Aerosol pH and its driving factors in Beijing

Abstract. Aerosol acidity plays a key role in secondary aerosol formation. The high-temporal-resolution PM 2.5 pH and size-resolved aerosol pH in Beijing were calculated with ISORROPIA II. In 2016–2017, the mean PM 2.5 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, Ca 2+ 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 2.5 pH. In the North China Plain (NCP), the common driving factors affecting PM 2.5 pH variation in all four seasons were SO 4 2 - , TNH 3 (total ammonium (gas  +  aerosol)), and temperature, while unique factors were Ca 2+ in spring and RH in summer. The decreasing SO 4 2 - and increasing NO 3 - mass fractions in PM 2.5 as well as excessive NH 3 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 2.5 pH and TNH 3 indicated that although NH 3 in the NCP was abundant, the PM 2.5 pH was still acidic because of the thermodynamic equilibrium between NH 4 + and NH 3 . To reduce nitrate by controlling ammonia, the amount of ammonia must be greatly reduced below excessive quantities.