Surface Ozone Variability in Two Contrasting Megacities, Cairo and Paris, and Its Observation from Satellites

With recognized adverse effects on human health and the environment, surface ozone constitutes a major problem within and downwind of urbanized areas. In this work, we first analyzed 5 years of hourly concentrations of ozone measured in two megacities with contrasting climates: Paris and Cairo. In both cases, the maximal daily concentrations were observed in summer and they exceeded the 35 ppb threshold recommended by the World Health Organization in 45% and 69% of the days, respectively. During periods of forced reduced activities, these concentrations decreased in Cairo but not in Paris. This indicates that low-emission zones are not necessarily effective to help curb the ozone problem. In a second stage, the ozone retrievals of two satellite-based atmospheric sounding methods (AIRS, and the multispectral approach IASI+GOME2) were compared to the surface measurements. A systematic overestimation, larger for AIRS than IASI+GOME2, was observed. This is likely linked to the fact that satellite approaches retrieve ozone concentrations at higher atmospheric levels than the surface. However, a significantly high linear correlation was obtained at the monthly temporal resolution. Therefore, shift adjustments of the satellite measurements provide efficient proxies of surface observations with significant monthly correlations. This may help complete lacunar surface measurements.