Advanced Air Purification Systems at the Age of COVID-19

COVID-19大流行导致对声称可以去除或灭活病毒和其他生物气溶胶的空气净化装置的需求增加。在这些设备中应用的技术不仅是高效过滤，如HEPA过滤器，还包括电子空气净化（EAC）系统，如紫外线，紫外线光催化氧化，等离子体和电离器。大多数eac会产生自由基和臭氧等氧化剂，这些氧化剂可以使生物气溶胶失活，并氧化空气中的气体和蒸汽。此外，自2019冠状病毒病大流行开始以来，建议经常进行表面消毒和手部卫生处理，作为减少病毒传播的预防措施；这些做法已被大多数建筑所采用，无论其类型如何。最近的一项研究评估了政府推荐/批准用于对抗冠状病毒的清洁和消毒产品的挥发性有机化合物（VOCs）排放。他们共鉴定出399种挥发性有机化合物；其中127种挥发性有机化合物被列为潜在有害化合物。这一新的室内VOCs来源加剧了室内空气质量，迫切需要开展基础研究，以更好地了解VOCs控制措施及其对建筑设计和运营的影响。PCO是一种很有前途的去除室内环境中挥发性有机化合物的技术。二氧化钛基光催化剂在紫外光作用下的PCO控制室内环境VOCs已被广泛研究。理论上，烃类VOCs完全PCO的最终产物应该是CO2和H2O。然而，反应物部分氧化产生的副产物可能比母体化合物对人体健康更有害。大量的参数会影响PCO系统的性能和副产物的产生，这一点尚未得到系统的研究。另一方面，PCO过程中产生的副产物可能会与原始污染物竞争在光催化剂表面吸附，从而对去除率产生不利影响。提出的研究工作旨在解决PCO技术的一些主要缺点，使其在不断变化的世界中应用于暖通空调系统中更加成熟和可靠。主要目标是为产生副产物开发一个全面的预测模型，并提出挥发性有机化合物混合物的降解途径。所建立的模型将用实验数据进行验证。该模型可用于提高PCO系统的去除效率，并最大限度地减少副产品产生对健康造成的风险。准确描绘挥发性有机化合物在PCO过程中的降解机理也有助于设计更高效的光催化剂，提高反应动力学和耐久性。此外，随着越来越多用于去除气相空气污染物的空气净化设备进入市场，制定一套评估其性能和开发设计工具的程序至关重要。

The COVID-19 pandemic has caused increased demand for air cleaning devices that claim to remove or inactivate virus and other bioaerosols. The technologies applied in such devices are not only high efficiency filtration like HEPA filters, but also electronic air cleaning (EAC) systems like UV, UV-photocatalytic oxidation, plasma and ionizers. Most EACs generate oxidizing agents like radicals and ozone, which can deactivate bioaerosols and oxidize gases and vapors in the air. Moreover, since the start of the COVID-19 pandemic, frequent surface disinfection and hand sanitization have been recommended as a preventative measure to reduce viral transmission; these practices have been adopted by most buildings regardless of the type.  A recent study evaluated cleaning and disinfecting products that government-recommended/approved to use against the coronavirus, for volatile organic compounds (VOCs) emissions. They identified a total 399 VOCs; among them 127 VOCs are classified as potentially hazardous compounds.  This new source of indoor VOCs has exacerbated the indoor air quality (IAQ), and created an urgent need for fundamental studies for better understanding of VOCs control measures and their effects on building design and operations. PCO is a promising technology for removing VOCs from indoor environment. PCO with TiO2 based photocatalyst in the presence of UV light has been widely studied for controlling indoor environment VOCs. Theoretically, the final products of complete PCO for hydrocarbon VOCs should be CO2 and H2O. However, partial oxidation of reactants generates by-products that can be more harmful to human health than the parent compound. A large number of parameters  can impact the performance of PCO system and by-product generation which have not been systematically investigated. On the other hand, the by-products generated during PCO might compete with the original pollutant for adsorption on the photocatalyst's surface and adversely affect the removal rate. The proposed research work aims to address some of the key shortcomings associated with PCO technology so as to make it a more mature and reliable option applied in HVAC system in the changing world. The primary goal is to develop a comprehensive predictive model for generation of by-products and to propose degradation pathways for VOC mixtures. The developed model will be validated using the experimental data. Such model can be employed to boost the removal efficiency of PCO systems and minimize the health risks posed by generation of by-products. Accurately mapping the degradation mechanism of VOCs during PCO would also be beneficial to designing more efficient photocatalysts with enhanced reaction kinetics and superior durability. Furthermore, as more air cleaning devices for the removal of gas-phase air pollutants enter the market, it would be essential to develop a procedure to evaluate their performance and develop design tools.