Chemical and optical properties of marine boundary layer aerosol particles of the mid-Pacific in relation to sources and meteorological transport

Incorporating the direct effect of tropospheric aerosol on climate into global climate models involves coupling the optical properties of the aerosol with its physical and chemical properties. This coupling is strengthened if the optical, physical, and chemical properties of the individual aerosol components are known as well as how these properties depend on the air mass source and synoptic scale meteorology. To relate properties of the aerosol components to air mass sources over a wide range of meteorological conditions, two long latitudinal cruises were conducted in the central Pacific Ocean from 55 degrees N to 70 degrees S. Submicron non-sea-salt (nss) SO4= aerosol averaged about 35 to 40% of the submicron ionic mass as analyzed by ion chromatography and 6% of the total ionic mass, while supermicron nss SO4= aerosol contributed about 1% to the total ionic mass. About 1% of the remaining total ionic mass was composed of methanesulfonate and 90% was sea salt. Ionic mass fractions of nss SO4= aerosol were highest in regions having the longest marine boundary layer residence times or the largest source of marine or continental gas phase precursors. The calculated scattering by nss SO4= aerosol was highest in these same regions due to the dependence of scattering on particle size and the concentration of nss SO4= in the submicron size range. The calculated scattering by submicron sea salt was similar to that of the nss SO4= aerosol, indicating that its contribution to scattering in the marine boundary layer can be significant or even dominant depending on its mass concentration. Mass scattering efficiencies for nss SO4= at 30% RH ranged from 4.3 to 7.5 m(2) g(-1) and for submicron sea salt from 3.5 to 7.7 m(2) g(-1). Mass backscattering efficiencies for nss SO4= ranged from 0.41 to 0.58 m(2) g(-1) and for submicron sea salt from 0.33 to 0.63 m(2) g(-1). These values fall within the same range as others reported previously for the marine atmosphere.