Observations of total RONO2 over the boreal forest: NOx sinks and HNO3 sources

In contrast with the textbook view of remote chemistry where HNO3 formation is the primary sink of nitrogen oxides, recent theoretical analyses show that formation of RONO2 (6ANs) 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 6ANs. We present observations of nitrogen oxides and organic molecules collected over the Canadian boreal forest during the summer which show that 6ANs 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 6ANs as a control over the lifetime of NOx (NOx =NO+NO2) in remote, continental environments. However, HNO3 is generally present in larger concentrations than 6ANs indicating that the atmospheric lifetime of 6ANs is shorter than the HNO3 lifetime. We investigate a range of proposed loss mechanism that would explain the inferred lifetime of 6ANs 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 NOx from the carbon backbone and/or (2) hydrolysis of particulate organic nitrates with HNO3 as a product. Implications of these ideas for our understanding of NOx and NOy budget in remote and rural locations are discussed.