Why is the air quality index of some cities negative today?

AQI

Air quality index (AQI) is a nonlinear dimensionless index that quantitatively describes air quality. The larger the value, the higher the grade and category, and the darker the color, indicating that the more serious the air pollution, the greater the harm to human health.

Because there is no hourly concentration standard for particulate matter, the hourly change of AQI calculated according to the 24-hour average concentration will lag behind the air quality. Therefore, when the primary pollutants are PM2.5 and PM 10, we should consider the real-time concentration data when looking at AQI.

AQI is quite different from the original air pollution index (API). The reference standard for AQI classification calculation is GB 3095-20 12 ambient air quality standard (current), and the pollutants involved in the assessment are SO2, NO2, PM 10, PM2.5, O3 and CO, which are discharged once an hour; The reference standard of API grading calculation is GB 3095- 1996 "Ambient Air Quality Standard" (outdated), and the pollutants evaluated are only SO2, NO2 and PM 10, which are published once a day. Therefore, AQI has stricter standards, more pollutant indicators and higher release frequency, and its evaluation results will be closer to the real feelings of the public.