to be generally representative of the unperturbed backgroundatmosphericconditionssincethere was no indication that there had been significant near-term perturbations to this air from the other major types of air masses.

Influence of BiomassBurning

Fires were commonin Canada duringthe summerof 1990, and the plumes from these fires were easily detected in the airborne lidar data from the enhanced aerosol scattering in the plumes. The relative aerosol scatteringfrom the plumes is usually more than 6 times larger than the scatteringfrom nearby "clean" regions that are relatively aerosol free.

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AIRBORNE DIAL OZONE MEASUREMENT

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, i i i I i i i i I i i [ i

7/09/90 14.5141 -155152

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8/05/90 140.5.58 -14.5441

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50

75

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150

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OZONE, PPBV

Figure 2. Average 03 profile for the lower troposphere Figure 4.

Example of averagebackground0 3 profilein the upper troposphere on August 5, 1990, with a tropopause system on July 9, 1990. This covers the 03 distributions height of -• 11 km. shown in Plates 1 and 2. The horizontal bars represent the standarderror of the averageprofile derivedfrom DIAL 03 profile statisticsthat go into calculatingthe averageprofile. fire which produces the plume. In situ measurementsof nonmethane hydrocarbons can aid in identifying whether a Plate 3 showsan example of the enhancedaerosolscattering plume is only a few hours old by examining the amount of observedin associationwith fire plumes. Severalplumescan highly reactive speciesremaining in the plume [Blake et al., be seen in the free troposphere above 2 km (note the 1992; Wofsy et al., 1992, this issue]. Also, if the plume is enhancedscatteringin the plumesrelative to the clean region producedfrom a smolderingfire, there may not be any detectin the top left-hand corner of the plate). The increased able03 productionevenafter severaldaysbecauseof the low obtained by the airborne differential absorptionlidar (DIAL)

scattering within the BL (below 2 km) was also from a portion of the fire plume advecting within the BL. In this

case there was a general increase in 03 above the BL compared to within the BL; however, by comparing03 values inside and outside the aerosol-enhancedregions, there was no indication that additional 03 had been produced photochemicallyin the plumes. The low 03 production observed in the fire plumes can be due to either the recent emissionof the plume or the smolderingnature of the

AIRBORNE DIAL OZONE MEASUREMENT ,

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levelsof NOx (NO and NO2) emittedfrom the fire [Wofsyet al., 1992,this issue].A dramaticexampleof a youngfire plume is shown in Plate 4. The full cross section of the plume is

readily seen in the aerosol data, and there is no apparent

changein the 03 distributionover the samearea. Aged fire plumes from Alaska were observed on some flights during ABLE 3B [Shipham et al., this issue], and in addition to enhanced aerosol scattering they were found to

have enhanced03 due to photochemical03 productionin the plumes. Plate 5 shows a case of aged fire plumes with strongaerosol scatteringand 03 that is in the 47-61 ppbv range, which is generally 10-15 ppbv above unperturbed backgroundlevels in the lower troposphere. The increased noise in the 03 measurementbelow 1.5 km is due to the attenuationof the lidar signalsby the plumes. Most of the fire plumes observed during ABLE 3A had negligible03 production;however, a few of the plumesdid have enhanced03 up to 20 ppbv above backgroundlevels [Browell et al., 1992; Wofsyet al., 1992]. A similar maximum level of 03 enhancementwas measuredin conjunctionwith someof the agedplumesobservedduringABLE 3B. This is also comparableto the maximumlevels of 03 enhancement (20-30 ppbv) observed in plumes from fires during the dry season over the Amazon

basin of Brazil in the 1985 Amazon

Boundary Layer Experiment 2A [Browell et al., 1988; Andreae et al., 1988]. I

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Low Ozone Air

150

Large regions of air containing low aerosol loading and unusuallylow 03 were observedin the free troposphereon Figure 3. Average 0 3 profile in the upper troposphere many flights during ABLE 3B. The case shown in Plate 4 along the flight track of Plate 2. shows a clean region (very low relative scattering levels) OZONE, PPBV

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I II I I I I I I -58.41

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Plate 3. Aerosol and 0 3 distributionsacrossfire plumeson August 5, 1990, east of Goose Bay, Labrador.

with lower than usual 0 3 (-•30 ppbv) above 3.5 km near point2. This 03 level is morethan 15ppbvbelowthe general unperturbedbackground03 levelsobservedat this altitude. This region also had the lowest CO (55 ppbv) and CH 4 (1696 ppbv) seen during ABLE 3B [Harriss et al., this issue (b)]

extend into the upper troposphere(Plate 6) to an altitude

above9.0 km. Thelowlevelsof 03, CO, CH4, andNOy are

similar to those found in the tropical Pacific [Harriss et al., this issue(b); Talbot et al., this issue;Andersonet al., this issue; GTE/CITE, Journal of Geophysical Research, 90(D7), 1985], and a backtrajectory analysis-confirmedthat alongwithlowNOy(135partspertrillionby volume(pptv)) [Talbot et al., this issue]. This unique air masswas found to this air mass had come from low latitudes in the central

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NITROGEN BUDGET FLUX SURVEY SCHEFFERVlLLE, QUEBEC ABLE-3B

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RELATIVE AEROSOL SCATTERING X 1000 (IR)

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Plate 4. Aerosoland 0 3 distributionsacrossa youngfire plumein the boundarylayer and acrossa region of low 03 air (above 3.5 km and near point 2) on August7, 1990, near Schefferville,Quebec. Pacific [Shiphamet al., this issue].This type of air masswas 20 ppbvlower than the background03 profilefrom 5.5 to 8.6 again detected over Canada on a flight the next day. Figure km. 5 showsa comparisonof the average03 profilefrom the low Air with low 03 and low aerosolscatteringwas alsofound 03 case seen in Plate 6 with the background03 profiles in the lower troposphere(1-4 kin) above the BL on several shownin Figures3 and 4. The 03 in the tropicalair is almost missions.Plate 7 showsthe distributionof aerosolsand 03

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16:50 ,

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Plate 5. Aerosol and 0 3 distributionsacrossaged fire plumes on July 26, 1990, over the JamesBay lowlands

west of Moosonee.

obtained on the flight from Frobisher to Goose Bay. The relative aerosol scattering scale is enhanced to show the distributionof the residual aerosol scatteringassociatedwith this relatively clean air mass. The BL top was below 500 m, and the regionof low 03 extendedfrom the BL top to about

2.4 km. The average0 3 distributionfor this crosssectionis plotted in Figure 6 with the background03 profile given in Figure 2. The 03 in this air masswas 30% lower than the

background profileat 2 km. Low valuesof NOy (