Pandemic Restrictions in 2020 highlight the significance of non-road NO_x sources in central London

Abstract. Fluxes of nitrogen oxides (NO_x = NO + NO₂) and carbon dioxide (CO₂) were measured using eddy covariance at the BT Tower in central London during the coronavirus pandemic. Comparing fluxes to those measured in 2017 prior to the pandemic restrictions and the introduction of the Ultra-Low Emissions Zone (ULEZ) highlighted a 75 % reduction in NO_x emissions between the two periods but only a 20 % reduction in CO₂ emissions and a 32 % reduction in traffic load. Use of a footprint model and the London Atmospheric Emissions Inventory (LAEI) identified transport and heat and power generation to be the two dominant sources of NO_x and CO₂ but with significantly different relative contributions for each species. Application of external constraints on NO_x and CO₂ emissions allowed the reductions in the different sources to be untangled identifying that transport NO_x emissions had reduced by > 75 % since 2017. This was attributed in part to the success of air quality policy in central London, but crucially due to the substantial reduction in congestion that resulted from pandemic reduced mobility. Spatial mapping of the fluxes suggests that central London was dominated by point source heat and power generation emissions during the period of reduced mobility. This will have important implications on future air quality policy for NO₂ which until now, has been primarily focused on the emissions from diesel exhausts.