Abstract
Abstract. The hygroscopicity of organic aerosol (OA) is important for investigation of its climatic and environmental impacts. However, the hygroscopicity parameter κOA 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 κOA based on f(RH) and bulk aerosol chemical-composition measurements was developed. We found that κOA varied in a wide range with significant diurnal variations. The derived κOA ranged from almost 0.0 to 0.25, with an average (±1σ) of 0.08 (±0.06) for the entire study. The derived κOA was 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=0.79), and the relationship can be parameterized as κOA=1.04×f44-0.02 (κOA=0.3×O:C-0.02, based on the relationship between the f44 and O∕C ratio for CV-ToF-ACSM). On average, κOA reached 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 κOA were highly and positively correlated with those of mass fractions of oxygenated OA (R=0.95), indicating that photochemical processing played a dominant role in the increase in κOA in 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 κOA 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.
Highlights
Aerosol hygroscopic growth plays significant roles in different atmospheric processes, including atmospheric radiation transfer, cloud formation, visibility degradation, atmospheric multiphase chemistry, and even air-pollution-related health effects, and is crucial for studies on aerosol climatic and environmental impacts
Measured σsp at 525 nm of PM1 and PM10 ranged from 11 to 1875 Mm−1 and from 18 to 2732 Mm−1, with average values of 550 and 814 Mm−1, respectively. These results demonstrate that this campaign was carried out at a site that is overall highly polluted, where quite clean conditions as well as extremely polluted conditions were experienced during the measurement period
The mass contributions of ammonium, nitrate, sulfate, and organics to NR-PM2.5 and NR-PM1 are listed in Table 3, with organics being the major fraction of NR-PM1 and NR-PM2.5
Summary
Aerosol hygroscopic growth plays significant roles in different atmospheric processes, including atmospheric radiation transfer, cloud formation, visibility degradation, atmospheric multiphase chemistry, and even air-pollution-related health effects, and is crucial for studies on aerosol climatic and environmental impacts. The hygroscopicity parameter κ (Petters and Kreidenweis, 2007) of organic aerosol (κOA) is a key parameter for investigating the roles of organic aerosol in radiative forcing, cloud formation, and atmospheric chemistry. Rastak et al (2017) reported that global average aerosol radiative forcing would decrease about 1 W m−2 should κOA increase from 0.05 to 0.15, which is of the same order of the overall climate forcing of anthropogenic aerosol particles during the industrialization period. ΚOA has not yet been characterized well due to the extremely complex chemical compositions of organic aerosol. It is important to conduct more research on the spatiotemporal variation in κOA and its relationship with aerosol chemical compositions to reach a better characterization and come up with more appropriate parameterization schemes in chemical, meteorological, and climate models
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