Aerosol mass closure and reconstruction of the light scattering coefficient over the Eastern Mediterranean Sea during the MINOS campaign

摘要

As part of the Mediterranean Intensive Oxidant Study (MINOS) performedduring August 2001 in the Eastern Mediterranean Region, intensivemeasurements of chemical and radiative properties of atmospheric aerosolswere performed at two remote sites on Crete Island, located in the marineboundary layer (MBL), and in the lower free troposphere (FT), respectively.Gravimetric particulate mass, as well as chemically-derived masses of watersoluble ions, organic and elemental carbon, and tracer elements for dustaerosols were measured for fine (<1.2 µm) and coarse (>1.2 µm)particles at the two sampling sites. Although strongly bound water, mainlyassociated with inorganic species, could have slightly altered our results(10% of the reconstructed mass), chemical mass closure was achieved mostof the time for the fine and coarse size fractions and at both sites. Ourconversion factor of 2.1 for organic carbon (OC) to particulate organicmatter (POM) is at the upper end of those reported in the literature, butfits with the aged smoke particles collected during the campaign. Theresults indicate that this conversion factor changed during the campaignalong with the BC/TC ratio.

The particulate mass (PM) concentration for fine aerosols at the MBL and FTsites averaged 17.4±4.7 µg/m3 and 11.2±3.2 µg/m3, respectively, and is among the highest reported in the literaturefor remote sites; more than 90% of this PM was composed equally ofammonium sulfate and carbonaceous aerosols. Comparison between the MBL andFT sites showed a slight vertical gradient for PM that was not observed fordust 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 scatteringcoefficient (σ_sp) that was measured at ambient RelativeHumidity (RH) for fine particles at the MBL site. Reconstruction of σ_sp was achieved using ratios of wet to dry scattering, f(RH), thatdepend on RH for ammonium sulfate, but are kept equal to 1 for POM. Thisresults in a low water adsorption for our organic-rich carbonaceousaerosols, although these aged biomass smoke aerosols are supposed to behighly oxidized. Mass scattering efficiencies of the main aerosol componentswere 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 reconstructedσ_sp was due to water uptake by ammonium sulfate aerosols,demonstrating their major role in the radiative aerosol properties in theeastern Mediterranean.