Reducing uncertainties associated with filter-based optical measurements of soot aerosol particles with chemical information

Abstract

Abstract. Of the many identified and potential effects of atmospheric aerosol particles on climate, those of soot particles are the most uncertain, in that analytical techniques concerning soot are far from satisfactory. One concern when applying filter-based optical measurements of soot is that they suffer from systematic errors due to the light scattering of non-absorbing particles co-deposited on the filter, such as inorganic salts and mineral dust. In addition to an optical correction of the non-absorbing material this study provides a protocol for correction of light scattering based on the chemical quantification of the material, which is a novelty. A newly designed Particle Soot Absorption Photometer was constructed to measure light transmission on particle accumulating filters, which includes an additional sensor recording backscattered light. The choice of polycarbonate membrane filters avoided high chemical blank values and reduced errors associated with length of the light path through the filter. Two protocols for corrections were applied to aerosol samples collected at the Maldives Climate Observatory Hanimaadhoo during episodes with either continentally influenced air from the Indian/Arabian subcontinents (winter season) or pristine air from the Southern Indian Ocean (summer monsoon). The two ways of correction (optical and chemical) lowered the particle light absorption of soot by 63 to 61%, respectively, for data from the Arabian Sea sourced group, resulting in median soot absorption coefficients of 4.2 and 3.5 Mm-1. Corresponding values for the South Indian Ocean data were 69 and 97% (0.38 and 0.02 Mm-1). A comparison with other studies in the area indicated an overestimation of their soot levels, by up to two orders of magnitude. This raises the necessity for chemical correction protocols on optical filter-based determinations of soot, before even the sign on the radiative forcing based on their effects can be assessed.