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. The Chinese government has exerted strict emission controlsto mitigate air pollution since 2013, which has resulted in significantdecreases in the concentrations of air pollutants such as SO2 . Strictpollution control actions also reduced the average PM 2.5 concentrationto the low level of 39.7  µg m−3 in urban Beijing during the winterof 2017. To investigate the impact of such changes on the physiochemicalproperties of atmospheric aerosols in China, we conducted a comprehensiveobservation focusing on PM 2.5 in Beijing during the winter of 2017.Compared with the historical record (2014–2017), SO2 decreased to thelow level of 3.2 ppbv in the winter of 2017, but the NO2 level was stillhigh (21.4 ppbv in the winter of 2017). Accordingly, the contribution of nitrate(23.0  µg m−3 ) to PM 2.5 far exceeded that of sulfate (13.1  µg m−3 ) during the pollution episodes, resulting in a significantincrease in the nitrate-to-sulfate molar ratio. The thermodynamic model(ISORROPIA II) calculation results showed that during the PM 2.5 pollution episodes particle pH increased from 4.4 (moderate acidic) to 5.4(more neutralized) when the molar ratio of nitrate to sulfate increased from1 to 5, indicating that aerosols were more neutralized as the nitratecontent elevated. Controlled variable tests showed that the pH elevationshould be attributed to nitrate fraction increase other than crustal ion andammonia concentration increases. Based on the results of sensitivity tests, future prediction for the particle acidity change was discussed. Wefound that nitrate-rich particles in Beijing at low and moderate humidconditions (RH: 20 %–50 %) can absorb twice the amount of water thatsulfate-rich particles can, and the nitrate and ammonia with higher levels havesynergetic effects, rapidly elevating particle pH to merely neutral (above5.6). As moderate haze events might occur more frequently under abundantammonia and nitrate-dominated PM 2.5 conditions, the major chemicalprocesses during haze events and the control target should be re-evaluatedto obtain the most effective control strategy.

Nitrate-dominated PM&lt;sub&gt;2.5&lt;/sub&gt; and elevation of particle pH observed in urban Beijing during the winter of 2017

Nitrate-dominated PM<sub>2.5</sub> and elevation of particle pH observed in urban Beijing during the winter of 2017