Airborne particle emissions from wastewater treatment plants (WWTP) have been associated with health repercussions but particulate quantification studies are scarce. In this study, particulate matter (PM) number concentrations and size distributions in the ultrafine range (7-300 nm) were measured from two different sources: a laboratory-scale aerobic bioreactor and the activated sludge aeration basins at Orange County Sanitation District (OCSD). The relationships between wastewater parameters (total organic carbon (TOC), chemical oxygen demand (COD), and total suspended solids (TSS)), aeration flow rate and particle concentrations were also explored. A significant positive relationship was found between particle concentration and WWTP variables (COD: r(10) = 0.876, p <.001, TOC: r(10) = 0.664, p <.05, TSS: r(10) = 0.707, p <.05, aeration flow rate: r(8) = 0.988, p <.0001). A theoretical model was also developed from empirical data to compare real world WWTP aerosol number emission fluxes with laboratory data. Aerosol number fluxes at OCSD aerated basins (9.8 × 104 lbs/min·cm2) and the bioreactor (7.95 × 104 lbs/min·cm2) were calculated and showed a relatively small difference (19%). The ultrafine size distributions from both systems were consistent, with a mode of ∼48 nm. The average mass concentration (7.03 μg/cm3) from OCSD was relatively small compared to other urban sources. However, the in-tank average number concentration of airborne particles (14 480 lbs/cm3) was higher than background ambient concentrations.