A new method to assess the climate effect of mitigation strategies for road traffic: The fast chemistry-climate response model TransClim

Emissions of road traffic crucially influence Earth’s climate. The vehicle fleet emits not only carbon dioxide (CO2), but also nitrogen oxides (NOx), volatile organic compounds (VOC) and carbon monoxide (CO) which produce ozone (O3) and destroy methane (CH4) in the troposphere. As the demand of mobility is expected to further increase in future, a reduction of the climate effect from road traffic emissions is indispensable. Therefore, it is essential to assess the climate impact of emission changes caused by technological trends and mitigation strategies for road traffic. Several studies have already quantified the impact of road traffic emissions on climate. But climate simulations with complex chemistry climate models are still computational expensive hampering the assessment of many road traffic emission scenarios. Consequently, an efficient method for quantifying the climate impact and contribution of mitigation options is required. Within the scope of this thesis, a unique chemistry-climate response model called TransClim (Modelling the effect of surface Transportation on Climate) was developed. Using an efficient interpolation algorithm, it assesses the impact and the contribution of road traffic emission scenarios on O3 and CH4 concentration as well as their corresponding radiative forcings. Comparing the results delivered by TransClim with simulations of the complex global chemistry climate model EMAC reveals very low deviations (0.02 – 6 %). To determine not only the impact but also the contribution of road traffic emissions to O3, OH and CH4 in TransClim, a so-called tagging method is applied. It attributes the concentrations of trace gases to emission sources such as road traffic. This thesis presents an improved tagging method for the short-lived species OH and HO2 as well as a new method for CH4. Within the scope of this thesis, TransClim enabled to assess the climate effect of two scientific questions: first, the effect of three prospective mitigation options of German road traffic and second, two scenarios describing the cases that European vehicles use fuel blends containing a low and a high proportion of biofuels. Summing up, TransClim offers a new method to quickly assess the climate impact and the contribution of mitigation strategies for road traffic in a sufficiently accurate manner. As TransClim simulates about 6000 times faster than a complex chemistry climate model, it enables to quantify the effect of many emission scenarios in different regions.