CLIMATE CO-BENEFITS OF REDUCING DIESEL VEHICLE EMISSIONS IN THAILAND

Abstract:

Particulate matter (PM) and ozone are climate forcing agents. Black carbon, in particular, is very strongly absorbing, and each kilogram warms the atmosphere hundreds of times more than a kilogram of carbon dioxide, despite its short atmospheric lifetime. Measures that improve urban air quality by reducing emissions of particulate matter and ozone also have climatic impacts. We quantify the radiative impact of on-road diesel vehicles in Thailand by combining measured emission data, country information on vehicle fleet and driving conditions, and a global climate model.

We develop emission estimates for input to climate models by leveraging a vehicle characterization project implemented by the World Bank (DIESEL). We added our own measurements of climate-relevant properties for the particles: chemical composition, light absorption and water activity. We estimate emissions of particulate matter, nitrogen oxide, carbon monoxide, and hydrocarbons for vehicles in Thailand (Winijkul et al, submitted to BAQ). We find that diesel emissions are about 40% elemental carbon, and that this fraction is not significantly different for high-emitting vehicles. Average emission factors for normal vehicles are 2.2 g/kg fuel. The absorption of the elemental carbon is about 40% higher than that assumed in radiative-transfer models. Despite the high-sulfur diesel used in Thailand, the particles are not hygroscopic.

We demonstrate how reducing diesel emissions would affect climate, using the Community Atmosphere Model housed at the National Center for Atmospheric Research. We present four model runs: (1) with all emissions [the reference case]; (2) with particulate diesel emissions from Thailand halved; (3) with gaseous diesel emissions from Thailand halved; and (4) with both PM and gaseous emissions halved. The resulting change in radiative forcing from these short-lived pollutants is small compared to the global average from all emissions, but it is comparable to the magnitude of other single actions to reduce climate forcing.