Atmospheric Sciences

1] The distributions of NO x and O 3 are analyzed during TOPSE (Tropospheric Ozone Production about the Spring Equinox). In this study these data are compared with the calculations of a global chemical/transport model (Model for OZone And Related chemical Tracers (MOZART)). Specifically, the effect that hydrolysis of N 2 O 5 on sulfate aerosols has on tropospheric NO x and O 3 budgets is studied. The results show that without this heterogeneous reaction, the model significantly overestimates NO x concentrations at high latitudes of the Northern Hemisphere (NH) in winter and spring in comparison to the observations during TOPSE; with this reaction, modeled NO x concentrations are close to the measured values. This comparison provides evidence that the hydrolysis of N 2 O 5 on sulfate aerosol plays an important role in controlling the tropospheric NO x and O 3 budgets. The calculated reduction of NO x attributed to this reaction is 80 to 90% in winter at high latitudes over North America. Because of the reduction of NO x , O 3 concentrations are also decreased. The maximum O 3 reduction occurs in spring although the maximum NO x reduction occurs in winter when photochemical O 3 production is relatively low. The uncertainties related to uptake coefficient and aerosol loading in the model is analyzed. The analysis indicates that the changes in NO x due to these uncertainties are much smaller than the impact of hydrolysis of N 2 O 5 on sulfate aerosol. The effect that hydrolysis of N 2 O 5 on global NO x and O 3 budgets are also assessed by the model. The results suggest that in the Northern Hemisphere, the average NO x budget decreases 50% due to this reaction in winter and 5% in summer. The average O 3 budget is reduced by 8% in winter and 6% in summer. In the Southern Hemisphere (SH), the sulfate aerosol loading is significantly smaller than in the Northern Hemisphere. As a result, sulfate aerosol has little impact on NO x and O 3 budgets of the Southern Hemisphere.

The budget of nitrogen oxides (NO x ) in the arctic free troposphere is calculated with a constrained photochemical box model using aircraft observations from the Tropospheric O 3 Production about the Spring Equinox (TOPSE) campaign between February and May. Peroxyacetic nitric anhydride (PAN) was observed to be the dominant odd nitrogen species (NO y ) in the arctic free troposphere and showed a pronounced seasonal increase in mixing ratio. When constrained to observed acetaldehyde (CH 3 CHO) mixing ratios, the box model calculates unrealistically large net NO x losses due to PAN formation (62 pptv/day for May, 1-3 km). Thus, given our current understanding of atmospheric chemistry, these results cast doubt on the robustness of the CH 3 CHO observations during TOPSE. When CH 3 CHO was calculated to steady state in the box model, the net NO x loss to PAN was of comparable magnitude to the net NO x loss to HNO 3 (NO 2 reaction with OH) for spring conditions. During the winter, net NO x loss due to N 2 O 5 hydrolysis dominates other NO x loss processes and is near saturation with respect to further increases in aerosol surface area concentration. NO x loss due to N 2 O 5 hydrolysis is sensitive to latitude and month due to changes in diurnal photolysis (sharp day-night transitions in winter to continuous sun in spring for the arctic). Near NO x sources, HNO 4 is a net sink for NO x ; however, for more aged air masses HNO 4 is a net source for NO x , largely countering the NO x loss to PAN, N 2 O 5 and HNO 3. Overall, HNO 4 chemistry impacts the timing of NO x decay and O 3 production; however, the cumulative impact on O 3 and NO x mixing ratios after a 20-day trajectory is minimal. r

Nonmethane hydrocarbons (NMHCs) and selected halocarbons were measured in whole air samples collected over the remote Pacific Ocean during NASA's Global Tropospheric Experiment (GTE) Pacific Exploratory Mission-Tropics B (PEM-Tropics B) in March and early April 1999. The large-scale spatial distributions of NMHCs and C2C14 reveal a much more pronounced north-south interhemispheric gradient, with higher concentrations in the north and lower levels in the south, than for the late August to early October 1996 PEM-Tropics A experiment. Strong continental outflow and winter-long accumulation of pollutants led to seasonally high Northern Hemisphere trace gas levels during PEM-Tropics B. Observations of enhanced levels of Halon 1211 (from developing Asian nations such as the PRC) and CH3C1 (from SE Asian biomass burning) support a significant southern Asian influence at altitudes above 1 km and north of 10øN. By contrast, at low altitude over the North Pacific the dominance of urban/industrial tracers, combined with low levels of Halon 1211 and CH3C1, indicate a greater influence from developed nations such as Japan, Europe, and North America. Penetration of air exhibiting aged northern hemisphere characteristics was frequently observed at low altitudes over the equatorial central and western Pacific south to -5øS. The relative lack of southern hemisphere biomass burning sources and the westerly position of the South Pacific convergence zone contributed to significantly lower PEM-Tropics B mixing ratios of the NMHCs and CH3C1 south of 10øS compared to PEM-Tropics A. Therefore the trace gas composition of the South Pacific troposphere was considerably more representative of minimally polluted tropospheric conditions during PEM-Tropics B.

During the First Aerosol Characterization Experiment (ACE 1) field campaign, 1419 whole air samples were collected over the Southern Ocean, of which approximately 700 samples were collected in the marine boundary layer (MBL), 300 samples were taken in the free troposphere (FT), and the remainder were collected in the buffer layer (BuL), the layer between the MBL and FT. Concentrations of tetrachloroethene, ethane, ethyne, and propane decayed over the 24 day duration of the intensive portion of the field campaign, which began November 18, 1995. This decline was consistent with what is known about seasonal increase of HO and the seasonal decrease in biomass burning. Using a simple empirical model, the best fit to the observations was obtained when the average [HO] was 6.1 + 0.3 x 105 HO cm -3, and an average [C1] of 720 + 100 C1 cm -3. The corresponding exchange times were 14 + 2 days between the MBL and FT, and 49 +40/-13 days between the MBL in the intensive campaign region and the MBL region to the north (nMBL). 1. Introduction Knowledge of the oxidative capacity of the atmosphere, and the contributions to it from various short-lived free radicals such as hydroxyl (HO), atomic chlorine (C1), and nitrate (NO3), is crucial in assessing the lifetimes and impacts of many greenhouse and/or ozone-depleting gases, such as methane and methyl chloroform. In the troposphere, many important trace gases, such as hydrocarbons, hydrohalocarbons, carbon monoxide, and dimethyl sulfide (DMS), are primarily removed from the atmosphere by reaction with HO radicals. In some polluted air masses, oxidation by CI and NO 3 can be significant [Blake et al., 1993; Wingenter et al., 1996]. The oxidation of DMS and its products can lead to the formation of sulfate aerosols, which affect the Earth's radiative budget [Charlson e; al., 1987]. Several articles addressing the oceanic and atmospheric distribution of DMS in the First Aerosol Characterization

Canister sampling for the determination of atmospheric mixing ratios of nonmethane hydrocarbons (NMHCs), selected halocarbons, and methyl nitrate was conducted aboard the National Center for Atmospheric Research (NCAR) C-130 aircraft over the Pacific and Southern Oceans as part of the First Aerosol Characterization Experiment (ACE 1) during November and December 1995. A latitudinal profile, flown from 76øN to 60øS, revealed latitudinal gradients for most trace gases. NMHC and halocarbon gases with predominantly anthropogenic sources, including ethane, ethyne, and tetrachloroethene, exhibited significantly higher mixing ratios in the northern hemisphere at all altitudes. Methyl chloride exhibited its lowest mixing ratios at the highest northern hemisphere latitudes, and the distributions of methyl nitrate and methyl iodide were consistent with tropical and subtropical oceanic sources. Layers containing continental air characteristic of aged biomass burning emissions were observed above about 3 km over the remote southern Pacific and near New Zealand between approximately 19øS and 43øS. These plumes originated from the west, possibly from fires in southern Africa. The month-long intensive investigation of the clean marine southern midlatitude troposphere south of Australia revealed decreases in the mixing ratios of ethane, ethyne, propane, and tetrachloroethene, consistent with their seasonal mixing ratio cycle. By contrast, increases in the average marine boundary layer concentrations of methyl iodide, methyl nitrate, and dimethyl sulfide (DMS) were observed as the season progressed to summer conditions. These increases were most appreciable in the region south of 44øS over Southern Ocean waters characterized as subantarctic and polar, indicating a seasonal increase in oceanic productivity for these gases. 4permanently at ; Wang et al., 1995; Blake et al., 1997]. The main sink of NMHC gases with an extractable hydrogen atom, such as ethane, as well as organic gases with an unsaturated bond, such as tetrachloroethene, is reaction with hydroxyl radicals (HO). Because HO is generated photochemically during daylight, its concentration varies diurnally and seasonally. Minimum HO levels are found in winter at high latitudes, resulting in significantly reduced removal rates for many trace gases. In the remote atmosphere, the most abundant NMHC compounds are the longest-lived group of light hydrocarbons (i.e., ethane, ethyne, and propane).

Measurements of the atmospheric mixing ratios of ozone, peroxyacetylmtrate (PAN), hydrocarbons. and alk3q rotrates were made in a boreal forest ecosystem in the interior of Alaska from March 15 to May 14, 1993. During this period the mixing ratios of PAN, alkyl nitrates, and nonmethane hydrocarbons (NMHCs) generally decreased due to the influence of both meteorology and OH removal. Mean mixing ratios of ozone, PAN, C2 -C6 alk•4 rotrates, and total C2 -C5 NMHC during southerly flow periods were 24.4 parts per billion (ppbv), 132.1 parts per trillion (pptv), 34 pptv, and 8.2 ppbCv, respectively. During a short period of northerly flow, mixing ratios of PAN and total NMHC were approximately 2 times the southerly flow mixing ratios. PAN is correlated with ozone, and alk3q rotrates are correlated with alkanes. PAN and ozone mixing ratios exhibit similar diurnal variations on a number of days with an early morning mimmuln and afternoon maximum. This is likely due to a diurnal cycle in the boundar3 • layer -free troposphere exchange and loss processes in the boundary layer for both 03 and PAN. Higher molecular weight (mw) hydrocarbons and alk34 rotrates are observed to decrease more quickly than the lower mxv hydrocarbons, consistera with removal by OH as the primal' loss process. been reported to contribute 70-80% to NOy during spring in the high Arctic [Bottenheim et al., 1993; Muttmramu et aL, 1994]. In addition to the odd nitrogen reservoir species, elevated levels of NMHC are present in the high latitudes during spring [Rasmussen et al., 1983; Blake cmd Rowland, 1985; Pe•ffett, 1993; dobson et al., 1994b]. Furthermore, the decay of N(,)> during spring has been found to coincide with a pulse in NO mixing ratios [Honrath and daffe, 1992]. The combination of enhanced N()x and NMHC from anthropogenic sources may influence the ozone budget in the Arctic troposphere during spring.

Surface observations of several nitrogen oxides in the Canadian high Arctic during the period March-April 1988 are reported. These include data on NO2, the inorganic nitrates HNO3 and particulate nitrate, and the organic nitrates PAN and C3–C7 alkyl-nitrates. It is found that the organic nitrates make up 70–80% of the sum of the measured nitrogen oxides. Based on concurrently measured sulphur oxides, the period of observation was divided into two halves with the first half representing less polluted, more aged air than the second. The preponderance of the organic nitrates was less in the first period than the second. In contrast, there was little difference in the inorganic nitrates and NO2 concentrations. The dominant inorganic nitrate shifted from particulate nitrate in the first period towards gaseous HNO3 in the second. No correlation between the nitrates (inorganic or organic) and O3 was observed; although some indication of a positive correlation between NO2 and O3 has been reported earlier (Bottenheimet al., 1990). Possible explanations for these observations are proposed. A survey of other potential nitrogen oxides that may be present in the Arctic air but not measured in these experiments suggests that the nitrogen oxides not measured here constitute a minor fraction of the total reactive nitrogen (NOy ).

Results are presented from the 2-month Polar Sunrise Experiment 1988, which was undertaken to further investigate the cause of ozone destruction during spring in the lower Arctic atmosphere. A strong anticorrelation between the decrease in ozone and concurrent increase in bromine compounds collected on filters observed during a 21-day period in 1986 was confirmed. It is shown that the reason for this observation is a meteorological modulation which alternately brings to the sampling location lower boundary layer air, depleted in ozone and enriched in filterable bromine, and free tropospheric air with abundant ozone and few bromine compounds. Several other compounds that may have a bearing on the chemical reactions leading to ozone depletion were measured. Bromoform, a potential source for Br atoms, was found to be present at levels between 1 and 10 parts per trillion by volume (pptv); good evidence was obtained that, as with filterable bromine, it had increased in ozone depleted boundary layer air. The mean NO 2 level was 85 pptv with an estimated uncertainty of a factor less than 2. However, this measurement of NO 2 may have been the sum of NO 2 + N205. The possibility that Br atoms are formed from an N20 5 + NaBr interaction is therefore not ruled out. There was a good indication that apparent NO 2 was depleted during episodes of low ozone. An upper limit for the mixing ratio of formaldehyde was found to be 39 pptv, while acetaldehyde was observed at mixing ratios of ca. 65 pptv. Ethylene and acetylene were also found to be depleted concurrently with 0 3. These compounds do react with Br radicals in air (as do the aldehydes) and it is speculated that they may create a link with HO x chemistry. Data on several other chemical compounds are also presented. Their participation in the 03 depletion chemistry is less clear. The implications of the measurements with respect to the hypothesis that the ozone depletion are due to a BrOx-O3 destruction cycle are explored.

The Tropospheric Ozone Production about the Spring Equinox (TOPSE) experiment was conducted during February-March, 2000 to study the evolution of tropospheric chemistry at mid to high latitudes over North America. The experiment used airborne in situ and remote sensing measurements of trace gases, radiation, and aerosols, combined with model simulations, to describe the major processes that control evolution of ozone and oxidants during the winter-spring transition. This paper introduces the major scientific questions of the TOPSE experiment, describes the conduct of the mission, and introduces the scientific results discussed in a series of companion papers in this special section. Among the significant findings of the experiment was that tropospheric ozone increases from winter to spring were dominated by in situ photochemical production in the troposphere, rather than by transport from the stratosphere. Also unique observations of widespread surface ozone depletion events, and of seasonal evolution of trace gases and aerosols as a function of latitude and altitude, were obtained over the course of the mission.

We report measurements of light (C 2-C 4) nonmethane hydrocarbons (NMHCs) and C 1-C 4 alkyl nitrates made at Summit, Greenland, over a full annual cycle (June 1997-1998). The remoteness of the Summit camp from industrial source regions resulted in trends of these trace gases that showed a clear seasonal variation with low variability. Variability (calculated as the percentage 1-sigma deviation of a running 10-point mean) averaged 7-9% for ethane over the entire study. The shorter-lived species (ethyne, propane, and the butanes) exhibited variability from 12-20% in winter to 20-100% in summer. The best fit curve to the annual cycle of ethane is a sinusoidal oscillation, but each of the shorter-lived NMHCs exhibited flat periods of low concentrations during the summer months. The C 2-C 4 NMHCs peaked between 8 and 24 February, with the longer-lived NMHCs maximizing latest. These data were broadly consistent with literature values, confirming that high-latitude Northern Hemisphere (NH) emissions are similar year to year. Employing their linear fall accumulation rates, we calculated average NMHC ratios versus ethane of 0.59(±0.10):0.33(±0.05):0.26(±0.08):0.14(±0.04) for propane, ethyne, n-butane, and ibutane, respectively. We suggest that these ratios represent useful quantities with which to compare averaged mid-and high-latitude NH emission ratios. We also report the first yearround observations of the seasonal cycle of light C 1-C 4 alkyl nitrates. Similar to the NMHCs, the seasonal trend of these gases shows primary dependence on transport to Summit during winter, and photochemical removal during summer. Unlike their parent NMHCs, alkyl nitrate concentrations did not asymptote to low levels during summer and they exhibited winter maxima later with decreasing photochemical lifetime.

Local ozone production and loss rates for the arctic free troposphere (58-85 • N, 1-6 km, February-May) during the Tropospheric Ozone Production about the Spring Equinox (TOPSE) campaign were calculated using a constrained photochemical box model. Estimates were made to assess the importance of local photochemical ozone production relative to transport in accounting for the springtime maximum in arctic free tropospheric ozone. Ozone production and loss rates from our diel steady-state box model constrained by median observations were first compared to two point box models, one run to instantaneous steady-state and the other run to diel steady-state. A consistent picture of local ozone photochemistry was derived by all three box models suggesting that differences between the approaches were not critical. Our model-derived ozone production rates increased by a factor of 28 in the 1-3 km layer and a factor of 7 in the 3-6 km layer between February and May. The arctic ozone budget required net import of ozone into the arctic free troposphere throughout the campaign; however, the transport term exceeded the photochemical production only in the lower free troposphere (1-3 km) between February and March. Gross ozone production rates were calculated to increase linearly with NO x mixing ratios up to ∼300 pptv in February and for NO x mixing ratios up to ∼500 pptv in May. These NO x limits are an order of magnitude higher than median NO x levels observed, illustrating the strong dependence of gross ozone production rates on NO x mixing ratios for the majority of the observations. The threshold NO x mixing ratio needed for net positive ozone production was also calculated to increase from NO x ∼ 10 pptv in February to ∼25 pptv in May, suggesting that the NO x levels needed to sustain net ozone production are lower in winter than spring. This lower NO x threshold explains how wintertime photochemical ozone production can impact the build-up of ozone over winter and early spring. There is also an altitude dependence as the threshold NO x needed to produce net ozone shifts to higher values at lower altitudes. This partly

During the Tropospheric Ozone Production about the Spring Equinox (TOPSE) program, aircraft flights during April 7-11, 2000 revealed a large area air mass capped below ∼500 m altitude over Hudson Bay, Canada in which ozone was reduced from normal levels of 30-40 ppbv to as low as 0.5 ppbv. From some of the in-situ aircraft measurements, backtrajectory calculations, the tropospheric column of BrO derived from GOME satellite measurements, and results from a regional model, we conclude that the event did not originate from triggering of reactive halogen release in the sub-Arctic region of Hudson Bay but resulted from such an event occurring at higher latitudes over the islands of the northern Canada Archipelago and nearby Arctic Ocean with subsequent transport over a distance of 1,000-1,500 km to Hudson Bay. BrO x remained active during this transport despite considerable changes in the conditions of the underlying surface suggesting that chemical recycling during transport dominated any local halogen input from the surface. If all of the tropospheric column density of BrO is distributed uniformly within the surface layer, then the mixing ratio of BrO derived from the satellite measurements is at least a factor of 2-3 larger than derived indirectly from in situ aircraft measurements of the NO/NO 2 ratio.

Observations over the tropical Pacific during the Pacific Exploratory Mission (PEM)-Tropics B experiment (March-April 1999) are analyzed. Concentrations of CO and long-lived nonmethane hydrocarbons in the region are significantly enhanced due to transport of pollutants from northern industrial continents. This pollutant import also enhances moderately 03 concentrations but not NOx concentrations. It therefore tends to depress OH concentrations over the tropical Pacific. These effects contrast to the large enhancements of 03 and NOx concentrations and the moderate increase of OH concentrations due to biomass burning outflow during the PEM-Tropics A experiment (September-October 1996). Observed CH3I concentrations, as in PEM-Tropics A, indicate that convective mass outflux in the middle and upper troposphere is largely independent of altitude over the tropical Pacific. Constraining a one-dimensional model with CH31 observations yields a 1 O-day timescale for convective turnover of the free troposphere, a factor of 2 faster than during PEM-Tropics A. Model simulated HO2, CH20, H202, and CH3OOH concentrations are generally in agreement with observations. However, simulated OH concentrations are lower (~25%) than observations above 6 km. Whereas models tend to overestimate previous field measurements, simulated HNO3 concentrations during PEM-Tropics B are too low (a factor of 2-4 below 6 km) compared to observations. Budget analyses indicate that chemical production of 03 accounts for only 50% of chemical loss; significant transport of 03 into the region appears to take place within the tropics. Convective transport of CH3OOH enhances the production of HOx and 03 in the upper troposphere, but this effect is offset by HOx loss due to the scavenging of H202. Convective transport and scavenging of reactive nitrogen species imply a necessary source of 0.4-1 Tg yr-1 of NO., in the free troposphere (above 4 km) over the tropics. A large fraction of the source could be from marine lightning. Oxidation of DMS transported by convection from the boundary layer could explain the observed free tropospheric SO2 concentrations over the tropical Pacific. This source of DMS due to convection, however, would imply in the model free tropospheric concentrations much higher than observed. The model overestimate cannot be reconciled using recent kinetics measurements of the DMS-OH adduct reaction at low pressures and temperatures and may reflect enhanced OH oxidation of DMS during convection.

We present here results for reactive nitrogen species measured aboard the NASA DC-8 aircraft during the Transport and Chemical Evolution over the Pacific (TRACE-P) mission. The large-scale distributions total reactive nitrogen (NO y,sum = NO + NO 2 + HNO 3 + PAN + C 1-C 5 alkyl nitrates) and O 3 and CO were better defined in the boundary layer with significant degradation of the relationships as altitude increased. Typically, NO y,sum was enhanced over background levels of $260 pptv by 20-to-30-fold. The ratio C 2 H 2 /CO had values of 1-4 at altitudes up to 10 km and as far eastward as 150°E, implying significant vertical mixing of air parcels followed by rapid advection across the Pacific. Analysis air parcels originating from five principal Asian source regions showed that HNO 3 and PAN dominated NO y,sum. Correlations of NO y,sum with C 2 Cl 4 (urban tracer) were not well defined in any of the source regions, and they were only slightly better with CH 3 Cl (biomass tracer). Air parcels over the western Pacific contained a complex mixture of emission sources that are not easily resolvable as shown by analysis of the Shanghai mega-city plume. It contained an intricate mixture of pollution emissions and exhibited the highest mixing ratios of NO y,sum species observed during TRACE-P. Comparison of tropospheric chemistry between the earlier PEM-West B mission and the recent TRACE-P data showed that in the boundary layer significant increases in the mixing ratios of NO y,sum species have occurred, but the middle and upper troposphere seems to have been affected minimally by increasing emissions on the Asian continent over the last 7 years.

During the Tropospheric Ozone Production about the Spring Equinox (TOPSE) aircraft program, ozone depletion events (ODEs) in the high latitude surface layer were investigated using lidar and in situ instruments. Flight legs of 100 km or longer distance were flown 32 times at 30 m altitude over a variety of regions north of 58°between early February and late May 2000. ODEs were found on each flight over the Arctic Ocean but their occurrence was rare at more southern latitudes. However, large area events with depletion to over 2 km altitude in one case were found as far south as Baffin Bay and Hudson Bay and as late as 22 May. There is good evidence that these more southern events did not form in situ but were the result of export of ozone-depleted air from the surface layer of the Arctic Ocean. Surprisingly, relatively intact transport of ODEs occurred over distances of 900-2000 km and in some cases over rough terrain. Accumulation of constituents in the frozen surface over the dark winter period cannot be a strong prerequisite of ozone depletion since latitudes south of the Arctic Ocean would also experience a long dark period. Some process unique to the Arctic Ocean surface or its coastal regions remains unidentified for the release of ozone-depleting halogens. There was no correspondence between coarse surface features such as solid ice/snow, open leads, or polynyas with the occurrence of or intensity of ozone depletion over the Arctic or subarctic regions. Depletion events also occurred in the absence of long-range transport of relatively fresh ''pollution'' within the high latitude surface layer, at least in spring 2000. Direct measurements of halogen radicals were not made. However, the flights do provide detailed information on the vertical structure of the surface layer and, during the constant 30 m altitude legs, measurements of a variety of constituents including hydroxyl and peroxy radicals. A summary of the behavior of these constituents is made. The measurements were consistent with a source of formaldehyde from the snow/ice surface. Median NO x in the surface layer was 15 pptv or less, suggesting that surface emissions were substantially converted to reservoir constituents by 30 m altitude and that ozone production rates were small (0.15-1.5 ppbv/d) at this altitude. Peroxyacetylnitrate (PAN) was by far the major constituent of NO y in the surface layer independent of the ozone mixing ratio.

The budgets of ozone and nitrogen oxides (NOx = NO + NO2) ill the tropical South Pacific troposphere are analyzed by photochemical point modeling of aircraft observations at 0-12 km altitude from the Pacific Exploratory Mission-Tropics A campaign flown in September-October 1996. The model reproduces the observed NO2/NO concentration ratio to within 30% and has similar success in simulating observed concentrations of peroxides (H202, CH3OOH), lending confidence in its use to investigate ozone chemistry. It is found that chemical production of ozone balances only half of chemical loss in the tropospheric column over the tropical South Pacific. The net loss is 1.8 x 1011 molecules cm-2 S '1. The missing source of ozone is matched by westerly transport of continental pollution into the region. Independent analysis of the regional ozone budget with a global three-dimensional model corroborates the results from the point model and reveals the importance of biomass burning emissions in South America and Africa for the ozone budget over the tropical South Pacific. In this model, biomass burning increases average ozone concentrations by 7-8 ppbv throughout the troposphere. The NOx responsible for ozone production within the South Pacific troposphere below 4 km can be largely explained by decomposition of peroxyacetylnitrate (PAN) transported into the region with biomass burning pollution at higher altitudes. Narragansett. Equatorial Atlantic (TRACE A)campaign conducted in Sep-8School of Earth and Atmospheric Sciences, Georgia Institute of tember 1992 [Fishman et al., 1996] demonstrated that Technology, Atlanta.