| Summary: | 碩士 === 國立中興大學 === 環境工程學系所 === 96 === Ozone is a significant air pollution problem in central Taiwan. Those secondary aerosols derived via photochemical reactions were hazardous to human health. Mostly, Secondary aerosols were in fine particle size range. In the past, only a few studies addressed the contribution of secondary aerosols to the fine particles, and the characteristics of aerosol particles、acidic gases and basic gases. Therefore, this study intended to use an annular denuder sampling system to measure the ambient PM2.5 and the acidic and basic gaseous pollutants at National Chung Hsing University in Taichung during the period from 2005 to 2007. The purpose of this study was to investigate the chemical compositions of the ambient particulates and to estimate the fraction of secondary aerosol during the ozone episodes (O3-MAX > 100 ppb) and non- ozone episodes (O3-MAX < 60 ppb) in Taichung area.
Experimental results showed that the weather conditions during the ozone episodes were higher temperature、lower wind velocity, and lower relative humidity (RH). More pollutants (PM2.5、SO42-、NO3-、NH4+) were generated by photochemical reactions in daytime, so that the concentrations of secondary aerosols were higher. Similarly, the pollutants were influenced by higher relative humidity in nighttime, the concentration of
(NH4)2SO4 was increased. On the contrary, only the concentration of NO3- was higher from 10:00 to 14:00 am during the non-ozone episodes. These results showed the concentration of HNO3 was increased because of NH4NO3 evaporated easily in the ozone episode, and the concentrations of SO2 and HNO2 during the nighttime before ozone episodes were higher than non-ozone episodes. The photolysis of HNO2 in daytime generated more OH free radicals enhanced photochemical reactions and caused more ozone accumulation. The concentration of NH3 was reduced because of NH3 combined with HNO3 and H2SO4 easily in the ozone episode.
This study also estimated the concentration of secondary aerosols during the ozone episodes and non-ozone episodes. The results showed that there are 18-24% and 37-41% secondary aerosols of PM2.5 during non-ozone episodes and ozone episodes, respectively. And there were 57% and 38% of the secondary aerosols were in the form of (NH4)2SO4 and NH4NO3, respectively. In addition, the case of May 2007 was influenced by the weather conditions of high pressure systems and low pressure systems and regional sea land wind during midday. Especially, SO42- was major pollutant of this case. Model for PM2.5 concentration of ozone episodes daytime obtained using multiple linear regression analysis is:
[PM2.5] =0.09[O3-MAX]+0.47[NOx]+0.95[SO2]-6.6[W.S.]+26.9
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