以環形擴散採樣器採集大氣中之硝酸、亞硝酸、二氧化硫與氨氣濃度之採樣誤差探討

Abstract

環形擴散採樣器(ADS)是目前最廣泛應用於大氣中之酸鹼性氣體的採樣方法，其對酸鹼性氣體的採樣效率，經前人在實驗室內所作的研究顯示，均證實其有頗高之採樣效率，但實際在戶外作採樣時，其採樣濃度之真確性卻受到質疑。由環形擴散採樣器的採樣機制可發現，由於其係利用擴散吸收原理來吸收氣體污染物，使得採樣系統易因其他機制作用，而造成採樣誤差的發生。如環形擴散管吸收干擾氣體，以及微的沈降、擴散及揮發作用等，均會對採樣誤差結果造成正誤差。 本研究根據美國EPA之標準採樣方法，分別針對硝酸、亞硝酸、二氧化硫及氨氣氣體進行採樣誤差探討。實驗結果發現，環形擴散採樣器對硝酸、亞硝酸及氨氣等三種酸鹼性氣體的採樣，均會有採樣誤差的發生，且以硝酸氣體的室外採樣誤差百分率為最大，其誤差值可能高達85%以上。亞硝酸及氨氣的採樣誤差百分率則在工10%以下。 台灣地區大氣中之硝酸氣體濃度普遍小於1μg/m3以下，因此若採用美國EPA之標準採樣方法，進行大氣中硝酸氣體濃度採樣，將會出現明顯的採樣誤差。因此本研究建議，在硝酸氣體濃度＞0.5μg/m3情況下，若欲利用既有之商業化環形擴散採樣系統，使各項目標污染物之採樣誤差降至最低值，可採取以兩根環形擴散管吸收硝酸與二氧化硫氣體，一管吸收亞硝酸及二氧化硫氣體，以及一管吸收氨氣的組裝方式進行大氣採樣。

Annular denuder system (ADS), owning to its excellent efficiency on acidic and basic gas sampling as proved by many laboratory studies, is the most widely used sampling method. But its sampling accuracy was also questioned while detecting acidic and basic gaseous concentrations during field sampling. Sampling errors may occur due to the adsorption, diffusion and evaporation mechanisms of particles and the interference of other gases. These would result in positive errors to the sampling results. In this study, sampling errors for HNO3, HNO2, SO2 and NH3 gases are examined exprimentally. The results indicated that sampling errors many exist for HNO3, HNO2 and NH3 gas sampling. The sampling error of ADS for nitric acid gas could be as large as 85% , and the highest sampling error of ADS for nitrous acid gas and ammonia gas is around 10%. The concentration of nitric acid gas in Taiwan area is often lower than 1μg/m3 in the atmosphere. Therefore apparent sampling errors many take place if sampling nitric acid gas using standard ADS sampling method of USEPA. It is suggested that while employing the commercially available ADS, the sampling errors can be significantly reduced using two denuder tubes for nitric acid and sulfur dioxide gas sampling, one denuder tube for nitrous acid and sulfur dioxide gas sampling and one denuder tube for ammonia gas sampling.