A new apparatus, Steam Jet Aerosol Collector (SJAC) combined with on-line Mobile Ion Chromatograph provides a reliable way to measure both aerosol components and related gas species simultaneously. The SJAC was applied in the field campaigns in Beijing and Guangzhou to investigate aerosol chemical compositions and related gaseous pollutants. The concentration levels and diurnal variations are discussed, which implies differences in atmospheric chemistry of gas-to-particle conversions in Beijing and Guangzhou.