On the effects of fast chemical reactions on the vertical fluxes of NO and NO2 in the atmospheric surface layer

Rapid chemical reactions in the atmospheric surface layer can cause fluxes measured by the eddy correlation technique to be unrepresentative of the emission or deposition at the actual surface. Results from experiments involving eddy covariance and field chamber measurements at a field of soybeans in the coastal plain of North Carolina indicated that the emission rates of nitric oxide were considerably larger than the fluxes measured a few meters above the surface. By reactions involving ambient ozone and possibly hydrocarbons, much of the emitted nitric oxide was apparently converted to nitrogen dioxide within a few meters of the surface. This conversion produced an upward flux of nitrogen dioxide above the surface, despite the likely uptake of nitrogen dioxide by the soybean canopy. A numerical model that describes both turbulent mixing and rapid in-air chemical reactions was used to interpret these results. A large portion of the conversion from nitric oxide to nitrogen dioxide evidently occurred within the soybean canopy. With a numerical model tested with these field results, micrometeorological measurements of fluxes of highly reactive substances above the surface can be related to the surface fluxes, and parameterizations of the effects of rapid in-air chemical reactions can be developed for application in atmospheric chemistry models to allow more accurate estimates of the sources and fates of nitrogen oxides in the lower atmosphere.