PM2.5 is not directly measured by satellite observation. PM2.5 is primarily calculated from AOD (Aeorosol Optical Depth). Several NASA missions have PM2.5 derived products including MODIS, MISR, and SeaWIFS. Other relevant datasets include CALIPSO and VIIRS.
The user may be interested in a yearly, global PM2.5 data product derived from multiple missions available here:
https://sedac.ciesin.columbia.edu (/data/set/sdei-global-annual-gwr-pm2-5-modis-misr-seawifs-aod)
For PM and related data access and visualization, the user might find useful the ARSET training provided here:
https://arset.gsfc.nasa.gov/airquality/webinars/advanced-AOD-PM
For global circulation modeling, they should refer to the GEOS model or the MERRA-2 model at
https://gmao.gsfc.nasa.gov/
Other upcoming missions with relevant data/sensors include:
MAIA: To be launched in 2022 (which will provide PM2.5 product)
TEMPO: To be launched in 2022
Korean: GEMS sensor onboard GEO-KOMPSAT 2B (to be launched in 2019):
Also, some airborne field campaigns measure air quality parameters (e.g. DISCOVER-AQ, KORUS-AQ), but those are limited in spatial and temporal ___domain because of the nature of airborne campaigns. For direct measurements of PM2.5, government ground sensor networks collect direct measurement in the USA. Data sources from the EPA can be found at
www.airnow.gov. There are private sensor networks as well.