So, pm2.5 and pm10 are typically measured in the unit microgram per cubic meter, μg/m3. From what I've read about clean room standards, the clean room classifications are measured as # of particles per cubic meter. However, some clean room standards are measured as # of particles per cubic foot. So first, figure out if the particle counts you have are per m3 or per ft3.
To convert from a particle count into a mass concentration, you have to make several assumptions about the particle:
- All particles are spherical
- All particles have a density of 1.65E12 μg/m3 
- The radius of a particle in the PM2.5 channel is 0.44 μm 
- The radius of a particle in the PM10 channel is 2.60 μm 
caution: convert the aforementioned radius values into m3 before doing this calculation
With those assumptions, solve for the volume of each particle in m3:
then solve for the mass of each particle in micrograms, μg:
and then solve for the concentration in μg/m3 by multiplying:
= (# of particles per cubic meter) * mass
If you're particle count is actually per ft3 then you'll need an extra conversion factor:
(# of particles per cubic meter) = 0.0283168*(# of particles per cubic feet)
 Tittarelli, T. et al., “Estimation of particle mass concentration in ambient air using a
particle counter,” Atmospheric Environment, vol. 42, pp. 8543-8548, 2008.
 Lee, J. et al., “Seasonal variations of particle size distributions of PAHs at Seoul,
South Korea,” Air Quality Atmospheric Health, vol. 1, pp. 57-68, 2008.