Photochemical impacts of haze pollution in an urban environment

Rapid economic growth in China over the past 30 years has resulted in significant increases in the concentrations of small particulates (PM_2.5) over the city of Beijing. In addition to health problems, high aerosol loading can impact visibility and thus reduce photolysis rates over the city, leading to potential implications for photochemistry. Photolysis rates are highly sensitive not only to the vertical distribution of aerosols but also to their composition, as this can impact how the incoming solar radiation is scattered or absorbed. This study, for the first time, uses aerosol composition measurements and lidar optical depth to drive the Fast-JX photolysis scheme and quantify the photochemical impacts of different aerosol species during the Air Pollution and Human Health (APHH) measurement campaigns in Beijing in November-December 2016 and May-June 2017. This work demonstrates that severe haze pollution events (PM_2.5 > 75 µg m^-3) occur during both winter and summer, leading to reductions in O₃ photolysis rates of 27 %-34 % (greatest in winter) and reductions in NO₂ photolysis of 40 %-66 % (greatest in summer) at the surface. It also shows that in spite of much lower PM_2.5 concentrations in the summer months, the absolute changes in photolysis rates are larger for both O₃ and NO₂. In the winter, absorbing species such as black carbon dominate the photolysis response to aerosols, leading to mean reductions in J[O¹D] and J[NO₂] in the lowest 1 km of 24 % and 23 %, respectively. In contrast, in the summer, scattering aerosol such as organic matter dominate the response, leading to mean decreases of 2 %-3 % at the surface and increases of 8 %-10 % at higher altitudes (3-4 km). During these haze events in both campaigns, the influence of aerosol on photolysis rates dominates over that from clouds. These large impacts on photochemistry can have significant implications for concentrations of important atmospheric oxidants such as the hydroxyl radical. Idealized photochemical box model studies show that such large impacts on photochemistry could lead to a 12 % reduction in surface O₃ (3 % for OH) due to haze pollution. This highlights that PM_2.5 mitigation strategies could have important implications for the oxidation capacity of the atmosphere both at the surface and in the free troposphere.