Modelling the global tropospheric molecular hydrogen cycle

Abstract

Would urban air quality and climate improve if we replaced the fossil fuels by molecular hydrogen (H2) as an energy carrier? A quantitative answer to this question requires a thorough understanding of the current role of H2 in the Earth’s atmosphere. On its own, H2 does not impact climate, as for example carbon dioxide or methane. However, increasing levels of H2 in the stratosphere can lead to increased ozone loss due to the formation of polar stratospheric clouds. Additionally, the atmospheric lifetime of methane could increase because both H2 and methane are removed by photochemical oxidation with the hydroxyl radical. Consequently, the lifetime of the strong greenhouse gas methane could be prolonged. During the last two decades, more and more experimental data have become available to put tighter constraints on the different sources and sinks that contribute to the global H2 cycle. However, the main removal process, dry deposition due to microbial/enzymatic decomposition of H2 in the soils, still has a rather large uncertainty between 40-99 Tg/yr globally. This is a highly uncertain number compared to the estimated overall amount of 136-166 Tg present in the troposphere. The photochemical removal of H2 from the atmosphere is estimated at 14-24 Tg/yr. Together with the estimates for the burden and dry deposition, this implies a tropospheric lifetime of H2 between 1.1-3.1 years. The atmospheric H2 is replenished by emissions from the Earth’s surfaces due to fossil fuel burning (5-25 Tg/yr), biomass burning (7-21 Tg/yr) and nitrogen fixation processes in the oceans (1-11 Tg/yr) and soils (0-11 Tg/yr). H2 is photochemically produced from methane (15-21 Tg/yr) and non-methane hydrocarbons (10-25 Tg/yr) in the atmosphere. These uncertainties suggest that at present, the global hydrogen cycle is poorly understood. However, this statement would do little justice to the scientific quality of most studies so far. The main purpose of the research in this thesis is to show that the global tropospheric budget of H2 can be constrained quite well with available measurements. The study starts with the derivation of a full hydrogen isotope chemistry scheme to use the measured deuterium content in atmospheric H2 as an additional constraint for the global budget. This new chemistry scheme is subsequently evaluated and the most important parameters in the photochemistry are identified. A condensed version of the new chemistry scheme is implemented in the global TM5 model. The new model results are verified using available measurements of H2 mixing ratios and isotopic compositions from two global flask sampling networks and the EuroHydros network. Finally, a new tropospheric budget is derived for H2. The tropospheric burden is estimated at 165±8 Tg and the removal of H2 by deposition and photochemical oxidation are estimated at 53±4 and 23±2 Tg/yr, respectively. The main (photochemical) source is estimated at 37±4 Tg/yr. From these numbers, a tropospheric lifetime of 2.2±0.2 yr for H2 is derived. These new ranges of uncertainty allow for a much more accurate evaluation of the impact of future increases in H2 emissions on air quality and climate.