In many cities worldwide, modern fleets have been introduced to reduce gaseous and particle emissions from city buses. To date, most emission studies are limited to a few vehicles, making a statistically significant assessment of control options difficult, especially under real-world driving conditions. Exhaust emissions of 234 individual city buses were measured under real-world stop-and-go traffic conditions at a bus stop in Gothenburg, Sweden. The buses comprised models fulfilling Euro III-VI standards with different engine technologies, fuels, and exhaust after-treatment systems, and also included hybrid-electric buses (HEV). Both gaseous (NOx, CO, HC, and SO2) and size-resolved particle number (PN) and mass (PM) emission factors (EF) were calculated. The potential effect of changing to a non-fossil fuel vehicle fleet (vehicles using compressed natural gas (CNG), diesel (DSL), Rapeseed Methyl Ester (RME) and Hydro-treated Vegetable Oil (HVO)) was also investigated by measuring primary EF and secondary gas and particle formation downstream a Gothenburg Potential Aerosol Mass (Go:PAM) reactor. The results suggest that the potential for forming secondary mass should be considered in future fuel shifts since the environmental impact is different when only considering the primary emissions. The inclusion of secondary pollutant sources in emissions regulation may lead to a more accurate portrayal of the potential impact of mobile sources or emissions control procedures on regional air quality.