Characterisation of aerosol properties and radiative forcing at an anthropogenically perturbed continental site

Measurements of optical, chemical and physical properties of ambient aeroso l particles were obtained at a continental anthropogenically perturbed envi ronment. For aerosol sampled at the site from January to December 1995, the mean gravimetric mass concentration for particles with diameter d(p) less than or equal to 1 mu m on 24-hr filter samples was 11.0 +/- 7.6 mu g/m(3). Ion chromatographic (IC) analysis chemically identified 50 +/- 15% of the total gravimetric mass for fine particles (d(p) less than or equal to 1 mu m). Most of the IC identified material (88 +/- 14%) consisted of NH4+ and S O42-. Thermal optical reflectance (TOR) analysis of small number of samples showed that the material not identified with IC consisted of elemental and organic carbon. To examine the implications of the measurements and effect of their variation, the mean annual direct radiative forcing (<(Delta)over bar>(F) over bar(R)) due to aerosol particles at the site was estimated by a simple model using as input measured total light scattering coefficients , hemispheric backscatter ratio and hygroscopic growth factor. For a mean a nnual height of the mixed layer at 800 m, an assumed single scattering albe do (omega) value of 0.85 and wavelength 550 nm,<(Delta)over bar>(F) over ba r(R) as function of relative humidity (RH) ranged between -0.51 +/- 0.47 (R H = 40%) and -0.63 +/- 0.58 (RH = 80%) W/m(2). Sensitivity tests pointed to the importance of accurate quantification of omega. (C) 1999 Elsevier Scie nce Ltd. All rights reserved.