The performance of particle size separators that are characterized by their cutpoint (d50) and the steepness of their penetration curves (σg) is a critical feature of measurement methods for ambient PM2.5 and PM1 monitoring. The parameters d50 and σg for PM2.5 size separators are regulated in the official measurement methods of many countries and regions. However, those parameters for PM1 size separators have not been previously specified. The interaction of particle size distributions (PSDs) and the particle size separator performance characteristics has significant influence on the PM sampling results. Atmospheric PSDs for each of the four seasons covering a range of particulate pollution concentrations were measured in urban Beijing. The relative differences (RD) between estimated PM2.5/PM1 sampler concentrations and actual concentrations associated with the interaction between performance characteristics of size separator and actually measured urban PSDs, as well as for three idealized ambient PSDs, have been systematically investigated. For the PM2.5 size separator, when tolerance of cutpoint is 2.5 ± 0.2 µm and σg is no greater than 1.3, RD values for most typical urban atmospheric aerosol conditions were within the permissible error range (± 5%). For the PM1 size separator, when tolerance of cutpoint and sharpness is 1.00 ± 0.02 µm and σg no greater than 1.2, respectively, the RD values for typical urban atmospheric aerosol conditions are within the permissible range. Those parameters could also meet the requirements for sampling of the idealized ambient aerosol distribution.