Abstract
Abstract. The water uptake of aerosol influences its optical depth and capacity for cloud formation, depending on the vertical profile of aerosol hygroscopicity because of different solar radiation received and supersaturation (SS) conditions at different atmospheric levels. Such information is lacking over the polluted East Asian region. This study presents aircraft-based in situ measured aerosol size distributions and chemical compositions by a series of flights over the Beijing area in wintertime. Under high relative humidity (hRH) conditions (surface RH > 60 %), a significant enhancement of aerosol hygroscopicity parameter (κ) in the planetary boundary layer (PBL) was observed to increase by 50 % from 0.20 up to 0.34 from the surface to the top of the PBL (vertical gradient of ∼0.13 km−1), along with the dry particle effective diameter (Deff) being increased by 71 % and activation ratio up to 0.23 (0.64) at SS =0.05 % (0.1 %). However, a lower vertical gradient of κ (0.05 km−1) and smaller Deff was exhibited under low RH (lRH, surface RH < 60 %). This suggests that the aqueous processes played an important role in promoting the enhancement of particle hygroscopicity in the PBL. The κ in the lower free troposphere (LFT) was relatively stable at 0.24±0.03 with a slight increase during regional transport. The enhancement of aerosol optical depth (AOD) due to water uptake ranged 1.0–1.22 for the PBL under lRH and LFT, but it reached as high as 6.4 in the PBL under hRH. About 80 % and 18 % of the AOD were contributed to by aerosol hygroscopic growth under hRH and lRH, respectively. These results emphasize the important evolution of aerosol water-uptake capacity in the PBL, especially under the high RH condition.
Highlights
Water growth on particles can increase particle size and modify its refractive index, thereby affecting its radiative effects
The in situ measured size distribution and hygroscopic growth factor are combined to evaluate the influence of water uptake on the ambient aerosol optical depth (AOD) and cloud condensation nuclei (CCN) activation ratio under different moisture conditions
By including the ammonium chloride, a mass fraction of 3.6 % ± 2.0 % was found throughout the experiment, and the chloride concentration was mostly lower than the lower AMS detection limit; its contribution to bulk aerosol hygroscopicity could be ignored during the observation
Summary
Water growth on particles can increase particle size and modify its refractive index, thereby affecting its radiative effects. This impact could be exacerbated, especially under the high moisture condition, as evidenced by a number of studies that showed that over 25 % of the polluted days with significantly reduced visibility in megacities were associated with high RH (Deng et al, 2013; Zhong et al, 2018; Qiang et al, 2015; Quan et al, 2014; Liu et al, 2013) These results emphasize the importance of studying the vertical characteristics of particle hygroscopicity, but such information is still lacking due to limited airborne measurements over the East Asian region. The in situ measured size distribution and hygroscopic growth factor are combined to evaluate the influence of water uptake on the ambient aerosol optical depth (AOD) and CCN activation ratio under different moisture conditions
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