Assessment of Air Pollutant Emissions from Road Transport in Thailand

This study aimed to estimate air pollution emissions from road transportation in Thailand, supporting policymakers in planning and implementing mitigation measures, especially for fine particulate matter (PM2.5) in the Bangkok Metropolitan Region (BMR), by using the Tier 3 method, which allows for detailed analysis and reflects the country’s specific conditions. Additionally, this study seeks to evaluate the impact of older vehicles and biofuel blends on particulate matter (PM) exhaust emissions. For the first time in Thailand, a high-resolution gridded emission inventory was created at a 1 km × 1 km scale, using Geographic Information Systems (GIS) and official traffic data. This spatially detailed inventory distributes PM emissions by vehicle type and road classification, enabling more accurate air quality assessments and control strategies. The results showed that in 2019, road transportation in Thailand emitted 72.2 megatons (Mt) of CO₂, 470 kilotons (kt) of NO_X, 888 kt of CO, 34.9 kt of NMVOC, 4.4 kt of NH₃, and 97.1 kt of NMVOC from gasoline evaporation. For particulate matter, road transportation produced 28.3 kt of PM2.5, with 21.7 kt (77%) coming from hot exhaust emissions. PM2.5 exhaust sources were identified as diesel vehicles, including light-duty and heavy-duty trucks, accounting for 8.9 kt (41%) and 8.0 kt (37%), respectively. Using the 2019 road-transport emission inventory as a baseline, older diesel vehicles primarily contribute to tailpipe particulate matter. Therefore, scrappage alone offers limited, short-term improvements if replacements continue to meet Euro 3/4 standards. The most significant and lasting reductions happen when Euro-5 fuel quality (≤10 ppm sulfur) is combined with rapid adoption of Euro-5/6 new vehicles and targeted deployment of zero-emission vehicles in light-duty trucks, urban buses, and heavy-duty trucks. Although sales of BEV passenger cars are increasing quickly, their overall system PM impact remains modest because gasoline-powered cars already emit minimal PM per kilometer under similar Euro standards. Policy timing is crucial: further delays in adopting Euro-5/6 could lock in higher fleet-average emissions.