Observations of HNO3, Sigma AN, Sigma PN and NO2 fluxes: evidence for rapid HOx chemistry within a pine forest canopy
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 NO2, and downward fluxes of total peroxy and peroxy acyl nitrates (Sigma PNs), total gas and particle phase alkyl and multifunctional alkyl nitrates (Sigma ANs((g+p))), and the sum of gaseous HNO3 and semi-volatile NO3- particles (HNO3(g+p)). We use calculations of the vertical profile and flux of NO, partially constrained by observations, to show that net midday Sigma NOyi fluxes in winter are -4.9 ppt m s(-1). The signs and magnitudes of these wintertime individual and Sigma NOyi fluxes are in the range of prior measurements. In contrast, during summer, we observe downward fluxes only of Sigma ANs((g+p)), and upward fluxes of HNO3(g+p), Sigma PNs and NO2 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 NOy fluxes, at least at this site, are much more complex than previously recognized. We show that the observations of upward fluxes of HNO3(g+p) and Sigma PNs during summer are consistent with oxidation of NO2 and acetaldehyde by an OH x residence time of 1.1 x 10(10) molec OH cm(-3) s, corresponding to 3 to 16 x 10(7) molecules cm(-3) OH within the forest canopy for a 420 to 70 s canopy residence time. We show that Sigma AN((g+p)) fluxes are consistent with this range in OH if the reaction of OH with Sigma ANs produces either HNO3 or NO2 with a 6-30% yield. Calculations of NO fluxes constrained by the NO2 observations and the inferred OH indicate that NOx fluxes are downward into the canopy because of the substantial conversion of NOx to HNO3 and Sigma PNs in the canopy. Even so, we derive that NOx emission fluxes of similar to 15 ng(N) m(-2) s(-1) at midday during summer are required to balance the NOx and NOy 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 NOx 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 HNO3 and Sigma 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.