Derivation of the Density and Refractive Index of Organic Matter and Elemental Carbon from Closure between Physical and Chemical Aerosol Properties

Information on the density (p) and refractive index m(=n-ik) of elemental carbon (ECa) and organic matter (OMa), the main carbon components of atmospheric aerosols, has frequently been obtained from closure calculations between physical and chemical aerosol properties. However, this approach has suffered from large uncertainties since there were more unknown (or poorly known) parameters than defining equations. In this study, we propose a method that avoids this ambiguity mainly by considering both optical and mass closure and by expressing the three ECa parameters (p(Eca), n(Eca), k(ECa)) by a single (unknown) parameter. This allows mathematically rigorous determination of p(Eca), m(Eca), p(OMa) and m(OMa) from standard physico-chemical aerosol data and rigorous error analysis. The results are unambiguous and self-consistent, i.e., there is no difference between the chemically and physically derived p and m values of the atmospheric aerosol. Application of this method to our previously published data on biomass burning particles from Amazonia yields p(Eca) = 1.8(+/- 0.2) g/cm(3), m(ECa) = 1.9(+/- 0.1)-i0.20(-0.04/+0.02), p(OMa) = 1.39(+/- 0.13) g/cm(3) and m(OMa)=1.46(+/- 0.02), where the 1 sigma uncertainty limits given in parenthesis are based on the principles of error propagation. The relatively low imaginary part of mEca indicates the presence of only partially graphitized elemental carbon, which is consistent with biomass burning aerosol dominated bysmoldering combustion conditions.