Dominant aerosol chemical components and their contribution to extinction during the Aerosols99 cruise across the Atlantic

The Aerosols99 cruise crossed the Atlantic Ocean from Norfolk, Virginia, to Cape Town, South Africa, during January and February of 1999. On the basis of back trajectories, aerosol number concentrations and size distributions , and trace gas concentrations, seven "air mass" regions were encountered. These included North America, Northern Hemisphere (NH) marine, African dust , a mixture of dust and biomass burning from Africa, biomass burning from A frica, Southern Hemisphere SH) marine tropics, and SH marine temperate. Sim ultaneous measurements of aerosol chemical composition, number size distrib ution, scattering and absorption coefficients, vertical profiles, and optic al depth allowed for a thorough characterization of the aerosol. Presented here are the concentrations and mass fractions of the aerosol chemical comp onents that were dominant in each region and the aerosol scattering and abs orption coefficients, single scattering albedos, Angstrom ngstrom exponents , and optical depths measured in each region. Also presented is the percent of the extinction measured at the surface due to each chemical component a nd mass extinction efficiencies of the individual aerosol components estima ted from Mie calculations and a multiple linear regression. All results are reported at the measurement relative humidity of 55 +/- 5%. Non-sea-salt ( nss) SO aerosol was a significant contributor to the submicron mass concent ration in all air mass regions (mean mass fractions ranged from 20 to 67%). It made the largest contribution to submicron extinction in the North Amer ica region (45 +/- 30%, mean and 1 sigma standard deviation). Sea-salt mean submicron mass fractions ranged from 9 to 49% with the lowest value in the biomass burning region and highest values in the NH marine and dust region s. Its contribution to submicron extinction ranged from a mean of 29 to 66% . Sea-salt mean supermicron mass fractions ranged from 52 to 98% with the h ighest values in the marine regions. Its contribution to supermicron extinc tion ranged from 60 to 98%. Mean submicron and supermicron mass fractions o f dust in the dust region were 22 +/- 3.3% (mean and 95% uncertainty) and 2 6 +/- 3.9%, respectively. Corresponding submicron and supermicron extinctio n contributions were 24 +/- 7.5 and 18 +/- 4.0%, respectively. Submicron ma ss fractions of particulate organic matter (POM) ranged from below detectio n limits in the dust region to 18 +/- 11% in the biomass burning region. Co ntributions to submicron extinction ranged from below detection limits to 2 4% in the North America region. In the biomass burning region the black car bon mean submicron mass fraction was 6.7 +/- 1.3% with a contribution of 6. 4 +/- 2.7% to the submicron extinction. Extinction fractions of each compon ent for particles with aerodynamic diameters less than 10 mum also are repo rted in the paper. Sea salt dominated the extinction measured at the surfac e due to sub-10 mum aerosol for all air mass regions, even those influenced by continental sources. The fraction of the measured column aerosol optica l depth due to aerosol within the 'boundary layer was estimated for the NH marine, dust, biomass burning, and SH marine tropics regions. Mean values r anged from 35 +/- 15% for the biomass burning region to 95 +/- 46% for the NH marine region.