Abstract
Abstract. 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 (ANs) 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 (ΣANs) 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 on the order of 2 h due to efficient uptake to aerosol (and dry deposition), resulting in the transfer of reactive nitrogen from anthropogenic sources to the forest ecosystem.
Highlights
Alkyl nitrates (ANs; R−CH2ONO2) are formed in chainterminating reactions that limit photochemical cycling of organic and HOx radicals and represent an important sink for atmospheric nitrogen (Liu et al, 2012; Lee et al, 2016; Huang et al, 2019)
During a 2016 field experiment in the boreal forest (IBAIRN – Influence of Biosphere-Atmosphere Interactions on the Reactive Nitrogen budget), we showed that on average more than 20 % of NO3 radicals formed during the day were lost due to reaction with biogenic volatile organic compounds (BVOCs) (Liebmann et al, 2018a)
We examine the contribution of the NO3-initiated oxidation of volatile organic compounds (VOCs) to the formation of ANs both during the daytime and night-time during IBAIRN and compare this to AN formation initiated by reactions of OH and O3
Summary
Alkyl nitrates (ANs; R−CH2ONO2) are formed in chainterminating reactions that limit photochemical cycling of organic and HOx radicals and represent an important sink for atmospheric nitrogen (Liu et al, 2012; Lee et al, 2016; Huang et al, 2019). Terpene to form a primary ozonide (POZ; Reaction R3) which rapidly decomposes (Reaction R4) via Criegee intermediates to form OH and (in the presence of O2) RO2 The latter reacts via Reactions (R1) and (R2) to form alkyl nitrates: O3 + R = R → POZ, POZ(+O2) → OH + RO2, POZ → other products. The branching ratio to AN formation via NO3 oxidation is generally much larger than that for organic peroxy radicals reacting with NO and, for biogenic VOCs (BVOCs), can approach 80 % (Ng et al, 2017; IUPAC, 2019). Recent measurements of NO3 reactivity in forested regions (Ayres et al, 2015; Liebmann et al, 2018a, b) suggest that, even during the day, a significant fraction of the nitrate radicals generated in Reaction (R5) can react with BVOCs rather than undergoing photolysis or reaction with NO to reform NOx(NOx=NO + NO2). Using calculations of the overall production rate of ANs and measurements of the summed mixing ratio of ANs ( ANs), we derive a lifetime for ANs in this environment
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