Characteristics of Traffic-related Emissions: A Case Study in Roadside Ambient Air over Xi'an, China

Traffic-related emissions were measured at a heavy-traffic roadside in Xi'an at the end of April, 2008 to survey the exposure levels of freshly emitted carbon monoxide (CO), mass and chemical composition (OC, EC, inorganic ions, and PAHs) of PM10, and ozone (O-3). In general, high levels of mass concentration and chemical species in PM10 were observed during daily heavy traffic periods (morning, noon, and evening traffic rush hours), especially in the morning traffic rush hour. OC and EC accounted for 9.3% and 3.0% of PM10 mass, respectively, and the ratios of OC/EC were constant during the heavy traffic periods (with a mean value of 3.2). Water-soluble ions constituted 14.0% of PM10 and the dominant ionic species were Ca2+, SO42-, and NO3- in the roadside samples. Re-suspended road dust constituted a large fraction of PM10, causing the PM10 samples to become more alkaline. The concentrations of 12 prior-controlled PAHs revealed the potential high health risk of this roadside area in Xi'an. The diurnal variation in CO and O-3 exhibited a different trend, which indicated that CO level was largely influenced by the instantaneous emission of vehicles while O-3 was closely associated with the overall traffic conditions and meteorological status.