TROPOSPHERIC NITROGEN - A 3-DIMENSIONAL STUDY OF SOURCES, DISTRIBUTIONS, AND DEPOSITION

We simulate the global cycle of reactive nitrogen in a three-dimensional model of chemistry, transport, and deposition. Our model is based on the Lagrangian tracer model described by Walton et al. [1988] and uses winds and precipitation fields calculated by the Livermore version of the NCAR Community Climate Model. The model includes the basic chemical reactions of NO, NO2, and HNO3. For this study, we use prescribed OH and O3 concentrations and calculate the concentrations of NO, NO2, and HNO3 for a perpetual January and a perpetual July. The sources of reactive nitrogen due to fossil-fuel combustion (22 Mt N/yr), lightning discharges (3 Mt N/yr), soil microbial activity (10 Mt N/yr), biomass burning (6 Mt N/yr), and the oxidation of N2O in the stratosphere (1 Mt N/yr) are included. Model-predicted concentrations of NO, NO2, and HNO3 are compared to available measurements. In general, we find reasonable agreement between model predictions and measurements except for concentrations of HNO3 in the remote Pacific. At these latter locations, we require a larger source of reactive nitrogen to fit the observations. This may be supplied by lightning discharges, although increasing this source degrades our agreement with measured HNO3 abundances in the free troposphere. Alternatively, a local marine source could contribute to the measured abundances. Predictions for nitrate deposition by precipitation are within a factor of 2 of measured deposition rates in the northern hemisphere in the summer and in both seasons at remote locations. The model underpredicts nitrate deposition in winter in Europe, due primarily to the excessively strong winds generated by the general circulation model. Model simulations for NO(x) and HNO3 surface mixing ratios from calculations including only the fossil-fuel source, only natural sources, and all sources acting together, are compared. Anthropogenic sources have substantially increased the concentrations of NO(x) and HNO3 throughout all continents during both January and July. Fossil-fuel sources are responsible for most of this increase in the northern hemisphere, while both biomass burning and fossil-fuel combustion contribute in the southern hemisphere.