Atmospheric Deposition Total Nitrogen from Dry Deposition in 1998

The role of urban vegetation on NOx-derived dry deposition fluxes was investigated for the arid Phoenix (Arizona, USA) metropolitan area using the Community Multiscale Air Quality Model (CMAQ) (9-13 June 1998). A new land cover classification and updated land cover data were introduced in the model to account for spatial extent and heterogeneity of urban land cover. Adjustments were made in the deposition velocity calculations to consider the adaptation of local plants to the environmental conditions of Central Arizona. According to the simulations 25 percent of the NOx derived dry deposition fluxes in the urban area were deposited on vegetation. When urban vegetation was excluded from the simulations NO2 deposition was reduced by 57 percent because of the significantly lower deposition velocities of impervious compared to vegetated surfaces; nitric acid deposition was relatively unchanged. Using a diagnostic model with input data from urban air quality monitoring sites, hourly NO and NO2 dry N deposition fluxes were simulated for the entire year 1998 to ~6 kg ha-1 yr-1. Dry deposition declined during the summer months, due to lower pollutant concentrations and temperature-induced closure of the plant stomata during afternoon hours.