Table 21. Trends in the changes of six major air pollutants from 2013 to 2021.

Time	PM2.5, μg/m3	PM10, μg/m3	SO2, μg/m3	NO2, μg/m3	O3, μg/m3	CO, mg/m3
2013	72	118	40	44	139	2.5
2014	62	105	35	38	140	2.2
2015	50	87	25	30	134	2.1
2016	47	82	22	30	138	1.9
2017	43	75	18	31	149	1.7
2018	39	71	14	29	151	1.5
2019	36	63	11	27	148	1.4
2020	33	56	10	24	138	1.3
2021	30	54	9	23	137	1.1