Decomposing the Temporal Signature of Nitrogen Dioxide Declines during the COVID-19 Pandemic in UK Urban Areas
Alessia Calafiore; Jacob L. Macdonald; Alex Singleton (2022). Applied Spatial Analysis and Policy, 15(4), 1167-1191. DOI: 10.1007/s12061-022-09438-2
Abstract
On March 23, 2020, a national lockdown was imposed in the UK to limit interpersonal contact and the spread of COVID-19. Human mobility patterns were drastically adjusted as individuals complied with stay-at-home orders, changed their working patterns, and moved increasingly in the proximity of their home. Such behavioural changes brought about many spillover impacts, among which the sharp and immediate reduction in the concentration of nitrogen-based pollutants throughout the country. This work explores the extent to which urban Nitrogen Dioxide (NO2) concentration responds to changes in human behaviour, in particular human mobility patterns and commuting. We model the dynamic and responsive change in NO2 concentration in the period directly following national lockdown and respective opening orders. Using the national urban air quality monitoring network we generate a synthetic NO2 concentration series built from a time series of historic data to compare expected modelled trends to the actual observed patterns in 2020. A series of pre- and post-estimators are modelled to understand the scale of concentration responsiveness to human activity and varying ability of areas across the UK to comply with the lockdown closing and response to openings. Specifically, these are linked to workday commuting times and observed patterns of human mobility change obtained from Google mobility reports. We find a strong and robust co-movement of air pollution concentration and work-related mobility – concentrations of NO2 during typical weekday commuting hours saw a higher relative drop, moving in tandem with patterns of human mobility around workplaces over the course of lockdowns and openings. While NO2 concentrations remained relatively low around the time of reopening, particularly during commuting hours, there is a relatively fast responsiveness rate to concentrations increasing quickly in line with human activity. With one of the key Government advice for workers to take staggered transportation into work and lessen the burden of rush hours and adopting more flexible work-home arrangements, our results would suggest that reductions in NO2 in urban areas are particularly responsive to broader human patterns and dynamics over time as we transitioned towards new working routines.
Extended Summary
This research examines how nitrogen dioxide pollution levels in UK cities responded to changes in human mobility during the COVID-19 pandemic lockdowns and reopening phases. The study used data from 133 urban air quality monitoring stations across the UK, distinguishing between traffic-based stations near major roads and background stations measuring ambient pollution levels. To establish what pollution levels would have been without lockdown measures, the research created synthetic control data using ARIMA time series models based on five years of historical pollution and meteorological data from 2015-2020. This approach allowed researchers to compare actual observed pollution levels during 2020 with predicted levels had normal patterns continued. The methodology employed a difference-in-difference analysis to isolate the specific impacts of lockdown and reopening policies from other factors affecting air quality. Google Community Mobility Reports provided detailed data on changes in visits to workplaces, residential areas, retail locations, parks, public transport hubs, and grocery stores across different local authority areas. The research found substantial reductions in NO2 concentrations following the March 2020 lockdown, with particularly pronounced decreases during typical commuting hours (7-9 AM and 4-6 PM). Daily background pollution levels dropped by approximately 40-50% in the weeks following lockdown, whilst commuting hour concentrations showed even larger reductions of up to 60%. Traffic monitoring stations, located near major city junctions, showed similar patterns with strong correlations between reduced workplace mobility and lower pollution levels. Importantly, the study revealed significant spatial variation in pollution responses across the UK. Areas with higher increases in residential mobility and greater reductions in workplace visits - particularly Greater London and parts of southern England and Scotland - experienced larger pollution decreases. This suggests these regions had greater capacity for home working and compliance with stay-at-home orders. Conversely, areas where public transport usage declined less showed smaller pollution reductions, potentially indicating shifts towards private car use that partially offset overall emission decreases. When restrictions began easing from June 2020, NO2 concentrations responded rapidly to increased human activity, with areas showing the greatest returns to workplace and retail mobility experiencing the largest pollution increases. The research demonstrates the intimate connection between daily human activity patterns and urban air quality, with nitrogen dioxide proving highly responsive to changes in commuting and work-related travel. These findings have important policy implications for post-pandemic work arrangements, suggesting that flexible working patterns and staggered commuting could provide ongoing air quality benefits in urban areas.
Key Findings
- NO2 concentrations dropped by 40-50% during lockdown, with commuting hours showing reductions up to 60%.
- Areas with higher home working capacity, particularly Greater London, experienced larger pollution decreases during lockdown.
- Nitrogen dioxide levels responded rapidly to policy changes, increasing quickly when restrictions were eased.
- Traffic monitoring stations showed stronger correlations between workplace mobility reductions and pollution decreases than background stations.
- Spatial variation across the UK revealed unequal environmental benefits linked to different workforce compositions and mobility patterns.
Citation
@article{calafiore2022decomposing,
author = {Alessia Calafiore; Jacob L. Macdonald; Alex Singleton},
title = {Decomposing the Temporal Signature of Nitrogen Dioxide Declines during the COVID-19 Pandemic in UK Urban Areas},
journal = {Applied Spatial Analysis and Policy},
year = {2022},
volume = {15(4)},
pages = {1167-1191},
doi = {10.1007/s12061-022-09438-2}
}