Gas-particle partitioning of nitric acid modulated by alkaline aerosol

The role of alkaline mineral aerosol in controlling HNO3 partitioning betwe en gas and aerosol phases is explored using a comprehensive, process orient ed three-dimensional model. Simulation results for March 1994, a period fro m the PEM West B experiment, are presented. It is found that in the dust im pacted regions of the boundary layer and free troposphere, more than 50% of HNO3 is partitioned onto dust particles; while 10 similar to 50% of HNO3 i n the boundary layer and 10 similar to 30% of HNO3 in the free troposphere is partitioned onto sea-salt particles. This higher capacity of mineral dus t to uptake HNO3 is due to the fact that carbonate in the dust particles is more volatile (thus easily replaced by nitrate) than chloride in the sea-s alt particles. When this process of nitric acid partitioning onto alkaline particles is included in the analysis, model predicted HNO3-to-NOx ratios a re much closer to observed values that typically range between 1 and 9.