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Significant knowledge gaps persist in the understanding of forest–atmosphereexchange of reactive nitrogen oxides, partly due to a lack of directobservations. Chemical transport models require representations of drydeposition over a variety of land surface types, and the role of canopyexchange of NOx (= NO + NO2) is highly uncertain.Biosphere–atmosphere exchange of NOx and NOy(= NOx + HNO3 + PANs + RONO2 + pNO3− + ...)was measured by eddy covariance above a mixed hardwood forest in centralOntario (Haliburton Forest and Wildlife Reserve, or HFWR), and a mixedhardwood forest in northern lower Michigan (Program for Research on Oxidants:Photochemistry, Emissions and Transport, or PROPHET) during the summers of2011 and 2012 respectively. NOx and NOy mixing ratioswere measured by a custom-built two-channel analyser based onchemiluminescence, with selective NO2 conversion via LED photolysis andNOy conversion via a hot molybdenum converter. Consideration ofinterferences from water vapour and O3, and random uncertainty of thecalculated fluxes are discussed. NOy flux observations werepredominantly of deposition at both locations. In general, the magnitude ofdeposition scaled with NOy mixing ratios. Average midday(12:00–16:00) deposition velocities at HFWR and PROPHET were0.20 ± 0.25 and 0.67 ± 1.24 cm s−1 respectively. Averagenighttime (00:00–04:00) deposition velocities were0.09 ± 0.25 cm s−1 and 0.08 ± 0.16 cm s−1respectively. At HFWR, a period of highly polluted conditions(NOy concentrations up to 18 ppb) showed distinctly differentflux characteristics than the rest of the campaign. Integrated daily averageNOy flux was −0.14 mg (N) m−2 day−1 and−0.34 mg (N) m−2 day−1 (net deposition) at HFWR and PROPHETrespectively. Concurrent wet deposition measurements were used to estimatethe contributions of dry deposition to total reactive nitrogen oxide inputs,found to be 22 and 40% at HFWR and PROPHET respectively

Observations of reactive nitrogen oxide fluxes by eddy covariance above two midlatitude North American mixed hardwood forests