The particulates monitor measures airborne particles 2.5 microns in diameter or smaller. This size range eliminates many natural particulates (e.g. sea salt, dust) and focuses on those considered to have a negative affect on human health: airborne respirable particulate matter (PM2.5). This is made up of primary particles, which are emitted directly into the air by natural and manmade activities (i.e. dust from unpaved roads, construction and agricultural activities), and secondary particles which form in the atmosphere through physical and chemical interactions amongst precursor gases such as sulphur dioxide (SO2), ammonia, SOx and VOCs