Quantifying Molecular Hydrogen Emissions and an Industrial Leakage Rate for the South Coast Air Basin of California

The poorly understood atmospheric budget and distribution of molecular hydrogen (H₂) have invited further research since the discovery that emissions from a hydrogen-based economy could have negative impacts on the global climate system and stratospheric ozone. The burgeoning fuel cell electric vehicle industry in the South Coast Air Basin of California (SoCAB) presents an opportunity to observe and constrain urban anthropogenic H₂ emissions. This work presents the first H₂ emissions estimate for the SoCAB and calculates an upper limit for the current rate of leakage from production and distribution infrastructure within the region. A top-down method utilized whole air samples collected during the Student Airborne Research Program (SARP) onboard the NASA DC-8 research aircraft from 23-25 June 2015 to estimate H₂ emissions from combustion and non-combustion sources. H₂:carbon monoxide (CO) and H₂:carbon dioxide ratios from airborne observations were compared with experimentally established ratios from pure combustion sources and scaled with the well-constrained CO emissions inventory to yield H₂ emissions of 24.8 ± 3.6 Gg a^-1 (1σ) from combustion engines and 8.2 ± 0.5 Gg a^-1 from non-combustion sources. Total daily production of H₂ in the SoCAB was compared with the top-down results to estimate an upper limit leakage rate (5%) where all H₂ emissions not accounted for by incomplete combustion in engines were assumed to be emitted from production and distribution infrastructure. For bottom-up validation, the NOAA Hybrid Single Particle Lagrangian Integrated Trajectory dispersion model was run iteratively with all known stationary sources in attempt to vary leakage rates until the model output closely matched observations. While this investigation determined that H₂ emissions from non-combustion sources in the SoCAB are likely significant, more in-depth analysis is required to better predict the atmospheric implications of a hydrogen economy.