Abstract
Alkyl nitrates (RONO2) are an important class of nitrogen oxides reservoirs in the atmosphere and play a key role in tropospheric photochemistry. Despite the increasing concern for photochemical air pollution over China, the knowledge of characteristics and formation mechanisms of alkyl nitrates in this region is limited. We analyzed C1-C5 alkyl nitrates measured in Beijing at a polluted urban site in summer 2008 and at a downwind rural site in summers of both 2005 and 2008. Although the abundances of NOx and hydrocarbons were much lower at the rural site, the mixing ratios of RONO2 were comparable between both sites, emphasizing the regional nature of alkyl nitrate pollution. Regional transport of urban plumes governed the elevated RONO2 levels at the rural site. The concentrations of C1-C2 RONO2 were significantly higher at the rural site in 2008 compared to 2005 despite a decline in NOx and anthropogenic VOCs, mainly owing to enhanced contributions from biogenic VOCs. The photochemical formation regimes of RONO2 were evaluated by both a simplified sequential reaction model and a detailed master chemical mechanism box model. The observed C4-C5 RONO2 levels can be well explained by the photochemical degradation of n-butane and n-pentane, while the sources of C1-C3 RONO2 were rather complex. In addition to the C1-C3 alkanes, biogenic VOCs and reactive aromatics were also important precursors of methyl nitrate, and alkenes and long-chain alkanes contributed to the formation of C2-C3 RONO2. This study provides insights into the spatial distribution, inter-annual variation and photochemical formation mechanisms of alkyl nitrate pollution over the Beijing area.
Highlights
Alkyl nitrates (RONO2) are an important class of organic nitrates in the troposphere, and play key roles in the atmospheric carbon and nitrogen cycles (Bertman et al, 1995; Clemitshaw et al, 1997; Russo et al, 2010), and aerosol formation (Yan et al, 2016)
The alkyl nitrate levels were comparable at both sites even though the abundances of NOx and VOCs were much lower at the rural site
The C1-C2 alkyl nitrates measured at the rural site in 2008 were significantly higher than in 2005, despite a decrease in some anthropogenic VOCs
Summary
Alkyl nitrates (RONO2) are an important class of organic nitrates in the troposphere, and play key roles in the atmospheric carbon and nitrogen cycles (Bertman et al, 1995; Clemitshaw et al, 1997; Russo et al, 2010), and aerosol formation (Yan et al, 2016). They serve as temporary reservoirs of nitrogen oxides (NOx = NO+NO2) due to their low reactivity and slow photolysis rate in the troposphere (Atkinson et al, 2006; Jenkin and Clemitshaw, 2000; Seinfeld et al, 1998). The detailed chemical processes of alkyl nitrates can be described by the following reactions: RH + OH⋅ → R⋅ + H2O k1, α1
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