Unraveling the global response of secondary atmospheric pollutants to emission reductions imposed during the COVID-19 pandemic

This study employs the global chemistry–climate model EMAC, combined with emission adjustment factors (CONFORM), to evaluate the global impacts of the 2020 COVID-19 lockdowns on secondary air pollutants. Unlike earlier studies limited to short periods or specific regions, this work captures seasonal, spatial, and sectoral variability in emissions and pollutant formation. It examines both primary pollutants (e.g., NO_x, SO₂) and secondary pollutants such as ozone and PM_2.5 components. Significant NO_x reductions were simulated, with peak decreases of 31 % in Europe and 25 % in Eastern Asia during lockdowns. SO₂ dropped by up to 20 % in Southern Asia, while Eastern Asia saw a 14 % post-lockdown rebound due to increased industrial activity. Ozone responses varied regionally, with increases of up to 4 % in urban areas in Eastern Asia due to reduced titration, while Southern Asia saw decreases up to 1.4 %. Aerosol concentrations followed similar regional trends. Sulfate declined by 17 % in Southern Asia while nitrate and ammonium dropped by 27 % and 33 % in Europe, respectively. Global primary organic aerosol decreased by 4 %, with South Asia showing an 8 % reduction. Secondary organic aerosol fell by 4 %, with the largest drop (8 %) in Latin America and the Caribbean. Overall, PM_2.5 over land declined by 2 %, with Europe showing the largest regional decrease (6 %). These results highlight the substantial but regionally varied effects of pandemic-related emission changes. However, even substantial short-term reductions had limited impact on PM_2.5 levels, underscoring the need for sustained, large-scale emission cuts to meet WHO air quality guidelines.