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

According to reports, according to China's current air quality classification standards, sulfur dioxide should be less than 0.050 (mg/m3, the same below), nitrogen dioxide should be less than 0.08, and inhalable particulate matter should be less than 0.050; Sulfur dioxide, nitrogen dioxide and inhalable particulate matter in secondary weather should be less than 0. 150, 0. 120 and 0. 150 respectively. Sulfur dioxide, nitrogen dioxide and inhalable particulate matter should be less than 0.625, 0.240 and 0.350 respectively in Grade III weather. Zhu Yuqiang introduced that although the first-class weather in this city only appeared for five days this year, air quality statistics show that the first-class weather in this city was only six days in 2002, and it has been increasing almost every year since then. In 2008, the first-class weather in this city reached 52 days, and environmental management 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 first-class weather and the second-class weather is 0. 1, while the difference between the second-class weather and the third-class weather is as high as 0.475. If the sulfur dioxide index reaches 0. 152 (very close to Grade I) on the same day and 0.5 (close to Grade III) on the next day, it should belong to Grade II weather according to the defined standards, but people will feel completely different.