Aerosol Mass Scattering Efficiency: Generalized Treatment of the Organic Fraction

Atmospheric aerosols are complex mixtures of organic and inorganic compounds. Current efforts to provide a simplified parameterization to describe the RH dependence of water uptake and associated optical properties lack the capability to include any dependence on the composition of the organic fraction. Using laboratory generated aerosol we have investigated the validity of such simplified treatment of organic fraction and estimated potential biases. In this study, we use cavity ring-down aerosol extinction photometry (CRD-AEP) to study the relative humidity (RH) dependence of the light extinction of aerosols, σep, simultaneously considering the influence of particle size, chemical composition, and mixing state (internal and external mixtures). We have produced internally mixed aerosol systems including; ammonium sulfate, ammonium nitrate, sodium chloride, dicarboxylic acids, sugars, amino acids and humic acid. These aerosols are produced with an atomizer and size-selected with a Differential Mobility Analyzer (DMA). The particles then enter into a CRD-AEP to measure dry extinction, σep(Dry), after which they travel into a RH conditioner and another CRD-AEP to measure the humidified aerosol extinction, fσ(ep)RH. The ratio of the humidified extinction to the dry extinction is fσ(ep)RH. Representative organic compounds were found to have fσ(ep)RH values that are much smaller than pure salts; though the fσ(ep)RH values vary little within the organic compounds studied. In addition, we have found that treating the inorganic/organic aerosols as external mixtures is generally correct to within ~10%, indicating appropriate simplified treatment of the RH dependence of atmospheric aerosol according to inorganic/organic fraction. In this presentation, we include recommendations for the generalized treatment of the organic fraction, exceptions to this generalized behavior, and estimates of the potential bias caused by generalized treatment.