Observations of total RONO<sub>2</sub> over the boreal forest: NO<sub>x</sub> sinks and HNO<sub>3</sub> sources

Abstract

Abstract. 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 (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 NOx (NOx = NO + NO2) in remote, continental environments. However, HNO3 is generally present in larger concentrations than ΣANs indicating that the atmospheric lifetime of ΣANs is shorter than the HNO3 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 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.

Variations of OH radical in an urban plume inferred from NO2 column measurements

“…Results that neglect this effect such as those of Beirle et al [2011] should be treated with caution. Furthermore, NO 2 columns are not just affected by OH, but also by the formation and the decay of PAN (e.g., Valin et al, submitted 2013) and RONO 2 molecules [e.g., Perring et al, 2010;Browne et al, 2013] as well as by vertical and horizontal transport.…”

The changing role of organic nitrates in the removal and transport of NO<sub><i>x</i></sub>

“…However, the increase in fractional concentrations as NO2R/VOCR decreases is much less than the increase in fractional production. At low NO2R/VOCR ratios, the dominant RONO 2 species are typically short lived and can undergo heterogeneous hydrolysis to produce HNO 3 (e.g., Browne et al, 2013). This indirect source of HNO 3 can be the greatest source of HNO 3 in forested environments, and it leads to the relatively weak dependence of fractional concentration on NO2R/VOCR.…”

“…Historically, direct HNO 3 production was thought to be the only important NO x loss pathway, with RONO 2 chemistry playing at most a minor role. However, several studies have shown that the formation rate of RONO 2 in cities or forested regions can be competitive with or greater than the direct production rate of nitric acid (Rosen et al, 2004;Farmer et al, 2011;Browne et al, 2013;Romer et al, 2016;Sobanski et al, 2017).…”

Review response

“…Information is still emerging on the fate of organic nitrates, and that information is just beginning to be included in models. Hydrolysis of particle-phase organic nitrates is one such process more recently considered with impacts for both O 3 and PM in models (Hildebrandt Ruiz and Yarwood, 2013;Browne et al, 2014;Pye et al, 2015;Fisher et al, 2016).…”