Almost all stations are showing ZERO READINGS
I don’t know what effect cold air has on PurplrAir stations, but at this time of year, air can be relatively clean, especially crisp, cold Arctic air on clear days. So, that’s ONE reason for the very clean air pollution readings. If you get a bad air inversion that holds on tight, any factory smog will build up and hang around, raising the air pollution readings. That is rare. Usually, your pollution problems happen in warmer times of the year due to ozone on hot days, and factory smog from smokestacks, wildfires and such. There also can be air inversions in the warmer months.
bumchamp7 has a good point, but see previous message stream on the subject:
I installed a Zen last month in New Hampshire and am displaying its PM2.5 AQI along with my weather station value. On nights with a (cold) NW wind we don’t get an air inversion and the air is clean. On clear nights we get an inversion, but there is a down slope draft of cooled air from the hill north of me that keeps the AQI low. On nights that start clear but cloud over radiational cooling stops, the down slope flow stops, and things can get really bad, like last night. Cooling stopped around midnight due to cloud cover from a low pressure system moving by to our south.
Wind and weather info is pretty much vital to understand air quality data.
See Today's Weather Conditions at Sutton Mills, NH