Abstract
Abstract. Isoprene is the largest single VOC emission to the atmosphere. Although it is primarily oxidized photochemically during daylight hours, late-day emissions that remain in the atmosphere at sunset undergo oxidation by NO3 in regionally polluted areas with large NOx levels. A recent aircraft study examined isoprene and its nocturnal oxidants in a series of night flights across the Northeast US, a region with large emissions of both isoprene and NOx. Substantial amounts of isoprene that were observed after dark were strongly anticorrelated with measured NO3 and were the most important factor determining the lifetime of this radical. The products of photochemical oxidation of isoprene, methyl vinyl ketone and methacrolein, were more uniformly distributed, and served as tracers for the presence of isoprene at sunset, prior to its oxidation by NO3. A determination of the mass of isoprene oxidized in darkness showed it to be a large fraction (>20%) of emitted isoprene. Organic nitrates produced from the NO3+isoprene reaction, though not directly measured, were estimated to account for 2–9% of total reactive nitrogen. The mass of isoprene oxidized by NO3 was comparable to and correlated with the organic aerosol loading for flights with relatively low organic aerosol background. The contribution of nocturnal isoprene oxidation to secondary organic aerosol was determined in the range 1–17%, and isoprene SOA mass derived from NO3 was calculated to exceed that due to OH by approximately 50%.
Highlights
Isoprene is the most important biogenically emitted VOC (440–660 Tg C yr−1 globally, Guenther et al, 2006) and is important in regulating levels of tropospheric oxidants such as ozone and OH (Lelieveld et al, 2008; Poisson et al, 2000; Trainer et al, 1987) and in the formation of secondary organic aerosol (Claeys et al, 2004; Henze and Seinfeld, 2006)
The remaining consumption of NO3 radicals is not defined here, possibilities include reactions with other biogenic VOC, second generation isoprene oxidation products, peroxy radicals and heterogeneous uptake of either NO3 or N2O5
This paper has examined nighttime oxidation of isoprene in air masses sampled from an aircraft within the residual daytime boundary layer during the summer of 2004 in the Northeast US
Summary
Isoprene is the most important biogenically emitted VOC (440–660 Tg C yr−1 globally, Guenther et al, 2006) and is important in regulating levels of tropospheric oxidants such as ozone and OH (Lelieveld et al, 2008; Poisson et al, 2000; Trainer et al, 1987) and in the formation of secondary organic aerosol (Claeys et al, 2004; Henze and Seinfeld, 2006). Measurements at forested sites impacted by urban NOx emissions have in several instances found evening isoprene loss rates consistent with consumption by NO3 radicals (Starn et al, 1998; Steinbacher et al, 2005; Stroud et al, 2002), while analysis of data from more remote locations have shown the importance of either transport or, possibly, additional oxidants such as nighttime OH (Biesenthal et al, 1998; Sillman et al, 2002). These nighttime flights sampled in urban and forested regions across the Northeast US, an area with both large isoprene and NOx emissions, where NO3driven isoprene oxidation is likely to be important
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