Looking into this again. Investigating particulate sensors, especially whether PPD42 is worth using. The aqicn.org website has some views on the matter. They seem to like the "Plantower" PMS series because it is a laser based sensor (not simply optical cf PPD42) and can properly distinguish between 2.5 and 10 micron particles. The Plantower PMS 7003 (they quote $20 USD) appears to be the latest.