A three-dimensional model of molecular hydrogen in the troposphere

The global distribution and budget of atmospheric molecular hydrogen (H-2)is simulated with a global Chemistry-Transport Model (CTM). Surface emissions include technological sources (industry, transportation and other fossil fuel combustion processes), biomass burning, nitrogen fixation in soils, and oceanic activity and totals 39 Tg/yr. The photochemical production (31 Tg/yr) from formaldehyde photolysis accounts for about 45% of the total source of H-2. Soil uptake (55 Tg/yr) represents a major loss process for H-2 and contributes for 80% to the total destruction. H-2 oxidation by OH in the troposphere contributes the remainder. The global burden of H-2 in the atmosphere is 136 Tg. Its overall lifetime in the atmosphere is 1.9 years. H-2 is rather well-mixed in the free troposphere. However, its distribution shows a significant seasonal variation in the lower troposphere where soil uptake dominates. This loss process shows a strong temporal variability and is maximum over the northern hemisphere landmass during summer. Strong vertical gradients result from this surface uptake. In these regions, H-2 varies by more than 30% between the maximum mixing ratio in winter and the summer minimum. Our results stress the important role played by the tropics in the budget of H-2. In these regions a strong seasonal cycle is also predicted due to the annual variation in biomass burning emissions, soil uptake, and rapid transport by convection of H-2 depleted air masses from the boundary layer to the upper troposphere. A comparison with the observed H-2 distribution allows to test some of the model predictions. Good agreement is found for the global burden and the annually averaged latitudinal gradient in the southern hemisphere and the tropics. A detailed comparison of the seasonal cycles of H-2 in surface air indicates that the use of the net primary productivity to prescribe the seasonal and geographical pattern of soil uptake in the model leads to an underestimate of the deposition velocity during winter and spring over the continents in the northern hemisphere.