Gas-particle partitioning of nitric acid modulated by alkaline aerosol

The role of alkaline mineral aerosol in controlling HNO3 partitioning between gas and aerosol phases is explored using a comprehensive, process oriented three-dimensional model. Simulation results for March 1994, a period from the PEM West B experiment, are presented. It is found that in the dust impacted 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 in 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 dust 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-salt particles. When this process of nitric acid partitioning onto alkaline particles is included in the analysis, model predicted HNO3-to-NOx ratios are much closer to observed values that typically range between 1 and 9.