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 (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.
Highlights
The upper troposphere (UT) is distinguished from the lower troposphere (LT) by low water vapor mixing ratios (10– 300 ppmv) and cold temperatures (210–235 K)
Formaldehyde was measured by the Difference Frequency Generation Absorption Spectrometer (DFGAS, Weibring et al, 2010), nitric oxide (NO) and NO2 were measured by the NCAR Chemiluminescence instrument (Carroll et al, 1992), and all other observations are from the Trace Organic Gas Analyzer (TOGA) measurements
Several dramatic events were observed in which isoprene and its oxidation products were detected at hundreds of pptv at altitudes higher than 8 km
Summary
The upper troposphere (UT) is distinguished from the lower troposphere (LT) by low water vapor mixing ratios (10– 300 ppmv) and cold temperatures (210–235 K). The primary source of HOx radicals from O(1D) + H2O is one to two orders of magnitude smaller than at low altitudes. Studies over the past 15 yr have shown oxygenated volatile organic compounds (OVOCs) to be ubiquitous in the atmosphere, reaching significant mixing ratios throughout the troposphere, even in remote regions (Singh et al, 1995). These species typically include acetone, methanol, and aldehydes, which can be transported from the LT to the UT or may be formed within the UT. There is discussion in the community about the reliability of background tropospheric measurements of some of the short-lived species such as acetaldehyde (Millet et al, 2010), there is no controversy about the presence of methanol and acetone, relatively long-lived species, in the UT; a number of studies have observed these species, especially acetone, with different techniques and reasonable agreement (Wohlfrom et al, 1999; Karl et al, 2007)
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