Evidence of Heating-Dominated Urban NOx Emissions.

Road transport NOx emissions in many high-income countries have steadily reduced due to improved exhaust after-treatment technology. However, ambient concentrations of NO2, O3 and PM2.5 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 NOx 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 NOx 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 NOx 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 NOx 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.