Global atmospheric hydrogen chemistry and source-sink budget equilibrium simulation with the EMAC v2.55 model

Abstract

In this study, we use an earth system model with detailed atmospheric chemistry (EMAC v2.55.2) to undertake simulations of hydrogen (H2) atmospheric dynamics. Extensive global equilibrium simulations were performed with a horizontal resolution of 1.9°. The results of this simulation are compared with observational data from 56 stations in the National Oceanic and Atmospheric Administration (NOAA) Global Monitoring Laboratory (GML) Carbon Cycle Cooperative Global Air Sampling Network. We introduced H2 sources and sinks, the latter inclusive of a soil uptake scheme, that accounts for bacterial consumption. The model thus accounts for detailed H2 and methane (CH4) flux boundary conditions. Results from the EMAC model are accurate and predict the magnitude, amplitude and interhemispheric seasonality of the annual H2 cycle at most observational stations. Time series comparison of EMAC and observational data produces Pearson correlation coefficients (r) in excess of 0.9 at eight remote stations located in polar regions and on high mid-latitude islands. A further 23 stations yielded correlation coefficients between 0.7–0.9, predominantly located in remote marine stations across all latitudes and also in polar regions. The quality of model predictions (r<0.5, 9 stations) is reduced in anthropogenically highly polluted stations in east Asia and the Mediterranean region and stations impacted by peat fire emissions in Indonesia, as local and incidental emissions are difficult to capture. Our H2 budget corroborates bottom-up estimates in the literature in terms of source and sink strengths and overall atmospheric burden. By simulating hydroxyl radicals (OH) in the atmosphere leading to a CH4 lifetime in agreement with observationally constrained estimates, we show that the EMAC model is a capable tool for undertaking high accuracy simulation of H2 at global scale. Future research applications could target the impact of potentially significant natural and anthropogenic H2 sources on air quality and climate, reducing uncertainties in the H2 soil sink and impacts of H2 release on the future oxidising capacity of the atmosphere.