Aerosol acidity in a megacity with high ambient temperature and relative humidity of Central China: temporal variation, determining factors and pollution transition effect

Abstract. Aerosol acidity affects the chemical transformation of aerosols and subsequent haze formation. High resolution (1-h) observation of water-soluble inorganic ions in fine particles, gaseous pollutants, and meteorological parameters was conducted from September 2015 to August 2016 at Wuhan, a megacity of Central China with high relative humidity and ambient temperature, compared with north Chinese cities. By adopting thermodynamic model ISOROPPIA-II, the aerosol acidity for different time scales, pollution episodes, and air mass directions was calculated. Aerosols in Wuhan were moderate acidic, with pH averaged as 3.30 ± 0.49. The aerosol acidity was higher in July (pH as 2.64 ± 0.31), September (pH as 2.75 ± 0.30) and August (pH as 2.79 ± 0.29), and lower in January (pH as 3.77 ± 0.28) and March (pH as 3.70 ± 0.16). It decreased with the air pollution increasing, with the pH values of 3.07 ± 0.45, 3.63 ± 0.27 and 3.84 ± 0.22 for clean, transition and polluted episodes, respectively. The air masses in Wuhan transported from North China exhibited higher aerosol acidity, with pH averaged as 3.17–3.22. The unique environmental and meteorological conditions (high humidity, annual averaged RH as 0.74 ± 0.13) lead to excess ammonium (on average of 6.06 ± 4.51 μg m⁻³) and abundant aerosol water content (AWC, on average of 71.0 ± 82.8 μg m⁻³) in Wuhan, which can explain the lower PM_2.5 acidity in Wuhan than other megacities of China. At lower AWC level (less than ~ 15 μg m⁻³), the particle pH showed a decreasing trend with AWC increased. When the AWC continuous increased from ~ 15 to ~ 380 μg m⁻³, there was an obvious increase of particle pH. Then no significant growth of pH was found when AWC was higher than ~ 380 μg m⁻³. With atmospheric RH increasing, the aerosol pH exhibited decreasing trend firstly and then increased, with the turning point RH as about 0.48. There was a logarithmic growth of aerosol pH with total NH_x (NH₃ + NH₄⁺) increasing. From the fitted logarithmic curve, the aerosol pH of Wuhan was at the range of pH rapid growth stage with NH_x increasing, indicating that the control of ammonia emission in Wuhan could be an effective way to reduce the aerosol pH and further mitigate air pollution. This paper firstly obtained the aerosol acidity properties at a megacity under abundant ammonium and high humidity with high time-resolution, which is an important supplementary for the current aerosol acidity research around the world.