Challenges in quantification of photocatalytic NO 2 abatement effectiveness under real world exposure conditions illustrated by a case study.

Health risks due to NO ₂ exposure commonly exceed acceptable levels in modern societies. Among the measures to reduce such risks, photocatalytic materials present a promising technology. However, while the pollutant remediation of such materials has been extensively validated in laboratory studies, the performance under real world environmental exposure conditions is still subject to controversy. Indeed, a comparison of available in-situ monitoring studies manifests non-conclusive and highly scattered results regarding the photocatalytic effectiveness observed. The reasons for this behaviour must be carefully explored in order to prevent non-efficient photocatalytic applications from being put into practice on a larger scale. This paper presents a comprehensive large-scale study for assessing the photocatalytic NO ₂ remediation by active pavements in a street of Madrid (Spain), comprising different in-situ monitoring techniques. The discussion is enriched by relating the obtained results to those of other large-scale studies. The discrepancies between these results may be traced back to different circumstances, among them the distance between the active pavement and the pollutant concentration sampling inlet, as well as to significant site-specific and time-dependent variations of pollutant concentrations and climatic parameters. Under due consideration of these influences, for materials with relatively high initial effectiveness, it was concluded that in most such applications, the average NO ₂ removal effectiveness, if evaluated at a typical inlet height of Air Quality Stations (3 m), will not exceed a value of 4% (averaged over a sufficiently large number of measurement points in the area of application and a sustained amount of time, i.e. several months). When considering more realistic human exposure conditions (lower heights and daytime), it might be justified to assume somewhat higher average effectiveness.
Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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