Impact of the deep convection of isoprene and other reactive trace species on radicals and ozone in the upper troposphere

Abstract

Abstract. Observations of a comprehensive suite of inorganic and organic trace gases, including non-methane hydrocarbons (NMHCs), halogenated organics and oxygenated volatile organic compounds (OVOC), obtained from the NASA DC-8 over Canada during the ARCTAS aircraft campaign in July 2008 illustrate that convection is important for redistributing both long and short-lived species throughout the troposphere. Convective outflow events were identified by the elevated mixing ratios of organic species in the upper troposphere relative to background conditions. Several dramatic events were observed in which isoprene and its oxidation products were detected at hundreds of pptv at altitudes higher than 8 km. Two events are studied in detail using detailed experimental data and the NASA Langley Research Center (LaRC) box model. One event had no lightning NOx (NO + NO2) associated with it and the other had substantial lightning NOx (LNOx). When convective storms transport isoprene from the boundary layer to the upper troposphere and LNOx is present, there is a large effect on the expected ensuing chemistry. The model predicts a dominant impact on HOx and nitrogen-containing species; the relative contribution from other species such as peroxides is insignificant. The isoprene reacts quickly, resulting in primary and secondary products, including formaldehyde and methyl glyoxal. The model predicts enhanced production of alkyl nitrates (ANs) and peroxyacyl nitrate compounds (PANs). PANs persist because of the cold temperatures of the upper troposphere resulting in a large change in the NOx mixing ratios, compared to the case in which no isoprene is convected, a scenario which is also explored by the model. This, in turn, has a large impact on the HOx chemistry. Ozone production is substantial during the first few hours following the event, resulting in a net gain of approximately 10 ppbv compared to the scenario in which no isoprene is present aloft. In the case of isoprene being present aloft but no LNOx, OH is reduced due to scavenging by isoprene, which serves to slow the chemistry resulting in longer lifetimes for species that react with OH.