pm2_5_to_aqi.py

"""
PM2.5 to AQI (Air Quality Index) conversion

This implements the formula in
https://www.airnow.gov/sites/default/files/2020-05/aqi-technical-assistance-document-sept2018.pdf
and uses category names and colors from there.

PurpleAir notes:

In the JSON blob returned from the PurpleAir API, the key 'pm2_5Value' appears
once per sensor. PurpleAir averages the two values when calculating AQI,
unless is determines that one of the sensors is malfunctioning. (See
https://www2.purpleair.com/community/faq#hc-what-does-downgraded-mean-on-the-graph-1)

"""

import sys


def pm2_5_to_aqi(pm2_5):
    """Convert a PM2.5 reading in ug/m^3 to AQI

    Returns AQI, the name of the category the range falls in, and the RGB
    color for the range.
    """
    c = round(pm2_5, 1)
    if c <= 12.0:
        i = (( 50.0 -   0.0) / ( 12.0 -   0.0)) * (c -   0.0) +   0.0
        category = 'Good'
        color = '#00e400'
    elif c <=  35.4:
        i = ((100.0 -  51.0) / ( 35.4 -  12.1)) * (c -  12.1) +  51.0
        category = 'Moderate'
        color = '#ffff00'
    elif c <=  55.4:
        i = ((150.0 - 101.0) / ( 55.4 -  35.5)) * (c -  35.5) + 101.0
        category = 'Unhealthy for Sensitive Groups'
        color = '#ff7e00'
    elif c <= 150.4:
        i = ((200.0 - 151.0) / (150.4 -  55.5)) * (c -  55.5) + 151.0
        category = 'Unhealthy'
        color = '#ff0000'
    elif c <= 250.4:
        i = ((300.0 - 201.0) / (250.4 - 150.5)) * (c - 150.5) + 201.0
        category = 'Very unhealthy'
        color = '#993f97'
    elif c <= 350.4:
        i = ((400.0 - 301.0) / (350.4 - 250.5)) * (c - 250.5) + 301.0
        category = 'Hazardous'
        color = '#7e0023'
    else:
        i = ((500.0 - 401.0) / (400.4 - 350.5)) * (c - 350.5) + 401.0
        category = 'Hazardous'
        color = '#7e0023'
    return int(i), category, color


if __name__ == '__main__':
    try:
        pm2_5 = float(sys.argv[1])
        epa, category, color = pm2_5_to_aqi(pm2_5)
        print("{} -> {} {} {}".format(pm2_5, epa, category, color))
    except:
        print("usage: {} pm2_5_value".format(sys.argv[0]))
        print("  e.g., {} 11.6".format(sys.argv[0]))