SO2 is Sulphur Dioxide

Sulfur dioxide (SO2) is a criteria pollutant. Short-term exposure to SO2, ranging from 5 minutes to 24 hours, is linked with adverse respiratory effects including broncho constriction and increased asthma symptoms. SO2 is also a major precursor to fine particulate soot and acid rain.

The EPA (USA) has set an annual mean SO2 standard of 30 ppb and a 24-hour average of 140 ppb 

 

NO2 is Nitrogen Dioxide

Nitrogen dioxide (NO2) is a criteria pollutant and contributes to the formation of photochemical smog, with significant impact on human health. Breathing raised levels of NO2 inflames the lining of the lungs and reduces immunity to lung infections

The EPA (USA) set an annual mean NO2 standard of 53 ppb

and a new hourly standard of 100 ppb.

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O3 is Ozone

At ground level ‘bad’ ozone is a criteria pollutant that is a significant health risk, especially for people with asthma. It also damages crops, trees and other vegetation and is the main component of smog. Ground level ozone is not emitted directly; it is created by chemical reactions between oxides of nitrogen (NOx) and volatile organic compounds (VOC) in the presence of sunlight

The EPA (USA) set an 8-hour mean O3 standard of 75ppb

 See, https://ballotpedia.org/Air_pollutants 

https://ballotpedia.org/Ground-level_ozone_standards