Divergent air pollution response and rebound across the Asian Monsoon region during and after COVID-19 restrictions

COVID-19 restriction measures created an opportunity to evaluate the influence of reduced anthropogenic activity on regional air quality across the Asian Monsoon region. This study investigated the spatiotemporal variability of nitrogen dioxide (NO2) and aerosol optical depth (AOD) across 14 Asian Monsoon cities during 4 pandemic phases: pre-restriction (2019), Phase I restrictions (2020), Phase II restrictions (2021), and post-restriction (2022). Satellite observations from TROPOMI (NO2) and MODIS (AOD) were integrated with ground-based NO2 and PM2.5 measurements and meteorological reanalysis data (MERRA-2). Spatial analysis, anomaly assessment, percentage change analysis, and multivariate statistics were applied to evaluate pollutant responses under different restriction intensities represented by the Average Stringency Index (ASI). During Phase I, most cities experienced substantial pollutant reductions, particularly in higher-stringency countries (ASI ≥ 50), with satellite-derived NO2 reductions reaching −37.5%, ground-based NO2 reductions up to −67.6%, and PM2.5 reductions up to −34.6%. In contrast, AOD showed a heterogeneous aerosol-related response, decreasing in 9 of 14 cities by 3.3–15.7% but increasing in 5 by 0.9–17.7%. Corresponding satellite-derived NO2 rebounded in 13 of 14 cities during Phase II (up to 57.3%); 12 increased further post-restriction, and 6 exceeded the pre-restriction baseline. Ground-based NO2 rebounded in 3 of 8 cities with available data (up to +160.6%); 2 increased further post-restriction, and 1 exceeded the baseline. Ground-based PM2.5 rebounded in 9 of 12 cities with available data (up to +72.2%); 4 increased further post-restriction, and 4 exceeded the baseline. Satellite anomaly analyses demonstrated consistent temporal responses between satellite-derived and ground-based observations, although surface measurements exhibited stronger local variability. Multivariate PCA–k-means analysis identified 3 distinct pollution-response regimes (R2 = 0.69; p < 0.05) corresponding to combustion-dominated environments, aerosol-dominated regions, and restriction-responsive mixed-source cities. The results indicate that while restriction measures contributed to short-term air quality improvements, regional pollutant transport, biomass-burning influence, and continued emissions from traffic and urban activities limited the sustainability of these reductions. These findings highlight that the short-term emission reductions alone are insufficient to sustain air quality improvement in the rapidly urbanizing Asian Monsoon region, particularly when regional pollutant transport and biomass burning continue to influence aerosol-related pollution.