Characterization of ozone precursors in the Pearl River Delta by time series observation of non-methane hydrocarbons

Continuous hourly observation of more than 50 non-methane hydrocarbons (NMHCs) from C₃ to C₁₂ was performed between 16 October and 5 November 2004 in the Pearl River Delta (PRD) area by two automated gas chromatographs deployed separately at an urban site in downtown Guangzhou and a rural site in Xinken. Distinct differences in diurnal patterns of NMHCs are found between the two sites. In Guangzhou, peak concentrations were usually seen after dusk at the traffic rush hour, whereas the maxima in Xinken occurred between late night and early morning. High correlation between NO_x, CO, and volatile organic compounds (VOCs) in Guangzhou suggests that motor vehicle exhaust is the dominant source of NMHCs in Guangzhou. It is also found that propane, iso-butane, and n-butane accounted for nearly 40% of the total NMHC concentration in Guangzhou, possibly due to the wide use of liquefied petroleum gas (LPG) as motor vehicle fuel in Guangzhou. Principal component analysis (PCA) of Xinken's dataset reveals that reactive alkenes are more depleted than alkanes and aromatics, providing evidence that the air masses arriving at Xinken are more aged in terms of photochemistry compared to the source area of Guangzhou. Using the ratio of ethylbenzene to m,p-xylenes as an effective age indicator, it is also found that the air in Xinken is substantially more aged than that in Guangzhou, consistent with the relatively smaller percentage of alkenes found in Xinken than in Guangzhou. Furthermore, the daily peak of the age indicator occurs in the afternoon between 2 and 5 p.m., concurrent in time with the ozone peak. The peak values of this indicator of Xinken are generally higher than those of Guangzhou by about a factor of 3, consistent with the much higher ozone levels found in Xinken.