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 NOₓ (NO + NO₂) associated with it and the other had substantial lightning NOₓ (LNOₓ > 1 ppbv). When convective storms transport isoprene from the boundary layer to the upper troposphere and no LNOₓ 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 LNOₓ is present, there is a large effect on the expected ensuing chemistry: isoprene exerts a dominant impact on HOₓ and nitrogen-containing species; the relative contribution from other species to HOₓ, 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 which, in turn, has a large impact on the HOₓ 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 LNOₓ is present but no isoprene is present aloft.