Layer enhancements of tropospheric ozone in regions of biomass burning

Abstract

Several field expeditions were organized in recent years to study modifications introduced into the lower atmosphere of the Amazon region, as a result of anthropogenic influence. In a simplified way, the legal Brazilian Amazon is formed by the rainforest ecosystem in the North and west, and the savanna ( cerrado) in the central region. The frontier region between these two quite different types of vegetation is where most burning occurs, and due to the large areas and amounts of phytomass involved these burnings may represent sources of Global importance. This report describes enhancements of atmospheric ozone concentrations in the remote cerrado troposphere which are believed to result from biomass burning activity. The measurements were obtained in two field expeditions to the Brazilian cerrado region of central Brazil, an area subject to cyclic burning, during the local dry season. Ozone concentrations are known to increase in the burning season. However, this process is normally a gradual increase over the season and proportional at all heights. Occasionally, large layer enhancements are seen. Three such cases are discussed, showing concentrations of ozone between 80 and 120 ppbv (parts per billion by volume), when normal averages are about 60 ppbv. To our knowledge this is the first time that such large concentrations were observed in areas of biomass burning. In one event the ozone mixing ratio was increased in a layer near 800 mbar (2.0 km above ground) to a maximum of 112 ppbv, when the known maximum observed near this height was 80 ppbv. Other events show a layer near 250 mbar (10.9 km above ground) reaching 120 and 122 ppbv at the peak. It is believed that these are special events which could be produced by a coincidence of chemical production and dynamical layering effects.