Volker W. J. H. Kirchhoff

Convective transport of biomass burning emissions over Brazil during TRACE A

A series of large mesoscale convective systems that occurred during the Brazilian phase of GTE/TRACE A (Transport and Atmospheric Chemistry near the Equator-Atlantic) provided an opportunity to observe deep convective transport of trace gases from biomass burning. This paper reports a detailed analysis of flight 6, on September 27, 1992, which sampled cloud- and biomass-burning-perturbed regions north of Brasilia. High-frequency sampling of cloud outflow at 9-12 km from the NASA DC-8 showed enhancement of CO mixing ratios typically a factor of 3 above background (200- 300 parts per billion by volume (ppbv) versus 90 ppbv) and significant increases in NOx and hydrocarbons. Clear signals of lightning-generated NO were detected; we estimate that at least 40% of NO x at the 9.5-km level and 32% at 11.3 km originated from lightning. Four types of model studies have been performed to analyze the dynamical and photochemical characteristics of the series of convective events. (1) Regional simulations for the period have been performed with the NCAR/Penn State mesoscale model (MM5), including tracer transport of carbon monoxide, initialized with observations. Middle-upper tropospheric enhancements of a factor of 3 above background are reproduced. (2) A cloud-resolving model (the Goddard cumulus ensemble (GCE) model) has been run for one representative convective cell during the September 26-27 episode. (3) Photochemical calculations (the Goddard tropospheric chemical model), initialized with trace gas observations (e.g., CO, NO x, hydrocarbons, 03) observed in cloud outflow, show appreciable 0 3 formation postconvection, initially up to 7-8 ppbv O3/d. (4) Forward trajectories from cloud outflow levels (postconvective conditions) put the ozone-producing air masses in eastern Brazil and the tropical Atlantic within 2-4 days and over the Atlantic, Africa, and the Indian Ocean in 6-8 days. Indeed, 3-4 days after the convective episode (September 30, 1992), upper tropospheric levels in the Natal ozone sounding show an average increase of -30 ppbv (3 Dobson units (DU) integrated) compared to the September 28 sounding. Our simulated net 0 3 production rates in cloud outflow are a factor of 3 or more greater than those in air undisturbed by the storms. Integrated over the 8- to 16-km cloud outflow layer, the postconvection net 0 3 production (-5-6 DU over 8 days) accounts for -25% of the excess 03 (15-25 DU) over the South Atlantic. Comparison of TRACE A Brazilian ozonesondes and the frequency of deep convection with climatology (Kirchhoff et al., this issue) suggests that the late September 1992 conditions represented an unusually active period for both convection and upper tropospheric ozone formation.

Where did tropospheric ozone over southern Africa and the tropical Atlantic come from in October 1992? Insights from TOMS, GTE TRACE A, and SAFARI 1992

The seasonal tropospheric ozone maximum in the tropical South Atlantic, first recognized from satellite observations (Fishman et al., 1986, 1991), gave rise to the IGAC/ STARE/SAFARI 1992/TRACE A campaigns (International Global Atmospheric Chemistry/South Tropical Atlantic Regional Experiment/Southern African Fire Atmospheric Research Initiative/Transport and Atmospheric Chemistry Near the Equator- Atlantic) in September and October 1992. Along with a new TOMS-based method for deriving tropospheric column ozone, we used the TRACE A/SAFARI 1992 data set to put together a regional picture of the 0 3 distribution during this period. Sondes and aircraft profiling showed a troposphere with layers of high O3 (->90 ppbv) all the way to the tropopause. These features extend in a band from 0 o to 25oS, over the SE Indian Ocean, Africa, the Atlantic, and eastern South America. A combination of trajectory and photochemical modeling (the Goddard (GSFC) isentropic trajectory and tropospheric point model, respectively) shows a strong connection between regions of high ozone and concentrated biomass burning, the latter identified using satellite-derived fire counts (Justice et al., this issue). Back trajectories from a high-O3 tropical Atlantic region (column ozone at Ascension averaged 50 Dobson units (DU)) and forward trajectories from fire- rich and convectively active areas show that the Atlantic and southern Africa are supplied with O3 and O3-forming trace gases by midlevel easterlies and/or recirculating air from Africa, with lesser contributions from South American burning and urban pollution. Limited sampling in the mixed layer over Namibia shows possible biogenic sources of NO. High-level westerlies from Brazil (following deep convective transport of ozone precursors to the upper troposphere) dominate the upper tropospheric 03 budget over Natal, Ascension, and Okaukuejo (Namibia), although most enhanced O3 (75% or more) equatorward of 10oS was from Africa. Deep convection may be responsible for the timing of the seasonal tropospheric 0 3 maximum: Natal and Ascension show a 1- to 2-month lag relative to the period of maximum burning (cf. Baldy et al., this issue; Olson et al., this issue). Photochemical model calculations constrained with TRACE A and SAFARI airborne observations of O3 and 03 precursors (NOx, CO, hydrocarbons) show robust ozone formation (up to 15 ppbv O3/d or several DU/d) in a widespread, persistent, and well-mixed layer to 4 km. Slower but still positive net 03 formation took place throughout the tropical upper troposphere (cf. Pickering et al., this issue (a); Jacob et al., this issue). Thus whether it is faster rates of 0 3 formation in source regions with higher turnover rates or slower 03 production in long-lived stable layers ubiquitous in the TRACE A region, 10-30 DU tropospheric 03 above a -25-DU background can be accounted for. In summary, the 03 maximum studied in October 1992 was caused by a coincidence of abundant 03 precursors from biomass fires, a long residence time of stable air parcels over the eastern Atlantic and southern Africa, and deep convective transport of biomass burning products, with additional NO from lightning and occasionally biogenic sources.

Southern Hemisphere Additional Ozonesondes (SHADOZ) 1998–2000 tropical ozone climatology 2. Tropospheric variability and the zonal wave-one

(1) The first view of stratospheric and tropospheric ozone variability in the Southern Hemisphere tropics is provided by a 3-year record of ozone soundings from the Southern Hemisphere Additional Ozonesondes (SHADOZ) network (http://croc.gsfc.nasa.gov/ shadoz). Observations covering 1998-2000 were made over Ascension Island, Nairobi (Kenya), Irene (South Africa), Reunion Island, Watukosek (Java), Fiji, Tahiti, American Samoa, San Cristobal (Galapagos), and Natal (Brazil). Total, stratospheric, and tropospheric column ozone amounts usually peak between August and November. Other features are a persistent zonal wave-one pattern in total column ozone and signatures of the quasi-biennial oscillation (QBO) in stratospheric ozone. The wave-one is due to a greater concentration of free tropospheric ozone over the tropical Atlantic than the Pacific and appears to be associated with tropical general circulation and seasonal pollution from biomass burning. Tropospheric ozone over the Indian and Pacific Oceans displays influences of the waning 1997-1998 El Nino, seasonal convection, and pollution transport from Africa. The most distinctive feature of SHADOZ tropospheric ozone is variability in the data, e.g., a factor of 3 in column amount at 8 of 10 stations. Seasonal and monthly means may not be robust quantities because statistics are frequently not Gaussian even at sites that are always in tropical air. Models and satellite retrievals should be evaluated on their capability for reproducing tropospheric variability and fine structure. A 1999- 2000 ozone record from Paramaribo, Surinam (6� N, 55� W) (also in SHADOZ) shows a marked contrast to southern tropical ozone because Surinam is often north of the Intertropical Convergence Zone (ITCZ). A more representative tropospheric ozone climatology for models and satellite retrievals requires additional Northern Hemisphere tropical data. INDEXTERMS: 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry; 1640 Global Change: Remote sensing; 3309 Meteorology and Atmospheric Dynamics: Climatology (1620); 9305 Information Related to Geographic Region: Africa; 9325 Information Related to Geographic Region: Atlantic Ocean; KEYWORDS: Free-words-ozone, tropospheric ozone, ozonesondes, satellite ozone, tropical climatology, wave-one, biomass burning, El Nino, satellite retrievals

NASA GTE TRACE A experiment (September–October 1992): Overview

An overview of the Transport and Atmospheric Chemistry near the Equator-Atlantic (TRACE A) field mission is presented. TRACE A was conducted to provide a comprehensive investigation of the chemical composition, transport, and chemistry of the atmosphere over the tropical South Atlantic Ocean and the adjacent South American and African continents. Measurements for TRACE A consisted of a remote sensing component to derive tropospheric ozone and biomass burning patterns, an airborne atmospheric chemistry component to determine the composition of the air in the most pristine areas of our research domain as well as to characterize the photochemistry and transport of trace gas emissions from both fire and biogenic sources, a series of ozonesonde observations, and an enhanced radiosonde network and airborne meteorological measurements that provided information about the transport of trace gases and the physical processes that were responsible for their observed distributions. The data were interpreted through the use of both photochemical and meteorological numerical models. The picture that emerges from TRACE A is that widespread biomass burning in both South America and southern Africa is the dominant source of the precursor gases necessary for the formation of the huge amounts of ozone over the South Atlantic Ocean. In addition, however, the meteorology in this region of the world is favorable for the accumulation of these pollutants over the tropical Atlantic basin so that photochemical processes produce large quantities of ozone in situ. The generation of ozone occurs over scales of thousands of kilometers and is unusually enhanced in the upper troposphere where relatively high concentrations of nitrogen oxides (NOx) prevail. This latter finding suggests that convective processes (or other lifting mechanisms) may play an important role in the generation of tropospheric ozone or that there may be an additional significant upper tropospheric source of NOx, such as from lightning.

Ozone climatology at Natal, Brazil, from in situ ozonesonde data

Results are presented from analysis of a large ozone profile data set obtained from balloon ozonesonde soundings made at Natal, Brazil (6°S, 35°W) during the last 10 years (1978–1988). The measurements have been made through an Instituto Nacional de Pesquisas Espaciais (INPE)/NASA long-term collaboration program. The balloons were released by the Brazilian Air Force at the Natal rocket range. The data set is sufficiently large to provide useful climatology on the average ozone concentration behavior and its seasonal variation. The day-to-day ozone concentration variability in the troposphere is rather large, giving standard deviations of about 30–40% for seasonal averages. Maximum ozone concentrations occur during local spring, September–October, and minimum concentrations during late autumn, April-May. The seasonal variation in the troposphere is much larger than in the stratosphere. If there were no seasonal variation at all in the stratosphere, the seasonal variation observed in the troposphere alone would be strong enough to drive a total ozone column variation of about 5%, which is about one half the size of the variation observed in the Natal Dobson spectrophotometer data. The ozone concentration at Natal increases with height between the surface and about 500 mbar, almost linearly, from about 15 parts per billion by volume (ppbv) to about 38 ppbv, in autumn. For the spring average the ozone concentration increases from about 25 ppbv at the surface to about 66 ppbv at 500 mbar. The sonde data suggest that limitations in aneroid pressure sensors used until 1986 caused the Natal sondes to indicate too much ozone above 6 mbar. Because of the relative sparsity and uneven distribution in time of the ozone soundings, the data are not adequate to study ozone trends. The Dobson data time series shows no definitive ozone trend but displays a pronounced quasi-biennial oscillation in ozone.

Evidence for an ozone hole perturbation at 30° South

Abstract A relatively strong stratospheric column ozone decrease was observed in the south of Brazil (29.5‡S) in 1993, at the end of October. This ozone decrease was observed when the normal behavior of the ozone column at low latitudes, in Brazil, reaches its yearly maximum, so that a decrease of ozone during this time period is unexpected. The local observations were made by two different measurement techniques. Two independent ground-based Brewer spectrophotometers documented strong column ozone decreases in the south of Brazil, in 1993. The vertical distribution of ozone was observed with ozone-soundings, showing a uniform ozone decrease at all heights in the stratosphere, and very low ozone in the lower stratosphere, which has been shown to be a characteristic of Antarctic ozone in Spring. TOMS ozone data, representing a third observational technique, averaged over small latitude-longitude bands, correlate very well with the local observations, which leaves no doubt that the observed ozone decreases in the south of Brazil during October 1993 were real. Also, the observed ozone decreases may be considered large, since for comparison, the local seasonal variation is at most of the order of 30 Dobson Units (DU), whereas one of the October decreases measured about 60 DU. There seems to be no physical or chemical mechanism at low latitudes, that could account for such a large and fast perturbation in the stratosphere. On the other hand, inspection of the TOMS total ozone data maps, on the days of the above observations, show a distinct link between the Antarctic ozone hole latitudes, reaching out to the north in a curved path, and touching tropical latitudes over a narrow belt. Trajectory analyses confirm that for days of low ozone in Santa Maria, Brazil, the air masses at 20 and 25 km height have an Antarctic origin.

Observations of ozone concentrations in the Brazilian cerrado during the TRACE A field expedition

The Transport and Atmospheric Chemistry Near the Equator-Atlantic (TRACE A) experiment, sponsored by the NASA GTE program, was a multinational field mission that took place simultaneously in Brazil, Africa, and the South Atlantic region, between the African and the Brazilian coasts. The general objective of the field mission was to investigate the tropospheric minor constituent composition, known to be disturbed by biomass burning practices. This report describes ozone measurements that were made by the Brazilian component. Two field missions in central Brazil were made with the objective of investigating ozone concentrations in the biomass burning source region: one smaller mission in the wet season period, April, and a major mission in the dry season, September/October 1992. The main field expedition during the dry season obtained data over a period of about 20 days in September and a few days in October 1992, in a savanna environment of central Brazil. Simultaneous surface ozone and ozone soundings were made. In the wet season the observation site was Goiânia (16°S, 49°W); and in the dry season, two other sites were added: Cuiaba (16°S, 56°W) and Porto Nacional (11°S, 48°W). In addition, measurements were also made at an Atlantic coast site, Natal (6°S, 35°W), outside of the savanna region, and not affected directly by the biomass burning source areas, used as a control site. The average behavior of the ozone concentrations at the different sites suggests that surface ozone concentrations tend to be rather uniform, despite different precipitation rates, but slightly larger at the drier sites. However, other factors, such as burning fuel, for example, or cloudiness, may be also important to determine ozone concentrations. This is reflected by large day-to-day variabilities that are common in the source region. The diurnal variation of the surface ozone concentrations maximize around 1600 LT. Hourly averages in September, at this time, amount to 47 parts per billion by volume (ppbv) at Porto Nacional and 40 ppbv at Cuiaba. For this station the values are lower than those of previous years (55 ppbv in 1991 and 48 ppbv in 1990). Only small differences, of the order of 5 ppbv, are observed between the source (burning) sites and Natal (the control site) in the wet season. In April, only 16 ppbv are observed at Natal. Much larger concentrations may be observed occasionally in the source areas, in the dry season. For example, at Porto Nacional, 80 ppbv have been measured at the surface and in the lower troposphere. In comparison with the coastal site, near the surface, large scatter in concentration values at Porto Nacional (20–80 ppbv) contrast with the smaller concentration range seen at Natal (20–40 ppbv). In addition, at Natal the ozone mixing ratios below about 600 hPa are distributed around a vertical gradient in which the mixing ratios increase with height, whereas at Porto Nacional in the same height region, larger concentrations and a large scatter of the data are apparent. In the upper troposphere, perhaps surprisingly, the ozone concentrations at Natal and Porto Nacional are about equal, 70 ppbv at 10 km (with larger scatter at Natal than at Porto Nacional), probably reflecting a net production of ozone along the pathways from the source regions, coupled with its longer lifetime at the higher altitudes. This data set is consistent with the hypothesis that tropical ozone in the troposphere is produced photochemically from biomass burning products in the dry season. These products are exported from the source regions to the upper atmospheric levels by dry and wet convection and once in the upper atmospheric levels are taken eastward to the South Atlantic by the prevailing winds, where they contribute to local ozone formation. The air masses at Natal, Brazil, in the lower atmospheric levels, i.e., below about 500 hPa, originate from the South Atlantic. This allows one to classify Natal air masses as pristine over most of the year and justifies the stations use as a control station. However, the data now presented, combined with the detailed analyses of the other TRACE A studies, allows one to conclude that in the dry season, Natal ozone concentrations below about 500 hPa are perturbed by combustion products consistent with long-range transport from Africa.

Ozone in the Pacific tropical troposphere from ozonesonde observations

Ozone vertical profile measurements obtained from ozonesondes flown at Fiji, Samoa, Tahiti, and the Galapagos are used to characterize ozone in the troposphere over the tropical Pacific. There is a significant seasonal variation at each of these sites. At sites in both the eastern and western Pacific, ozone mixing ratios are greatest at almost all levels in the troposphere during the September-November season and smallest during March-May. The vertical profile has a relative maximum at all of the sites in the midtroposphere throughout the year (the largest amounts are usually found near the tropopause). This maximum is particularly pronounced during the September-November season. On average, throughout the troposphere, the Galapagos has larger ozone amounts than the western Pacific sites. A trajectory climatology is used to identify the major flow regimes that are associated with the characteristic ozone behavior at various altitudes and seasons. The enhanced ozone seen in the midtroposphere during September-November is associated with flow from the continents. In the western Pacific this flow is usually from southern Africa (although 10-day trajectories do not always reach the continent) but also may come from Australia and Indonesia. In the Galapagos the ozone peak in the midtroposphere is seen in flow from the South American continent and particularly from northern Brazil. High ozone concentrations within potential source regions and flow characteristics associated with the ozone mixing ratio peaks seen in both the western and eastern Pacific suggest that these enhanced ozone mixing ratios result from biomass burning. In the upper troposphere, low ozone amounts are seen with flow that originates in the convective western Pacific.

Analysis of the distribution of ozone over the southern Atlantic region

Tropospheric ozone data measured by ozonesondes during the Transport and Atmospheric Chemistry Near the Equator-Atlantic (TRACE A) field mission and the multiyear pre-TRACE A program are analyzed jointly with tropospheric ozone amounts derived from remote satellite data (“residuals”). We present here the first detailed analysis of the entire Ascension Island pre-TRACE A data set. Data from the three pre-TRACE A ozonesonde sites are used to establish a coherent spatial and temporal climatology of ozone in the southern tropical Atlantic region. This analysis shows a significant ozone seasonality over the Atlantic region, with a period of maximum values that extends from the austral winter through at least October at Natal, Brazil, and Ascension Island. Concentrations begin to decline somewhat earlier at Brazzaville, Congo, especially at lower altitudes. Although Natal exhibits a significantly lower annual average than Ascension Island or Brazzaville by about 4 Dobson Units (DU), the magnitude of the seasonal amplitude at Natal is the largest of the three stations. Additionally, more of the seasonal amplitude at Natal is due to a contribution from ozone in the middle and upper troposphere than at either Ascension Island or Brazzaville. Amplitudes as large as 15 DU are measured at individual sites, and the residuals show an average amplitude over the southern tropical Atlantic region of 10–12 DU. Statistical comparison of the residuals to the ozonesonde climatology show that while the residuals tend to underpredict both the means and the seasonal amplitudes compared to the in situ data, they provide a good representation of the variance of ozone in this region and predict the local annual and seasonal means to within better than 10% and seasonal amplitudes to within 15%.

Operational Overview of the NASA GTE/CITE 3 Airborne Instrument Intercomparisons for Sulfur Dioxide, Hydrogen Sulfide, Carbonyl Sulfide, Dimethyl Sulfide, and Carbon Disulfide

This paper reports the overall experimental design and gives a brief overview of results from the third airborne Chemical Instrumentation Test and Evaluation (CITE 3) mission conducted as part of the National Aeronautics and Space Administrations Global Tropospheric Experiment. The primary objective of CITE 3 was to evaluate the capability of instrumentation for airborne measurements of ambient concentrations of SO2, H2S, CS2, dimethyl sulfide, and carbonyl sulfide. Ancillary measurements augmented the intercomparison data in order to address the secondary objective of CITE 3 which was to address specific issues related to the budget and photochemistry of tropospheric sulfur species. The CITE 3 mission was conducted on NASAs Wallops Flight Center Electra aircraft and included a ground-based intercomparison of sulfur standards and intercomparison/sulfur science flights conducted from the NASA Wallops Flight Facility, Wallops Island, Virginia, followed by flights from Natal, Brazil. Including the transit flights, CITE 3 included 16 flights encompassing approximately 96 flight hours.