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

Abstract

Use of micrometeorological methods for measurement of surface NO and NO2 fluxes is problematic due to chemical reactions of the O3–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 (<1 m) 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 2 Hz from two heights of 0.3 and 0.5 m. 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±0.04 ng N m−2 s−1) and a net sink for NO2 (–1.76±0.06 ng N m−2 s−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±0.17 mm s−1), which was mostly comprised by a surface resistance (daytime mean 436 s m−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.