Vertical ozone distribution over southern Africa and adjacent

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 101, NO. D19, PAGES 23,823-23,833, OCTOBER 30, 1996

Vertical ozone distribution over southern Africa and adjacent oceans during SAFARI-92

R.D. Diab, 1 A.M. Thompson, 2 M. Zunckel, 3 G.J.R. Coetzee, 4 J. Combrink, •.5 G.E. Bodeker, 6.7 J. Fishman, 8 F. Sokolic, • D.P. McNamara, ø C.B. Archer, 4 and D. Nganga •ø

Abstract. A set of four ozonesonde stations located at Ascension Island, Brazzaville, Okau- kuejo, and Irene, operational during the TRACE A and SAFARI-92 experiments has provided an opportumty to investigate the vertical distribution of ozone over southern Africa and adjacent oceans. All stations display a springtime maximum in tropospheric ozone. Enhanced tropospheric ozone, which occurs between June and September at Brazzaville and between July and October at Ascension Island, is linked to dry season biomass burning. The influence of tropical biomass burning is delayed until September at Okaukuejo when a sharp increase in tropospheric ozone is experienced. The biomass burning influence at Irene is less because of its more southerly location. A general tropospheric enhancement is observed at all stations. It is manifest as an enriched layer in the upper troposphere at Okaukuejo (9-12 km) and Brazzaville (11-14 kin) and in the lower troposphere (2-8 km) at Ascension Island. At Ascension Island lower tropospheric ozone values are about 20 parts per billion by volume greater than elsewhere and the tropospheric component here accounts for about 18% of the total column ozone. A series of tethersonde soundings conducted at hourly intervals at Okaukuejo revealed ozone to be well mixed in the lower boundary layer during the day, but to display marked vertical stratification at night.

Introduction

Observations of dramatically elevated levels of tropospheric ozone over the southern tropical Atlantic Ocean between South America and equatorial Africa in September and Octo- ber each year [Fishman, 1991; Fishman et al., 1991] led to the establishment of the Southern Tropical Atlantic Regional Ex- periment (STARE). Under the umbrella of STARE the Southem African Fire-Atmosphere Research Initiative (SAFARi) was imtiated to investigate the role that emissions from biomass burning over southem Africa play in supplying precursor gases which lead to ozone enhancement.

A set of four ozonesonde stations located at Ascension

Island in the Atlantic Ocean, Brazzaville in the Congo, Okau- kuejo in Namibia, and Irene in South Africa were operational

•Department of Geographical and Environmental Sciences, University of Natal, Durban, South Africa.

2NASA Goddard Space Flight Center, Greenbelt, Maryland. 3Council for Scientific and Industrial Research, Pretoria, South Africa. 4South African Weather Bureau, Pretoria. •Now at Department of Environmental Health, Technikon Natal,

Durban, South Africa. 6Space Physics Research Institute, University of Natal, Durban, South

Africa.

ß 1ow at National Institute of Water and Atmospheric Research Ltd., Omakau, New Zealand.

ß IASA Langley Research Center, Hampton, Virginia. 9Applied Research Corporation, NASA Goddard Space Flight Center,

Greenbelt, Maryland. •øDepartment of Atmospheric Physics, Marien Ngouabi University,

Brazzaville, Congo.

Copyright 1996 by the American Geophysical Union.

Paper number 96JD01267. 0148-0227/96/96JD-01267509.00

during the experiment. The former two formed part of the Transport and Atmospheric Chemistry near the Equator - Atlantic (TRACE A) program, which addressed source regions in Brazil and southern Africa and the long-range transport of pollutants to the southern tropical Atlantic. Since the ozonesonde ascents were coordinated between the four

stations, these data sets provide a unique opportumty to exam- ine the continental scale processes operating at the time. Each of the data sets has formed the basis of an independent analysis, which has been published either separately in this special issue [Diab et al,, this issue; Zuncke! et al., this issue] or elsewhere [Cros et al., 1992; Fishman et al., 1992; Nganga et al., this issue].

The objective of this paper is to bring together the ozonesonde results to reveal new insights and interpretations. The southern African stations are located in regions where few in situ vertical profile ozone measurements have been conducted. A general tropospheric ozone enhancement is noted at all locations. The tropospheric component accounts for up to 18% of total ozone at Ascension Island (13% at Irene), compared with the expected level of 10% at a background site. Although all stations display a September/October tropospheric ozone maximum, differences in the magnitude, commencement, and duration of the enhancement at each station reflect their lati-

tudinal differences. It is also noted that Okaukuejo is one of the few ozonesonde stations located in the 10ø-20øS latitude

band, in which notably low total ozone values were detected in October 1992 [Gleason et al., 1993].

Data

The analysis of ozonesonde data from four stations is included in this paper. They are Brazzaville (4ø17'S, 15ø15'E), Ascension Island (8øS, 14øW), Irene (25ø52'S, 28ø13'E), and

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Figure 1. Map showing location of ozonesonde stations.

Okaukuejo (19ø11'S, 15ø55'E). The locations of the stations are shown in Figure 1. Their altitudes above mean sea level are as follows: Brazzaville (314 m), Ascension Island (91 m), Irene (1523 m), and Okaukuejo (1102 m).

At all stations, Science Pump Type 5A electrochemical cell (ECC) sondes were interfaced with Vaisala RS 80-15NE radiosondes to provide pressure (hPa), temperature (øC), humidity (%), and ozone partial pressure (mPa) measurements. Temporal resolution of data at Brazzaville and Ascension Island is every minute (approximately every 350 m). Data at these stations are also reduced to standard pressure levels. At Irene and Okaukuejo, data are given for standard and significant pressure levels, but data at 10-s intervals are normally available throughout the ascent.

Ascent times were coordinated to facilitate comparison of data from the four stations. At Ascension Island the time of

ascent was 0900 UT (0800 local time), at Brazzaville 0900 UT (1000 local time), at Okaukuejo 0700 UT (0900 local time), and at Irene 0600 UT (0800 local time). A sounding log of dates during the SAFARI period and maximum heights to which sondes reached at each location is presented in Table 1.

A measure of tropospheric ozone was also obtained by inte- grating the ozone profiles between the surface and the major point of inflection in the ozone profile in the lower stratosphere where ozone concentration showed a sudden, sharp increase, in accordance with the method adopted by Browell et al. [this issue] and Thompson et al. [this issue(b)]. This level was at an altitude of approximately 16 km at Brazzaville and Ascension Island and closer to 15 km at both Okaukuejo and Irene.

A series of 137 tethersonde soundings were conducted at Okaukuejo. An Atmospheric Instrumentation Research Inc. (AIR) tethersonde system measuring wind speed and direction, dry and wet bulb temperature, atmospheric pressure, and ozone mixing ratio was used. Soundings typically commenced at 1700 LT (local time (LT) at Okaukuejo is umversal time plus two hours) and were repeated hourly until 0900 LT the following morning. Prior to the onset of the low- level jet, usually around 1900 LT [Zunckel et al., this issue], soundings reached in excess of 1000 m above the surface. The tethered balloon could not penetrate the low-level jet, and sounding heights during these episodes at night were limited

to about 200 m above the surface. Nevertheless, the controlled ascent rate enabled a more detailed study of the vertical distribution of ozone in the boundary layer than that achieved by the standard balloon-borne ozonesonde.

Tropospheric Ozone Climatology

All stations except Okaukuejo have benefitted from longer- term vertical profile ozone data, which will be summarized here to provide a seasonal reference against which to view the SAFARI-92 data. Ozonesonde launches were initiated in

Brazzaville and Ascension Island in June and July 1990, respectively, to further investigate the seasonal enhancement of tropospheric ozone which was detected in satellite data sets over the tropical Atlantic Ocean.

The results of the first year of Ascension Island and Brazza- ville data have been summarized by Fishman et al. [ 1992] and Cros et al. [1992], respectively. Nganga et al. [1996] have ex- tended the discussion of the Brazzaville ozonesonde data to

embrace the period from June 1990 to October 1992. Ozone- sonde ascents at Irene commenced in August 1990 and con-

Table 1. Maximum Heights (in Kilometers) Reached by Ozonesondes During the SAFARI-92 Experiment

Ascension Brazzaville Okaukuejo Irene

Sept. 9 32.0 30.4 11.0 38.5 Sept. 10 Sept. 11 Sept. 12 Sept. 13 Sept. 14 Sept. 15 34.9 28.1 32.5 37.4 Sept. 16 Sept. 17 Sept. 18 Sept. 19 Sept. 20 Sept. 21 34.0 31.3 36.5 Sept. 22 Sept. 23 33.3 33.7 20.8 Sept. 24 32.1 Sept. 25 Sept. 26 30.7 Sept. 27 31.7 29.7 21.4 Sept. 28 32.7 Sept. 29 Sept. 30 34.4 36.0 Oct. 1

Oct. 2 Oct. 3 30.9 19.7 37.4 Oct. 4

Oct. 5 31.0 31.9 26.0 38.2 Oct. 6 28.2 37.1 Oct. 7 32.2 37.2 Oct. 8

Oct. 9 31.3 34.9 37.0 Oct. 10 15.5 Oct. 11 25.6 37.6 Oct. 12 Oct. 13 31.7 36.8 26.3 Oct. 14 32.8 17.5 Oct. 15 32.5 32.1 33.4 35.5 Oct. 16 Oct. 17 35.4 18.9 36.1 Oct. 18 30.3 31.6 31.5 27.0

DIAB ET AL.' VERTICAL OZONE DISTRIBUTION 23,825

Brazzaville a Ascension o Irene ß Okaukuejo

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San Feb Mar Apr May Sun $ul Aug Scp Oct Nov D½½

Months

Figure 2. Monthly integrated tropospheric o7one arno•mt.• derived from ozonesondes at Brazzaville, Ascension Island, and Irene (1990-1993). At Okaukuejo monthly tropospheric ozone values are derived from the tropospheric residual tech- nique of Fishman et al. [1990] and based on version 6 total ozone mapping spectrometer (TOMS) data for the period 1979-1990.

tinued on a weekly launch schedule through to September 1993. Aspects of the vertical ozone distribution at Irene have been described by Zunckel [1992] and Zunckel et al. [1992]. Irene data for the period August 1990 to July 1992 are in- eluded here for discussion.

At Brazzaville the seasonal peak (52 Dobson units (DU)) in integrated tropospheric ozone derived from ozonesondes occurs in September, although enhanced ozone is present from June to September (Figure 2). A weak secondary peak is evident in Febmary/March and a minimum of the order of 30 DU occurs in January [Cros et al., 1992]. Longer-term results discussed by Nganga et al. [this issue] conEh'm the June to September enhancement but do not reveal a well-defmed Febmary/March secondary maximum.

There appear to be several reasons for the seasonal pattern of tropospheric ozone at Brazzaville. Both Fishman et al. [1992] and Nganga et al. [this issue] have drawn attention to the presence of enhanced tropospheric ozone in the dry season, supporting the earlier findings of Cros et al. [1987, 1988]. The implication that dry season biomass burning is respon- sible for the observed ozone enhancement is confirmed by Thompson et al. [this issue (b)], who show good correlation between the soundings with the highest integrated tropospheric ozone and the location of fires in southern Africa [Justice et al., this issue]. These fires maximize in June and July and wane by September and October. Indeed, by mid- October 1992, two Brazzaville soundings are at background (clean) levels (< 30 DU). The corresponding trajectories pass over regions that are heavily cloud-covered and have reduced burning [Thompson et al., this issue (a)]. Precipitation statistics for Brazzaville, though not specific to 1992, suggest that the latter part of SAFARI-92 was already in the wet

season (Table 2). Widespread deep convection in Brazzaville is confirmed by visible advanced very high resolution radiometer (AVHRR) cloud imagery and by total ozone mapping spectrometer (TOMS) reflectivity. Satellite-derived tropospheric column ozone is approximately 25 DU in east central Africa [Kim et al., this issue], also suggesting that burning impact is greatly reduced.

It is not possible to eliminate the contribution of biogenic emissions to ozone enhancement. Indeed, Swap et al. [1996] and Harris et al. [this issue] provide a strong argument in favor of a biogenic contribution. Enhanced nitrogenous emissions from wetted soils [Levine et al., 1996] and hydro- carbon production from new vegetation growth follow the onset of rains. However, biogenic emissions are likely to be important only in the latter part of SAFARI (October) and are unlikely to be significant outside of the mixed layer. During the Amazon Boundary Layer Expedition ABLE 2A experiment in Brazil, observations showed that significant amounts of ozone were produced in the boundary layer because of nitric oxide emissions from soils and isoprene emissions from vegetation, but this contribution was not detected aloft because of rapid ozone loss in the boundary layer [Kaplan et al., 1988; Jacob and Wofsy, 1988].

The seasonal cycle in integrated tropospheric ozone at Ascension appears to lag that at Brazzaville by 1 month (Figure 2). Elevated ozone levels (> 45 DU) occur from July to October. A secondary maximum in February, which was earlier noted by Fishman et al. [ 1992] is also present.

The maximum in tropospheric ozone at Ascension lags the maximum in southern Africa savanna burning even though the latter supplies Ascension with most of its excess ozone through the easterlies leaving Africa [Thompson et al., this issue (a); Olson et al., this issue]. Indeed, Ascension Island (like Natal, Brazil) is consistently elevated in tropospheric ozone during SAFARI-92, with only one sounding less than 40 DU [Thompson et al., this issue (a)]. Thompson et al. [this issue (b)] and Olson et al. [this issue] give several reasons for this. First, subsidence at Ascension Island (estimated at a rate of 225 m/day [Swap et al., this issue]) brings high ozone from the middle and upper troposphere toward the surface. The

Table 2. Monthly Rainfall Totals (mm) and Average Number of Days of Rain at the Brazzaville Station of Maya-Maya (4ø15'S, 15ø15'E)

Normal, mm Number of Rain Days

January 158 11 February 123 6 March 188 8

April 203 8 May 126 4 June 5 0

July 1 0 August 3 0 September 37 2 October 129 7 November 238 10 December 171 14

Monthly Climate Data for World, National Oceanic and Atmospheric Administration / World Meteorological Organization.

23,826 DIAB ET AL.: VERTICAL OZONE DISTRIBUTION

lower troposphere at Ascension has greater ozone thickness than any of the six AriScan or South American sounding sites operative in SAFARI-92/TRACE A. Second, trajectories show that Ascension Island is approximately 1/2-2 weeks downwind of the biomass burning regions of South America and Africa [Swap et al., this issue] and is enriched by ozone transport and in-transit ozone formation from both sources [Thompson et al., this issue (a)]. Third, as Olson et al. [this issue] have noted, westerly flow bringing upper tropospheric ozone from South America contributes more to South Atlantic

ozone in the latter part of the dry season and the early part of the wet season. In 1992 at least (i.e., during SAFARI-92), upper tropospheric ozone formed following deep convection over South America. This led to ozone enhancements of up to 6 DU [Thompson et al., this issue (b)] in the upper tropospheric segments of the Ascension soundings. The peak in South American savanna burning occurs in late August- early September, coincident with sporadic rains of the wet season onset. These two factors combine to augment high concentrations of lower and middle tropospheric ozone, already present from African burning, to give maximum tropospheric ozone column in September after the African savanna burning season has peaked. Similar seasonal behavior is noted in the Natal ozone soundings [Thompson et al., 1996, this issue (a)].

At Okaukmejo there are no long-term ozonesonde measurements to provide a seasonal perspective. However, tropospheric ozone residuals, defined as the difference between TOMS total ozone and Stratospheric Aerosol and Gas Experi- ment (SAGE) stratospheric ozone [Fishman et al., 1990; Fishman, 1991 ], were calculated for Okaukuejo by centering a box of 10 ø latitude and 20 ø longitude on the station in accordance with the method adopted by Fishman et al. [1992]. Al- though data from individual ozonesondes are not strictly com- parable with the computed tropospheric residual, it is a useful quantity for describing the seasonal tropospheric ozone climatology in the absence of other data. J. Fishman and coworkers

[Fishman and Browell, 1988; Fishman et al., 1991, 1992; Cros et al., 1992; Olson et al., this issue] found that ozonesondes at a particular site and the corresponding tropospheric residual tend to track one another within 20%.

The sharper September peak in tropospheric ozone at Okaukuejo reflects, first, its more southerly location with respect to the major areas of fire activity and, second, a different transport pattern, that is, more variable flow relative to the predominant location of fires. Tyson et al. [1996] show com- plex recirculation patterns at Okaukuejo. Furthermore, Diab et al. [this issue] show that ozone in the lower midtroposphere tends to alternate between clean and polluted levels, depend- ing on which of two flow patterns is operative. The lower ozone column values correspond with westerly flows that bring cleaner air from the Atlantic. The higher variability of source regions (and presumably of ozone pollution from biomass burning and/or urban areas) is supported by statistical analysis of back trajectories from all the SAFARI ozone soundings. Thompson et al. [this issue (b)] show that low and mid tropospheric air masses in Ascension and Brazzaville soundings are > 70% from African regions affected by burning, whereas at Okaukuejo that fraction is < 40%, with many trajectories of marine origin.

The secondary tropospheric ozone maxima in March/April

Brazzaville and Ascension Island, to be due to the transport of biomass bum products from northem hemisphere dry season burning in north and west Africa [Fishman et al., 1992]. However, on the basis of evidence of the origin of trajectories it is unlikely that northem hemisphere biomass buming would account for the secondary tropospheric ozone maximum observed at Okaukuejo.

The seasonal distribution in tropospheric ozone at Irene (Figure 2) reveals values that are lower than at Brazzaville, Ascension Island, or Okaukuejo. A spring maximum, where values approximate 35 DU, extends from August to October. The minimum is observed between April and June. The lower tropospheric ozone values at Irene compared with those at the other stations in this study result from the location of Irene to the south of the major regions of biomass buming, as shown in the satellite images of Cahoon et al. [1992] and, like Okaukuejo, from highly variable transport pattems [Garstang et al., this issue].

The seasonal pattern appears to be a product of two origins. First, it reflects the seasonal burning pattern over Africa detected by satellite [Cahoon et al., 1992] in which the fire season commences earlier (May, June) in the western and interior countries (Angola, southern Zaire, Zambia and Zimbabwe) and spreads to the eastern coast of southern Africa (Tanzania, Mozambique) to peak in October, while simul- taneously declining in the west. Similar conclusions about the seasonality of fire were drawn by dustice et al. [this issue], pertaining specifically to the SAFARI period. Second, the ex- tension of the ozone enhancement into the summer period at Irene may be indicative of an urban-industrial photochemical origin of ozone, as suggested by Zunckel et al. [1992].

A clear latitudinal dependence in ozone concentration is evident in Figure 2. Tropospheric ozone in the equatorial region (Brazzaville and Ascension Island) is higher than in the subtropics to the south, as represented by Irene. The latitudinal differences are at a maximum (---10 DU) in winter and spring at the time of peak tropospheric ozone and only of the order of 4 DU at the time of minimum tropospheric ozone. This pattern is most likely linked to biomass buming, which has been shown to be greater equatorward of 20øS [Cahoon et al., 1992; dustice et al., this issue]. The latitudinal gradient is confirmed by Thompson et al. [1996], where UV-DIAL data from the flights over Africa during the SAFARI-92 period are presented.

Comparison of Mean Ozone Profiles

Mean ozone profiles during the SAFARI-92 period at the four locations are shown in Figure 3. Whilst it is recognized that substantial stratification in ozone exists in the troposphere [Browell et al., this issue], smoothed profiles (averaged over 200-m intervals) are presented here to overcome the noise in the data sets and to facilitate some generalized observations. Individual profiles have been presented by Thompson et al. [this issue (a)]. Although the number of profiles and the days on which they occur differ for each station, some general features are nevertheless apparent.

There is a sharp increase from a surface ozone value around 30 parts per billion by volume (ppbv) (20 ppbv in the case of Irene) to approximately 60 ppbv within the lowest 2-3 km above sea level. Thereafter values generally increase through-

observed at all three stations are thought, in the case of out the troposphere. A sharp increase in gradient occurs in the

DIAB ET AL.' VERTICAL OZONE DISTRIBUTION 23,827

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Figure 3. Mean vertical ozone concentration (parts per billion by volume (ppbv)) for the SAFARI-92 period at (a) Ascension Island, (b) Brazzaville, (c) Okaukuejo, and (d) Irene. Standard deviation bars for 200-m layers are given at 2-km intervals.

vicinity of the tropopause at an altitude of between 14 and 16km.

Both Brazzaville and Ascension Island show a low tropospheric maximum near 6 km. At Ascension Island this enhancement appears to be of the order of 20 ppbv compared with the other stations. Brazzaville and Okaukuejo display an elevated enriched ozone layer in the upper troposphere. The layer is situated between 11-14 km at Brazzaville and 9-12 km at Okaukuejo. Concentrations are of the order of 90 ppbv and 80 ppbv, respectively. These features are endorsed by the values presented in Table 3, which represent the percent frequency that ozone along the vertical profile is above an arbitrarily selected threshold of 80 ppbv in the troposphere on each day. Daily variability is evident but there is clearly a trend toward higher ozone values at Ascension Island. The enriched layer observed at Brazzaville and Okaukuejo in the upper troposphere and at Ascension Island in the lower troposphere is absent at Irene. At the latter station, although upper tropospheric concentrations are generally similar to other stations (around 80-90 ppbv), lower to mid tropospheric ozone concentrations are lower. Table 3 shows that the extent to

which tropospheric ozone exceeds a threshold of 80 ppbv is approximately 30% at Irene, Brazzaville, and Okaukuejo, compared with 50% at Ascension Island.

Variability, as indicated by the standard deviation bars, is generally greater in the upper than in the lower troposphere. The considerable increase in variability observed at Irene in the region of the tropopause and lower stratosphere is indicative of a more dynamic atmosphere closer to the midlatitudes. Injections of ozone-rich stratospheric air in association with the passage of midlatitude frontal systems accounts for the enhanced variability in ozone concentration. These synoptic scale events have been discussed by Bruinties et al. [1990], Barsby and Diab [ 1995], and Zunckel [ 1992] and are further investigated here in the section on dynamics of vertical ozone distribution. Considerable variability is also evident in the upper troposphere at Brazzaville and is explored in greater detail below.

Comparison Among Tropospheric, Stratospheric, and Total Ozone

Integrated stratospheric (> 16 km) and tropospheric (< 16 km) ozone are computed from ozonesonde data at each of the four locations. Where sondes burst prematurely, ozone was extrapolated to 34 km using the mean profile which was estimated from remaining ascents. The sum of the tropospheric and stratospheric components was generally lower

23,828 DIAB ET AL.' VERTICAL OZONE DISTRIBUTION

Table 3. Percent Frequency That Ozone Concentration Along a Vertical Profile in the Troposphere is Above a Threshold of 80 Parts Per Billion by Volme (ppbv) on Each Day

Date Percent Frequency, %, Ozone > 80 ppbv

Ascension Brazzaville Irene Okaukuejo

Sept. 9, 1992 75 28 68 41 Sept. 15, 1992 33 6 13 31 Sept. 21, 1992 20 50 50 Sept. 23, 1992 6 15 66 Sept. 24, 1992 50 Sept. 26, 1992 60 Sept. 27, 1992 79 29 0 Sept. 28, 1992 59 Sept. 20, 1992 62 48 Oct. 3, 1992 64 25 Oct. 5, 1992 55 54 22 46 Oct. 6, 1992 65 49 Oct. 7, 1992 57 22 Oct. 9, 1992 63 60 10 Oct. 10, 1992 59 Oct. 11, 1992 54 10 Oct. 13, 1992 62 30 12 Oct. 14, 1992 21 16 Oct. 15, 1992 31 15 33 16 Oct. 17, 1992 8 10 16 Oct. 18, 1992 33 6 17 31 Mean 50 31 30 32

The tropopause is taken as 13 km at Irene, 14 km at Okaukuejo, and 16 km at Brazzaville and Ascension Island.

13% at Irene. In all cases the percentage exceeds the commonly quoted figure of 10% [e.g., Fishman and Larsen, 1987] and is a reflection of general tropospheric ozone enhancement in these regions.

Dynamics of Vertical Ozone Distribution

Time-height cross sections of ozone (ppbv) for the SAFARI-92 period are presented for the four stations of interest in Figures 5a to 5d. The base of the stratospheric ozone layer (represented approximately by the 100 ppbv isoline) is marked by a sharp increase in gradient. The generally horizontal alignment of the isolines thereafter reflects the marked atmospheric stability of the stratosphere. Ozone in the troposphere generally lies between 50 and 100 ppbv. The ozone enhancement in the lower troposphere over Ascension Island where values sometimes exceed

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10 Sop 20 Sop 1 Oct 10 Oct

than TOMS total ozone, since few sondes reached above

34 km. Variations in TOMS total column ozone, integrated stratospheric ozone and integrated tropospheric ozone during the SAFARI period are displayed in Figure 4. Total ozone at Okaukuejo is consistently lower than elsewhere, averaging 272 DU compared with 287 DU at Irene, 292 DU at Brazza- ville, and 287 DU at Ascension Island (Figure 4a). This is contrary to the expectation established in the literature, that

total ozone increases from the tropics to peak in the midlatitude regions. However, it is most likely a reflection of a band of anomalously low TOMS values which occurred during 1992 in the band from 10øS to 20øS [Gleason et al., 1993]. Unmodified version 6 TOMS data were utilized in these comparisons. It is noted that correction of TOMS re-

trieved ozone over the marine stratocumulus region west of Africa reduces TOMS total ozone over Ascension Island on

, most days [Thompson et al. this issue (a)]. The integrated stratospheric ozone amount is approximately 200 DU at Brazzaville, Ascension Island, and Irene and only

180 DU at Okaukuejo, representing a 10% reduction compared with other stations (Figure 4b). The stratospheric ozone maximum at Okaukuejo during the SAFARI period is of the

order of 12 mPa, compared with a value greater than 15 mPa at Ascension Island.

Mean integrated tropospheric ozone amounts below 16 km are greatest at Ascension Island (53 DU) and least at Irene (38 DU) (Figure 4c). This latter value may even be a slight overestimate, as the tropopause is situated at a lower altitude than 16 km. The percent contribution of the tropospheric component to total column ozone ranges from 18% at Ascension Island, 15% at Brazzaville, 15% at Okaukuejo, to

Co) 250

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-- Ascension .... Brazzaville Okaukuejo ...... Irene Island

Figure 4. Daily variations in (a) Version 6 TOMS total column ozone (Dobson units (DU)), (b) integrated stratospheric ozone (> 16 km) (DIS) derived from ozonesondes, and (e) integrated tropospheric ozone (< 16 km) 0DU) derived from ozonesondes for the SAFARI-92 period at four locations.

DIAB ET AL.: VERTICAL OZONE DISTRIBUTION 23,829

(b) Ascension Island (c) Okaukuejo

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2O

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I .... ••_•8.•x•.•¾..__> 500 ppbv _.__.._/••,•E•J •_ > 500 ppbv -J

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Figure 5. Time-height cross sections of ozone (ppbv) for the SAFARI-92 period derived from ozonesondes at (a) Ascension Island, (b) Brazzaville, (c) Okaukuejo, and (d) Irene.

100 ppbv is evident in Figure 5a. Two periods of relatively low ozone in the lower troposphere at Okaukuejo are discussed in greater detail in the paper by Diab et al. [this issue].

Interchange between the stratosphere and troposphere or vice versa is suggested by the perturbations in the 100 ppbv isoline. Occasional protrusions of high ozone extend into the troposphere, for example, at Irene, where in early October there is clearly some disturbance in the upper troposphere. In contrast, the early part of SAFARI is characterized by a markedly urnform boundary between stratosphere and troposphere, confmning the observations of Garstang et al. [this issue] that the early SAFARI period was dominated by stable anticyclonic flow.

An interesting disturbance was evident around September 21, 1992, at Brazzaville, when considerable upper tropospheric enrichment occurred (Figure 5b). Data were checked for possible sources of error, but evidence suggests that the enrichment of above 200 ppbv is real. A similar enhancement, though of smaller magnitude, was observed 6 days later at Ascension Island (Figure 5a), which lies to the west of Brazzaville, suggesting that this is an anomaly associated with easterly winds. Indeed, back trajectories from Brazzaville indicate a north-east north AfTican origin for the high-ozone air mass, a unique source among the 11 soundings sampled between September 15 and October 20, 1992. Other evidence of anomalous conditions over this period is presented in the time-height cross section of the co component of the wind


20

14

]0 ]5 20 25 30 5 ]0 ]5 10ppbv increased systematically in the stable lower

'ø•"'""""•:':•••":':;'"::'" '' ";:''"' '" "•' '"'"•"'•'.-"••••....;••}ii'"--'"""• '• .'"'"'---"• 16 inversion. Figure 9b clearly shows the stratified ozone ]4 structure at 0700 LT up to the top of the inversion at 650 m k-'k.• ) / f:;i•ii:iiii•ii:ii'•/5 •------• -%'j•x,'d • •,o •,jq.•,__•_.• ]2 above the surface. With the collapse of the temperature

:.½i:::•:::i::;:.?... '•--•••2 a __•;•---• -• 10 ø '•:'•.'.:-::•:•...-:•:•:•:•. O _ o_• inversion after sunrise, mixing recommenced and the low-

ii•½::-•:;::•. 8 level structure in ozone concentration exhibited during the ' 6 noctumal period dissipated. •_• 4 Comparison of integrated ozone in sunset and early •_Pf/-/•'-'-••••••:...•••• .:.•--(-i•f'-'""•'"" •:i•:• '5+ 2 morning profiles in Figure 9 implies a loss of--,1 ppbv O3/h or 0

]0 ]5 20 25 30 5• ]0 ]5 a deposition velocity of 1.7 cms 4 in the surface layer. This September 1992 October 1992

Figure 6. Time-height cross section of the vertical component of the wind (o)) (x 100 Pa s -1 ) at the location of Brazzaville derived from European Centre for Medium-Range Weather Forecasts data during the SAFARI-92 period.

(Figure 6). In addition, there is some evidence of possible stratospheric-tropospheric exchange (STE). Potential vorticity (PV) plots, which have been used by many authors as indi- cators of STE, show relatively high PV values (> 100 10'?K hPa4s 'l) centered on longitude 20øE just to the east of Brazzaville on September 19 and 20 (Figure 7). Such injections in the tropics have been little researched but have been mentioned by Krishnamurti et al. [1993] as a possible source of ozone enhancement.

Time-height cross sections of stratospheric ozone (above 16 km above sea level asl) at each of the four stations are plotted in Figure 8. Units of milliPascals are used here, in preference to ppbv, for ease of graphical representation, as the range in values is far smaller. The ozone maximum is situated at approximately 26 km (24 km at Irene). A broad band of ozone greater than 12 mPa is present at Irene, Brazzaville, and Ascension Island. At Okaukuejo this band is poorly defined. Values seldom rise above 12 mPa, and indeed this deficiency in ozone at this level is most likely the cause of lower total ozone values observed.

Vertical Ozone Profiles Measured by Tethersondes

Tethersonde soundings conducted at Okaukuejo provide a more detailed resolution of vertical ozone disthbution in the

boundary layer. Figure 9 shows temperature and ozone profiles at 1710 LT on September 19, 1992, and 0700 LT on September 20. In the thermally convective atmosphere before sunset, ozone concentration exhibited little vertical structure

(Figure 9b). Efficient mixing ensured a nearly homogeneous ozone distribution from near ground level to the top of the sounding. Concentrations in this layer were about 30 ppbv. The standard ozonesonde showed the mean tropospheric concentration to increase to about 60 ppbv in the free troposphere.

The onset of the surface temperature inversion at 1800 LT was associated with the collapse of thermal convection. Higher ozone concentrations in the free troposphere were thus prevented from rmxmg downward to the Earth's surface. Ozone trapped within the inversion layer was steadily depleted through deposition. As a result, strong stratification in the ozone mixing ratio developed. Concentrations of less than

compares favorably with published rates of surface ozone deposition and is approximately equal to the rate of photochemical formation in haze layers just above the surface layer sampled by aircraft during SAFARI [Zenker et al., 1996]. Thus the pattem of diumal mixing, which exposes ozone- forming air to surface destruction, maintains an equilibrium in the ozone mixing ratio in the layer below the first absolutely stable layer identified by Tyson et al. [ 1996].

This low-level structure in ozone is likely to be represen- tative of conditions at Okaukuejo for most of the year. Noctumal surface inversion frequencies at Windhoek, which

----12

1

1

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i i

• 14

10

8

4

2

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-20 -15 -10 -5 0

-20 - 15 - 10 -5 0 5 10 15 20 25 30 35 40 45 50

Longitude

(b)

6 0.• I••!1• I /•' I[I I/ •. vi I 16 2 12

Longitude

-'-" 12

8 8

..• 6 6 .? 4 4

0 I I I I• I I I I I I I I 0 -20 - 15 - 10 -5 0 5 10 15 20 25 30 35 40 45 50

Longitude

Ei•re 7. Ve•ical cross sections of potential vo•ici• -(•+•/• [10-7K •a4s '•] t•ough 5øS for (a) September 19, 1992, •) September 20, 1992, and (c) September 21, 1992.

(c)


a) Ascension Island (c) Okaukuejo

.30 .30 30 I-- •o---•_ • • • / \ • •o.. / I 30

:,,,, ,,,,,,,,,,,,,,, ,,,,,

36

34

32

30

• 28

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_c: 24

22

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15 Sep 1 Oct 15 Oct 15 Sep 1 Oct 15 oct

b) Brazzaville

II I•1 ' I'1•1 I•1 I I III I I/l"l-d/I I\l•.•,Xl 't,\'1,,",1, NIIIII•IlfiN. IXI•/

.... •-X 0 ' :---• •___./-:T•-, • _ '-•-•' ' , -•"- .-" 'x•: - , • - _:•: ' .

lB Sep 1 Oct 15 Oct

(d) Irene

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16 16 •J I(I I I I I I I I I I I I Iq I•'t' 15 Sep • Oct 15 Oct

36

34

32

30

28

26

24

22

20

18

16

Figure 8. Time-height cross sections of stratospheric ozone (mPa) for the SAFARI-92 period at (a) Ascension Island, (b) Brazzaville, (c) Okaukuejo, and (d) Irene.

is situated 380 km south in a similar semidesert environment, are over 80% on average and 97% in the winter months [Tyson et al., 1976]. Similar diurnal variations in vertical structure of boundary layer ozone were observed at Victoria Falls during SAFARI-92 (F. Meixner, Personal. communi- cation, 1993). Early morning (0500-0600 LT) aircraft measurements showed marked vertical stratification, with surface values of approximately 20 ppbv, increasing to 40 ppbv at the top of the inversion (- 1700 m above ground level agl). By 0800 LT, surface values increased to 40 ppbv and a low-level maximum of 50 ppbv was observed at 1500 m. In the early afternoon, values were approximately 50 ppbv throughout a well-mixed 3000-m layer.

The suite of nocturnal tethersonde soundings emphasizes the potential importance of convection in transporting ozone

and other fire products from ground level, where they are formed, up to the free troposphere.

Conclusion

The vertical distribution of ozone over the scuthem African

subcontinent and adjacent oceans has been the focus of this paper. Data from four ozonesonde stations (Brazzaville, Ascension Island, Irene, and Okaukuejo), which were operational during the SAFARI-92 experiment, have been analyzed together with a series of tethersonde soundings at Okaukuejo, which provided enhanced resolution of ozone vertical distribution in the lower boundary layer. Longer-term seasonal perspectives were provided by the approximately weekly launches of ozonesondes which commenced in 1990 at Brazzaville,


(a) 1000

9OO

800

•,E 700 • 600

• 500

ß 400

300

200

lOO

•5

(b) 1 ooo

i

Temperature (øC)

35

9OO

800

700 600 500

400

300

200

100

3O 0 i i

0 10 20 40 50

Ozone (ppbv)

19 September, 17'10 20 September, 07:00

Figure 9. Vertical profiles of (a) temperature (øC) and (b) ozone (ppbv) at 1710 LT on September 19 and 0700 LT on September 20, 1992, at Okaukuejo.

Ascension Island,. and Irene and long-term satellite-derived tropospheric residual ozone climatology at Okaukuejo.

All stations display a September maximum (October at Irene) in tropospheric ozone. There is evidence of enhanced ozone (> 45 DU) between June and September at Brazzaville and between July and October at Ascension Island. A sharply def'med September maximum is present at Okaukuejo, while the tropospheric ozone values at Irene are generally lower than at the other stations but remain elevated through to late austral summer.

The role of biomass burning is emphasized first through the coincidence of the elevated ozone values at Brazzaville and

Ascension Island with the dry season biomass burning period. Second, the abrupt appearance of an ozone maximum at Okaukuejo reflects its more southerly location with respect to the major areas of fire activity and its more variable flow patterns. Third, tropospheric ozone values at Irene are lower than the other stations and peak later, reflecting its more southerly location and the fire seasonality of Africa. An urban- industrial photochemical contribution to enhanced tropospheric ozone may also be present at Irene, where elevated ozone levels extend further into summer than at the other

stations.

During the SAFARI-92 period, general tropospheric enhancement evident in mean ozone profiles is consistently present at all stations. It is manifest as an enriched ozone layer

situated in the upper troposphere at Okaukuejo and Brazza- ville. Here concentrations exceed 80 ppbv. The mean Ascen- sion Island profiles generally show a lower tropospheric (2-8 km) enhancement of about 20 ppbv compared with the other stations. The percent contribution of the tropospheric component (derived from integrated ozonesonde profiles) to total column ozone is also greater here, averaging 18% compared with 13% at Irene.

It is noted that the stratospheric ozone maximum at Okau- kuejo is lower than the other stations. Generally, lower total ozone values are consequently found at Okaukuejo, in line with evidence of global ozone depletion in the 10ø-20øS band [Gleason et al., 1993].

Tethersonde soundings at Okaukuejo provide evidence of a low-level vertical ozone structure linked to the diurnal heating cycle. During the daytime, ozone was well mixed throughout the convective boundary layer with concentrations in the region of 30 ppbv. After the onset of the nocturnal surface inversion, strong stratification in the ozone mixing ratio developed. Higher ozone concentrations in the free troposphere were prevented from mixing downward and the ozone trapped within the inversion was depleted to values around 10 ppbv.

Acknowledg!nents. Acknowledgments are due to the Department of Environment Affairs and the Foundation for Research Develop- ment, both of Pretoria, for providing funding which enabled the South African team to participate in SAFARI-92. The South African Weather Bureau provided the equipment for the ozonesonde station which was operated at Okaukuejo during SAFARI-92. NASA provided funding for consumables at Okaukuejo and for additional launches at Irene. Students of the Universities of Natal and Virginia and staff of the CSIR, Pretoria, are thanked for their assistance in the field. We also wish to acknowledge the Etosha Ecological Research Institute for use of their facilities whilst in the field. A. Thompson is grateful for support from the NASA Tropospheric Chemistry and Atmospheric Chemical Modeling and Analysis Programs and the U.S. Environ- mental Protection Agency (DW 80935678). The University of Natal Student Visiting Lecturers' Trust Fund facilitated collaboration on the manuscript.

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A.M. Thompson, NASA Goddard Space Flight Center, Code 916, Greenbelt, MD 20771. (e-mail: thompson(•gatorl .gsfc.nasa.gov)

M. Zunckel, Council for Scientific and Industrial Research, P.O. Box 395, Pretoria 0001, South Africa. (e-mail: mzunckel•csir.co.za)

(Received July 21, 1994; revised April 23, 1996; accepted April 23, 1996.)

Vertical ozone distribution over southern Africa and adjacent

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