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

摘要

Measurements of exchange of reactive nitrogen oxides between the atmosphereand a ponderosa pine forest in the Sierra Nevada Mountains are reported.During winter, we observe upward fluxes of NO, and downward fluxes oftotal peroxy and peroxy acyl nitrates (ΣPNs), total gas and particlephase alkyl and multifunctional alkyl nitrates (ΣANs), andthe sum of gaseous HNO and semi-volatile NO particles(HNO). We use calculations of the vertical profile and flux ofNO, partially constrained by observations, to show that net midday ΣNO fluxes in winter are –4.9 ppt m s. The signs andmagnitudes of these wintertime individual and ΣNO fluxes arein the range of prior measurements. In contrast, during summer, we observedownward fluxes only of ΣANs, and upward fluxes ofHNO, ΣPNs and NO with signs and magnitudes that areunlike most, if not all, previous observations and analyses of fluxes ofindividual nitrogen oxides. The results imply that the mechanismscontributing to NO fluxes, at least at this site, are much morecomplex than previously recognized. We show that the observations of upwardfluxes of HNO and σPNs during summer are consistent withoxidation of NO and acetaldehyde by an OH x residence time of1.1×10 molec OH cm s, corresponding to 3 to16×10 molecules cm OH within the forest canopy for a 420 to70 scanopy residence time. We show that ΣAN fluxes areconsistent with this range in OH if the reaction of OH with ΣANsproduces either HNO or NO with a 6–30% yield. Calculations ofNO fluxes constrained by the NO observations and the inferred OHindicate that NO fluxes are downward into the canopy because of thesubstantial conversion of NO to HNO and σPNs in thecanopy. Even so, we derive that NO emission fluxes of~15 ng(N) m s at midday during summer are required to balance theNO and NO flux budgets. These fluxes are partly explained byestimates of soil emissions (estimated to be between 3 and6 ng(N) m s). One possibility for the remainder of the NO source is largeHONO emissions. Alternatively, the 15 ng(N) m s emissionestimate may be too large, and the budget balanced if the deposition ofHNO and σPNs is slower than we estimate, if there are largeerrors in either our understanding of peroxy radical chemistry, or ourassumptions that the budget is required to balance because the fluxes do notobey similarity theory.