Theoretical analysis of evaporative losses from impactor and filter deposits

The pressure drop in aerosol samplers can lead to evaporation of the collected particles. A theoretical analysis for the evaporative losses from both impactor and filter deposits is developed. Quantitative expressions for the losses are expressed in terms of three dimensionless parameters: dimensionless pressure drop, ratio of the equilibrium vapor density of the aerosol species to its mass concentration, and dimensionless sampling time. In typical atmospheric aerosol sampling, the first two parameters are the dominant factors affecting evaporative loss rates. The results that are presented account for evaporative losses of pure, single component aerosols that are saturated at the inlet of the samplers. The analysis shows that evaporative losses will typically be substantial (10% or more) when the vapor-to-particle mass ratios of the aerosol are larger than about 1.0. It follows that for species present at mass concentrations of 10 μg m^-3, evaporative losses will be substantial when vapor pressures exceed about 10^-9 atm. The analysis also suggests that evaporative losses from single-stage impactor samplers may be less than from typical filter media. It follows that impactors may offer some advantages over filters for sampling volatile aerosol species.