| 英文摘要 |
The objective of this project sponsored by EPA of Taiwan is to operate and maintain the three photochemical assessment stations (PAMS) deployed in central Taiwan to form a monitoring network. The formation of such a network is essential for investigating the high ozone problem frequently encountered in suburban areas by in-situ monitoring 56 critical non-methane hydrocarbons. The first of the three stations was setup in early 2001 in Taichung city to serve as an up-wind site monitoring elevated ozone precursors typically observed in an urbanized metropolitan area. Approximately a year later, the other two down-wind stations were deployed south east of the Taichung station within Nantou county area, namely the Chouton station and the Chu-shan station, respectively. Simultaneous hourly measurement of 56 target compounds has been made for the three stations since May 2002. A series of procedures and criteria have been developed to quality control the speciation and calibration in terms of accuracy, precision, and long-term stability.
We employed the ratio of ethylbenzene versus m, p-xylene in the dataset to indicate the photochemical age of an air parcel. Because the atmospheric lifetime of ethylbenzene is significant longer than that of xylene, and both compounds are likely to be released from an upwind common source, i.e., car exhaust, a larger ratio would translate into a more aged air parcel and hence more depletion in shorter-lived precursors and, therefore, more ozone, and vise versa. As expected, higher ethylbenzene/xylene ratios are indeed accompanied by higher ozone values observed around noon when the prevailing wind is in the northeast direction. It is also found that slower wind speed with the northeastlies often correspond to higher ozone values and higher ethylbenzene/xylene ratios. As a result, based on the striking photochemistry evidence deduced from the PAMS dataset along with ozone and wind data, it is reasonable to postulate that the high ozone level observed at Chushan is the result of photochemical degradation of ozone precursors transported from the northeast upwind areas. Along the transport passage the reactive compounds such as xylenes are subject to greater loss to HO reaction to form ozone than the less reactive compounds, such as ethylbenzene. Concentration difference between ethylbenzene and xylene gradually increase and more ozone is produced as the air mass moves towards the downwind areas such as the Chushan area.
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