Validation of RegCM-CHEM4 model by comparison with surface measurements in the Greater Cairo (Egypt) megacity

The densely populated Greater Cairo (GC) region suffers from severe air quality issues caused by high levels of anthropogenic activities, such as motorized traffic, industries, and agricultural biomass burning events, along with natural sources of particulate matter, such as wind erosion of arid surfaces. Surface-measured concentrations of particulate matter (PM₁₀), sulfur dioxide (SO₂), and ozone (O₃) and its precursor's gases (nitrogen dioxide, NO₂; carbon monoxide, CO) were obtained for the GC region. The PM₁₀ concentrations were found to exceed remarkably the Egyptian guidelines (150 μg/m³). These high levels of PM₁₀ were recorded throughout 68% of the period of measurement in some industrial areas (El-Kolaly). The measured data of pollutants were used for both the evaluation of environmental pollution levels and the validation of the online-integrated regional climate chemistry model "RegCM-CHEM4." Calculation of the bias between the model results and the measured data was used to evaluate the model performance in order to assess its ability in reproducing the chemical species over the area. The model was found to reproduce the seasonal cycle of the pollutants successfully, but with a large underestimation of the PM₁₀ values. Validation of the RegCM-CHEM4 indicated that the emission inventories of mobile sources and anthropogenic activities need to be improved especially with respect to local and regional activities in order to enhance air quality simulations over the GC region.