The AQMD website has tested devices, and any they found accurate is probably a good place to start. My limited research suggests the most affordable and reliab

The AQMD website has tested devices, and any they found accurate is probably a good place to start. My limited research suggests the most affordable and reliable gaseous measurements like NO2 want a metal oxide sensor, which you don’t see on consumer grade equipment but is available affordably from commercial sources. It’s more hassle though so I gave up on measuring NO2 for now. Particulates seem to be readily measured with suitable accuracy by consumer grade equipment.