Pilot-scale NOx removal efficacy of photocatalytic roofing granules under urban environmental conditions

Photocatalytic building envelope materials harness sunlight to activate self-cleaning and de-polluting properties, which can help mitigate the urban heat island effect and abate urban pollution. The performance of roofing membranes manufactured with bright-white photocatalytic (BWP) granules, and with a non-photocatalytic bright-white control (BWC), were evaluated. The solar reflectance of unexposed specimens was 0.56 for BWP and 0.60 for BWC. Small changes in solar reflectance with respect to the initial values were observed during 22 months of field exposure between March 2020 and January 2022, indicating good resistance to soiling. Changes in the de-pollution capacity during the same period were quantified with a bench-scale method adapted from the ISO Standard 22197-1, in which unexposed and aged specimens were placed horizontally inside a flow chamber admitting clean air enriched with nitric oxide (NO) under UV irradiation. In addition, a pilot-scale approach was developed to evaluate the photocatalytic performance of the same materials under more realistic urban environmental conditions. An outdoor exposure chamber was designed and built with a rectangular stainless-steel main body topped with a UV-transparent fluorinated ethylene propylene (FEP) thin film. The area of the 30 cm × 90 cm BWP specimen tested in the outdoor chamber was more than one order of magnitude larger than those evaluated in the benchtop apparatus. The BWP membrane was pre-activated by field exposure for 75 days. The pilot-scale chamber operation was validated in the laboratory using UV lamps to irradiate the BWP membrane and a reference TiO₂-coated aluminum plate (P25, 1 g m⁻²) of the same dimensions. Tests showed comparable or better performance of the BWP membrane with respect to the P25 film. Its de-pollution capacity was tested with direct sunlight irradiation in Berkeley, California (37.88° N, 122.25° W) during June 2020. Urban air was enriched with NO, or with a NO/NO₂ mixture, and circulated through the pilot-scale chamber at two different flows: 1.1 and 440 L min⁻¹. The gas phase concentrations of NO and NO₂ were measured upstream and downstream of the tested specimen with a chemiluminescence analyzer. Upstream levels of the challenge gases were high but realistic, not exceeding 200 ppb. The UV irradiance outdoors (15–30 W m⁻²) was higher and more variable than in indoor tests (11.5 W m⁻²), leading to overall higher NO deposition velocity under direct sunlight. In outdoor measurements, the NO deposition velocity increased at higher irradiances. When NO was the only challenge gas, a fraction of NO was oxidized to NO₂ resulting in lower NO and higher NO₂ downstream concentrations. However, when tested with NO/NO₂ mixtures, the concentration of both compounds decreased during irradiation, and the surface showed a faster loss of photoactivity for NO₂, compared with NO. These effects are consistent with those observed in the laboratory. The NO_x deposition velocity outdoors was 0.7 and 105 μmol h⁻¹ m⁻² for the low and high air flow, respectively.