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

Abstract. Significant knowledge gaps persist in the understanding of forest–atmosphere exchange of reactive nitrogen oxides, partly due to a lack of direct observations. Chemical transport models require representations of dry deposition over a variety of land surface types, and the role of canopy exchange of NO_x (= NO + NO₂) is highly uncertain. Biosphere–atmosphere exchange of NO_x and NO_y (= NO_x + HNO₃ + PANs + RONO₂ + pNO₃⁻ + ...) was measured by eddy covariance above a mixed hardwood forest in central Ontario (Haliburton Forest and Wildlife Reserve, or HFWR), and a mixed hardwood forest in northern lower Michigan (Program for Research on Oxidants: Photochemistry, Emissions and Transport, or PROPHET) during the summers of 2011 and 2012 respectively. NO_x and NO_y mixing ratios were measured by a custom-built two-channel analyser based on chemiluminescence, with selective NO₂ conversion via LED photolysis and NO_y conversion via a hot molybdenum converter. Consideration of interferences from water vapour and O₃, and random uncertainty of the calculated fluxes are discussed. NO_y flux observations were predominantly of deposition at both locations. In general, the magnitude of deposition scaled with NO_y mixing ratios. Average midday (12:00–16:00) deposition velocities at HFWR and PROPHET were 0.20 ± 0.25 and 0.67 ± 1.24 cm s⁻¹ respectively. Average nighttime (00:00–04:00) deposition velocities were 0.09 ± 0.25 cm s⁻¹ and 0.08 ± 0.16 cm s⁻¹ respectively. At HFWR, a period of highly polluted conditions (NO_y concentrations up to 18 ppb) showed distinctly different flux characteristics than the rest of the campaign. Integrated daily average NO_y flux was −0.14 mg (N) m⁻² day⁻¹ and −0.34 mg (N) m⁻² day⁻¹ (net deposition) at HFWR and PROPHET respectively. Concurrent wet deposition measurements were used to estimate the contributions of dry deposition to total reactive nitrogen oxide inputs, found to be 22 and 40% at HFWR and PROPHET respectively.