Aerosol acidity plays a key role in secondary aerosol formation. The high-temporal-resolution PM sub2.5/sub pH and size-resolved aerosol pH in Beijing were calculated with ISORROPIA?II. In 2016–2017, the mean PM sub2.5/sub pH (at relative humidity (RH)??30?%) over four seasons was 4.5±0.7 (winter)? 4.4±1.2 (spring)? 4.3±0.8 (autumn)? 3.8±1.2 (summer), showing moderate acidity. In coarse-mode aerosols, Casup2+/sup played an important role in aerosol pH. Under heavily polluted conditions, more secondary ions accumulated in the coarse mode, leading to the acidity of the coarse-mode aerosols shifting from neutral to weakly acidic. Sensitivity tests also demonstrated the significant contribution of crustal ions to PM sub2.5/sub pH. In the North China Plain (NCP), the common driving factors affecting PM sub2.5/sub pH variation in all four seasons were SO 4 2 - , TNHsub3/sub (total ammonium (gas + aerosol)), and temperature, while unique factors were Casup2+/sup in spring and RH in summer. The decreasing SO 4 2 - and increasing NO 3 - mass fractions in PM sub2.5/sub as well as excessive NHsub3/sub in the atmosphere in the NCP in recent years are the reasons why aerosol acidity in China is lower than that in Europe and the United States. The nonlinear relationship between PM sub2.5/sub pH and TNHsub3/sub indicated that although NHsub3/sub in the NCP was abundant, the PM sub2.5/sub pH was still acidic because of the thermodynamic equilibrium between NH 4 + and NHsub3/sub . To reduce nitrate by controlling ammonia, the amount of ammonia must be greatly reduced below excessive quantities.
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