Alkyl nitrates in the boreal forest: formation via the NO₃-, OH- and O₃-induced oxidation of biogenic volatile organic compounds and ambient lifetimes

The formation of alkyl nitrates in various oxidation processes taking place throughout the diel cycle can represent an important sink of reactive nitrogen and mechanism for chain-termination in atmospheric photo-oxidation cycles. The low volatility alkyl nitrates formed from biogenic volatile organic compounds (BVOCs), especially terpenoids, enhance rates of production and growth of secondary organic aerosol. Measurements of the NO3-reactivity and the mixing ratio of total alkyl nitrates in the Finnish boreal forest enabled assessment of the relative importance of NO3−, O3− and OH-initiated formation of alkyl-nitrates from BVOCs in this environment. The high reactivity of the forest air towards NO3 resulted in reactions of the nitrate radical with terpenes contributing substantially to formation of ANs not only during the night but also during daytime. Overall, night-time reactions of NO3 accounted for 49 % of the local production rate of ANs, with contributions of 21 %, 18 % and 12 % for NO3, OH and O3, during the day. The lifetimes of the gas-phase ANs formed in this environment were of the order of 2 hours implying that the lifetime of NOx is strongly controlled by biogenic emissions from the forest. As the organic nitrates are lost to the particle phase and via dry-deposition to foliar surfaces, the overall result is transfer of reactive nitrogen from anthropogenic sources to the forest ecosystem