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 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 (HNO3(g+p)). We use calculations of the vertical profile and flux of NO, partially constrained by observations, to show that net midday ΣNOyi fluxes in winter are –4.9 ppt m s−1. The signs and magnitudes of these wintertime individual and ΣNOyi 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 HNO3(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 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 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 NOx fluxes are downward into the canopy because of the substantial conversion of NOx to HNO3 and σPNs in the canopy. Even so, we derive that NOx emission fluxes of ~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 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.
Highlights
Forests and other ecosystems constantly interact with the atmosphere, both emitting and removing chemicals including long-lived greenhouse gases (CO2, N2O, CH4) as well as more reactive carbon compounds and nitrogen oxides
peroxy acyl nitrates (PNs), ANs(g+p), HNO3(g+p), and of the sum of NO2+ PNs+ ANs(g+p)+HNO3(g+p) (Farmer et al, 2006). These results are qualitatively what were expected from these measurements, namely an upward flux of NO2 resulting from www.atmos-chem-phys.net/8/3899/2008/
Much more research is required to assess the global effects of high within-canopy OH, the high OH we show is present within the canopy airspace at University of California-Blodgett Forest Research Station (UC-BFRS), and suggest might be present in many pine forests, will no doubt have interesting and possibly important consequences for the mechanisms of regional atmospheric chemistry, biosphereatmosphere exchange of nitrogen, production of secondary organic aerosols, and determining the identity of volatile organic compounds (VOC) that are emitted at the ecosystem scale
Summary
Forests and other ecosystems constantly interact with the atmosphere, both emitting and removing chemicals including long-lived greenhouse gases (CO2, N2O, CH4) as well as more reactive carbon compounds and nitrogen oxides. Natural terrestrial systems release 1150 Tg C/yr of volatile organic compounds (VOC) (Guenther et al, 1995) and 27 Tg/yr of N compounds (12, 9 and 6 Tg/yr of NOx, NH3 and N2O respectively) (Schlessinger, 1997) With few exceptions, these VOC and nitrogen compounds have been thought to be emitted from leaves and soils and transported out of the ecosystem canopy and into the boundary layer above on time scales of minutes, with oxidation occurring throughout the boundary layer. These VOC and nitrogen compounds have been thought to be emitted from leaves and soils and transported out of the ecosystem canopy and into the boundary layer above on time scales of minutes, with oxidation occurring throughout the boundary layer Effects associated with these large scale oxidation processes that have been of particular recent interest include the effects of biogenic isoprene mixing with anthropogenic NOx on surface ozone Cohen: NOyi fluxes imply rapid HOx chemistry in a pine forest al., 2005), the role of biogenic VOC on secondary organic aerosol (e.g. Larsen et al, 2001; Kanakidou et al, 2005), the effects of some of the longer-lived VOC such as acetone on the greenhouse forcing due to their effects on the global distribution of tropospheric ozone (e.g. Collins et al, 2002), the contribution of nitrogen oxides to the natural background of ozone and secondary organic aerosol (e.g. Lelieveld et al, 2004; Kroll et al, 2005) and the contribution of nitrogen oxide deposition to soil nutrient levels (e.g. Takemoto et al, 2001)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
