Nitrogen dioxide, tropospheric water

Only a small percentage of the chlorine released by photolysis of CFCs is present in the active forms as Cl or CIO, however. Most of it is bound up in reservoir compounds such as hydrogen chloride and chlorine nitrate, formed respectively by hydrogen abstraction (equation 10) from methane and addition (equation 11) to nitrogen dioxide. Slow transport of these reservoir species across the tropopause, followed by dissolution in tropospheric water and subsequent rain-out, provide sink processes for stratospheric chlorine. 

The temperature and density structure of the troposphere, along with the concentrations of major constituents, are well documented and altitude profiles have been measured over a wide range of seasons and latitudes for the minor species water, carbon dioxide, and ozone. A few profiles are available for carbon monoxide, nitrous oxide, methane, and molecular hydrogen, while only surface or low-altitude measurements have been made for nitric oxide, nitrogen dioxide, ammonia, sulfur dioxide, hydrogen sulfide, and nonmethane hydrocarbons. No direct measurements of nitric acid and formaldehyde are available, though indirect information does exist. The concentrations of a number of other important species, such as peroxides and oxy and peroxy radicals, have never been determined. Therefore, while considerable information concerning trace constituent concentrations is available, the picture is far from complete. 

At our level in the troposphere, air is a mixture of gases of uniform composition, except for water vapor, which composes l%-3% of the atmosphere by volume, and some of the trace gases, such as pollutant sulfur dioxide. On a dry basis, air is 78.1% (by volume) N2, 21.0% O2, 0.9% argon, and 0.04% carbon dioxide. Trace gases at levels below 0.002% in air include ammonia, carbon monoxide, helium, hydrogen, krypton, methane, neon, nitrogen dioxide, nitrous oxide, ozone, sulfur dioxide, and xenon. 

Atmospheric measurements are also challenging because they must deal with low to extremely low concentrations of trace chemical species. The major components (>99.999%) of the lowest portions of the atmosphere (the troposphere up to 10 km in altitude and the stratosphere between 10 and 50 km) are molecular nitrogen, molecular oxygen, argon, water vapor, and carbon dioxide. Chemists will recognize that all of these species are very stable, strongly bonded molecules or atoms that are essentially inert gases at normal atmospheric temperatures (190-310 K). Indeed, without solar photons to break up selected molecules, atmospheric chemistry would be very dull indeed. 

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