Advanced Air Purification Systems at the Age of COVID-19

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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.