空氣負離子控制室內空氣污染物之研究

Abstract

本研究探討空氣負離子控制室內空氣污染物之研究，以瞭解空氣負離子去除室內空氣中揮發性有機污染物、懸浮微粒與生物氣膠之效能與機制。本研究經空氣負離子基礎特性研究以電極型態0.1 mm之不鏽鋼針尖於低起始放電電壓下，以誘導電極輔助進行負極針尖放電產生空氣負離子，並控制適當之放電電壓(30.0 kV以下)與低放電電流(0.01 mA以下)以dark discharge進行負極針尖放電產生穩定之空氣負離子濃度，且於部分空氣負離子反應器中以15 kV以下之放電電壓進行放電產生空氣負離子，以避免放電過程中生成臭氧與氮氧化物干擾空氣負離子與室內空氣污染物之反應。實驗結果顯示，相對溼度影響空氣負離子在室內空間中之分布，且空氣負離子濃度隨著空間距離之增加呈現對數線性遞減之趨勢(logarithmic linear tendency)，在一特定之對數線性距離(log-linear distance)之後空氣負離子濃度隨著距離之增加呈現一固定濃度，本研究依據實驗數據之分析建立空氣負離子在室內空間中分布之經驗模式NAIDI Model，用以推估室內空間中空氣負離子之濃度分布。 本研究探討空氣負離子與揮發性有機污染物(chloroform、toluene 與 1,5-hexadiene)之化學反應動力機制。利用ppm濃度等級之揮發性有機污染物與不同相對濕度下之空氣負離子(相對溼度由0 到 70%之空氣負離子濃度由 1.34E6降低至1.24E6 ion cm-3)進行反應。結果顯示空氣負離子與chloroform，以及toluene 的反應級數為零級反應，而1,5-hexadiene之反應級數則為0.433級。chloroform、toluene 與1,5-hexadiene在相對溼度0到 70%之間的反應速率常數分別為1.74 至 3.07 ppb min-1, 1.07至2.66 ppb min-1, 以及 0.463至0.478 ppb0.567 min-1 。此外，空氣負離子與1,5-hexadiene的反應，會生成一個相對穩定的中間產物4-pentenal。研究結果顯示空氣負離子與揮發性有機污染物之反應屬於緩慢的氧化反應，對於低濃度範圍之揮發性有機污染物具有去除控制之效能，惟需注意是否有危害性之反應副產物產生。 本研究探討空氣負離子控制室內懸浮微粒之研究，實驗結果顯示空氣負離子有效去除氣相中懸浮微粒PM2.5與次微米微粒，於操作空氣負離子產生裝置54分鐘之後，次微米氣膠粒徑分布之個數濃度大部分皆降至1000 number cm-3以下，其中粒径835 nm由4599降至495 number cm-3，去除效率約89.2%，1.5小時後PM2.5由0.164 mg m-3降為0.084 mg m-3，去除效率約50%。空氣負離子控制多粒徑分布鹽類氣膠之實驗結果顯示，懸浮微粒之粒徑影響空氣負離子控制懸浮微粒之效能，且懸浮微粒之粒徑越小去除效能越佳。測試箱表面材質特性影響空氣負離子控制懸浮微粒之實驗結果顯示，不鏽鋼、木板、PVC板、壁紙與水泥漆等表面材質中表面電阻(surface resistivity)越高與電導度(electrical conductivity)越低的表面材質(例如木板)其有效清淨速率ECR (effective cleaning rate)越高，此外材質表面之粗糙度亦為空氣負離子控制懸浮微粒之影響因子。空氣負離子對懸浮微粒之去除機制，在於空氣負離子致使懸浮微粒表面帶負電荷，並藉由電移動度將帶電氣膠自氣相中移動至帶正電荷或電中性之物體表面去除。 本研究探討空氣負離子對生物氣膠之移除與殺菌效能，實驗結果顯示空氣負離子對四種生物氣膠(Escherichia coli (E. coli), Bacillus subtilis (B. subtilis) endospores, spores of Penicillium citrinum (P. citrinum), yeast cells of Candida famata (C. famata) var. flareri)之移除效率隨生物氣膠在空氣負離子反應器中的停留時間增加而增加，例如E. coli在停留時間為3.1, 6.2, 7.8, 10.4與15.5分鐘之移除效率分別為12.5, 33.8, 42.1, 68.2與80.1%。實驗結果顯示空氣負離子對生物氣膠之移除效能為B. subtilis endospores > E. coli > spores of P. citrinum = yeast cells of C. famata。本研究以survival factor (SF)評估空氣負離子對生物氣膠之殺菌效能，SF小於1或趨近於1，分別代表殺菌效能顯著或不具殺菌效能。實驗結果顯示整體實驗之SF為0.96 +- 0.19，表示空氣負離子對生物氣膠不具有效的殺菌效能。本研究結果顯示空氣負離子對生物氣膠的控制機制，主要為空氣負離子使生物氣膠帶電而加以移除。

This study investigated the control of volatile organic compounds (VOCs), particles, and bioaerosols with the aid of negative air ions (NAIs) in indoor environment. The NAIs were generated by negative electric discharge; the discharge was kept at less than 0.01 mA and 30.0 kV. The reaction chamber was designed as a dark discharge to prevent the generation of O3 and NOx. This study investigated the concentration gradient of NAI at various temperatures, relative humidities and distances in indoor air, and developed an empirical model for concentration gradient of NAI as well. Experimental results indicate that the concentration gradient of NAI was little affected by temperatures between 23.9C and 25.7C. However, the influence of relative humidity on the concentration gradient of NAI was complicated. There were four trends for the relationship between NAI concentration and relative humidity at different distances from the discharge electrode. Additionally, the regression analysis of NAI concentrations and distances from the discharge electrode indicated a logarithmic linear (log-linear) relationship; the distance of log-linear tendency decreased with an increase in relative humidity. Moreover, an empirical model for the concentration gradient of NAI generated in indoor air was developed for estimating the NAI concentration. This study investigated the reactions of NAI and VOCs in a batch reactor. Three species of VOCs - chloroform, toluene and 1,5-hexadiene - were selected to react with NAI at relative humidity of 0, 25 and 70%. The NAI was generated by a negative electric discharge at 15.0 kV. The NAI concentrations were 1.34E6 to 1.24E6 ion cm-3 at relative humidities between 0 and 70%. The results indicate that the order of the reactions of chloroform and toluene with NAI was zero, and 0.433 order for 1,5-hexadiene. The reaction rate constants of chloroform, toluene and 1,5-hexadiene were 1.74 - 3.07 ppb min-1, 1.07 - 2.66 ppb min-1 and 0.463 - 0.478 ppb0.567 min-1 at relative humidity from 0 to 70%. The effect of relative humidity on the reaction kinetics was obvious for chloroform and toluene but not for 1,5-hexadiene. The reaction between 1,5-hexadiene and NAI generated a relatively stable intermediate species, 4-pentenal. The oxidation of chloroform, toluene and 1,5-hexadiene by NAI proceeded slowly. This study investigated the control of particles with the aid of NAIs in an indoor environment. The results indicated that the PM2.5 and submicron particle were obviously removed by NAIs. Moreover, this work studied how wall surface materials influence the removal of airborne particles with NAIs. Five wall surface materials – stainless steel, wood, PVC (polyvinyl chloride), wallpaper and cement paint – were applied to the inner surface of a test chamber. The results indicated that NAI could remove particles from the wood and PVC wall surfaces substantially more effectively than from other wall materials. The various electrical characteristics and roughness of the wall materials may have been responsible for the associated of the various ECRs (effective cleaning rate) with the various wall surface materials. Although negative air ionizers have been used in indoor air cleaning, little study have been done on the elimination of bioaerosols by NAIs. This study investigated the removal and germicidal effects of NAIs on bioaerosols. Bioaerosols, Escherichia coli (E. coli), Bacillus subtilis (B. subtilis) endospores, spores of Penicillium citrinum (P. citrinum), and yeast cells of Candida famata (C. famata) var. flareri, were produced by six-jet Collison nebulizer, which aerosolized the suspension of microorganisms in DI water and PBS (phosphate buffer solution). NAIs were generated at a concentration of 5E5 ions cm-3 in an experimental chamber (9.32E-2 m3) by negative electric discharge at 10 kV. The removal and germicidal efficiencies of bioaerosols were measured by aerodynamic particle sizer (APS) and high velocity impinger (AGI-30), respectively. Bioaerosols collected by AGI-30 was cultured for colony forming unit (CFU) counting. The results indicated that the removal efficiency of bioaerosols was enhanced by NAIs and increased with the retention time of bioaerosol in the experimental chamber. The germicidal efficiency of bioaerosols was evaluated by the survival factor (SF). The SF less than 1 and approach of 1, respectively, showed that the NAIs with and without germicidal efficiency of bioaerosols. The results indicated that the SF was 0.96 +- 0.19 at different retention time and relative humidity, therefore, the germicidal function of NAIs on bioaerosols was invalid. However, the removal effect of NAIs on bioaerosols was the major mechanism for eliminating the bioaerosols using negative air ionizers.