Summertime C 1 -C 5 alkyl nitrates over Beijing, northern China: Spatial distribution, regional transport, and formation mechanisms

Abstract Alkyl nitrates (RONO 2 ) 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 C 1 -C 5 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 NO x and hydrocarbons were much lower at the rural site, the mixing ratios of RONO 2 were comparable between both sites, emphasizing the regional nature of alkyl nitrate pollution. Regional transport of urban plumes governed the elevated RONO 2 levels at the rural site. The concentrations of C 1 -C 2 RONO 2 were significantly higher at the rural site in 2008 compared to 2005 despite a decline in NO x and anthropogenic VOCs, mainly owing to enhanced contributions from biogenic VOCs. The photochemical formation regimes of RONO 2 were evaluated by both a simplified sequential reaction model and a detailed master chemical mechanism box model. The observed C 4 -C 5 RONO 2 levels can be well explained by the photochemical degradation of n -butane and n -pentane, while the sources of C 1 -C 3 RONO 2 were rather complex. In addition to the C 1 -C 3 alkanes, biogenic VOCs and reactive aromatics were also important precursors of methyl nitrate, and alkenes and long-chain alkanes contributed to the formation of C 2 -C 3 RONO 2 . This study provides insights into the spatial distribution, inter-annual variation and photochemical formation mechanisms of alkyl nitrate pollution over the Beijing area.