Atmospheric Chemistry and Physics (Jan 2012)
Impact of the deep convection of isoprene and other reactive trace species on radicals and ozone in the upper troposphere
Abstract
Observations of a comprehensive suite of inorganic and organic trace gases, including non-methane hydrocarbons (NMHCs), halogenated organics and oxygenated volatile organic compounds (OVOCs), 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 NO<sub>x</sub> (NO + NO<sub>2</sub>) associated with it and the other had substantial lightning NO<sub>x</sub> (LNO<sub>x</sub> > 1 ppbv). When convective storms transport isoprene from the boundary layer to the upper troposphere and no LNO<sub>x</sub> is present, OH is reduced due to scavenging by isoprene, which serves to slow the chemistry, resulting in longer lifetimes for species that react with OH. Ozone and PAN production is minimal in this case. In the case where isoprene is convected and LNO<sub>x</sub> is present, there is a large effect on the expected ensuing chemistry: isoprene exerts a dominant impact on HO<sub>x</sub> and nitrogen-containing species; the relative contribution from other species to HO<sub>x</sub>, 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 NO<sub>x</sub> mixing ratios which, in turn, has a large impact on the HO<sub>x</sub> chemistry. Ozone production is substantial during the first few hours following the convection to the UT, resulting in a net gain of approximately 10 ppbv compared to the modeled scenario in which LNO<sub>x</sub> is present but no isoprene is present aloft.
