It is an area weighted mean radius of the aerosol particles.
Aerosol effective radius (microns) is defined as:
r = particle radius
n(r) = particle size distribution (number of particles per cm2
with radius in the range r and r+dr microns)
To characterize the size of atmospheric particles, aerosols are usually divided into three modes: the Aitken mode (diameter less than 0.5 μm), the accumulation mode (0.5 to 2.5 μm), and the coarse mode (greater than 2.5 μm). The Aitken and accumulation modes are collectively referred to as fine particles. Human produced emissions (e.g biomass burning in southern Africa and South America and Industrial pollutions) are examples of small mode aerosols. Example of the coarse mode particles are the large Africa's Sahara Desert dust particles that cross Atlantic ocean and reach America
In air pollution studies PM-2.5 aerosols are referred to particulate matter smaller than 2.5 μm that is suspended in the air. In general, submicron aerosols (particles smaller than 1 micron in diameter) affect human health by penetrating to the deepest part of human lungs.