Evaluation of WRF and CHIMERE models for the simulation of PM_2.5 in large East African urban conurbations

Urban conurbations of East Africa are affected by harmful levels of air pollution. The paucity of local 12 air quality networks and the absence of capacity to forecast air quality make difficult to quantify the real level of air pollution in this area. The chemistry-transport model CHIMERE has been used along with the meteorological model WRF to run simulations at high spatial resolution (2×2 km) of hourly concentrations of Particulate Matter PM_2.5 for three East African urban conurbations: Addis Ababa in Ethiopia, Nairobi in Kenya, and Kampala in Uganda. Two existing emission inventories were combined to test the performance of CHIMERE as an air quality model for a target monthly period of 2017 and the results compared against observed data from urban, roadside, and rural sites. The results show that the model is able to reproduce hourly and daily temporal variability of aerosol concentrations close to observations in urban, roadside and in rural environments. CHIMERE’s performance as a tool for managing air quality was also assessed. The analysis demonstrated that despite the absence of high-resolution data and up-to-date biogenic and anthropogenic emissions, the model was able to reproduce 66 – 99% of the daily PM_2.5 exceedances above the WHO 24-hour mean PM_2.5 guideline (25 μg m^-3) in the three cities. An analysis of the 24-hour average levels of PM_2.5 was also carried out for 17 constituencies in the vicinity of Nairobi. This showed that 47% of the constituencies in the area exhibited poor air quality index for 25 PM_2.5 in the unhealthy category for human health exposing between 10,000 to 30,000 people/km² to harmful levels of air contamination.