Closure study on measured and modeled optical properties for dry and hydrated laboratory inorganic aerosols with mixtures of dicarboxylic acids

A laboratory-based closure study was completed to compare measured and modeled optical properties and their dependence on controlled relative humidity (RH) for inorganic salts, dicarboxylic acids, and their mixtures. The closure between measured and modeled values of the light scattering coefficients were evaluated by calculating the average relative difference (ARD) values, which revealed agreement within 8.0% for the total scattering (σ_sp) and 14.8% for the back scattering (σ_bsp) values at dry RH conditions for all test aerosols. These ARD values were less than the total relative uncertainty based on the measurement and modeling approaches, indicating the achievement of closure for σ_sp and σ_bsp. Optical properties derived from σ_sp including: (1) the hygroscopic growth factor, fσsp, (2) the backscatter ratio, b, and (3) the Ångström exponent, å, were also compared with measured values. The ARD values between corresponding measured and modeled results for these derived optical parameters ranged from 0.1% to 30.8%. The impact of particulate organic matter (POM) on optical and hygroscopic properties of the aerosols tested here was also compared to the aerosol optical and composition measurements that occurred during the New England Air Quality Study-Intercontinental Transport and Chemical Transformation field campaign. Such comparison confirmed that a larger POM mass fraction resulted in less hygroscopicity for both the ambient and the laboratory aerosols. This study evaluated closure between laboratory measurements and model calculations and validated the reliability of the measured and modeled results with the closure analysis. Therefore, Mie-Lorentz model can be used to calculate the optical properties and their dependence on RH for other aerosols with more confidence.