A take-home message from COVID-19 on urban air pollution reduction through mobility limitations and teleworking

The COVID-19 pandemic prompted unprecedented mobility restrictions globally, leading to significant reductions in urban air pollution. This study uses the experience of the Barcelona lockdown as a large-scale experiment to assess the potential of teleworking to improve air quality. High levels of urban air pollution are a major public health concern, with significant contributions from traffic emissions (30-45%). Studies from China and Europe showed dramatic decreases in pollutants like NO₂ and PM2.5 during lockdowns, highlighting the connection between mobility and air quality. The study focuses on the potential of teleworking to sustainably reduce these emissions, considering that many jobs, particularly in the service sector, are adaptable to remote work. Prior research has indicated positive correlations between teleworking and air pollution reductions, but large-scale, real-world data were largely lacking before the pandemic. The COVID-19 lockdown provided an opportunity to analyze the impact of widespread, enforced teleworking on air quality, informing future urban planning and policy decisions.

Existing literature demonstrates the significant health impacts of long-term exposure to air pollutants like PM2.5, NO₂, and O₃. Studies have linked these pollutants to premature deaths and exacerbated respiratory illnesses. Pre-COVID-19 research highlighted the contribution of traffic emissions to urban air pollution and explored the potential benefits of teleworking to reduce emissions. However, the large-scale, enforced experiment provided by the pandemic offered a unique opportunity to empirically assess the efficacy of widespread teleworking on air quality, surpassing the limitations of previous small-scale studies and simulations. The review of the energy impacts of teleworking by Holocher et al. showed reductions in air pollution, specifically in PM, O₃, and SO₂ concentrations. Another study in the United States demonstrated that increased teleworking significantly decreased various air pollutants, emphasizing the potential for substantial reductions.

The study used the Metropolitan Area of Barcelona (AMB) as a case study. Data from urban and suburban air pollution monitoring stations during the spring 2020 lockdown were analyzed, comparing NO₂, PM10, and SO₂ concentrations to those of 2019. The analysis considered meteorological factors to isolate the effects of reduced mobility. Six scenarios were formulated to simulate different levels of teleworking (from 2 to 4 days a week) and corresponding traffic reductions. The chemistry transport model WRF-Chem with a multi-layer urban scheme was used to simulate the impact of each scenario on NO₂ and O₃ concentrations. The model was evaluated against observational data from the AMB monitoring network. The simulations covered the AMB, considering its complex topography and meteorological influences. Emissions inventories from CAMS-REG-APV were used, with modifications to reflect the different teleworking scenarios. Traffic emissions were reduced in proportion to the teleworking levels, and air traffic reductions mirrored those of road traffic. The RADM2 gas-phase chemical scheme and MADE/SORGAM aerosol emissions were used in the model. The model's horizontal resolution was 3x3 km, with 45 vertical layers up to 100 hPa.

Observational data showed significant reductions in NO₂ concentrations during the lockdown, with reductions correlating with the degree of mobility reduction. The highest reduction (75%) was observed during the week with the most stringent mobility restrictions (84.4% reduction). However, O₃ concentrations increased, likely due to the decrease in NO which normally titrates O3. WRF-Chem simulations corroborated these findings, showing that increasing teleworking to 2, 3, and 4 days per week resulted in average NO₂ reductions of 4%, 8%, and 10% across the AMB, respectively. These reductions were more pronounced during peak pollution hours (morning and evening). The simulations also showed that higher teleworking scenarios led to a slight increase in O₃, but within acceptable limits for most scenarios. The model simulations for severe lockdown scenarios (75% emission reduction) showed substantial reductions in NO₂ but also significant increases in O₃. The simulations indicated that a 25% emission reduction (through increased teleworking and reduced other work-related travel) is achievable without excessively increasing O3.

The findings demonstrate a strong link between teleworking, reduced traffic emissions, and improvements in air quality. The observed and simulated reductions in NO₂ concentrations, particularly during peak hours, highlight the potential of teleworking as a powerful tool for mitigating urban air pollution. While the increase in O₃ is a concern, it remains within manageable levels in most scenarios of increased teleworking. The study provides empirical evidence and model simulations to support the promotion of teleworking as a key component of urban air pollution control strategies. The results suggest that targeted policies promoting teleworking could lead to relatively swift and substantial improvements in air quality, especially when complemented by other measures like Low Emission Zones (LEZs). Teleworking offers a socially equitable approach to emission reduction compared to measures that disproportionately affect lower-income groups.

This study provides strong evidence that promoting teleworking can effectively reduce urban air pollution, particularly NO₂ levels. The COVID-19 lockdown acted as a real-world experiment demonstrating this potential. While increases in O₃ levels were observed, they were largely manageable within the scenarios of increased teleworking. Future research could investigate the long-term economic and social impacts of widespread teleworking, as well as explore the optimal combination of teleworking and other pollution mitigation strategies to maximize air quality benefits while minimizing negative consequences.

The study focuses on the AMB during a specific period, and the generalizability to other urban areas might be limited by differences in topography, meteorology, and socio-economic factors. The model relies on emission inventories, which may contain uncertainties. The study primarily focuses on NO₂ and O₃; further research could explore the impact on other pollutants. While meteorological conditions are considered, the complexity of atmospheric processes may introduce limitations to the direct attribution of emission reductions to pollution changes. Finally, the study considers only the impacts of teleworking on emissions; other potential impacts on energy consumption from increased home energy usage, for example, are not fully investigated.