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Hygroscopicity is an important property of aerosols that has significant influences on environmental quality and atmospheric chemical processes. Based on field observations in Beijing, the influences of the measured water-soluble inorganic ions (WSII) in PM2.5 on aerosol hygroscopicity were analyzed. Changes in abundances of WSII in PM2.5 (nitrate & ammonium-dominated) under polluted conditions significantly influenced the aerosol hygroscopic capacity. The measured aerosol light scattering hygroscopic growth factors (f(RH)) of PM2.5 at 80 ± 0.5 % RH was 1.65 during the clean period and 1.65–1.69 during haze events I–III. The values of hygroscopic parameter (κ) under haze events (0.20 ± 0.05 to 0.26 ± 0.07) were 18 %–53 % higher than during clean period (0.17 ± 0.07). The κ of PM2.5 generally increased with increasing mass concentration of PM2.5 and RH. Linear relationships between κ and the mass fractions of sulfate, nitrate, and ammonium (SNA) were also observed. Furthermore, the values of κ showed a diurnal fluctuations similar to the diurnal fluctuations of SNA, especially nitrate. This work showed that hygroscopicity of PM2.5 was mainly influenced by SNA, in the order NH4+ > NO3− > SO42−. This work indicated that emissions of NOx (x = 1, 2) and NH3 should be controlled to mitigate their influences on aerosols in the atmospheric environment and climate change.
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•Influences of the main water-soluble ions on PM2.5 hygroscopicity were analyzed.•The κ of PM2.5 during haze events were 18 %–53 % higher than that under clean period.•The diurnal variation of aerosol κ is similar to that of SNA, especially nitrate.•Emission reduction of NH3 may mitigate the effects of hygroscopic particles on atmospheric environment.