Indirect Footprint of the 8 September 2023 Al Haouz (Morocco) Earthquake on Air Quality in Distant Cities

The Al Haouz earthquake (8 September 2023, Morocco) caused severe ground damage and initiated widespread human behavioural responses, raising concerns about possible indirect impacts on urban air quality. This study investigates the long-distance air quality response of near-surface particulate matter (PM10, PM2.5, and PM1) and selected gaseous pollutants (NO2 and CO), using high-time-resolution measurements from urban (Kenitra and Salé) and background (Maamoura) monitoring sites located hundreds of kilometres away from the epicentre. Baseline comparison, change-point detection, and correlation analyses consistently indicate the absence of an immediate response of PM levels at the exact arrival of the seismic waves, suggesting that direct mechanical effects of ground shaking were negligible at these sites. However, a multiday disruption of diurnal cycles was observed across all PM size fractions, causing up to an 111% increase in PM2.5 levels at the background station. These post-seismic changes are likely driven by intensified human mobility promoting traffic-related emissions propagating over local urban scales. The behaviour of gaseous pollutants further supports these findings: NO2, a short-lived tracer of traffic activity, showed pronounced and repeated anomalies during the night and early morning following the earthquake, closely tracking changes in human mobility. CO, by contrast, displayed weaker and smoother variations, consistent with its longer atmospheric lifetime. Therefore, the responses of short-lived (NO2) and longer-lived (CO) pollutants provide a diagnostic framework for unravelling rapid behavioural emissions changes from background atmospheric variability during extreme events. Together, these observations provide the first evidence that strong earthquakes can induce complex and multi-phase perturbations in air quality over regional scales, far from epicentres.