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Measurements and characteristics of atmospheric ammonia and particulate ammonium
Lin, Wei Hsiang
為確認NH3濃度量測結果之準確性，分別以靛酚/分光光度計法與離子層析儀法分析大氣中NH3濃度，前者量測結果平均較後者低估11.8 %，然變化趨勢非常一致（R2 = 0.99），此外，本研究以ADS採集NH3氣體收集效率平均亦高達99.2 %，顯示台中都會區NH3濃度的確較國外相關文獻高出許多。揮發之NH4+微粒濃度量測方面，質量平衡推估結果平均較實際量測濃度高估11.8 %，尤其當相對濕度低及溫度高時，質量平衡推估法更大幅高估NH4+揮發量。PM2.5中NH4+微粒有34 %為NH4NO3結合態，14 %為NH4Cl結合態，52 %為 (NH4)2SO4結合態，本地區在大量的NH3存在下，酸性氣體完全被中和，約有71 %的NH3氣體未轉化成NH4+微粒，HNO3及HCl等酸性氣體不足是影響NH4+微粒生成的主要因子，NH3與CO濃度呈現中度正相關，顯示NH3污染物可能來自於裝有觸媒轉化器的交通車輛。
An annular denuder system was used to sample ambient gaseous pollutants and PM2.5 in the campus of the National Chung Hsing University from August 2004 to January 2005. A total number of eighty daytime and nighttime samples were collected. This study intends to understand the characteristics of these pollutants and the variation between daytime and nighttime, as well as their differences between ozone and non-ozone episodes. The study also investigates the characteristics of atmospheric ammonia and particulate ammonium, and their affecting factors.
According to the experiments, the average concentrations of PM2.5, SO42-, NO3-, Cl-, NH4+, HCl, HNO3, HNO2, SO2 and NH3 were respectively 39.3, 3.9, 4.6, 0.7, 2.8, 0.3, 0.8, 2.8, 4.5 and 7.1 μg/m3. In the daytime, the concentrations of HNO2, HNO3 and NH3 were respectively 0.4, 3.0 and 0.6 times higher than those in the nighttime. It showed that the life time of these acidic or alkaline gases in the atmosphere was short, and they were easily converted to particulates. During the sampling period, the concentrations of HCl, HNO3, SO2, NH3, PM2.5, SO42- and NH4+ in ozone episodes were respectively 2.4, 2.8, 1.5, 1.7, 1.9, 1.7 and 2.1 times higher than those in non-ozone episodes. The result indicated that the strong photochemical reaction and the poor air dispersion might cause the accumulation of the air pollutants.
This study also assured the accuracy of NH3 measurements by measuring the NH3 concentrations using the indophenol-blue method and the ion chromatography. The results showed that the NH3 concentrations determined by indophenol-blue method was approximately 11.8 % lower than those by ion chromatography. However both results were well correlated (R2 = 0.99). Besides these results, the collection efficiency of NH3 by annular denuder system was determined and the efficiency could be as high as 99.2 %. All of these measurements suggested that the ambient NH3 concentrations in Taichung urban area is much higher than those with other urban areas as described in the literatures. The amount of evaporation from the NH4+ particulates can be either measured through a back-up Teflon filter or estimated by using the mass balance method. Comparing these two methods, we find that the amount derived from the mass balance method is 11.8 % higher than the results measured from the back-up filter. This phenomenon particularly occurred under the conditions of lower humidity and higher temperature.
Based on the experimental results, there were about 34 % of NH4+ associated with NO3- to form NH4NO3, and 14 % forming NH4Cl and 52 % forming (NH4)2SO4. Due to significant amount of NH3 in this area, the acidic gases were fully neutralized by NH3. It resulted in about 71 % of NH3 in the gaseous phase. Insufficient HNO3 and HCl affected the formation of the particulate ammonium. Furthermore, NH3 was found positively correlated well with CO. It indicated that NH3 might be generated from vehicles equipped with catalytic converters.
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