Methane, tropospheric mixing ratio

Dhar and Ram (50) found formaldehyde in rain water and estimated a tropospheric mixing ratio of 0.7 ppb, while Cauer (36) measured a mean value of 0.4 ppb. Lodge and Pate (160) obtained an average value of 1.1 ppb for the total aliphatic aldehydes in surface air in the tropics. Levy (152) proposed the formation of formaldehyde via the tropospheric oxidation of methane and calculated (155) an upper limit of 1 ppb for the mixing ratio, with an altitude profile for a summer midlatitude decreasing from 0.6 ppb at the ground to less than 0.01 ppb in the upper troposphere, where methane oxidation is very slow (154). 

One may conclude that approximately 600 Tg of methane are produced each year. Since the total atmospheric burden of methane is about 4900 Tg (corresponding to a mean tropospheric mixing ratio of about 1.75 ppmv), a global atmospheric lifetime of 8 years can be derived. 

Table 4-6. Recent Measurements of the Average Tropospheric Mixing Ratio m of Methane and the Ratio m /msfor the Mixing Ratios in the Northern and Southern Hemispheres...

Bainbridge and Heidt (8) estimated from the rate of decrease of the CH mixing ratio above the tropopause that at most 10% of the tropospheric CH was lost by transport into the stratosphere and suggested that a large tropospheric sink was necessary. Ehhalt and Heidt (53) found that the calculated sink for methane due to transport into the stratosphere was much too small. 

By comparison, the average CO mixing ratio in Earth s troposphere is —0.12 ppmv and it is produced from a variety of anthropogenic and biogenic sources such as fossil fuel combustion, biomass burning, and oxidation of methane and other hydrocarbons. Most of the CO in Earth s troposphere is destroyed by reaction with OH radicals, which are also important for the catalytic... 

Atmospheric chemistry is dominated by trace species, ranging in mixing ratios (mole fractions) from a few parts per million, for methane in the troposphere and ozone in the stratosphere, to hundredths of parts per trillion, or less, for highly reactive species such as the hydroxyl radical. It is also surprising that atmospheric condensed-phase material plays very important roles in atmospheric chemistry, since there is relatively so little of it. Atmospheric condensed-phase volume to gas-phase volume ratios range from about 3 x KT7 for tropospheric clouds to 3 x ICE14 for background stratospheric sulfate aerosol. 

Whether the methane oxidation cycle leads to a net production or consumption of ozone depends on the NO level. For a net production of 03 to exist, reaction (6), which propagates the chain, must compete effectively with reaction (15), which simply consumes an 03 molecule to regenerate N02. Reaction (6) competes with reaction (15) at tropospheric 03 levels, reaction (6) dominates over reaction (15) for NO mixing ratios... 

Bromine is potentially able to interact with stratospheric ozone in the same manner as chlorine (Wofsy et al., 1975). The catalytic cycle for bromine is expected to be quite efficient, because its reaction with methane is slower than that of Cl atoms in addition, the reaction of OH with HBr is faster than that of OH with HC1. The major bromine compound in the troposphere is methyl bromide, which has a natural origin and occurs with a mixing ratio of about 10 pptv (see Table 6-14). This seems small enough to neglect bromine to a first approximation. 

The various influences on stratospheric ozone caused by trace gases having their origin in the troposphere make the ozone layer susceptible to considerable perturbations, both natural and human. Currently of greatest interest are the effects of the chlorofluorocarbons, but there are others as well, such as variations in the mixing ratios of N20 and methane in the troposphere, or the rise of C02 in the atmosphere. The last constituent does not enter into chemical reactions, but it cools the stratosphere, thereby lowering the rates of important reactions such as that of OH with HCI. The various... 

Hydroxyl is unreactive toward the main atmospheric constituents N2, 02, H20, and C02, but reacts readily with many trace gases. Table 4-2 lists trace gases, their approximate mixing ratios, and rate coefficients for reaction with OH for three tropospheric conditions. The reactions with methane and... 

The stratosphere as a sink was first identified by Seiler and Junge (1969). The flux of CO into the stratosphere is caused by a decline of CO mixing ratios above the tropopause toward a steady-state level lower than that normally found in the upper troposphere (see Fig. 1-14). In the lower stratosphere, CO is produced from methane and other long-lived hydrocarbons, and it is consumed by reaction with OH as in the troposphere, but the rate of vertical mixing is much slower (Seiler and Warneck, 1972 Warneck et al., 1973). The flux of CO from the troposphere into the stratosphere can be derived from the observed gradient of the CO mixing ratio above the tropopause in a manner described in Section 4.3 for methane. The loss rate obtained, llOTg/yr, is small compared with that for the reaction of CO with OH radicals. 

Fig. 10-9. Flux diagram for sulfur in the unperturbed marine atmosphere. Fluxes are given in units of p.gS/m2day. Numbers in boxes indicate column densities in units of p.gS/m2. DMS, Dimethyl sulfide MSA, methane sulfonic acid (associated with the aerosol). The mixing ratio of S02 is 60 ng S/m3, independent of altitude. The mixing ratio of SOis 280 ng S/m3 in the boundary layer and 80 ng S/m3 in the free troposphere. Contrary to the model of Kritz (1982), the fluxes are confined to the boundary layer. There exists no significant net flux into or out of the free troposphere. The dry deposition velocity for S02 is 5mm/s.

Once the importance of DMS to the global sulfur cycle was established, numerous measurements of DMS concentrations in the marine atmosphere have been conducted. The average DMS mixing ratio in the marine boundary layer (MBL) is in the range of 80-1 lOppt but can reach values as high as 1 ppb over entrophic (e.g., coastal, upwelling) waters. DMS mixing ratios fall rapidly with altitude to a few parts per trillion in the free troposphere. After transfer across the air-sea interface into the atmosphere, DMS reacts predominantly with the hydroxyl radical and also with the nitrate (N03) radical. Oxidation of DMS is the exclusive source of methane sulfonic acid (MSA) in the atmosphere, and the dominant source of S02 in the marine atmosphere. We will return to the atmospheric chemistry of DMS in Chapter 6. 

Whereas the troposphere contains abundant water vapor, little H20 makes it to the stratosphere the low temperatures at the tropopause lead to an effective freezing out of water before it can be transported up (a cold trap at the tropopause). Mixing ratios of H20 in the stratosphere do not exceed approximately 5-6 ppm. In fact, about half of this water vapor in the stratosphere actually results from the oxidation of methane that has leaked into the stratosphere from the troposphere. Between 20 and 50 km the total rate of... 

In this model, for every methane molecule which reacts, the sequence leads to 4 ozone and 2 hydroxyl radicals, extra. Formation of ozone in the lower troposphere is therefore catalysed by photochemical oxidation of organic molecules, but it does require comparatively high levels of NO (mixing ratio > 5 — 10 x 10 ) to be present. If it goes to completion, OH can react further with CO to make CO2 thus completing the oxidation of methane (Scheme 5.2). At low NO levels, the net reaction is the destruction of ozone via the reaction with CO [Scheme 5.2b]. 

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