Integrated satellite and reanalysis assessment of primary and secondary pollutant dynamics in Tunisia during COVID-19 lockdown phases

This study quantifies air quality changes across Tunisia during the pre-lockdown, strict lockdown, and partial lockdown phases of the COVID-19 pandemic, using MERRA-2 reanalysis and Sentinel-5P TROPOMI satellite data for $${\text{SO}}_{2}$$ , $${\text{PM}}_{2.5}$$ , CO, $${\text{O}}_{3}$$ , and $${\text{NO}}_{2}$$ . Wind frequency analysis reveals a lockdown-associated shift from northwesterly to northerly/northeasterly flows, underscoring meteorology’s role in modulating pollution. As expected, $${\text{SO}}_{2}$$ and CO declined significantly during strict lockdown due to reduced industrial and vehicular activity. $${\text{PM}}_{2.5}$$ showed high spatial variability, influenced by dust transport and local enforcement. Notably, $${\text{O}}_{3}$$ concentrations increased nationwide—a counterintuitive outcome driven by reduced NOₓ titration and enhanced photochemistry. These divergent responses highlight that while emission controls effectively reduce primary pollutants, secondary pollutants like ozone require chemistry-aware strategies. This work provides actionable insights for Tunisian environmental policy: effective air quality management must integrate emission reductions with meteorological forecasting and regional transport modeling—especially in semi-arid climates where natural and anthropogenic drivers interact strongly.