Influence of Atmospheric Particulate Matter on Ozone in Nanjing, China: Observational Study and Mechanistic Analysis

Particulate matter with diameters of 2.5 μm or smaller (PM 2.5 ) and ozone (O 3 ) are major pollutants in the urban atmosphere. PM 2.5 can affect O 3 by altering the photolysis rate and heterogeneous reactions. However, these two processes and their relative importance remain uncertain. In this paper, with Nanjing in China as the target city, we investigate the characteristics and mechanism of interactions between particles and O 3 based on ground observations and numerical modeling. In 2008, the average concentrations of PM 2.5 and O 3 at Caochangmen station are 64.6±47.4 μg m −3 and 24.6±22.8 ppb, respectively, while at Pukou station they are 94.1 ± 63.4 μg m −3 and 16.9 ± 14.9 ppb. The correlation coefficient between PM 2.5 and O 3 is −0.46. In order to understand the reaction between PM 2.5 and O 3 , we construct a box model, in which an aerosol optical property model, ultraviolet radiation model, gas phase chemistry model, and heterogeneous chemistry model, are coupled. The model is employed to investigate the relative contribution of the aforementioned two processes, which vary under different particle concentrations, scattering capability and VOCs/NO x ratios (VOCs: volatile organic compounds; NO x : nitric oxide and nitrogen dioxide). Generally, photolysis rate effect can cause a greater O 3 reduction when the particle concentrations are higher, while heterogeneous reactions dominate O 3 reduction with low-level particle concentrations. Moreover, in typical VOC-sensitive regions, O 3 can even be increased by heterogeneous reactions. In Nanjing, both processes lead to O 3 reduction, and photolysis rate effect is dominant. Our study underscores the importance of photolysis rate effect and heterogeneous reactions for O 3 , and such interaction processes should be fully considered in future atmospheric chemistry modeling.