OBSERVATIONS OF OZONE CONCENTRATIONS IN THE BRAZILIAN CERRADO DURING THE TRACE-A FIELD EXPEDITION

The Transport and Atmospheric Chemistry Near the Equator-Atlantic (TRA CE A) experiment, sponsored by the NASA GTE program, was a multination al field mission that took place simultaneously in Brazil, Africa, and the South Atlantic region, between the African and the Brazilian coas ts. The general objective of the field mission was to investigate the tropospheric minor constituent composition, known to be disturbed by b iomass burning practices. This report describes ozone measurements tha t were made by the Brazilian component. Two field missions in central Brazil were made with the objective of investigating ozone concentrati ons in the biomass burning source region: one smaller mission in the w et season period, April, and a major mission in the dry season, Septem ber/October 1992. The main field expedition during the dry season obta ined data over a period of about 20 days in September and a few days i n October 1992, in a savanna environment of central Brazil. Simultaneo us surface ozone and ozone soundings were made. In the wet season the observation site was Goiania (16 degrees S, 49 degrees W); and in the dry season, two other sites were added: Cuiaba (16 degrees S, 56 degre es W) and Porto Nacional (11 degrees S, 48 degrees W). In addition, me asurements were also made at an Atlantic coast site, Natal (6 degrees S, 35 degrees W), outside of the savanna region, and not affected dire ctly by the biomass burning source areas, used as a control site. The average behavior of the ozone concentrations at the different sites su ggests that surface ozone concentrations tend to be rather uniform, de spite different precipitation rates, but slightly larger at the drier sites. However, other factors, such as burning fuel, for example, or c loudiness, may be also important to determine ozone concentrations. Th is is reflected by large day-to-day variabilities that are common in t he source region. The diurnal variation of the surface ozone concentra tions maximize around 1600 LT. Hourly averages in September, at this t ime, 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 tho se 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 Apr il, 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, SO ppbv have been measured at the surface and in the lower troposphere. In comparison with the coastal site, nea r the surface, large scatter in concentration values at Porto Nacional (20-80 ppbv) contrast with the smaller concentration range seen at Na tal (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 s ame height region, larger concentrations and a large scatter of the da ta are apparent. In the upper troposphere, perhaps surprisingly, the o zone concentrations at Natal and Porto Nacional are about equal, 70 pp bv at 10 km (with larger scatter at Natal than at Porto Nacional), pro bably reflecting a net production of ozone along the pathways from the source regions, coupled with its longer lifetime at the higher altitu des. This data set is consistent with the hypothesis that tropical ozo ne in the troposphere is produced photochemically from biomass burning products in the dry season. These products are exported from the sour ce regions to the upper atmospheric levels by dry and wet convection a nd once in the upper atmospheric levels are taken eastward to the Sout h Atlantic by the prevailing winds, where they contribute to local ozo ne formation. The air masses at Natal, Brazil, in the lower atmospheri c levels, i.e., below about 500 hPa, originate from the South Atlantic . This allows one to classify Natal air masses as pristine over most o f the year and justifies the stations use as a control station. Howeve r, the data now presented, combined with the detailed analyses of the other TRACE A studies, allows on to conclude that in the dry season, N atal ozone concentrations below about 500 hPa are perturbed by combust ion products consistent with long-range transport from Africa.