Several studies report high emission rates and serious pollution of carbonaceous aerosols in China, which may be related with the regional climate change and decreased crop production. Tianjin, one of the fastest growing areas in China, is the largest coastal city of north China with a total population of over 10 million and an area of 11919.7 km². The main energy source in Tianjin is coal accounting for about 70% of all energy consumed. The amount of vehicles now exceeds 1.2 million and private car is growing at a rate of nearly 20% in recent years. High consumption rates of fossil fuel along with low level of pollution control have brought about serious air pollution problems to the city, especially in winter due to greater emissions from heating sources and unfavorable meteorology for pollutant dispersion. Up to date, information on carbonaceous aerosol in this region of china is scarce. As a result, a field sampling campaign was conducted between October 2006 and January 2007 at a commercial-residential, an industrial and a coastal site with the aim to investigate the concentration level and distribution of carbonaceous aerosols. Ambient PM_2.5 aerosol samples were collected on quartz membrane filters using impact samplers at a flow rate of 100L/min, and analyzed for OC and EC by DRI Model 2001 implementing the IMPROVE TOR protocol.

Major meteorological parameters including temperature, relative humidity, wind speed and wind direction were also measured at each site. The local meteorology can be described as calm and dry during most of the study period. To identify the origin and transport pathway of air masses, 2-day backward air trajectories were calculated at 4:00 UTC for every sampling day using the HYSPLIT_4 model developed by NOAA/ARL. Results of the air mass backward trajectories indicated that atmosphere in Tianjin is a superposition of transported polluted air on locally produced pollutants.

The average concentrations of TC, OC and EC for the 24h winter samples were 47.8±33.3µg/m³, 44.1±32.5µg/m³ and 3.7±1.1µg/m³, accounting for 20.1%, 18.0% and 2.1% of the total PM_2.5 mass, respectively. Organic carbon dominated the carbonaceous compounds constituting about 88% of the TC. The OC concentrations were higher than those measured in other urban environment indicating severe pollution of organic aerosol in Tianjin. In contrast to OC, EC concentrations observed were very low. The EC levels at the three sites were similar, while OC exhibited apparent spatial distribution with an order of industrial site > commercial site > coastal site. About 77% of both OC and EC were measured to be associated with the fine particle mode. No significant differences were found between the 12h day- and night-time samples. The concentrations of PM_2.5 and OC in winter were significantly higher than those in autumn. The observation that EC levels were stable during the whole study period suggested constant emissions from vehicle sources over the city. The concentration ratios of OC to EC obtained in this study ranged from 2.7 to 19.5 with a mean of 7.7±4.3, which was higher than those reported for other urban environments. However, no evidence was found to support the presence of secondary organic aerosol. Further studies on the origin and composition of carbonaceous aerosols in Tianjin are especially needed.

Further investigation on the 8-carbon profile produced by the IMPROVE TOR protocol showed that pyrolytic carbon (OP) was the most abundant carbon fraction accounting for 45% of the TC. Therefore, the high OC and low EC values from this study may be partially attributed to a higher OP correction due to a delayed return of reflectance to its original value for these too dark samples.