Evidence of Heating-Dominated Urban NO_<i>x</i> Emissions

Road transport NO_<i>x</i> emissions in many high-income countries have steadily reduced due to improved exhaust after-treatment technology. However, ambient concentrations of NO₂, O₃ and PM_2.5 continue to exceed World Health Organization guidelines in many cities globally. The megacity of London has taken an international lead in mobility interventions through the use of low-emission zones. Using long-term air pollution flux measurements made from a communications tower, we show that the largest source of NO_<i>x</i> emissions in central London has transitioned from road transport to space heating. Observations and supporting consumption/mobility data indicated that natural gas combustion in boilers was responsible for 72 ± 17% of NO_<i>x</i> emissions in the measurement footprint (average years 2021-2023). Road transport has dominated air quality thinking on NO₂ for many decades. However, in urban environments that are reliant on natural gas, building heating may now be an effective sector to prioritize for further NO_<i>x</i> emissions intervention. With system-wide changes in the heat and power sector expected in the coming decades to achieve decarbonisation pledges, we project that very low urban emissions of NO_<i>x</i> are achievable. The trajectory will, however, depend on choices made around urban buildings and their associated infrastructure and whether low-carbon fuel combustion or electrification pathways are chosen. We estimate a damage cost penalty of up to £600 M in the U.K. should hydrogen combustion replace natural gas for heating rather than technologies such as heat pumps.