Photocatalytic-treated asphalt road in Copenhagen for urban NO x removal

Abstract

Atmospheric nitrogen oxides ( ${\text{NO}}_x=\text{NO}+{\text{NO}}_2$ ) are key pollutants and short-lived climate forcers contributing to acid rain, photochemical smog, aerosol formation and climate change. Exposure to nitrogen dioxide ( ${\text{NO}}_2$ ) emitted mainly from transportation, causes adverse health effects associated with respiratory illnesses and increased mortality even at low concentration. Application of titanium dioxide ( ${\text{TiO}}_2$ )-based photocatalysis in urban environment is a new air cleaning solution, activated by sunlight and water vapour to produce OH radicals, able to remove ${\text{NO}}_x$ and other pollutants from the planetary boundary layer. This study is a large-scale evaluation of ${\text{NO}}_x$ removal efficiency at a near-road environment with applied photocatalytic NOxOFF™ technology on an urban road west of Copenhagen, thus supporting local municipality in meeting their clean-air Agenda 2030. The photocatalytic NOxOFF™ granulate containing ${\text{TiO}}_2$ nanoparticles was applied on an asphalt road in July 2020 and ambient ${\text{NO}}_x$ was measured during a six-month monitoring campaign. It is the first ${\text{NO}}_x$ monitoring campaign carried out at this road and specific efforts have been devoted to evaluate the reduction in ambient ${\text{NO}}_x$ levels with NOxOFF™-treated asphalt. Several methods were used to evaluate the photocatalytic effect, taking into account analysis limitations such as the short reference period prior to application and the highly uncertain measurement period during which SARS-CoV-2 lockdown measures impacted air quality. There was no statistically significant difference in ${\text{NO}}_x$ concentrations between the reference period and the photocatalytic active period and NO removal efficiency resulted in $-0.17$ (± 1.27). An upper limit removal of 17.5% ${\text{NO}}_x$ was estimated using a kinetic tunnel model. While ${\text{NO}}_2$ comparison with COPERT V street traffic model projection was roughly estimated to decrease by 39% (± 38%), although this estimate is subject to high uncertainty. The observed annual mean ${\text{NO}}_2$ concentration complies with Frederiksberg clean-air Agenda 2030 and air quality standards.

Graphical abstract: A graphical abstract illustrating the air cleaning properties of ${\text{TiO}}_2$ -based photocatalytic-treated asphalt.

Keywords: Air quality; Clean-air technology; Nitrogen dioxide; Photocatalysis; Titanium dioxide.