3. Mean concentration and percentage of days exceeding limit of primary pollutants in outdoor air in China between 2015 and 2019*.
| Variables | 2015 | 2016 | 2017 | 2018 | 2019 |
| *, Source: Ministry of Ecology and Environment of the People’s Republic of China (http://www.mee.gov.cn/hjzl/sthjzk/zghjzkgb/). | |||||
| Compliance rate of air quality (%) | 21.6 | 24.9 | 29.3 | 35.8 | 46.6 |
| Mean concentration (μg/m3) | |||||
| PM2.5 | 50 | 47 | 43 | 39 | 36 |
| PM10 | 87 | 82 | 75 | 71 | 63 |
| O3 | 134 | 138 | 149 | 151 | 148 |
| SO2 | 25 | 22 | 18 | 14 | 11 |
| NO2 | 30 | 30 | 31 | 29 | 27 |
| CO | 2.1 | 1.9 | 1.7 | 1.5 | 1.4 |
| Percentage of days exceeding the limit (%) | |||||
| PM2.5 | 17.5 | 14.7 | 12.4 | 9.4 | 8.5 |
| PM10 | 12.1 | 10.4 | 7.1 | 6 | 4.6 |
| O3 | 4.6 | 5.2 | 7.6 | 8.4 | 7.6 |
| SO2 | 0.7 | 0.5 | 0.3 | <0.1 | <0.1 |
| NO2 | 1.6 | 1.6 | 1.5 | 1.2 | 0.6 |
| CO | 0.5 | 0.4 | 0.3 | 0.1 | <0.1 |