资源环境科技发展态势分析平台

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, Ca²⁺ 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 ${\mathrm{SO}}_{\mathrm{4}}^{\mathrm{2}-}$, TNH₃ (total ammonium (gas + aerosol)), and temperature, while unique factors were Ca²⁺ in spring and RH in summer. The decreasing ${\mathrm{SO}}_{\mathrm{4}}^{\mathrm{2}-}$ and increasing ${\mathrm{NO}}_{\mathrm{3}}^{-}$ mass fractions in PM_2.5 as well as excessive NH₃ 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 TNH₃ indicated that although NH₃ in the NCP was abundant, the PM_2.5 pH was still acidic because of the thermodynamic equilibrium between ${\mathrm{NH}}_{\mathrm{4}}^{+}$ and NH₃. To reduce nitrate by controlling ammonia, the amount of ammonia must be greatly reduced below excessive quantities.