Characteristics of PM_2.5 hygroscopicity and the influences of water-soluble ions during haze events in Beijing

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 (kappa) 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 kappa of PM2.5 generally increased with increasing mass concentration of PM2.5 and RH. Linear relationships between kappa and the mass fractions of sulfate, nitrate, and ammonium (SNA) were also observed. Furthermore, the values of kappa 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.