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

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Abstract
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: Cuiabá (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 Cuiabá. 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 station's 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.