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

The study examines whether enhanced teleworking and associated mobility limitations can sustainably reduce urban air pollution, using the COVID-19 lockdown as a natural experiment. The context is the global air pollution health crisis, with urban populations commonly exposed to pollutant levels exceeding WHO guidelines and substantial premature mortality attributed to PM2.5, NO₂, and O₃ in Europe. Traffic is a major contributor to urban air pollution, and during COVID-19, stringent mobility restrictions led to pronounced declines in NO₂ and particulate pollution globally. The authors focus on Barcelona’s Metropolitan Area (AMB), where air quality limits for NO₂ were routinely exceeded, to assess how different telework intensities and mobility reduction strategies could reduce emissions and improve air quality, particularly NO₂, and to understand implications for O₃. The purpose is to quantify air quality benefits from feasible teleworking scenarios, inform urban policy on mobility management, and gauge the potential for rapid, equitable, and effective pollution mitigation through telework.

The paper situates its research within evidence that mobility reductions during COVID-19 correlated with declines in PM2.5 and NO₂ across many regions. Prior studies documented large NO₂ emission decreases (e.g., ~30% in China) and European cities showing 30–50% reductions in contaminants versus 2019 during lockdowns. Reviews on teleworking indicate modest but meaningful pre-COVID reductions in pollutants attributable to telework (e.g., Switzerland: ~1.9% overall air pollution reduction; PM reductions of ~3.6%; O₃ and SO₂ 2.1–2.3%). U.S. scenario analyses suggested teleworking can lower CO₂ by 2–80%, NO₂ and PM10 by 10–100%, and CO by 20–100%, depending on uptake. The literature also discusses potential links between pollution and COVID-19 severity, underscoring the health urgency. The authors note that urban ozone chemistry is sensitive to NOx/VOC regimes, and NOx reductions in VOC-limited urban environments may raise O₃ due to reduced titration, a phenomenon observed during lockdowns.

Study area: Metropolitan Area of Barcelona (AMB), Spain, a densely populated coastal metropolis with complex topography (coastal mountains, river valleys of Llobregat and Besòs) and sea-breeze circulations that influence pollutant dispersion. Road traffic is the primary NO₂ source (~56% of emissions), with the airport contributing ~16%. The period analyzed mirrors strict COVID-19 confinement in spring 2020, with mobility reductions informed by Google Community Mobility Reports. Observational analysis: Hourly NO₂ and O₃ data from the urban air quality monitoring network (XVPCA) in urban and suburban stations, focusing particularly on a high-traffic site (Eixample). Comparisons are made between 2020 lockdown weeks and 2019 (monthly averages) while considering meteorological similarities and differences to isolate mobility effects. Scenario design: Six traffic emission reduction scenarios reflecting different degrees of teleworking and ancillary mobility reductions: - Low increase in telework (5% traffic emission reduction): 20% of service-sector workers telework 2 days/week, yielding ~5% overall traffic emission reduction. - Moderate increase in telework (10% reduction): 30% telework 3 days/week; base estimate ~7.2% rounded to 10% to reflect avoided ancillary work trips. - High increase in telework (15% reduction): 40% telework 4 days/week; base estimate ~12.8% rounded to 15% for additional avoided trips. - Low confinement (25% reduction): 45% reduction in work-related private vehicle use (incl. 4-day telework and 15% cuts in other work trips), 20% reduction from online education, 30% reduction in shopping trips; no reduction in leisure/personal travel. - Medium confinement (50% reduction): 55% reduction in work-related use (4-day telework plus 20% cut other work trips), 20% online education, 40% reduction in shopping, and 50–70% reductions in personal/leisure/health-related trips. - Severe lockdown (75% reduction): 55% reduction in work-related use (as above), 100% reduction from online education, 70% reduction in shopping; feasible only short-term. For simplicity, air traffic is reduced in line with road traffic in each scenario to reflect fewer business trips. Air quality modeling: WRF-Chem v4.1 is used to simulate base and scenario conditions. Configuration includes: - Chemistry: RADM2 gas-phase scheme; aerosols: MADE/SORGAM; photolysis: FAST-J. - Urban processes: BEP + BEM urban schemes. - Biogenic emissions: MEGAN. - Domain/resolution: 3 × 3 km horizontal grid; 45 vertical layers up to 100 hPa. - Initial/lateral conditions: Meteorology from NCEP FNL; chemical from MOZART4; 1-month chemistry spin-up. - Emissions: CAMS-REG-AP inventory; base case March 2016 used for model evaluation and scenario perturbations. Model evaluation: Base case evaluated against XVPCA NO₂ and O₃ at urban and low-traffic sites. The model shows reasonable agreement overall for March, with noted biases during some periods (up to ~35 µg m⁻³ for NO₂ and ~26 µg m⁻³ for O₃ at high-traffic sites; lower biases at low-traffic stations). Scenario simulations produce spatial maps and temporal profiles of NO₂ and O₃ during typical morning (7–9 h) and evening (21–23 h) peaks to assess percentage and absolute concentration changes relative to the base case. Meteorological considerations: Synoptic conditions (pressure patterns, advection, wind speeds) documented via supplementary meteorological summaries, acknowledging their role in dispersion and pollutant levels. The analysis avoids direct emission-concentration linear attribution due to chemical and dynamic non-linearities but interprets findings qualitatively and quantitatively where robust.

- Observations (Eixample, Barcelona): During the first two lockdown weeks, mobility reductions of ~75.4% and ~82.3% were associated with NO₂ concentration decreases of ~50% and ~59% versus 2019. In the third week (mobility reduction ~84.4%), morning peak NO₂ fell to ~21 µg m⁻³, about a 75% reduction relative to March 2019. Typical daily peaks (around 8 h and 21 h) dropped from ~80 to ~20 µg m⁻³ (8 h) and ~65 to ~10 µg m⁻³ (21 h). O₃ increased during lockdown, with a notable median peak ~150 µg m⁻³ at 16 h in week 4 (about double 2019), consistent with reduced NO titration in a VOC-limited urban regime and meteorological influences. - Modeling (WRF-Chem): - Morning peak (7–9 h): Severe lockdown scenario reduced city-wide NOx/NO₂ by up to ~52% (≈ −22 µg m⁻³). Medium confinement reduced NO₂ by ~33% (≈ −13 µg m⁻³). O₃ increased under strong reductions (e.g., up to ~6–11 µg m⁻³ depending on scenario and area), with some areas exceeding the 8-h daily max of 100 µg m⁻³ under extreme reductions. - Evening peak (21–23 h): Severe lockdown reduced NO₂ by ~53% (≈ −27 µg m⁻³) and increased O₃ by ~7% (≈ +5 µg m⁻³). Medium confinement reduced NOx by ~35% (≈ −17 µg m⁻³). Low confinement reduced NO₂ by ~13% and increased O₃ up to ~8 µg m⁻³. - Telework scenarios (daytime averages over AMB): - 2 days/week telework (low increase): NO₂ reduced by ~4% (≈ −1.5 µg m⁻³); small O₃ increase. - 3 days/week telework (moderate increase): NO₂ reduced by ~8% (≈ −3 µg m⁻³). - 4 days/week telework (high increase): NO₂ reduced by ~10% (≈ −6 µg m⁻³); O₃ increases moderately up to ~3 µg m⁻³. - Low confinement (25%): NO₂ reduced ~16% (≈ −6 µg m⁻³), O₃ increased ~+5 µg m⁻³. - Spatial patterns: Reductions occur across the AMB, with larger NO₂ decreases in the northern AMB where daytime pollution is typically higher due to plume transport and topography. - Meteorology: Periods of lower pressure and cyclonic flow enhanced dispersion, contributing to lowered concentrations beyond mobility effects; high-pressure periods had the opposite effect.

The findings corroborate that substantial teleworking and targeted mobility reductions can deliver rapid and meaningful improvements in urban NO₂ air quality, addressing persistent exceedances of health-based guidelines in Barcelona. The observed and modeled reductions during peak traffic hours translate into lower exposure during times of highest population activity. However, the chemistry of urban ozone implies that strong NOx reductions in VOC-limited regimes can increase O₃ due to reduced titration, particularly under extreme mobility cuts; hence, policies should balance emission reductions to avoid unintended O₃ elevations or complement telework with VOC control strategies and timing-based interventions. The COVID-19 experience demonstrates feasibility: large portions of the service sector can telework effectively, reducing commuting and work-related trips swiftly. Compared with measures like Low Emission Zones, teleworking can be deployed more rapidly and may be more socially equitable, primarily affecting higher-paid, telework-suitable occupations, while avoiding regressive impacts sometimes associated with road pricing or fuel taxes. Policymakers can leverage flexible schedules, mandatory telework during high-pollution episodes, and reduced work-related travel to achieve about a 25% emission reduction during episodes, which appears both impactful and feasible. Extreme reductions (50–75%) may not be necessary or advisable due to O₃ increases and feasibility constraints. Integrating telework with broader urban mobility and emissions management (public transport, LEZ, VOC controls, and space-use planning) can maximize benefits.

Enhanced teleworking is a practical, immediately implementable tool to reduce urban NO₂ pollution. Scenario modeling for Barcelona shows that increasing telework to 2, 3, and 4 days per week yields average daytime NO₂ reductions of approximately 4% (−1.5 µg m⁻³), 8% (−3 µg m⁻³), and 10% (−6 µg m⁻³), respectively, with modest O₃ increases. Observations during COVID-19 lockdown corroborate these effects, demonstrating swift air quality improvements aligned with large mobility reductions. Teleworking can complement other mitigation strategies such as LEZs and contributes to reduced fossil fuel use and carbon footprint, while potentially offering social equity and safety co-benefits. Future research should: (1) evaluate long-term productivity, economic, and equity implications of sustained teleworking; (2) analyze comprehensive pollutant responses including VOCs and secondary PM formation to manage O₃ trade-offs; (3) refine emissions inventories and urban chemical mechanisms for high-traffic environments; (4) assess generalizability to other cities with differing meteorology, topography, and emissions profiles; and (5) design integrated policy packages combining teleworking with targeted VOC controls and episode-specific measures.

- Modeling biases: The base-case WRF-Chem exhibited notable biases during certain periods at high-traffic stations (up to ~35 µg m⁻³ for NO₂ and ~26 µg m⁻³ for O₃), linking uncertainty to emissions inventories and urban chemistry representation. - Meteorological confounding: Differences in synoptic conditions (pressure, wind, dispersion) significantly affect concentrations, complicating attribution of changes solely to emission reductions. - Non-linear chemistry: Direct emission-to-concentration correlations are limited by atmospheric dynamics and chemistry; O₃ responses can be counterintuitive in VOC-limited urban regimes. - Scenario feasibility: The severe lockdown (75% reduction) is only feasible short-term and under extreme circumstances; results may not represent sustainable policy options. - Scope and generalizability: The study centers on Barcelona’s AMB; results may vary in cities with different emission mixes, VOC/NOx regimes, and meteorology. - Operational and social factors: The study does not fully assess long-term productivity, workspace energy use, equipment duplication, or rebound effects that could offset some benefits.