Influence of inlet concentration, UV intensity, and relative humidity on NO _x degradation in photocatalytic asphalt pavement

Road transportation significantly contributes to urban NOx pollution, with concentrations reaching 100–1000 ppb near high-traffic areas. This study provided a comparative assessment of TiO2 incorporation methods (mixing and surface coating) in asphalt pavement, while examining the effects of environmental factors on NOx and NO degradation. While asphalt mixtures modified with 4% TiO2 bitumen showed minimal activity (<1%), surface-coated specimens achieved substantial NOx reduction (22.7%). Performance analysis revealed NOx degradation decreased (23% to 7%) with increasing pollutant concentration (100–1000 ppb) and improved (9% to 23%) with higher UV intensity (5–15 W/m2) at 35% relative humidity. Low humidity around 35% RH performed better for improving NOx degradation. Response surface methodology identified optimal parameters (15 W/m2 UV, 100 ppb NO, 35% RH) achieving maximum degradation desirability. The coating maintained performance over five cycles (23% NO, 10% NOx removal), though traffic wear resistance requires investigation for practical implementation.