Gregg, D J; Tompkins, J; Cordell, R L; Brown, A S; Smallbone, K L; Vande Hey, J D; Wyche, K P; Monks, P S (2026) 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. Atmospheric Environment, 365. 121668 ISSN 13522310

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Abstract

Introduced in April 2019, the London Ultra Low Emission Zone (ULEZ) is designed to improve ambient air quality within Central London through reductions in both NOX and PM emissions, with COVID-19 related restrictions later superimposed upon the ULEZ throughout much of 2020 and 2021. However, little research has been undertaken to assess the impact of these interventions on O3 and UFP concentrations, or to account for variations in meteorological or anthropogenic influences. To assess these effects, NO2, O3, PM10, PM2.5 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. It was identified that the introduction of the ULEZ expedited reductions in NO2, PM10 and PM2.5 abundance, aligning with existing research, alongside reductions in non-nucleation mode particle (nNMP) abundance and concomitant increases in O3 and nucleation mode particle (NMP). The implementation of COVID-19 restrictions expedited an increase/decrease in NO2/O3 respectively through typical OX 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. PM10, PM2.5 and nNMP), increasing the abundance of the smallest nucleation mode particles by dampening the particle scavenging effect. The findings reinforce the necessity of examining the impact of interventions on atmospheric composition, including changes in the abundance of secondary pollutants.

Item Type:	Article
Keywords:	Air pollution interventions, ULEZ, Ultrafine particles, Particle size distribution, Secondary atmospheric pollution
Subjects:	Environmental Measurement > Air Quality and Airborne Particulates
Divisions:	Atmospheric Environmental Sciences
Identification number/DOI:	10.1016/j.atmosenv.2025.121668
Last Modified:	19 May 2026 10:12
URI:	https://eprintspublications.npl.co.uk/id/eprint/10411