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Methane, Nonmethane Hydrocarbons, Alkyl Nitrates, and Chlorinated Carbon Compounds including 3 Chlorofluorocarbons (CFC-11, CFC-12, and CFC-113) in Whole-air Samples

Graphics    Digital Data

Investigator

Donald Blake
Department of Chemistry,
University of California Irvine,
California, 92697 USA

Period of Record

April 1979 – December 2004

Methods

Whole-air samples are collected in conditioned, evacuated, 2-L stainless steel canisters; each canister is filled to ambient pressure over a period of about 1 minute (approximately 20 seconds to 2 minutes). These canisters are returned to the University of California at Irvine for chromatographic analysis.

Analysis for methane includes gas chromatography with flame ionization, as discussed in Simpson, et al.

Methane concentrations are reported for dry air, and are made relative to a primary standard purchased from the Matheson Gas Company in 1977 and to a National Bureau of Standards standard purchased in August 1982. Analytical precision is better than 2 parts per billion by volume. For additional details for methane measurements, and some discussion about methane trends, see Simpson, et al.

Analyses for the various other gases is discussed in Colman, et al.. Samples were cooled to allow the more volatile components (e.g., argon, nitrogen, and oxygen) to be pumped away; the less volatile components were then revolatilized by immersing the sample loop in hot (80°C) water and subsequently flushed into a helium carrier flow. After chromatographic analysis, air from each sample was subjected to electron-capture analysis to distinguish between organic nitrates and halocarbons, flame ionization which is sensitive to hydrocarbons, and mass spectroscopy for unambiguous compound identification and selected ion monitoring. Signal from electron-capture and flame-ionization detectors was routed to an integrator which produced hard copies of the analog responses, and signals from all systems were routed to computers for further analysis. Each resulting chromatogram was manually modified, and each peak was individually checked, to assure that the large range of compound abundances and potential coelutors did not adversely affect quantification.

Multiple standards were employed; working standards were run every 2 hours and calibrated standards were run twice daily. Standard gases used are discussed in Colman, et al. For the compounds included in this data set, precision can be taken as within 3%, and, in most cases, to within 2%. For specific details on measurements of each gas, see Colman, et al.

Trends

Concentrations of each substance for all locations are plotted vs time to show general temporal trends. The latitudinal distribution of concentrations, for all dates, are also plotted. The sampling locations are varied so a general picture of the global changes is given. The AGAGE site gives data about many of the same substances at a few latitudinally distributed sites where concentrations are measured much more frequently so as to provide continuous time series.

Methane concentrations had been rising prior to the mid 1990's when they began to level off. A tendency for reduced concentrations during the summer months in both hemispheres is related to increased production of hydroxyl (OH) radicals under conditions of extended sunlight and increased humidity, and associated destruction of methane and several nonmethane hydrocarbons that are subject to hydroxyl attack. The graphs of concentrations vs. latitude show that methane, as well as several nonmethane hydrocarbons, have their highest concentrations at mid-to-high latitudes in the Northern-Hemisphere. This largely reflects their source regions. However some substances (e.g., CFC-12) were largely produced in the Northern Hemisphere but have a rather flat distribution over all latitudes. This reflects discontinued emissions, so that the source regions are no longer clearly evident while the remaining amounts have been evenly mixed throughout the atmosphere.

Concentrations of CFCs 11 and 113 have declined in recent years, in response to the Montreal Protocol of the late 1980's. CFC-12 is also expected to decline, but the decline is expected to be delayed due to: (1) a longer atmospheric lifetime than CFC-11 or CFC-113, (2) its use in long-lasting appliances such as home refrigerators, and, possibly (3) particularly extensive stockpiling at the global scale, due to a perceived lack of suitable replacement at the time production decreases were being mandated. It is also difficult to find suitable replacements for some other substances. For example, Halon 1211 is a particularly effective fire extinguishing agent that has remained in use in many countries. Therefore, Halon 1211 concentrations have not declined. A large increase in spatial variance around 2001 may be at least partly due to a leak in one of the system lines. Checking with principal investigators to identify such problems is always advised before drawing conclusions from these data.

Since about 1990, trends in methyl chloroform and carbon tetrachloride have been sharply downward, reflecting the phasing out of these substances due to their toxic effects. Methyl chloroform has a much shorter atmospheric lifetime than carbon tetrachloride, so methyl chloroform has declined to about 15% of its pre-1990 levels and is currently in the 20-25 parts per trillion, by volume (pptv) range. Tetrachloroethene is a relatively short-lived compound that has been used as a dry cleaner and degreaser. It has also shown a recent decline in concentration. Chloroform data only go back to 1996, so effects of any earlier declines that may have occurred are not evident.

The alkyl nitrates (methyl, ethyl, and propyl nitrates) have a variety of natural and anthropogenic sources. Concentrations of these compounds were decreasing slightly until late 2003, when increases occurred, followed by more recent decreases. The cause of the 2003 increases has not been determined, but the graph of latitudinal distribution and the locations of the anomalously high concentrations suggest natural sources.

The alkanes (ethane, butane, and propane) and ethyne (acetylene) are used as fuel in torches, lighters and gas stoves. In general, concentrations of these substances have not changed appreciably since 1996. The atmospheric lifetimes of these substances vary throughout the year, being shortest in summer when oxidative reactions with hydroxyl radicals (OH) are particularly frequent due to the abundance OH at that time of year. In the table below, these lifetimes are simply given as "less than one year" due to the low likelihood of survival through a summer, regardless of time of emission.

¹CDIAC thanks Don Blake, University of California, Irvine, and Ray Wang, Georgia Institute of Technology for their help in updating the information in this table.

²8-9 years according to the usual definition of atmospheric lifetimes. However, in the literature, this figure is sometimes adjusted to account for some feedbacks involving global warming, and given as 12 years.

Reference

Blake, N. J., D. R. Blake, B. C. Sive, A. S. Katzenstein, S. Meinardi, O. W., Wingenter, E. L., Atlas F., Flocke, B. A., Ridley, and F. S. Rowland. 2003. The seasonal evolution of NMHC's and light alkyl nitrates at middle to high northern latitudes during TOPSE. J. Geophys. Res. 108 (D4), pp. 7-1 to 7-16. Special Section: Tropospheric Ozone Production about the Spring Equinox (TOPSE).

Citation

Please cite this Web page as:

D. Blake. 2005. Methane, Nonmethane Hydrocarbons, Alkyl Nitrates, and Chlorinated Carbon Compounds including 3 Chlorofluorocarbons (CFC-11, CFC-12, and CFC-113) in Whole-air Samples. In Trends: A Compendium of Data on Global Change. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge Tenn., U.S.A.

6/2005.