Ambient pressure and temperature can vary with location, which implies that classifying aerosol particle mass using APM might be influenced at high-altitude sites. On the other hand, when using the APM as a particle classifier coupled with inductively coupled plasma mass spectrometry, argon would be required as the carrier gas. Therefore, air, oxygen and carbon dioxide were selected as carrier gases to evaluate the effect of gas viscosity and the mean free path on the performance of APM.