| Summary: | 博士 === 國立中央大學 === 化學學系 === 104 === The establishment of the network of Photochemical Assessment Monitoring Stations (PAMS) in Taiwan began in 2002, and majority of the stations were set up in 2006. Up to date, there are nine fixed sites across the west side of the island. The functionalities of the PAMS network are as follows: 1. to understand the abundance and variations of major ozone precursors in the regions; 2. to provide the database for source-and-receptor analysis; 3. to validate emission profiles for air-quality models; 4. to facilitate elucidating the cause-and-effect of high ozone episodes. In order to serve these four purposes, the PAMS measurements were subject to a strict protocol of quality control and assurance to safeguard data quality. Afterａdecade-long operation, the PAMS data have become an asset for users to explore its true values. Of many possible aspects that could be explored in depth, two were pursued to form the main themes of this thesis.
Isoprene was selected to be the subject of investigation from the PAMS database owing to its mostly biogenic nature, significant emissions on the island, and its extremely high reactivity to hydroxyl radicals. Inter-annual variations of atmospheric isoprene in Taipei were reported based on two years of PAMS data to reveal the detailed interplay between the biogenic and vehicular sources throughout the year. To separate the vehicular contribution from the biogenic one for the ambient isoprene, ethylene was used as an indicator of traffic emissions. While dramatic surge of isoprene was observed at noontime in hot months with the highest average peak mixing ratio of 1.6 ppbv in August, its abundance decreased to 0.2 ppbv on average in cold months. The vehicular contribution to ambient isoprene was largely masked over by the noontime surge of isoprene in hot seasons, but was still able to be vaguely observed on the slopes of the isoprene peaks mimicking the rush-hour features of ethylene. In winter, the diurnal variations of isoprene were very similar to those of ethylene, suggesting the traffic dominance in cold months. This work of isoprene with the use of the PAMS dataset greatly enhances the key findings in previous flask studies. Compared to flask sampling, the highly time-resolved PAMS data was intended to reveal the evolution process from a biogenically overwhelmed condition in hot months to the condition where the biogenic source weakened to reveal the traffic source in cold months.
The decade-long PAMS dataset from 2006 to 2015 allowed trend analysis to respond to the decline observed by the air quality stations. Due to the large size of the dataset, a curve-fitting algorithm adopted by National Oceanic and Atmospheric Administration (NOAA) used for long-lived trace gases such as carbon dioxide was applied to our trend analysis of multi-sites and multi-compounds. With the curve-fitting algorithm the dataset for each target compound was de-seasonized to reveal the decadal trend and residuals. Three sites in the north, central and south regions of Taiwan were selected for trend analysis of ethylene, propylene, benzene, toluene and isoprene. The first four compounds have common sources of vehicles and industries, whereas isoprene is largely biogenic. The Wanhua site in Taipei showed decreased trends for all the aforementioned compounds, with isoprene showing the most dramatic decline of 95.7% over the decade. A significant change in vegetation coverage should be the cause for such a pronounced decline in isoprene. In the central region, the Taixi site showed more than 60% declines for propylene and toluene, and more than 30% for ethylene and benzene. In the south region, the Siaogang site, which is in an urban-industrial hybrid environment, showed an overall decline for all these five compounds with the highest decline of 69% for ethylene. It is speculated that the declines were related to the ever tightening regulations on vehicular and industrial emissions in the past.
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