Air pollution is the leading environmental concern in Macao. The temporal variations of particulate matter (PM) and black carbon (BC) mass concentrations and the size-resolved PM 2.5 chemical compositions, including trace elements and carbonaceous species, were measured in Macao during a near-road summer campaign. The average concentra tions for PM 10 , PM 2.5 and BC at a roadside measurement site were 37.8 g m -3 , 31.4 g m -3 and 5.6 g m -3 , respectively. Results showed that local emissions

more »

... ontributed at least 35% to PM 2.5 mass in Macao. Higher BC concentrations were observed in the daytime than at night, consistent with the diurnal variations of traffic flow. Factor analysis classified trace elements into three categories, representing crustal sources (Mg, Si, Al, Na, Fe, Ca and K), road traffic sources (Cu and Co) and secondary inorganic particle formation (S). Crustal elements and road traffic elements were enriched in the size ranges of >1.0 m and <0.2 m, respectively. The unimodal distribution pattern with a peak at <0.2 m for organic carbon (OC) was probably due to secondary organic aerosol formation, whereas about 70% of elemental carbon (EC) was in the size range of 0.2-1.0 m. Secondary organic aerosols were found to be a strong contributor to PM in the size ranges of <0.2 m and 1.0-2.5 m.