Real-time analysis of NOx emissions in heavy-duty diesel vehicles: Impact of speed and variations across vehicle groups

This study analyzes the NOx emission factors of heavy-duty diesel vehicles monitored in the Metropolitan Region of Rio de Janeiro (MRRJ) between February 2022 and April 2024, with the aim of assessing compliance with the PROCONVE (Air Pollution Control Program for Vehicles) program. To the best of our knowledge, this study is unprecedented in the country and one of the few studies conducted in Latin America. The results showed great variability in NOx emissions, reflecting differences in emission control technologies, vehicle age, maintenance conditions, and operational characteristics. The average NOx emissions for the fourteen vehicles were 3322 ± 106 ppm, ranging from 1.51 to 1257 ppm. The analysis indicated that vehicle weight influenced emissions, with greater variability in the lightest vehicles. Engine power was another relevant factor, with intermediate-power vehicles (188 kW and 191 kW) presenting the lowest emissions, challenging the assumption that more powerful engines always generate more emissions. Regarding the age of the vehicles, the newer ones emitted less NOx than the older ones. When converting the emission factors to g kWh⁻¹ using a theoretical conversion factor, more than 50 % of the vehicles exceeded the 2 g kWh⁻¹ limit established by the PROCONVE P-7 standard. It should be noted that the actual load conditions of the engine are different and therefore this comparison is qualitative only. Most of the monitored vehicles operated at low speeds (up to 40 km h⁻¹), typical of urban environments, highlighting the influence of traffic conditions on emissions. The analysis revealed different behaviors among vehicle groups: emissions increased with speed for WM vehicles, which operate between the MRRJ and the mountainous regions, while they decreased for M vehicles, which deliver dry cargo within the MRRJ, mainly in the southern zone of Rio de Janeiro. The results reinforce the need for environmental policies aimed at fleet renewal, adoption of advanced emission control technologies, and promotion of more efficient driving practices. Future studies should investigate the relationship between operating conditions, speed variability and NOx emissions in different contexts.