What caused the smog in Chengdu?

Topography; Located in the basin, the atmospheric environmental capacity is extremely limited, which is not conducive to the diffusion of pollutants; Quiet weather formed by geographical environment! (low wind speed) is not conducive to the diffusion and dilution of pollutants, which aggravates air pollution; Climate: Inversion weather often occurs in Chengdu, and pollutants can only accumulate near the ground and cannot spread, aggravating pollution; The high humidity and cloudy weather in Chengdu promoted the formation of haze weather, mainly due to the large amount of smoke and dust generated by burning straw around the autumn harvest season and the particularity of Sichuan Basin. I analyzed the data of Chengdu. Conclusion: PM2.5 in Chengdu is as serious as that in Beijing. I have to count to two decimal places before I find that Chengdu is slightly more serious than Beijing. How serious this is. 73 micrograms. Okay, this is serious. The average in Chengdu and Beijing is three times the WHO 24-hour limit. However, I should use the age value of WHO, because these data are all annual data, and some studies have found that even PM2 below 25 micrograms is harmful to people's health. By this standard, the average value of Beijing and Chengdu is 10 times higher than the upper limit, not to mention the particularly serious smog. I admit that I am dishonest. Compared with the above data, I am a little dishonest. Why? Because Chengdu is as bad as Beijing, not because Chengdu is getting worse, but because Beijing's PM2.5 has recently dropped: Beijing and Chengdu are the "champions" of smog, but no place in China's big cities meets the safety standards. No matter which city you are in, you should protect your body, such as wearing a mask! The reason may be that Sichuan is a basin with many factories, and these smog can't be dispersed. So in winter, under normal circumstances, we expect sunny days, but in order not to breathe smog, we hope it will rain, preferably strong winds.