Observations of HNO₃, ΣAN, ΣPN and NO₂ fluxes: evidence for rapid HO_x chemistry within a pine forest canopy

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 NO₂, and downward fluxes of total peroxy and peroxy acyl nitrates (ΣPNs), total gas and particle phase alkyl and multifunctional alkyl nitrates (ΣANs_(g+p)), and the sum of gaseous HNO₃ and semi-volatile NO₃⁻ 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 ΣNO_yi fluxes in winter are –4.9 ppt m s⁻¹. The signs and magnitudes of these wintertime individual and ΣNO_yi fluxes are in the range of prior measurements. In contrast, during summer, we observe downward fluxes only of ΣANs_(g+p), and upward fluxes of HNO_3(g+p), ΣPNs 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 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 σPNs during summer are consistent with oxidation of NO₂ and acetaldehyde by an OH x residence time of 1.1×10¹⁰ molec OH cm⁻³ s, corresponding to 3 to 16×10⁷ molecules cm⁻³ OH within the forest canopy for a 420 to 70 s canopy residence time. We show that ΣAN_(g+p) fluxes are consistent with this range in OH if the reaction of OH with ΣANs 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 NO_x fluxes are downward into the canopy because of the substantial conversion of NO_x to HNO₃ and σPNs 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 HNO₃ and σPNs 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.