自動PM10貝他監測器準確度之研究

Abstract

在台灣，自動貝他PM10監測器是一種廣泛被應用的即時監測器。由於台灣地區濕度偏高，常造成貝他監測器讀值偏高，因此，對於監測器本身因濕度而造成的讀值偏差問題則更顯得重要。在本文中，我們利用現場採樣與模式模擬的方式分別針對此項問題進行研究。 現場採樣的結果，我們發現當大氣濕度偏低時，貝他PM10監測器讀值與高流量採樣器濃度間的差距非常之小，然而，當大氣濕度增高甚至超過氣膠潮解點的時候，貝他PM10監測器讀值與高流量採樣器濃度間的差距會越來越大。為了要瞭解貝他PM10監測器讀值與高流量採樣器之間濃度差距的原因，我們更發展理論模式來求出貝他監測器濾紙上的微粒在採樣及監測室內含水量蒸發的情形。 模擬的結果顯示，當濕度低於RH = 85 % 時，所有吸附在濾紙微粒上的含水量在採樣或監測室內都會被完全蒸發掉，然而，當濕度高於RH = 85 % 時，部份濾紙微粒上的含水量會將留存而造成貝他PM10監測器讀值大於高流量採樣器濃度值的情況。在模擬日平均貝他監測器讀值時，我們發現，模擬值與實際貝他監測器讀值是非常接近的。 此外，為了瞭解濕度效應在貝他監測器讀值上的影響，我們將兩部貝他PM10監測器同時進行監測，其中，1號監測器以衝擊瓶加濕入口處的空氣，2號監測器則以平常濕度分別進行監測。結果數據顯示，當濕度高於RH = 80 %時，1號貝他PM10監測器的讀值則開始明顯高於2號監測器的讀值。另外，實驗顯示，1號貝他PM10監測器（RH較高）的壓降增加量較2號貝他PM10監測器（RH較低）的壓降增加量低，其中可能原因為粉塵餅內含水量的蒸發，造成粉塵餅的空孔而使得壓降增加量降低。

Automatic Wedding beta-gauge PM10 monitors are widely used in Taiwan. Due to the frequent occurrence of high relative humidity in Taiwan, which may increase the PM10 readings of the beta-gauge. Thus, it is of critical importance to study the humidity effect on the accuracy of the monitor. In this thesis, both field study and theoretical modeling were conducted. In the field study, the PM10 concentrations detected by the Wedding beta-gauge PM10 monitor and those measured by the manual hi-vol PM10 sampler were found to be quite close when the ambient relative humidity (RH) was lower than the deliquescence RH (DRH) of aerosols. However, when the deliquescent point was exceeded, PM10 concentrations of the beta-gauge were found to be higher and differences increases with increasing ambient RH. In addition, theoretical water mass calculated based on a thermodynamic model (ISORROPIA model (Nenes et al., 1998)) was found to be much higher than the actual values. To understand the differences in PM10 concentrations between the beta-gauge monitor and hi-vol sampler, models were developed to determine water evaporation loss from collected particles on the filter tape of the beta-gauge during sampling and in the monitoring room. Simulated results show that all absorbed water evaporates completely at RH lower than about 85 %. However, absorbed water does not evaporate completely at RH higher than about 85 %, and remaining water in particles accounts for higher beta-gauge readings than the hi-vol concentrations. The simulated daily beta-gauge PM10 concentrations are close to the actual beta-gauge readings obtained previously. Further study of the influence of relative humidity on the readings of the beta-gauge PM10 monitor was conducted using two collocated beta-gauge monitors, in which monitor #1 was conditioned with water vapor at the inlet while monitor #2 was not. Experimental data showed that PM10 readings of these two monitors were nearly identical when the relative humidity at the inlet of monitor #1 was less than about 80 %, although its relative humidity was higher than that of monitor #2. Higher PM10 readings were observed in monitor #1 when the relative humidity was over 80 %. The present models were found to fit the experimental data very well, and the thermodynamic model again overestimated the effect of relative humidity on the beta-gauge readings.