Convection and Redistribution of Tropospheric Ozone in Central Amazonia

The Amazon rainforest represents the largest terrestrial source of plant-emitted hydrocarbons. Once in the atmospheric boundary layer, hydrocarbons react with ozone, hydroxyl and nitrate radicals to form oxidants and secondary organic aerosols. Rates of oxidant formation from hydrocarbons strongly depend on levels of nitrogen oxides, which in the remote rainforest mostly prevail in the parts per trillion (pptv) levels. Two processes can influence the chemical cycles in the rainforest atmospheric boundary layer. First, mesoscale convective storms transport mid-tropospheric ozone-rich air to the atmospheric boundary layer. Second, as exemplified by the city of Manaus, Amazonas, Brazil, urbanization can enhance nitrogen oxides and carbon monoxide levels in forested, downwind areas. Results obtained as part of the GoAmazon 2014/15 project indicate that storms can enrich the regional atmospheric boundary layer with as much as 40 parts per billion (ppbv) of ozone. On average, ozone enhancements can last two hours following storms. Levels of ozone enhancements depend on the amounts of Convective Available Potential Energy (CAPE), which is a metric of the atmospheric convection associated with storms. Satellite-derived ozone profiles indicate that storms appreciably redistribute ozone in the lower troposphere. Due to sinks associated with surface deposition and chemical reactions in the atmospheric boundary layer, there is a measurable reduction of ozone in the lower troposphere sometime after convective storms. Emissions from the city of Manaus contribute to enhanced levels of nitrogen oxides and carbon monoxide, which reach maximum values of 5 ppbv and 400 ppb, respectively, at about 100 km downwind from sources. Such emissions of precursors result in enhanced formation of ozone whose mixing ratios can reach up to 90 ppbv. Anthropogenic emissions and the advection of polluted plumes can match and exceed ozone transport by convection. Both processes enhance the atmospheric boundary layer with ozone which subsequently react with plant-emitted hydrocarbons to produce large amounts of free radicals, thereby accelerating the air chemical cycles in the rainforest ecosystem.