I’m a retired environmental research scientist (27 years with EPA Office of Research and Development). After retirement, I spent some years working at the National Institute for Science and Technology (NIST) and most recently some time at Stanford. My main interest is human exposure to fine particles (PM2.5). For the last two years I’ve been looking at PurpleAir sensors and have developed a new (better, in my opinion) algorithm for calculating PM2.5 that does not depend on either of the two “black box” CF_1 and CF_ATM algorithms provided by Plantower. This algorithm is available on the PurpleAir map as an alternative conversion model. Also available for downloading from the API site–look for “PM2.5 alt”.