*5.2. Meteorological Factors*

Tropospheric O3 concentration was found to be positively correlated with ambient air temperature, and negatively correlated with wind speed and relative humidity. On the contrary, NOx was positively correlated with relative humidity, and negatively correlated with temperature [72]. Under normal conditions, temperature can affect the concentration of O3 by influencing the reaction rate, while wind speed can affect the dilution and diffusion of pollutants. Relative humidity has some influence on photochemical reaction processes, and higher relative humidity can cause wet deposition and even lead to the erosion of pollutants by rainwater. It was reported that aerosol could change the photolysis rate of trace gases [73]. Absorption of aerosols can reduce UV flux throughout the troposphere, resulting in a reduction in near-surface O3. Based on the analysis of the meteorological effect on atmospheric O3 in Tianjin from 2009 to 2015, it was concluded that the tropospheric O3 level was more dependent on temperature in the afternoon than in the morning since the daily maximum temperature usually occurred in the afternoon [74]. In spring and summer, the maximum daily O3 was less dependent on the solar radiation than the ambient temperature. In autumn and winter, solar radiation played a more important role in determining O3 level. The concentration of atmospheric O3 had a weak negative correlation with the wind speed in spring, summer, and autumn, but a weak positive correlation with the wind speed in winter. Moisture in spring and autumn also had an effect on atmospheric O3 concentration due to the compensation between water vapor and O3. Air with high humidity raised ·OH radicals and produced higher O3 concentration in the areas with high NOx. At the same time, a rise in water vapor also consumed excited

oxygen atoms and increased the loss of O3. The relationship between heat waves and the concentration of atmospheric O3 in the Yangtze River Delta was discussed [75]. The results showed that under the action of heat waves, the water vapor content and the cloud cover of the Yangtze River Delta were reduced because of the anticyclone controlled by the downdraft, which increased the concentration of atmospheric O3 in the presence of intense solar radiation. In the case of climate warming, the chemical reaction may cause the atmospheric O3 content to increase significantly, and the high temperature can also promote vertical turbulence and horizontal advection to some extent, which is beneficial to the removal of O3, but the extent is much less than that of the chemical action. Relevant studies have shown that the heat-island effect was directly or indirectly related to the increase in the emission of atmospheric O3 and its precursors. The pollution of atmospheric O3 in the Yangtze River Delta is becoming more and more serious, and the heat-island effect is the key factor affecting the atmospheric O3 level. There was a positive correlation between heat-island effect and the atmospheric O3 concentration in the Yangtze River Delta. The factors influencing the urban heat-island effect and atmospheric O3 include landscape, topography and population, but land surface temperature and vegetation index are the most important [76]. It was reported that the structure and evolution of weather was of great significance to the atmospheric photochemical pollution [77]. The level of ozone concentration affected by the surface and the boundary layer depends on the main weather conditions that are conducive to large-scale subsidence [78]. The eastern and central basins of the Mediterranean have obvious top-down ozone deposition, which is caused by adiabatic convection over the Persian Gulf during the Indian monsoon season [79]. The influence of Asian continental outflow on the regional background ozone level in the northern South China Sea was studied, and the results indicated that the Asian continental outflows brought about by the winter monsoon could be immense, and intense enough to affect regions from far south, at latitudes similar to Antarctica [80].
