Distinct diurnal variation of organic aerosol hygroscopicity and its relationship with oxygenated organic aerosol

Abstract. The hygroscopicity of organic aerosols (OA) is important for investigation of its climatic and environmental impacts. However, the hygroscopicity parameter κOrg remains poorly characterized, especially in the relatively polluted environment on the North China Plain (NCP). Here we conducted simultaneous wintertime measurements of bulk aerosol chemical compositions of PM2.5 and PM1 and bulk aerosol hygroscopicity of PM10 and PM1 on the NCP using a capture vaporizer time-of-flight aerosol chemical speciation monitor (ToF-ACSM) and a humidified nephelometer system which measures aerosol light scattering enhancement factor f(RH). A method for calculating κOrg based on f(RH) and bulk aerosol chemical composition measurements was developed. We found that κOrg varied in a wide range with significant diurnal variations. The derived κOrg ranged from almost 0.0 to 0.25 with an average (± 1σ) of 0.08 (± 0.06) for the entire study. The derived κOrg was highly correlated with f44 (fraction of m/z 44 in OA), an indicator of oxidation degree of OA (R = 0.79), and the relationship can be parameterized as κOrg = 1.04 × f44 − 0.02. On average, κOrg reached the minimum (0.02) in the morning near 07:30 and then increased rapidly reaching the peak value of 0.16 near 14:30. The diurnal variations of κOrg were highly and positively correlated with those of mass fractions of oxygenated OA (R = 0.95), indicating that photochemical processing played a dominant role for the increase of κOrg in winter on NCP. Results in this study demonstrate the potential wide applications of humidified nephelometer system together with aerosol composition measurements for investigating the hygroscopicity of OA in various environments, and highlight that the parameterization of κOrg as a function of OA aging processes needs to be considered in chemical transport models for better evaluating the impacts of OA on cloud formation, atmospheric chemistry and radiative forcing.