What are the air quality grades? What is each level like?

According to China's current air quality grading standards, sulfur dioxide, nitrogen dioxide and inhalable particulate matter in the first-class weather should be less than .5 (mg/m3, the same below), .8 and .5 respectively. Sulfur dioxide, nitrogen dioxide and inhalable particulate matter in secondary weather should be less than .15, .12 and .15 respectively. Sulfur dioxide, nitrogen dioxide and inhalable particulate matter in Grade III weather should be less than .625, .24 and .35 respectively. Zhu Yuqiang introduced that although the first-class weather in this city has only appeared for five days this year, the air quality statistics show that in 22, the first-class weather in this city was only six days, and it has been increasing almost every year since then. In 28, the first-class weather in this city reached 52 days, and environmental control has played an obvious role in improving air quality. The reason why citizens sometimes feel that the actual air quality is lower than the air quality grade announced by the relevant departments is mainly related to the boundary of the grade division. Taking the sulfur dioxide pollutant index as an example, the difference between the primary weather and the secondary weather is .1, while the difference between the secondary weather and the tertiary weather is as much as .475. If the sulfur dioxide index reached .152 (very close to Grade I) on that day, and the sulfur dioxide index reached .5 (close to Grade III) on the next day, it should be classified as Grade II weather according to the defined standards, but people's feelings will be completely different.