Photochemical reactions of methyl and ethyl nitrate: a dual role for alkyl nitrates in the nitrogen cycle

Environmental contextAlkyl nitrates are considered to be important intermediates in the atmospheric hydrocarbons–nitrogen oxides–ozone cycle, which significantly determines air quality and nitrogen exchange between the atmosphere and the Earth’s surfaces. The present laboratory study investigates reaction products of alkyl nitrates to elucidate their photochemical reaction mechanisms in the atmosphere. The results provide a better understanding of the role played by alkyl nitrates in the atmospheric environment. AbstractAlkyl nitrates (ANs) are important nitrogen-containing organic compounds and are usually considered to be temporary reservoirs of reactive nitrogen NOx (NO2 and NO) in the atmosphere, although their atmospheric fates are incompletely understood. Here a laboratory study of the gas-phase photolysis and OH-initiated reactions of methyl nitrate (CH3ONO2) and ethyl nitrate (C2H5ONO2), as models of atmospheric ANs, is reported with a focus on elucidating the detailed photochemical reaction mechanisms of ANs in the atmosphere. A series of intermediate and end products were well characterised for the first time from the photochemical reactions of methyl and ethyl nitrate conducted under simulated atmospheric conditions. Notably, for both the photolysis and OH-initiated reactions of CH3ONO2 and C2H5ONO2, unexpectedly high yields of HNO3 (photochemically non-reactive nitrogen) were found and also unexpectedly high yields of peroxyacyl nitrates (RC(O)OONO2, where R = H, CH3, CH3CH2,…) (reactive nitrogen) have been found as CH3C(O)OONO2 in the C2H5ONO2 reaction or proposed as HC(O)OONO2 in the CH3ONO2 reaction. Although the yields of HNO3 from the ANs under ambient conditions are likely variable and different from those obtained in the laboratory experiments reported here, the results imply that the ANs could potentially serve as a sink for reactive nitrogen in the atmosphere. The potential for this dual role of organic nitrates in the nitrogen cycle should be considered in the study of air quality and nitrogen exchange between the atmosphere and surface. Finally, an attempt was made to estimate the production of HNO3 and peroxyacyl nitrates derived from NOx by ANs as intermediates in the atmosphere.