Trace gases in the atmosphere

Forests can act as sources of some of the trace gases in the atmosphere, such as hydrocarbons, hydrogen sulfide, NO, and NH3. Forests have been identified as emitters of terpene hydrocarbons. In 1960, Went (10) estimated that hydrocarbon releases to the atmosphere were on the order of 108 tons per year. Later work by Rasmussen (11) suggested that the release of terpenes from forest systems is 2 x 10 tons of reactive materials per year on a global basis. This is several times the anthropogenic input. Yet, it is important to remember that forest emissions are much more widely dispersed and less concentrated than anthropogenic emissions. Table 8-2 shows terpene emissions from different types of forest systems in the United States. 

Tropospheric chemistry is strongly dependent on the concentration of the hydroxyl radical (OH), which reacts very quickly with most trace gases in the atmosphere. Owing to its short boundary layer lifetime ( 1 s), atmospheric concentrations of OH are highly variable and respond rapidly to changes in concentrations of sources and sinks. Photolysis of ozone, followed by reaction of the resulting excited state oxygen atom with water vapour, is the primary source of the OH radical in the clean troposphere ... 

Fourier Transform Infrared Analysis of Trace Gases in the Atmosphere... 

The principles behind these and other techniques used to measure a variety of trace gases in the atmosphere, including the criteria pollutants and free radicals such as N03, OH, H02, and ROz, are described in the following sections. In addition, typical tropospheric concentrations in regions from remote to urban areas are given. 

One motivation for studying CO2 equilibria is to understand the effect that trace gases in the atmosphere have on the acidity (pH) of rain. Acid rain has been a problem of national and international scope with major economic consequences. 

Reactions with hydroxyl radicals are considered one of the most efficient ways used by the atmosphere to remove natural and anthropogenic trace gases in the atmosphere [28,29]. Previous kinetics [27,30-32] and modeling studies [2, 33-35] have shown that the primary atmospheric sink of bromopropane is the reaction with OH and that it has an atmospheric lifetime of 10-16 days. Further studies have determined that the lifetime of short-lived species, such as... 

Tab. I - Some trace gases in the atmosphere and the estimated increase in global surface temperature for the indicated change in mixing ratio.

What has gone wrong This simple calculation tells us that gases in the atmosphere are not necessarily in equilibrium. This does not mean that atmospheric composition is especially unstable, but just that it is not governed by chemical equilibrium. Many trace gases in the atmosphere are in steady state. Steady state describes the delicate balance between the input and output of the gas to the atmosphere. The notion of a balance between the source of a gas to the atmosphere and sinks for that gas is an extremely important one. The situation is often written in terms of the equation ... 

Junge, C. (1963) A fr Chemistry and Radioactivity, Academic Press, New York. This classic text established air chemistry as a scientific discipline. It reviews what was known up to the lime on the behavior of aerosols and trace gases in the atmosphere. 

There are two ways to monitor the concentrations of trace gases in the atmosphere by FT-IR spectrometry. The first is to draw the atmosphere in the region of interest into a long-path gas cell, and the second is to measure the spectrum of the atmosphere in situ. The first approach, which is known as extractive monitoring [1], is covered in this section, and the second, known as open-path FT-IR spectrometry (OP/FT-IR) [2], is covered in Section 22.2. 

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