Measurement report: The effect of aerosol chemical composition on light scattering due to the hygroscopic swelling effect

Abstract

Liquid water in aerosol particles has a significant effect on their optical properties, especially on light scattering, whose dependence on chemical composition is investigated here using measurements made in southern Beijing in 2019. The effect is measured by the particle light scattering enhancement f(RH), where RH denotes the relative humidity, which is found to be positively and negatively impacted by the proportions of inorganic and organic matter, respectively. Black carbon is also negatively correlated. The positive impact is more robust when the inorganic matter mass fraction was smaller than 40% (RCombining double low line0.93, R: the Pearson's correlation coefficient), becoming weaker as the inorganic matter mass fraction gets larger (RCombining double low line0.48). A similar pattern was also found for the negative impact of the organic matter mass fraction. Nitrate played a more significant role in aerosol hygroscopicity than sulfate in Beijing. However, the deliquescence point of ambient aerosols was at about RHCombining double low line80% when the ratio of the sulfate mass concentration to the nitrate mass concentration of the aerosol was high (mostly higher than 1/44). Two schemes to parameterize f(RH) were developed to account for the deliquescent and non-deliquescent effects. Using only one f(RH) parameterization scheme to fit all f(RH) processes incurs large errors. A piecewise parameterization scheme is proposed, which can better describe deliquescence and reduces uncertainties in simulating aerosol hygroscopicity.