Atmos. Chem. Phys., 5, 2253-2265, 2005
© Author(s) 2005. This work is licensed under the
Creative Commons Attribution-NonCommercial-ShareAlike 2.5 License.
The particulate mass (PM) concentration for fine aerosols at the MBL and FT sites averaged 17.4±4.7 µg/m3 and 11.2±3.2 µg/m3, respectively, and is among the highest reported in the literature for remote sites; more than 90% of this PM was composed equally of ammonium sulfate and carbonaceous aerosols. Comparison between the MBL and FT sites showed a slight vertical gradient for PM that was not observed for dust aerosols, which averaged 10.5±4.8 and 11.7±5.0 µg/m3 for the MBL and FT sites, respectively.
The results were used to reconstruct the ambient light scattering
coefficient (σsp) that was measured at ambient Relative
Humidity (RH) for fine particles at the MBL site. Reconstruction of σsp was achieved using ratios of wet to dry scattering, f(RH), that
depend on RH for ammonium sulfate, but are kept equal to 1 for POM. This
results in a low water adsorption for our organic-rich carbonaceous
aerosols, although these aged biomass smoke aerosols are supposed to be
highly oxidized. Mass scattering efficiencies of the main aerosol components
were obtained by multivariate regression analysis, and were 2.66 and 4.19 m2/g
(at the 95% confidence level) for dry ammonium sulfate and POM,
respectively. The calculations indicate that one third of the reconstructed
σsp was due to water uptake by ammonium sulfate aerosols,
demonstrating their major role in the radiative aerosol properties in the