Particulate air pollution is a serious health concern, especially the particles small enough to remain in the air as it reaches the deepest, smallest areas in the lungs. These particles can cause health problems by themselves, and can act as "delivery devices" for adsorbed and far more dangerous chemicals from combustion (e.g. fossil-fuel burning engines, natural and human-induced fires) and other industrial processes.
The parameter PM2.5 is meant as a measure of these particles. The 2.5 corresponds to 2.5 microns, but it's not a hard cut-off, and actual particle size is less important than aerodynamic diameter. See https://diamondenv.wordpress.com/2010/12/10/particulate-pollution-pm10-and-pm2-5/
Local PM2.5 measurements are usually done using scatterometry; an airflow is directed past a laser beam and tiny pulses from photodetectors due to scattered light transients are analyzed and then counted. Size distribution is roughly inferred from the pulse height distribution.
Question: However, does NASA really measure ground-level PM2.5 concentrations from space, or anything even close to it? There are aerosols throughout the atmosphere, how could a satellite observation distinguish soot near ground level from small water droplets in the upper atmosphere?
below: From this 2014 NASAEarth tweet.
update: "This just in" at Earthobservatory.nasa.gov: Just Another Day on Aerosol Earth. Interesting article with links. Image has been downsized to 1 MB, (click for larger view)
caption: "A composite image using NASA satellite imagery shows the different kinds of aerosols in the earth's atmosphere."