Observations of total RONO_2 over the boreal forest: NO_x sinks and HNO_3 sources

Abstract

In contrast with the textbook view of remote chemistry where HNO_3 formation is the primary sink of nitrogen oxides, recent theoretical analyses show that formation of RONO_2 (ΣANs) from isoprene and other terpene precursors is the primary net chemical loss of nitrogen oxides over the remote continents where the concentration of nitrogen oxides is low. This then increases the prominence of questions concerning the chemical lifetime and ultimate fate of ΣANs. We present observations of nitrogen oxides and organic molecules collected over the Canadian boreal forest during the summer which show that ΣANs account for ~20% of total oxidized nitrogen and that their instantaneous production rate is larger than that of HNO3. This confirms the primary role of reactions producing ΣANs as a control over the lifetime of NO_x (NO_x = NO + NO_2) in remote, continental environments. However, HNO_3 is generally present in larger concentrations than ΣANs indicating that the atmospheric lifetime of ΣANs is shorter than the HNO_3 lifetime. We investigate a range of proposed loss mechanisms that would explain the inferred lifetime of ΣANs finding that in combination with deposition, two processes are consistent with the observations: (1) rapid ozonolysis of isoprene nitrates where at least ~40% of the ozonolysis products release NO_x from the carbon backbone and/or (2) hydrolysis of particulate organic nitrates with HNO_3 as a product. Implications of these ideas for our understanding of NO_x and NO_y budget in remote and rural locations are discussed.

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