A modified three-stage microwave-assisted digestion protocol for chromium analysis in airborne total suspended particulate matter (TSP) with recovery > 90%, which utilizes inductively coupled plasma dynamic reaction cell mass spectrometry (ICP-DRC-MS), has been developed and validated. Similar results were obtained for 52Cr and 53Cr analyses, with a method detection limit of 950 ng g(-1) (0.38 ng ml(-1)) for 52Cr. The method is applicable to the analyses of a suite of other elements in TSP, particularly for Al, Fe and Ti. A comparison of the method with various other extraction and digestion procedures has been given. Our results highlight the difficulties in Cr analyses. Concentrations of Cr in TSP at two urban sites in Hong Kong were determined in the range from 5.8-32.3 ng m(-3), with an average value of 13.2 ng m(-3). The mean concentration of Cr was found to be 10.7 ng m(-3) at a residential/commercial site and 15.6 ng m(-3) at a mixed industrial/residential/commercial site. Ambient Cr concentrations did not reach maxima at vehicular rush-hour times but peaked at combined vehicle and industrial activity. The Cr concentrations at each site were strongly correlated with the total PM10 (particulate matter with aerodynamic diameter < 10 microm) concentrations and inversely correlated with ambient relative humidity. Relationships of Cr with ambient pollutant gases and other elements in TSP suggest that there is no single source which dominates the concentration of Cr in aerosol. There are several contributing sources and their relative proportions vary at different locations.