Boundary layer ozone in the tropical america northern hemisphere region

Abstract

In recent field campaigns, boundary layer ozone concentrations were measuredat different sites in the Orinoco river basin (mainly savannahs) and nearbylocations (cloud forest and coastal sites), during dry and wet seasons. Theresults provide a comprehensive description of boundary layer ozone in atropical region north of the equator where scarce information is available.Ozone measurements were made using photometric ozone analyzers. In thesavannah region, ozone profiles (up to 1000 m) were also obtained usingelectrochemical concentration cell sondes, launched in a tethered balloon.The results, in comparison with other parts of the world, confirm the lowvalues for the concentration of ozone in the boundary layer of the studyregion. Throughout the entire region (marine, coastal, and continental areas),higher concentrations were produced during the dry season (20–30 ppbv)than during the wet season (13–17 ppbv), likely due to the photochemicalproduction of ozone from biomass burning emissions. This seasonal variationobserved in the boundary layer contrasts with satellite data, that show highertotal tropospheric ozone columns during the wet season in the 0–15°Nregion; this means that the ozone annual cycles in the boundary layer and freetroposphere are out of phase.At the most pristine continental sites in the southern part of Venezuela,quite a `peculiar' diurnal variation was observed, with maximum concentrationsin the morning and a steady decrease of ∼0.6 ppbv hr-1 duringthe hours of high solar irradiation. Although dynamic aspects cannot be ruledout, the daytime ozone decrease, which cannot be explained by thewell-established `classical' continental O3 sinks, suggests thatin this region part of the ozone consumption is caused byHOx cycles at low NOx levels. Futurestudies to elucidate the causes of the daytime ozone decrease at thesecontinental sites should be undertaken.