貝他衰減質量濃度監測儀性能評估

Abstract

貝他衰減監測技術是一種目前常見且可被接受用於連續監測環境氣懸浮微粒濃度的方法，透過計數貝他射線在穿透單位時間內採集到的氣懸浮微粒質量造成前後強度變化而得到環境質量濃度監測結果。最早開發的貝他衰減質量濃度監測儀是使用週期計數式的設計，但這種計數方法限制了該監測方法的時間解析度。因此，在近年開發了連續計數式的貝他衰減質量濃度監測儀，透過連續採樣過程中同時進行貝他強度計數，大幅改善了將濾紙移動至獨立設置的檢測區進行零點與樣本質量測定所造成的低時間解析度。此外，連續計數式設計中彎曲的採樣管道可能造成氣懸浮微粒在管道內沉積而造成結果的低估。目前對於這種連續式的貝他衰減質量濃度監測儀的性能評估數據十分有限。因此，本研究將對於連續式貝他衰減質量濃度監測儀進行包含微粒在管道沉積與其監測技術的優缺點評估與分析。 為降低探討過程中環境干擾，而先架設標準氣膠微粒產生系統，其中產生微粒方式為推送水溶液進入超音波霧化器中，並改變不同進流速度與溶液濃度，經過霧化後可產生不同的質量濃度與微粒分布。搭配使用0.2 L/min的稀釋空氣與經過10 mCi Am-241的中和器以降低霧化後的膠結現象與使氣膠達到波茲曼電中性平衡。接著將氣膠微粒引至固定大小(內徑13公分、高105公分)的系統內與固定流率90 L/min的稀釋空氣混和。後端量測其粒徑分佈及重量濃度則分別使用氣動粒徑分析儀與手動濾紙採樣進行分析。並發現本研究探討的連續式貝他射線衰減質量濃度監測儀ThermoFisher 5014i對質量濃度之反應時間約為60分鐘，無法反應即時濃度，因此本研究提出一套將貝他強度轉換質量濃度演算法進行討論。並透過架設穩定且準確的標準氣膠產生系統且運用在評估連續式貝他衰減質量濃度監測儀的性能，將可細緻的探討監測儀「準確度」、「精密度」、「偵測下限」、以及「反應時間」，可望能提升該儀器的快、準、穩、靈敏。 標準氣膠產生系統能產生質量濃度範圍在0至416.5 μg/m3、變異係數約5%且在固定稀釋空氣流率為90 L/min下反應時間約為12秒，且能控制產生氣膠微粒分布與成分。在連續式的設計中彎管對於大於PM2.5之大微粒有明顯的傳輸損失現象，進一步導致在評估大微粒的質量濃度時有低估的現象發生。質量衰減係數主要受到微粒成分之影響，在相同成分之氣膠微粒下評估質量濃度發現有良好的準確性。演算法可使連續式貝他衰減質量濃度監測儀時間解析度為一秒，並透過計算貝他強度轉換到質量濃度所使用的平滑時間，能使反應時間縮短但其代價為較不穩定而有較高的偵測下限。因此，利用縮小採樣面積在相同濃度下使單位時間、單位面積上累積的質量增加，發現能改善因為反應時間短而有較高偵測下限的問題，但受到在固定流量下縮小面積會導致壓降增大而會有所限制。最後，搭配平滑時間40分鐘且濾紙面積0.6 cm¬2，可使偵測下限小於5 μg/m3時，其反應時間約為30分鐘。

The beta attenuation (also referred to as beta gauge) is one of the few dynamic mass measurement methods that are commonly used for aerosol mass monitoring. Previously, most of the beta gauges are operated in periodic counting mode. The periodic counting approach apparently restricts the time resolution. In addition, there are uncertainties associated with periodic filter transport. Therefore, the continuous beta gauge is recently developed. In this new design, the dynamic filter loading is measured simultaneously by the attenuation of the beta rays. It is not necessary to move the filter spot from the sample position to a separate detector position for zero and mass determinations. However, the bent sampling train might create aerosol deposition loss. Moreover, the performance evaluation data of this type of relatively new instrument is still very limited. Therefore, the performance of a continuous counting beta gauge, including the sampling train, was thoroughly tested and analyzed in the present study. In this work, the challenge aerosol particles were generated using an ultrasonic atomizing nozzle. A syringe pump was used to transport the sodium chloride solution into the atomizer. The combination of the syringe pump and atomizing nozzle could be regarded as a standard aerosol generation system. The dispersing air of 0.2 L/min was used to reduce the aerosol coagulation, and to carry the atomized droplets through a radioactive source, 10 mCi Am-241, to neutralize the challenge aerosol particles. The aerosol output was then introduced into the mixing chamber (diameter 13 cm, height 105 cm) with a dilution air flow of 90 L/min. The aerosol mass concentration and size distribution could be controlled by adjusting the feeding rate of the syringe pump and the solution concentration, respectively. The particle size distribution and mass concentration were measured using an aerodynamic particle sizer. The filter samples were used to verify the aerosol mass concentration in the chamber. The performance of a beta gauge (ThermoFisher 5014i) was evaluated from four aspects: accuracy, precision, detection limit, and response time. The sampling train of the beta gauge was examined for potential aerosol deposition loss, as a function of particle size. The aerosol generation system, by changing the solution concentration or solution feeding rate, could produce aerosol mass concentrations ranging from 0 to 416.5 μg/m3 steadily, with a coefficient of variation about 5%. It took less than 12 seconds to reach stable aerosol concentration in the chamber, with the dilution air flow set at 90 L/min. The elbow shape sampling train, a necessary design for continuous beta rays, might cause aerosol deposition loss due to inertial impaction for aerosols larger than 2.5 μm, when operated under the default sampling flow of 16.7 L/min. The mass attenuation coefficient is dependent on chemical composition of the deposited aerosols. The response time of the tested beta gauge decreased with decreasing smoothing time, but at the cost of higher variation. With fixed sampling flow and aerosol concentration, the detection limit and response time decreased with decreasing filter size. However, smaller filter caused higher pressure drop and resulted in shorter filter life. Mass attenuation coefficient is mainly affected by the composition of the particles, and the mass concentration is evaluated under the same composition of the particles to find good accuracy. Then, the algorithm can make time resolution of mass concentration to one second. And the response time will decrease as the smoothing time decreases, but it will lead to unstable results and a higher detection limit. However, the results show that the problem can be solved by reducing the sampling area. It is considered that the reduced sampling area to increase the cumulative mass density per unit time at the same mass concentration. But the limit of reducing sampling area is the pressure drop. Finally, with a smoothing time of 40 minutes and the filter area of 0.6 cm2, the detection limit can be less than 5 μg/m3 and the response time is about 30 minutes.