An optical model for US aerosol consistent with observations of chemical speciation

The aerosol sampling network of the IMPROVE (Interagency Monitoring of Protected Visual Environments) program began operation in 1988, and has since routinely provided information on aerosol fine mass and chemical composition at 43 representative US visibility Class I areas. We develop an aerosol optics model derived from the IMPROVE data that allows for external or internal mixing of sulfates, nitrates, dust, and organic and elemental carbon. Particular attention is paid to cases where supplemental studies have provided such information as water activity, particle size distributions, and acidity. The optical properties derived from this model are incorporated into column climate forcing calculations in order to deduce the potential impacts of the ambient aerosol on instantaneous forcing trends in regions of the continental US. Diurnal variations are considered, as are variations in the vertical structure of the aerosol.