Dry deposition measured with a water surface sampler: a comparison to modeled results

In this study, a water surface sampler (WSS) and a knife-leading-edge surrogate surface (KSS) covered with both a Nylasorb filter, and a greased strip were used to directly measure nitrate, sulfate, and ammonium dry fluxes in Chicago between May and October 1997. Concurrently, an annular denuder system was used to measure the ambient concentration of gaseous nitric acid, sulfur dioxide, and ammonia gases. Water evaporation and relative humidity were also simultaneously measured. The measured fluxes and ambient concentrations were used to calculate the mass transfer coefficients of all four species. The measured MTCs of water vapor, ammonia gas, nitric acid and sulfur dioxide were normalized to the square root of their diffusion coefficient and correlated to the wind speed at 10 m above the WSS. The best-fit model was kA=DA0.5[(0.98±0.1)u10+(1.26±0.3)],where kA is the air side mass transfer coefficient (cm s−1), DA is the air side diffusion coefficient (cm2 s−1), u10 is the wind speed 10 m above the WSS (m s−1), and the ± is the 95% confidence interval. Field measured MTCs of water vapor, ammonia gas, nitric acid, and sulfur dioxide were used to examine the applicability of two models, the resistance model and the water evaporation model. Results showed that the resistance model predictions were statistically the same as the measured MTCs to the surrogate surfaces while the predictions of the water evaporation model was statistically smaller than the measured values.