Search

Measurements of the CH4 concentration over the North Atlantic and the Pacific are presented. The Pacific data indicate a difference of about 0.1 ppmv between Northern and Southern Hemispheres. From this difference and a plausible distribution of CH4 sources a tropospheric CH4 lifetime between 3.5 and 11 years is estimated.DOI: 10.1111/j.2153-3490.1978.tb00830.x

The literature on the distribution, budget and isotope content of molecular hydrogen (H2) in the troposphere is critically reviewed. The global distribution of H2 is reasonably well established and is relatively uniform. The surface measurements exhibit a weak latitudinal gradient with 3% higher concentrations in the Southern Hemisphere and seasonal variations that maximize in arctic latitudes and the interior of continents with peak-to-peak amplitudes up to 10%. There is no evidence for a continuous long-term trend, but older data suggest a reversal of the interhemispheric gradient in the late 1970s, and an increase in the deuterium content of H2 in the Northern Hemisphere from 80‰ standard mean ocean water (SMOW) in the 1970s to 130‰ today. The current budget analyses can be divided in two classes: bottom up, in which the source and sink terms are estimated separately based on emission factors and turnovers of precursors and on global integration of regional loss rates, respectively. That category includes the analyses by 3-D models and furnishes tropospheric turnovers around 75 Tg H2 yr-1. The other approach, referred to as top down, relies on inverse modelling or analysis of the deuterium budget of tropospheric H2. These provide a global turnover of about 105 Tg H2 yr-1. The difference is due to a much larger sink strength by soil uptake and a much larger H2 production from the photochemical oxidation of volatile organic compounds (VOC) in the case of the top down approaches. The balance of evidence seems to favour the lower estimates—mainly due to the constraint placed by the global CO budget on the H2 production from VOC. An update of the major source and sink terms yields: fossil fuel use 11 ± 4 Tg H2 yr-1; biomass burning (including bio-fuel) 15 ± 6 Tg H2 yr-1; nitrogen fixation (ocean) 6 ± 3 Tg H2 yr-1; nitrogen fixation (land) 3 ± 2 Tg H2 yr-1; photochemical production from CH4 23 ± 8 Tg H2 yr-1 and photochemical production from other VOC 18 ± 7 Tg H2 yr-1. The loss through reaction of H2 with OH is 19 ± 5 Tg H2 yr-1, and soil uptake 60+30−20 Tg H2 yr-1. All these rates are well within the ranges of the corresponding bottom up estimates in the literature. The total loss of 79 Tg H2 yr-1 combined with a tropospheric burden of 155 Tg H2 yields a tropospheric H2 lifetime of 2 yr. Besides these major sources of H2, there are a number of minor ones with source strengths

International audience; 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.