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

Abstract

The hygroscopicity of organic aerosol (OA) is important for investigation of its climatic and environmental impacts. However, the hygroscopicity parameter κ OA 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 (CV-ToF-ACSM) and a humidified nephelometer system which measures the aerosol light-scattering enhancement factor f .RH/. A method for calculating κ OA based on f .RH/ and bulk aerosol chemical-composition measurements was developed. We found that κOAvaried in a wide range with significant diurnal variations. The derived κOAranged from almost 0.0 to 0.25, with an average (± 1 σ) of 0.08 ± 0:06) for the entire study. The derived κOAwas highly correlated with f44 (fraction of m=z 44 in OA measured by CV-ToF-ACSM), an indicator of the oxidation degree of OA (R D 0:79), and the relationship can be parameterized as κOAD 1:04×f44-:02 (κOAD 0:3×O V C-0:02, based on the relationship between the f44 and O=C ratio for CV-ToF-ACSM). On average, κOAreached the minimum (0.02) in the morning near 07:30 local time (LT) and then increased rapidly, reaching the peak value of 0.16 near 14:30 LT. The diurnal variations in κOAwere highly and positively correlated with those of mass fractions of oxygenated OA (R D 0:95), indicating that photochemical processing played a dominant role in the increase in κOAin winter on the NCP. Results in this study demonstrate the potential wide applications of a humidified nephelometer system together with aerosol composition measurements for investigating the hygroscopicity of OA in various environments and highlight that the parameterization of κOAas 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.