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

Abstract

<p><strong>Abstract.</strong> 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<span class="thinspace"></span>±<span class="thinspace"></span>0.49. The aerosol acidity was higher in July (pH as 2.64<span class="thinspace"></span>±<span class="thinspace"></span>0.31), September (pH as 2.75<span class="thinspace"></span>±<span class="thinspace"></span>0.30) and August (pH as 2.79<span class="thinspace"></span>±<span class="thinspace"></span>0.29), and lower in January (pH as 3.77<span class="thinspace"></span>±<span class="thinspace"></span>0.28) and March (pH as 3.70<span class="thinspace"></span>±<span class="thinspace"></span>0.16). It decreased with the air pollution increasing, with the pH values of 3.07<span class="thinspace"></span>±<span class="thinspace"></span>0.45, 3.63<span class="thinspace"></span>±<span class="thinspace"></span>0.27 and 3.84<span class="thinspace"></span>±<span class="thinspace"></span>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<span class="thinspace"></span>±<span class="thinspace"></span>0.13) lead to excess ammonium (on average of 6.06<span class="thinspace"></span>±<span class="thinspace"></span>4.51<span class="thinspace"></span>μg<span class="thinspace"></span>m<sup>−3</sup>) and abundant aerosol water content (AWC, on average of 71.0<span class="thinspace"></span>±<span class="thinspace"></span>82.8<span class="thinspace"></span>μg<span class="thinspace"></span>m<sup>−3</sup>) in Wuhan, which can explain the lower PM<sub>2.5</sub> acidity in Wuhan than other megacities of China. At lower AWC level (less than ~<span class="thinspace"></span>15<span class="thinspace"></span>μg<span class="thinspace"></span>m<sup>−3</sup>), the particle pH showed a decreasing trend with AWC increased. When the AWC continuous increased from ~<span class="thinspace"></span>15 to ~<span class="thinspace"></span>380<span class="thinspace"></span>μg<span class="thinspace"></span>m<sup>−3</sup>, there was an obvious increase of particle pH. Then no significant growth of pH was found when AWC was higher than ~<span class="thinspace"></span>380<span class="thinspace"></span>μg<span class="thinspace"></span>m<sup>−3</sup>. 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<sub>x</sub> (NH<sub>3</sub><span class="thinspace"></span>+<span class="thinspace"></span>NH<sub>4</sub><sup>+</sup>) increasing. From the fitted logarithmic curve, the aerosol pH of Wuhan was at the range of pH rapid growth stage with NH<sub>x</sub> 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.</p>