Abstract: The NOx gasses (NO and NO2) are among the most important air pollutants due to the toxicity of NO2, as well as the role of NOx in the tropospheric oxidation of Volatile Organic Carbons (VOCs), contributing to the formation of other hazardous air pollutants. Air pollution is one of the biggest health threats worldwide; hence reducing NOx levels is an important objective of the UN sustainable development goals, e.g., #3, “Good health and well-being” and #11 “Sustainable cities and communities.”Photocatalysis using TiO2 and light is a promising technique for removing NOx along with other pollutants, as demonstrated on a laboratory scale. Furthermore, a long-range of real-life test studies of varying scales have been conducted during the past two decades.
The results of these studies have been conflicting, with some studies reporting no effect on the ambient air quality and others reporting significant reductions of NOx levels. However, the studies are very difficult to compare and assess due to the different approaches used, which consequently vary in quality. In this review, we aim to develop a set of objective evaluation criteria to assess the quality of the individual studies in order to simplify the interpretation and comparison of the existing studies.
Moreover, we propose some guidelines for future test studies. Furthermore, the approaches and main conclusion from 23 studies are independently assessed and discussed herein.