Observations of total RONO 2 over the boreal forest: NO x sinks and HNO 3 sources
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 HNO 3. 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.