Abstract
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 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 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. NOx and NOy mixing ratios were measured by a custom-built two-channel analyser based on chemiluminescence, with selective NO2 conversion via LED photolysis and NOy conversion via a hot molybdenum converter. Consideration of interferences from water vapour and O3, and random uncertainty of the calculated fluxes are discussed. NOy flux observations were predominantly of deposition at both locations. In general, the magnitude of deposition scaled with NOy 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 (NOy concentrations up to 18 ppb) showed distinctly different flux characteristics than the rest of the campaign. Integrated daily average NOy 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.
Highlights
Emissions of NOx from both anthropogenic and biogenic sources control tropospheric ozone production and the oxidizing capacity of the atmosphere through reactions involving hydrocarbons and OH radicals
Observations were made from 20 July to 11 October 2011 at Haliburton Forest and Wildlife Reserve (HFWR; 45◦17 11 N, 78◦32 19 W), located in central Ontario, and from 24 July to 14 August 2012 at the University of Michigan Biological Station (UMBS; 45◦33 32 N, 84◦42 52 W), located in northern Michigan
PROPHET and HFWR towers are both located within mixed forests that mark the transition between the deciduous and boreal forest, along 45◦ N
Summary
Emissions of NOx from both anthropogenic and biogenic sources control tropospheric ozone production and the oxidizing capacity of the atmosphere through reactions involving hydrocarbons and OH radicals. Mixing ratio observations are sparse and are rarely available for all the necessary species, requiring spatial interpolation and assumptions about the unknown contributions (Holland et al, 2005) Using the former approach applied to the US and Europe, dry deposition was calculated to contribute 40–60 % of total NOy deposition, this only includes contributions from HNO3(g) and particulate nitrate in the US, and HNO3(g), particulate nitrate, and NO2 in Europe (Holland et al, 2005). Shorter-term eddy covariance measurements of NOx flux were performed at each site to help elucidate the role of the forests as net sinks or sources of NOx. In this paper, the focus is on reporting the instrumental methods, summarizing the observations, discussing the results in the context of quantifying a total oxidized nitrogen deposition budget (by incorporating observations from national wet deposition monitoring networks) and identifying the influence of atmospheric transport. Results are compared to other NOy flux observations previously reported above forests across eastern North America
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