Modelled Emission Reduction Potentials from the Transport Sector in Nairobi City

The city of Nairobi, Kenya, is experiencing accelerated growth with increasing population. Expectedly, there is bound to be a rise in the vehicle population leading to rising pollution from the transport sector. This study proposes mitigation scenarios for pollutant reduction from the transport sector within Nairobi. Assuming that about 70% of the vehicles registered in Kenya are found within Nairobi, the vehicle population up to 2030 was projected using a growth rate of 10%. The vehicle kilometer traveled values were obtained from literature and the vehicle category distribution was determined from the values obtained in the vehicle traffic count and origin-destination data collection exercises carried out in this study. The pollutant levels were modeled using the Global Atmospheric Pollution Forum Air Pollutant Emission Inventory tool (GAPF). This tool is used to estimate emissions from various sectors and assist in the development of Emission Inventories, especially in developing countries that lack databases and relevant information. The air pollutants considered in this study are oxides of nitrogen (NOx), carbon monoxide (CO), non-methane volatile organic compounds (NMVOC), ammonia (NH3), and total particulate matter (PM10 and PM2.5). The projected emissions values for various pollutants had four scenarios including Scenario (a) Business as usual (BAU) where the vehicle growth rate was 10% per annum with only 10% having Euro IV, Scenario (b) 2023 as the base year with 10% of the vehicles having Euro IV technology and subsequent phase increase in this fraction is 5% for 2024 and 2025, 10% for 2026, 2027, and 2028 and no subsequent increase for 2029 and 2030, Scenario  (c) 2023 as the base year and all new vehicles were euro IV and Scenario (d) a combination of scenarios (b) and (c). The modeled results showed that the BAU scenario indicated a steady increase in pollutant emission levels over the period considered. For the second and third scenarios, there was an increase in the rising trend when the increase ratio was 5% followed by a sharp decline trend (especially for NMVOCs) when considered due to the increasing number of vehicles as a whole. The best intervention scenario combines the assumption that all new vehicles are meeting Euro IV standards and that there is a phase increase in the uptake of exhaust gas treatment technologies such as DPFs and catalytic converters for this scenario there was a projected decline in the emission levels of both NMVOCs and total PM. The reduction potentials were 28.5% for NMVOCs and 26.8% for total PM by 2030. This mitigation scenario could be recommended to the city government to inform the policies aimed at mitigating the pollution from the transport sectors.