Photoactive roadways: Laboratory, field and modeling insight on the impact of photocatalytic paving materials on urban tropospheric chemistry

Abstract

Photoactive roadways have been suggested as a mitigation method to improve air quality in urban areas. However, difficulties translating laboratory results to real world conditions has complicated a wider adoption of this technology. This work presents a methodology to determine first-order loss coefficients of ozone precursors on photoactive asphalt and concrete using a continuously-stirred tank reactor under different conditions of humidity and UV illumination. The experimental loss coefficients were used to determine uptake coefficients that can be incorporated in air quality models to represent photoactive surfaces.

The laboratory findings indicate that paving materials respond differently to variations in humidity, with concrete efficiency more influenced by the increase in water vapor. The uptake of NO on asphalt was found to have a power dependence with UV irradiance. The uptake of NO₂ was smaller than that of NO, suggesting that any impact on air quality would occur mainly through removal of NO. The uptake of VOC was found to decrease exponentially with the increase in the vapor pressure of the organic compound indicating a competitive adsorption mechanism driving the photocatalytic removal of mixtures. Molar yields of HONO, NO₂ and aldehydes were detected during the removal of pollutants in asphalt, while only NO₂ was detected from concrete, indicating potential disadvantages of TiO₂ treated surfaces.

The use of the CO/NOx molar ratio was explored to monitor changes in NOx levels in experiments performed in an outdoor chamber. CO removal was not observed during the time scale at which NOx removal occurs, suggesting that CO could be used as tracer in field studies to follow changes in vehicle emissions caused by photoactive roads.

The uptake coefficients for NO determined in this work were incorporated in a one-dimensional model to understand the overall impact of photoactive surfaces on atmospheric chemistry. Preliminary results indicate that a moderate reduction of up to 20% in NO could be achieved for a scenario simulating Houston conditions. No significant effect on ozone was observed as result of adding HONO yields. However, further investigation needs to address aldehyde and NO₂ yields to determine if these byproducts offset the benefits of this strategy.