Observations of HNO<sub>3</sub>, ΣAN, ΣPN and NO<sub>2</sub> fluxes: evidence for rapid HO<sub>x</sub> chemistry within a pine forest canopy

Abstract

Abstract. Measurements of exchange of reactive nitrogen oxides between the atmosphere and a ponderosa pine forest in the Sierra Nevada Mountains are reported. During winter, we observe upward fluxes of NO 2 , and downward fluxes of total peroxy and peroxy acyl nitrates ( PNs), total gas and particle phase alkyl and multifunctional alkyl nitrates ( ANs (g+p) ), and the sum of gaseous HNO 3 and semi-volatile NO − 3 particles (HNO 3(g+p) ). We use calculations of the vertical profile and flux of NO, partially constrained by observations, to show that net midday NO yi fluxes in winter are -4.9 ppt m s −1 . The signs and magnitudes of these wintertime individual and NO yi fluxes are in the range of prior measurements. In contrast, during summer, we observe downward fluxes only of ANs (g+p) , and upward fluxes of HNO 3(g+p) , PNs and NO 2 with signs and magnitudes that are unlike most, if not all, previous observations and analyses of fluxes of individual nitrogen oxides. The results imply that the mechanisms contributing to NO y fluxes, at least at this site, are much more complex than previously recognized. We show that the observations of upward fluxes of HNO 3(g+p) and PNs during summer are consistent with oxidation of NO 2 and acetaldehyde by an OH x residence time of 1.1×10 10 molec OH cm −3 s, corresponding to 3 to 16×10 7 molecules cm −3 OH within the forest canopy for a 420 to 70 s canopy residence time. We show that AN (g+p) fluxes are consistent with this range in OH if the reaction of OH with ANs produces either HNO 3 or NO 2 with a 6-30% yield. Calculations of NO fluxes constrained by the NO 2 observations and the inferred OH indicate that NO x fluxes are downward into the canopy because of the substantial converCorrespondence to: R. C. Cohen (rccohen@berkeley.edu) sion of NO x to HNO 3 and PNs in the canopy. Even so, we derive that NO x emission fluxes of ∼15 ng(N) m −2 s −1 at midday during summer are required to balance the NO x and NO y flux budgets. These fluxes are partly explained by estimates of soil emissions (estimated to be between 3 and 6 ng(N) m −2 s −1 ). One possibility for the remainder of the NO x source is large HONO emissions. Alternatively, the 15 ng(N) m −2 s −1 emission estimate may be too large, and the budget balanced if the deposition of HNO 3 and PNs is slower than we estimate, if there are large errors in either our understanding of peroxy radical chemistry, or our assumptions that the budget is required to balance because the fluxes do not obey similarity theory.

Observations of elevated formaldehyde over a forest canopy suggest missing sources from rapid oxidation of arboreal hydrocarbons

“…Bouvier-Brown et al (2009b) showed significant levels of sesquiterpene emissions that mostly react before escaping the forest canopy from branch enclosure experiments, supporting this argument. In addition, Farmer and Cohen (2008) argued that vigorous intra-canopy chemistry was evident at this site based on estimates of vertical gradients and fluxes of ANs, PNs, NO 2 and HNO 3 . Wolfe et al (2009) Fig.…”

Influence of isoprene chemical mechanism on modelled changes in tropospheric ozone due to climate and land use over the 21st century

“…To test this hypothesis, for each mechanism, box model runs were performed at a series of different NO x and isoprene emission rates, so that an O 3 isopleth plot could be constructed, similar to those found in Dodge (1977) and Sillman and He (2002). In general, all mechanisms produce the same overall behaviour of increasing O 3 with increasing isoprene and NO x emissions.…”

“…The degree to which NO x is regenerated from isoprene nitrate degradation remains uncertain (Fiore et al, 2012;Xie et al, 2013) and has a significant effect on the O 3 response to isoprene emission changes . Fourth, dry deposition of isoprene nitrates, which may represent an important NO x sink O. J. Squire et al: Influence of isoprene chemical mechanism on modelled tropospheric ozone 5125 in isoprene-rich regions, is also uncertain, with measured deposition velocities ranging from 0.4 cm s −1 (Shepson et al, 1996) to 2.7 cm s −1 (Farmer and Cohen, 2008). Finally, there is evidence for the importance of O 3 -initiated isoprene nitrate degradation (Lockwood et al, 2010) and fast photolysis of isoprene nitrates (Müller et al, 2014).…”

Effects of biogenic nitrate chemistry on the NO<sub>x</sub> lifetime in remote continental regions

“…Measurements of total RONO 2 by thermal dissociation laser induced fluorescence (Day et al, 2002) have shown that RONO 2 account for a substantial fraction of oxidized nitrogen (NO y = NO + NO 2 + NO 3 + total peroxy nitrates + total alkyl and multifunctional nitrates + HNO 3 + 2*N 2 O 5 + HONO +...) in urban (Rosen et al, 2004;Cleary et al, 2005;Perring et al, 2010;Farmer et al, 2011) and rural (Murphy et al, 2006;Farmer and Cohen, 2008;Day et al, 2009;Perring et al, 2009a;Beaver et al, 2012) locations as well as downwind of the continents . Isoprene nitrates are thought to be a large fraction of the organic nitrates.…”