Understanding the dynamics of global change requires an examination of trace gases other than CO2 and CH4 in the atmosphere and an analysis of the role played by aerosols suspended in the atmosphere.
Although less abundant than either CO2 or CH4, a number of so-called minor atmospheric trace gases are also radiatively active; that is, they are able to perturb the radiative energy balance of the earth-atmosphere system. Thus, they are potentially important contributors to global climate change. As the release of chemicals into the atmosphere from anthropogenic sources has grown over the last 60 years, atmospheric concentrations of nearly all of the radiatively active gases ("greenhouse gases") have increased. Extrapolations of current trends in the atmospheric concentrations, along with estimates of their relative abilities to alter the global energy balance, suggest that the collective contribution of the minor trace gases to any future global warming may be significant. In addition, some trace gases notably the chlorofluorocarbons (CFCs) and the halons rise into the stratosphere and then decompose, releasing halogen atoms that cause chain-reaction destruction of ozone (O3). Stratospheric O3, through its absorption of high-energy ultraviolet radiation from the sun, provides a protective shielding for the living organisms (including man) that inhabit the Earth's biosphere. Not surprisingly, the unprecedented seasonal reductions of stratospheric O3 in the Antarctic and more recently in the Arctic have captured the attention of the news media and the general public.
This chapter presents data for five man-made halocarbons: CCl3F (CFC-11), a propellant in aerosol sprays and a blowing agent in flexible and rigid foam products; CCl2F2 (CFC-12), an aerosol propellant and a common cooling agent in refrigerators; CHClF2 (HCFC-22), a refrigerant in home air-conditioners; and the halons CBrF3 (H-1301) and CBrClF2 (H-1211), the active agents in chemical fire extinguishers. In addition, data are presented for nitrous oxide (N2O), a gas whose atmospheric origin is not fully understood but may result from a combination of human influences, including groundwater pollution, use of nitrogen fertilizers, combustion, and deforestation. These six gases were selected because they are radiatively important and they are potential depletors of stratospheric O3. Furthermore, high-quality measurements of their atmospheric concentrations during several years are available from globally distributed sites. Readers familiar with Trends '91 may note that the data on atmospheric trace gases have been substantially increased in Trends: additional species (HCFC-22, H-1301, and H-1211) are presented, and additional monitoring sites are included.
Some trace gases have not been included in this compilation although they are potentially important in terms of radiative activity and ability to deplete stratospheric O3. Examples include CFC-113 (CCl2FCClF2), a cleaning solvent in the electronics industry; methyl- chloroform (CH3CCl3), an industrial solvent; and tropospheric O3, which results from photochemical action on other pollutants such as the oxides of nitrogen (NOx). At present, data on the atmospheric concentrations of these gases are limited, or the gases are thought to have less impact than those presented in this chapter.
Efforts to accurately estimate human influence on the Earth's radiative energy balance must address the effects of atmospheric aerosols, very small solid particles (from ~10-3 to >102 µm in radius) suspended in the troposphere or stratosphere. Aerosols are generated by natural processes, including volcanoes and windborne transit of soil or sea salt, and by human industrial activities. The latter consist primarily of fossil fuel combustion and metal smelting, which produce gases containing sulfur, carbon, and nitrogen. Chemical reactions in the atmosphere convert these gases to particulate matter, chiefly sulfate (SO42-) compounds and elemental carbon. Atmospheric aerosols can absorb and scatter solar radiation, and they can act as condensation nuclei in cloud formation. Recent studies suggest that anthropogenic aerosols produce a cooling effect on the earth-atmosphere system of perhaps the same order of magnitude as the warming effect produced by anthropogenic greenhouse gases.
The presentation of atmospheric aerosol data in this chapter is a new addition to the Trends series. Data are presented for two variables atmospheric solar transmission and aerosol optical depth each of which is measured over a broad range of the solar spectrum. These variables were selected because they are indices of the overall loss of solar radiation at the Earth's surface due to absorption and scattering by atmospheric aerosols. Furthermore, data for these two variables are available in continuous records of long duration from one or more sites located relatively far from significant sources of pollution.
Researchers have also recorded high-quality measurements for other aerosol-related variables, including cloud condensation nuclei concentration, aerosol scattering extinction coefficients (measured at a number of discrete wavelengths), and lidar backscattering. At present, however, the data records for these variables are of limited duration or are of relatively specialized use, so they are not included in this report. It is our hope to be able to present some of these data in future editions of Trends.
The following pages provide data for N2O concentrations found in ice cores; monthly averages for global and hemispheric concentrations of CFC-11 and CFC-12, as derived from a globally distributed monitoring network; monthly averages for atmospheric concentrations of several trace gases (selected from among the species CFC-11, CFC-12, HCFC-22, H-1301, H-1211, and N2O), measured at a number of individual monitoring sites (see site location figure below); estimates of annual atmospheric releases of CFC-11 and CFC-12; atmospheric solar transmission measurements obtained at Mauna Loa (Hawaii, U.S.A.); and aerosol optical depth anomalies recorded at several globally distributed sites. The atmospheric concentration data were obtained from two sources: the Atmospheric Lifetime Experiment/Global Atmospheric Gases Experiment (ALE/GAGE) and the National Oceanic and Atmospheric Administration Climate Monitoring and Diagnostics Laboratory's (NOAA/CMDL's) Halocarbon Flask Sampling Program. The figure on atmospheric concentrations for the past 3100 years (shown below) presents three representative N2O records: the Byrd ice core, showing atmospheric concentrations during the period 3100 150 years BP; the Law Dome BHD ice core, showing atmospheric concentrations during the period 1520 1966; and the ALE/GAGE record from Barbados, a site with a continuous record that is representative of the trends observed at other monitoring sites. A comparison of these records reveals a significant increase in atmospheric N2O concentrations during the past 100 years.
Readers may note that two apparently different systems of units have been used in presenting the atmospheric trace gas data in this chapter. For most of the modern atmospheric trace gas data provided by NOAA/CMDL and ALE/GAGE, values are given as mixing ratios in parts per trillion (1 × 1012); for N2O, units are in parts per billion (1 × 109). For data on N2O in ice cores, however, levels are presented as concentrations in parts per billion by volume. These differences in unit designations reflect the preferences of the researchers who have contributed their respective data sets for inclusion in Trends. In the context of atmospheric trace gases, concentration in parts per billion by volume refers to the number of volumes of trace gas per billion volumes of sample. In this same context, mixing ratio in parts per billion is derived by dividing the number of moles of trace gas by the total number of moles in the sample and then multiplying the quotient by 1 billion. Assuming that the volume of a gas is proportional to the number of moles contained within the volume (this assumption should be valid for trace gases in air under the conditions that atmospheric measurements are routinely carried out), we can expect that trace gas concentrations should be equivalent to trace gas mixing ratios. For all practical applications, therefore, users of Trends should consider the terms concentration and mixing ratio to be interchangeable.
Readers should credit the principal investigators and their organizations listed at the beginning of each Methods section when referring to the data in Trends. To facilitate correct referencing, each section provides a full citation by which contributions should be referenced. In addition, readers are encouraged to contact CDIAC before applying the data in specific model exercises or research exercises. Some of the data are preliminary and may be subject to change. All of the data presented here are available in digitized form from CDIAC at no cost.
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Date created 12/09/96 (jaw)