*5.3. Atmospheric Particulates*

Aerosols are small particles suspended in the atmosphere and play an important role in the earth's radiation balance, air quality and cloud microphysics. They directly affect the regional and global climate by absorbing and scattering solar and terrestrial radiation, and indirectly affect the global climate by altering cloud formation characteristics. Ambient aerosol particles are mainly derived from anthropogenic activities and natural sources, such as residential heating, automobile exhausts, open-air combustion and volcanic activities [81]. The Asian monsoon brought in aerosols from biomass burning in southeast Asia, which were mixed with moist air particles in southern China, eventually reaching high aerosol concentrations in the spring, which reached the lowest concentration in winter [82]. There was a significant negative correlation between O3 and particulate matter in the margin of Tarim Basin, indicating that the effect of dust on solar transmittance in the atmosphere lead to a decrease in net O3 productivity [83]. The concentration of O3 was influenced by the nonuniform chemical processes occurring on the surface of particles, so increasing the concentration of PM2.5 could weaken the atmospheric radiation. This would allow the O3 level to be suppressed by eliminating ultraviolet light, which was consistent with the conclusions of Wang et al. [22] and Qu et al. [84]. In 2017, 338 main cities in China were selected to sample ambient air for 365 days to compare the concentrations of O3, NO2, SO2, particulate matter and CO in the atmosphere [85]. The results showed that O3 concentrations were significantly correlated with PM10 in 238 cities, among which, the coefficients in 142 cities were positive whereas those in 96 cities were negative. Most cities with positive correlations were mainly located in the south and northeast, while most cities with negative correlations were mainly located in the north of China. There was no significant correlation between O3 concentration and PM10 concentration in 100 cities. O3 concentrations were significantly correlated with PM2.5 in 250 cities, among which, the coefficients in 117 cities were positive and those in 133 cities were negative. Most cities with positive correlations were mainly located in the south, while most cities with

negative correlations were mainly located in the north. There was no significant correlation between O3 concentration and PM2.5 concentration in 88 cities. The possible reason for the above results was that NOx and VOCs would simultaneously increase significantly on the particulate matter (PM) pollution days in many cities, and the increase in these precursors influences the atmospheric O3 concentration more than the particulates. Atmospheric O3 was usually used as a tracer for photochemical reactions. A large amount of O3 was used as an oxidant to enrich the secondary components of PM2.5 through a secondary photochemical process, so higher PM2.5/PM10 usually indicated the existence of more active photochemical reactions. To some extent, PM2.5/PM10 could be used as a reference index for the types of air pollution, that is, higher or lower PM2.5/PM10 indicated the complicated pollution types related to photochemical reaction [86].

The main fixed sources of PM2.5 and PM10 are smoke and dust produced by fuel combustion and gas oil during heating in industrial enterprises, such as power generation, oil and printing. The main moving source is exhaust gas emitted by road traffic vehicles into the atmosphere. The temporal characteristics of PM2.5 in Anhui province showed that PM2.5 decreased from January to July, and increased from July to December, that is, the concentrations of PM2.5 were lower in summer and higher in winter [87]. Some studies showed that PM2.5 and PM10 were positively correlated with NO2 and CO, and weakly correlated with O3. The high concentration of O3 in highly oxidized air in hightemperature seasons promoted the formation of secondary particulate matter, which made PM2.5 positively correlated with O3 [88]. Several studies have found that reducing PM2.5 might lead to an increase in atmospheric O3, and reducing emissions of NOx and VOCs is required to overcome this effect. A more important factor affecting O3 trends in the North China Plain (NCP) from 2013 to 2017 was the reduction in PM2.5, which slowed down the sink of hydroperoxy radicals, thus speeding up O3 production [89]. In addition, atmospheric particles could directly affect the scattering and absorption of radiation, consequently changing the intensity of incident ultraviolet radiation, and affecting the production of O3. The formation of O3 decreased with the decrease in UV radiation or light scattering associated with PM2.5 [90].
