大氣中硝酸氣體與硝酸鹽濃度採樣效率之探討

Abstract

環形擴散採樣器是目前應用最廣泛的採樣方法，其對酸、鹼性氣體及粒 狀物中之酸、鹼性鹽類的採集效率，經前人在實驗室研究中，均證實有很 高之採樣效率，但是在野外實際採樣時，其採樣效率卻也受到許多質疑。 如在偵測硝酸氣體濃度時，可能因含氮化合物的干擾，而使得採樣的結果 偏離了實際的濃度值。例如二氧化氮與亞硝酸氣體的干擾、含硝酸鹽微粒 藉由擴散作用而附著於管壁，以及硝酸銨鹽的揮發作用等，皆會形成硝酸 氣體採樣結果的正誤差。本研究由環形擴散採樣器及過濾器採樣結果的分 析比較發現，兩者對總硝酸根離子量的採集具有一致性，但在顆粒相硝酸 鹽及氣體硝酸的採集上，卻有些許的差異性。過濾器採集顆粒相的硝酸鹽 ，可能受硝酸或亞硝酸氣體的干擾而形成正誤差，由實驗結果發現過濾器 中之鐵弗龍濾紙上的硝酸鹽揮發量低於環形擴散採樣器中之鐵弗龍濾紙揮 發量。環形擴散採樣器收集顆粒相的硝酸鹽效果雖較過濾器佳，但是在採 集硝酸氣體時，卻容易受到干擾。由實驗發現， 以環形擴散採樣器採集 硝酸氣體的誤差比例，可能高達 100 ﹪以上，顯示以往單以一根擴散管 來吸收硝酸氣體的採樣結果，可能存在很大的誤差。而其中由粒狀物中之 硝酸鹽沈降或揮發所造成的比例約佔 10 ﹪左右， 因此來自含氮之氣狀 化合物（ HNO2 及NO2 ）之干擾可能高達 90 ﹪，因此有必要進一步以實 驗驗證，並定量含氮之氣狀化合物對硝酸氣體採樣結果之影響性。 Annular denuder system, owing to its excellent efficiency on acidic and basicgas and their particulate salts as proved by many laboratory studies, is themost widely used sampling method. But its sampling efficiency was alsoquestioned while detecting nitric acid gaseous concentration during fieldsampling. The interferences were suspected from nitrogen- containing compoundswhich cause positive errors from the real concentration. For example, theinterference of nitrogen dioxide and nitrous acid gas, the adsorption of nitratecontaining particle deposited to the denuder wall by diffusional process, andthe evaporation of ammonium nitrate etc., will produce positive errors to thesampling result.The comparison of Annular denuder sampling system （ ADS ） and Filter pack （FP ） sampling system was made in this study in terms of the sampling of nitricacid gas and particulate nitrate. The results show similar sampling efficienciesfor total nitrate ion, but there were some differences on particulate nitrateand gaseous nitric acid. The collection of particulate nitrate by FP isinterfered probably by nitric acid and nitrous acid gas. The nitrate evaporationrates from the Teflon filter of FP are lower than that from ADS.The ADS has a better efficiency for particulate nitrate than the FP method. Butfor collecting nitric acid gas the ADS is easily interfered by other compounds.According to experimental results, the sampling error of ADS for nitric acid gasusing only one denuder tube may be larger than 100 ﹪. The precipitation orevaporation of particulate nitrate accounts for around 10 ﹪ error, thereforemost of the error may be contributed by other nitrogen-containing gaseouscompounds such as nitrous acid and nitrogen dioxide.