硝酸鹽微粒對擴散採樣器採集硝酸氣體之採樣誤差理論探討

Abstract

本研究主要在探討含硝酸鹽微粒對環形擴散採樣器採集硝酸氣體所造成之採樣誤差。研究中以粒徑分段方法模擬大氣中粒徑分布狀況，並結合氣固相之質傳理論及氣體之質量平衡方程式，而建立氣固相數學模式(gas-particle phases model)來預測環形擴散採樣器(annual denuder System, ADS)中硝酸氣體之採樣誤差。此外本研究除發展考慮粒徑變化之氣固相模式外，另以較簡易之氣相模式(gaS phase model)來探討在微粒粒徑分布不變下之採樣誤差，模式結果並與文獻所發展之常態對數模式進行比較。 研究結果發現在分別以氣固相模式、氣相模式與對數常態模式探討真實大氣粗徑分布下硝酸氣體之採樣誤差時，對數常態模式有明顯的高估情形，導致高估的原因主要是對數常態模式假設大氣粗徑分布為對數常態分布，但是真實之大氣粗徑分布狀況並非為對數常態分布，因此無法正確的預測硝酸氣體的採樣誤差，而本研究發展的模式能模擬真實的大氣粗徑分布狀況，因此較可正確預估硝酸氣體在ADS中之採樣誤差。 本研究估計以擴散採樣器採集硝酸氣體之採樣誤差中，由含硝酸鹽之粒狀物擴散與揮發所造成之採樣誤差，約佔總誤差之15-95%。採樣誤差明顯受大氣中含硝酸鹽微粒、採樣溫度以及擴散採樣器型式所影響。當大氣中含硝酸鹽微粒(＜2.5μm)之濃度在52μg/m3，大氣溫度為25°C時，其造成之硝酸氣體採樣誤差約為0.051μg/m3，而溫度增高為30°C時，其所造成之採樣誤差可能達0.093μg/m3。若比較環形擴散採樣管與蜂巢式擴散採樣器之採樣誤差時，則發現蜂巢式擴散管之採樣誤差約為ADS之3倍，因此蜂巢式採樣管較不適合應用於台灣地區採集低濃度之硝酸氣體。

This study is presented to evaluate sampling artifact of HNO3 gas due to interferences from nitrate containing aerosols in an annular denuder system. Approximating the aerosol size distribution by a sectional approach function, a gas-particle phases model was developed from the general dynamic equation of aerosols and the mass balance equations of gaseous species, and it was used to predict sampling artifact of HNO3 gas. In addition, the gas-particle phases model was further simplified to a gas phase model without losing accurary on the estimation of sampling artifact of a denuder system. Both models were then compared to the numerical results of Lin's lognormal model. The results indicated that Lin's lognormal model overpredicted the sampling artifact of the HNO3 gas. This is because that Lin's model assumed the particle size distribution to be a lognormal function, which is not usually the case occurred in real atmosphere. The particle size distribution of real atmosphere can be properly modelled in this study to estimate the sampling artifact of HNO3 gas due to particle interferences in an ADS. The diffusional deposition and evaporation of nitrate containing aerosols account for about 15-95 % of HNO3 sampling artifacts in an ADS. The sampling artifact depends on the concentration of nitrate containing aerosols, the ambient temperature and the design of a denuder system. When the concentration of nitrate containing aerosols was 52μg/m3(< 2.5 μm) and the ambient temperature was 25°C, the sampling artifact concentration of HNO3 gas due to particle interferences was about 0.051μg/m3. If the temperature is increased to 30°C, the sampling artifact of HNO3 gas could be 0.093 μg/m3. The sampling artifact of a honeycomb denuder was about 3 times higher than that of an ADS. Therefore honeycomb denuder is not suitable to be applied to sample low HNO3 gas concentration.