Estimation of the PM2.5 effective hygroscopic parameter and water content based on particle chemical composition: Methodology and case study

Particle hygroscopicity plays a key role in understanding the mechanisms of haze formation and particle optical properties. The present study developed a method for predicting the effective hygroscopic parameter kappa and the water content of PM2.5 on the basis of the kappa-Kohler theory and bulk chemical components of PM2.5. Our study demonstrated that the effective hygroscopic parameter can be estimated using the PM2.5 mass concentration, water-soluble ions, and total water-soluble carbon. By combining the estimated kappa and ambient relative humidity, the water content of PM2.5 can be further estimated. As an example, the kappa and water content of PM2.5 in Beijing were estimated utilizing the method proposed in this study. The annual average value of kappa of PM2.5 in Beijing was 0.25+/-0.09, the maximum kappa value 0.26+/-0.08 appeared in summer, and the seasonal variation is insignificant. The PM2.5 water content was determined by both the PM2.5 hygroscopicity and the ambient relative humidity ( RH). The annual average mass ratio of water content and PM2.5 was 0.18+/-0.20, and the maximum value 0.31+/-0.25 appeared in summer. Based on the estimated water content of PM2.5 in Beijing, the relationship between the PM2.5 water content and RH was parameterized as: m (%) = 0.03 + ( 5.73x10(-8))xRH(3.72). This parametric formula helps to characterize the relationship between the PM2.5 mass concentration and atmospheric visibility.