Evaluating a flux-gradient approach for flux and deposition velocity of nitrogen dioxide over short-grass surfaces

Use of micrometeorological methods for measurement of surface NO and NO₂ fluxes is problematic due to chemical reactions of the O ₃-NO-NO₂ triad in the surface air layer resulting in violation of the assumption of flux constancy with height. Application of a flux-gradient approach close to a short-grass surface (<1m) minimises this problem, and allows for continuous monitoring of flux and deposition velocity under varying environmental conditions. In this study, we evaluate data collected using a flux-gradient approach for NO and NO₂ flux determination over turfgrass from May to August 2001 in Ontario, Canada. Gas concentrations were determined using two chemiluminescence analysers sampling at 2Hz from two heights of 0.3 and 0.5m. Deposition velocities for NO₂ were determined by dividing the calculated flux by the measured atmospheric concentration. The unfertilised grass field studied was a very small source of NO (mean hourly flux 0.45±0.04ngN m^-2s^-1) and a net sink for NO₂ (-1.76±0.06ngNm^-2s^-1). Largest NO₂ downward flux occurred during a period with ideal grass growing conditions, and lowest fluxes were measured during the driest period of the summer. This same seasonal pattern was also noted for the NO₂ deposition velocity (mean 2.7±0.17mms^-1), which was mostly comprised by a surface resistance (daytime mean 436sm^-1). The flux-gradient approach proposed yielded NO₂ flux and deposition velocities within the range of values presented in the literature, and responded to environmental variables in an expected manner.