DeNOxing the air in urban spaces by building and construction photocatalytic coverings

• ISO test method was used to rank the NO x -purifying ability of commercial materials. • The influence of environmental variables on NO x removal efficiency was studied. • Updated bibliographic review of NO x photocatalytic performance is presented. • Two numerical approaches have given similar NO surface deposition velocities. • Modelling real and ideal NO x sink effects: a urban canyon versus a promising device. A variety of air depolluting TiO 2 -based marketed products were applied on bituminous mixtures, sidewalk pavements and facades, giving NO x oxidation ratios under ISO 22197–1:2007 in the 35–9%, 56–2% and 28–2% ranges, respectively. Correspondingly, DeNO x toxicity indexes varied from −0.8 to 5.6, 0 to 14 and −4 to 1 μmol. The three most efficient photocatalytic products were selected: two TiO 2 -water dispersions, for road and sidewalk, and a TiO 2 -covering, for facade. NO x purifying ability of these materials were evaluated when key physical parameters were modified. The observed NO x conversion is positively correlated with UV-A irradiance up to 10 W/m 2 , reaching a plateau, and negatively correlated with relative humidity, with a more pronounced decrease above 35%. Inversely, no dependence with inlet NO x concentration is observed in the range of 0.14–1 ppm v . Further, two first-order kinetic approximations were followed to calculate NO surface deposition rates, giving 2 to 8 10 −3 m/s on the selected photocatalytic urban surfaces. Subsequently, the potential NO x sink effect induced in a photocatalytic urban canyon and a NO x -purifier was modelled taking NO x surface deposition rates from 10 −3 to 10 −1 m/s. Purifying devices could be utilised as a preferred option to help alleviate local atmospheric NO x in high-polluted areas.