Hi Jeff - My guess is that if Weather Underground and EPA and CARB have been involved the product is probably pretty good. And Weather Underground is a great idea for a host network. They presented their initial ideas for this at an EPA community sensor conference in RTP NC several years ago. Dylos also makes some pretty well-regarded instruments that measure pretty small airborne particles. Ultimately it would be most helpful to have relatively inexpensive ultrafine particle (UFP) sensors that could count particles per cubic centimeter in the 5 nm to 100 nm size range.
Our bench instruments, used in the Boston area by Tufts and community partners like Somerville Transportation Equity Partnership (STEP) both in stationary boxes and in mobile labs, generally count between 10,000 and 50,000 UFP per cubic centimeter but sometimes more, depending on meteorology and proximity to sources, especially mobile sources like highways, rail and aviation. Or some indoor sources like candles and cooking. These monitors are both accurate and pretty expensive. There are some very inexpensive particle sensors, such as Shinyeis, but reports are that they are not always accurate.
Joshua Apte and co-authors have done very fine grained mobile lab street mapping using Google vehicles (http://apte.caee.utexas.edu/google-air-mapping/). Their particle metric is Black Carbon, a pretty good mobile indicator, and they look at Nitrogen Oxides also. Mueller and others have done similar projects (http://www.sciencedirect.com.ezproxy.library.tufts.edu/science/article/pii/S1352231015305409#bib10) in Zurich using multi-pollutant monitor boxes on top of light rail vehicles, a brilliant idea. They use DiscMINIs for the ultrafine particles.
As you know, I would also love to see a bicycle sensor network that could calibrate off of stationary monitors from time to time. Could be done with bike share. But so far cost is a real barrier for measuring ultrafine particles. Maybe the new nano labs will come up with something soon. - Cheers, Wig