The impact of soil uptake on the global distribution of molecular hydrogen: Chemical transport model simulation
The global tropospheric distribution of molecular hydrogen (H-2) and its\nuptake by the soil are simulated using a model called CHemical AGCM\n(atmospheric general circulation model) for the Study of the Environment\nand Radiative forcing (CHASER), which incorporates a two-layered soil\ndiffusion/uptake process component. The simulated distribution of\ndeposition velocity over land is influenced by regional climate, and has\na global average of 3.3x10(-2) cm s(-1). In the region north of 30\ndegrees N, the amount of soil uptake shows a large seasonal variation\ncorresponding to change in biological activity due to soil temperature\nand change in diffusion suppression by snow cover. In the temperate and\nhumid regions in the mid- to low- latitudes, the uptake is mostly\ninfluenced by the soil air ratio, which controls the gas diffusivity in\nthe soil. In the semi-arid regions, water stress and high temperatures\ncontribute to the reduction of biological activity, as well as to the\nseasonal variation in the deposition velocity. A comparison with the\nobservations shows that the model reproduces both the distribution and\nseasonal variation of H-2 relatively well. The global burden and\ntropospheric lifetime of H-2 are 150 Tg and 2.0 yr, respectively. The\nseasonal variation in H-2 mixing ratios at the northern high latitudes\nis mainly controlled by a large seasonal change in the soil uptake. In\nthe Southern Hemisphere, seasonal change in net chemical production and\ninter-hemispheric transport are the dominant causes of the seasonal\ncycle, while large biomass burning contributes significantly to the\nseasonal variation in the tropics and subtropics. Both observations and\nthe model show large inter-annual variations, especially for the period\n1997-1998, associated with large biomass burning in the tropics and at\nNorthern Hemisphere high latitudes. The soil uptake shows relatively\nsmall inter-annual variability compared with the biomass burning signal.\nGiven that the thickness of biologically inactive layer plays an\nimportant role in the soil uptake of H-2, its value in the model is\nchosen to achieve agreement with the observed H-2 trends. Uncertainty of\nthe estimated soil uptake flux in the semi-arid region is still large,\nreflecting the discrepancy in the observed and modeled seasonal\nvariations.