Impact of a future H₂-based road transportation sector on the composition and chemistry of the atmosphere – Part 1: Tropospheric composition and air quality

Abstract. Vehicles burning fossil fuel emit a number of substances that change the composition and chemistry of the atmosphere, and contribute to global air and water pollution and climate change. For example, nitrogen oxides and volatile organic compounds (VOCs) emitted as byproducts of fossil fuel combustion are key precursors to ground-level ozone and aerosol formation. In addition, on-road vehicles are major CO₂ emitters. In order to tackle these problems, molecular hydrogen (H₂) has been proposed as an energy carrier to substitute for fossil fuels in the future. However, before implementing any such strategy it is crucial to evaluate its potential impacts on air quality and climate. Here, we evaluate the impact of a future (2050) H₂-based road transportation sector on tropospheric chemistry and air quality for several possible growth and technology adoption scenarios. The growth scenarios are based on the high and low emissions Intergovernmental Panel on Climate Change Special Report on Emissions Scenarios, A1FI and B1, respectively. The technological adoption scenarios include H₂ fuel cell and H₂ internal combustion engine options. The impacts are evaluated with the Community Atmospheric Model Chemistry global chemistry transport model (CAM-Chem). Higher resolution simulations focusing on the contiguous United States are also carried out with the Community Multiscale Air Quality Modeling System (CMAQ) regional chemistry transport model. For all scenarios future air quality improves with the adoption of a H₂-based road transportation sector; however, the magnitude and type of improvement depend on the scenario. Model results show that the adoption of H₂ fuel cells would decrease tropospheric burdens of ozone (7%), CO (14%), NO_x (16%), soot (17%), sulfate aerosol (4%), and ammonium nitrate aerosol (12%) in the A1FI scenario, and would decrease those of ozone (5%), CO (4%), NO_x (11%), soot (7%), sulfate aerosol (4%), and ammonium nitrate aerosol (9%) in the B1 scenario. The adoption of H₂ internal combustion engines would decrease tropospheric burdens of ozone (1%), CO (18%), soot (17%), and sulfate aerosol (3%) in the A1FI scenario, and would decrease those of ozone (1%), CO (7%), soot (7%), and sulfate aerosol (3%) in the B1 scenario. In the future, people residing in the contiguous United States could expect to experience significantly fewer days of elevated levels of pollution if a H₂ fuel cell road transportation sector were to be adopted. Health benefits of transitioning to a H₂ economy for citizens in developing nations, like China and India, will be much more dramatic, particularly in megacities with severe, intensifying air-quality problems.