Uptake of NO and VOCs to a Photoactive Roadway Surface

Creating photoactive roadway materials using TiO2 is being advocated as an inexpensive means to improve air urban quality. Photoactive roadways enhance dry depositional loss rates of NO, NO2, and volatile organic compounds (VOCs). This work presents results of measuring accommodation coefficients of NO and selected VOCs as a function of UV irradiance and humidity levels for two paving materials (asphalt and concrete) treated with a commercially available TiO2 formulation. The approach presented here is based on a continuously stirred tank reactor (CSTR). The paving materials responded differently to variations in water vapor concentration. HONO formation, with yields ranging from 1 to 14%, was observed for asphalt samples but not for concrete. Aromatic VOCs produced formaldehyde and acetaldehyde as major by-products with aldehyde yields ranging from 5% to 30%. While these photoactive materials are effective at removing NO and VOCs, by-product yields of photolabile species may enhance O3 formation rates in some urban areas.