The impacts of the Ultra Low Emission Zone (ULEZ) and COVID-19 restrictions on air quality in central London – evidence for an increase in small particles

Introduced in April 2019, the London Ultra Low Emission Zone (ULEZ) targets reductions in NO x and PM emissions to improve ambient air quality, with COVID-19 related restrictions superimposed throughout much of 2020 and 2021. However, little existing research assesses the impact of these interventions on O 3 and UFP (Ultrafine particles) concentrations, or accounts for variations in meteorological or anthropogenic influences. To assess these effects, NO 2 , O 3 , PM 10 , PM 2.5 and 51 size-channel UFP data collected between January 2015 and December 2022 were normalised using Boosted Regression Tree (BRT) models comprised of twelve predictor variables, including overall trend, time of day/year, wind speed/direction, temperature and traffic volume. The introduction of the ULEZ expedited reductions in NO 2 , PM 10 and PM 2.5 abundance, aligning with existing research, alongside reductions in nucleation mode UFP abundance and concomitant increases in O 3 , Aitken and accumulation mode UFP abundance. The implementation of COVID-19 restrictions expedited an increase/decrease in O 3 /NO 2 respectively through the typical O X couple chemistry. The use of BRT models accounts for changes in the predictor variables, thereby showing that changes in atmospheric composition are not wholly a reflection of seasonality, meteorology or anthropogenic activity. The findings indicate the introduction of both ULEZ and COVID-19 restrictions precipitated a reduction in ambient concentrations of larger particulate matter (i.e. PM 10 , and PM 2.5 ) and larger modes of UFPs (i.e Accumulation and Aitken), alongside increasing concentrations of nucleation mode particles. The findings reinforce the necessity of examining the impact of interventions on atmospheric composition, including changes in the abundance of secondary pollutants. • ULEZ and COVID-19 decreased larger particles and increased nucleation mode particles. • ULEZ accelerated declines in NO 2 , PM 10 , PM 2.5 and non-nucleation-mode particles. • Lockdowns increased O 3 and increased NO 2 via classical O x couple chemistry. • Adjusting for weather & traffic confirms policies—not seasonality—drive shifts. • Policy implications as traffic emission reductions can elevate O 3 and UFP abundance.