Abstract
Low molecular weight alkyl nitrates are produced in surface seawater from the photochemically initiated reaction of NO with organic peroxy radicals. In this study, methyl, ethyl, isopropyl, and n-propyl nitrate photochemical production rates were determined from shipboard incubation experiments. Surface water samples from a variety of different North Pacific water masses were irradiated in sunlight, with and without added nitrite. Production was observed in some, but not all unmodified waters. Production rates increased with increasing addition of nitrite. The relative rates of production of C 1, C 2, and C 3 alkyl nitrates were roughly constant in all production experiments, and similar to the concentration ratios in ambient waters. This constancy most likely reflects consistency in the ratios of the various organic peroxy radicals involved in the reaction ROO + NO. Alkyl nitrate production rates at 1 μM nitrite concentrations generally did not vary markedly by water type, indicating that distribution of nitrite, not organic matter may control photochemical alkyl nitrate formation in the oceans. At higher nitrite levels, differences in production rates may reflect variations in the concentration or reactivity of dissolved organics.
Highlights
Tropospheric ozone is an important greenhouse gas and hydroxyl radical (OH) precursor
Global atmospheric chemistry simulations based on these results show that oceanic alkyl nitrate emission is equivalent to 0.35 Tg of N per year, which contributes roughly 3% to the global oxidative capacity of the troposphere (Neu et al, 2008)
The production rate in the sample collected north of Hawaii continued to increase with additional nitrite up to 8 μM nitrite for methyl nitrate; the slope of C2–C3 production rate vs. nitrite decreased slightly between 2 and 4 μM
Summary
Tropospheric ozone is an important greenhouse gas and hydroxyl radical (OH) precursor. Atmospheric alkyl nitrates can account for up to 80% of NOy in the marine boundary layer (Jones et al, 1999; Talbot et al, 2000) These compounds undergo photolysis and reaction with OH to generate NOx, leading to the production of tropospheric ozone (Clemitshaw et al, 1997; Talukdar et al, 1997a,b). The free radical formation of alkyl nitrates in solution was observed in benzene (Shelton and Kopcaewski, 1967) and in water (Padmaja and Huie, 1993). This reaction has previously been observed during photochemical experiments with seawater (Dahl et al, 2003; Moore and Blough, 2002). Related measurements of alkyl nitrate saturation state and oceanic depth profiles were reported by Dahl et al (2005; 2007)
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