Table 1.

Descriptive statistics.

Variable	Observations	2018		2019		2020
		mean	Std. Dev.	mean	Std. Dev.	mean	Std. Dev.
PM2.5	2697	56.78	42.27	61.52	50.67	51.32	44.37
PM10	2697	90.73	61.32	84.92	62.03	63.37	48.43
SO2	2697	18.56	15.55	13.16	10.5	11.27	8.72
O3	2697	59.11	18.77	46.27	17.33	58.75	15.16
NO2	2697	38.68	17.96	36.18	19.61	22.69	12.8
Temperature	2697	13.34	8.39	8.82	8.3	8.62	7.7
Rain and Snow (ratio)	2694	18.80%		29.25%		18.35%
Wind Speed	2697	3.25	0.54	1.95	0.92	2.31	1.19

Note: Rain and Snow (ratio) statistic is the proportion of rain or snow days during the research period in each year.