Measurements of C2-C5 Hydrocarbons Over the North Atlantic

Latitudinal distributions of ethane, propane, propene, and acetylene in air over the Atlantic from 48°N to 4°S are reported. In addition, measurements of saturated and unsaturated hydrocarbons (C2-C5) at low latitudes and from 55°N to 80°N are presented. The mixing ratios of ethane, propane, and acetylene were found to vary systematically between a few tenths of a ppb and a few ppb. The alkene and higher alkane (C4 and C5) mixing ratios were found to be in the ppt range. The data are discussed with respect to the source distributions and atmospheric lifetimes of the different hydrocarbons

The dependence of soil H2 uptake on temperature and moisture: a reanalysis of laboratory data

In the past two types of laboratory experiments have been employed to determine the dependence of H2 uptake by soils on temperature and moisture: Head space and flow experiments. The former actually measure the rate constant of the H2 removal from the head space, kH, the latter the uptake rate of H2, UH2, both caused by a given volume of soil. From an analytical solution of the diffusion equation in the soil we derive a mathematical relation between kH and ks, the desired uptake rate constant of H2 in soil. Another equation relates UH2 with ks. Both types of experiments actually determine the product of ks with Θa, the air-filled pore volume fraction. ks.Θa for eolian sand and loess loam show zero uptake at very low and high moisture contents and a well defined maximum in between. Unlike soil moisture which also acts on the soil properties, the soil temperature, T, acts essentially on the enzyme activity only. Thus ks(T) is directly proportional to kH(T) or UH2(T) and the data of all experiments can be superimposed by scaling. The resulting average ks(T) shows a broad maximum around 30◦C with zero uptake below −20◦C and above 80◦C

The tropospheric cycle of H2: A critical review

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 TgH2 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 > 1 Tg H2 yr−1. Rough estimates for these are also given

DETERMINATION OF C2-C5 HYDROCARBONS IN THE ATMOSPHERE AT LOW PARTS PER 109 TO HIGH PARTS PER 1012 LEVELS

By far the most abundant hydrocarbon in unpolluted air is methane (mixing ratio cu. 1.6 ppm). The mixing ratios of other hydrocarbons are typically in the low parts per lo9 (ppb) and parts per 10” (ppt) ranges. Although methane is several orders of magnitude more abundant in clean air, it is conceivable that other hydrocarbons are still of considerable importance to clean air photochemistry, because their reaction with hydroxyl radicals proceeds much faster than that of methane. Owing to this high reactivity of many of the light non-methane hydrocarbons (NMHC), mixing ratios of NMHC as low as a few ppb or several ppt can have a considerable influence on the photochemistry of unpolluted air. For this reason a gas chromatographic method has been developed that permits the determination of several C2-C, hydrocarbons with detection limits of a few ppt from grab samples of 0.5-Z dm3 (STP). The samples are collected in evacuated 2-1 stainless-steel containers with metal bellows-sealed stainless-steel valves. These sample collection and storage cans are specially pre-treated and cleaned to avoid changes in sample composition during transport of the samples to the laboratory. In the laboratory the samples are analysed by enrichment of the hydrocarbons on a packed pre-column at sub-ambient temperatures (L’LI. - 35°C) and subsequent separation on a 7 m x 0.8 mm I.D. packed column (Spherosil XOB 075). A flame-ionization detector is used. This method allowed survey measurements on a global scale of C,-C, hydrocarbons. which gave an estimate of the contributions of light hydrocarbons to atmospheric photochemical reactions