How is Air Quality Index relevant to both PM2.5 and NO2

Question

I am comparing NO2 and PM2.5 levels in the pre-2020 Jan-April and 2020 Jan-April (representing air quality during lockdown and under no-lockdown conditions).

The data was from the aqicn website (https://aqicn.org/data-platform/covid19/). The data is not raw concentrations, it is converted into the US EPA standard, which I believe is represented as AQI values.

Can someone please explain why with PM2.5, the data reaches from 'Good' to 'Hazardous', whereas the NO2 is only within the 'Good' bracket. I'm hoping it is because PM2.5 is more harmful than NO2?

But if it is because I should be using a different scale for NO2 (e.g., 0-10 = Good, 10-20 = Moderate, etc.) I'd like to know - I haven't been able to find different AQI scales for different pollutants.

Answer 1

Most locations in the US don't see high concentrations of pollutants like NO₂ and CO anymore. They are largely no longer an air quality problem and air is "Good" in regards to those pollutants. Especially during "lockdown", when traffic is light. This is largely thanks to tailpipe emissions standards for vehicles and a much more fuel efficient fleet. On the other hand, PM2.5 has multiple sources that cause high concentrations, and we are much more likely to see the AQI vary beyond "Good" PM2.5 (e.g. outdoor fires, wood-stoves, secondary aerosol formation).

The AQI is based on health-based evidence, with a different concentration threshold for each pollutant. These are all put onto the AQI scale for simple communication to the public. If concentrations of NO2 had reached levels of the NAAQS, then you would see your graph plot into those AQI levels (note NO₂ doesn't go as far up in "AQI" values as PM2.5 on your graph because those NO₂ concentrations were never detected by the monitor).

The National Ambient Air Quality Standard (NAAQS) for nitrogen dioxide (NO₂) is a 1-hour NO₂ standard of 100 parts per billion (ppb). See https://www3.epa.gov/airnow/no2.pdf and page 2 here: http://njaqinow.net/App_Files/2016/AQI%202016.pdf which has the break levels for each pollutant, shown below: