Latitudinal Tropospheric Concentration Distribution of Selected Halocarbons and Hydrocarbons
Donald R. Blake and F. Sherwood Rowland
Department of Chemistry, University of California, Irvine, 92697
Measurements of the distribution of selected halocarbons, hydrocarbons, and carbon monoxide were carried out on tropospheric air samples collected at the surface in remote locations over 2-week periods every 3 months, covering the latitudinal range from 71ºN (Barrow, Alaska) to 47ºS (Bluff, New Zealand). Results for CCl4, CCl3F, CCl2F2, CH3CCl3, and CH4 form part of a continuing set of data from January 1978. Two halons, CBrF3 and CBrClF2, and methyl bromide (CH3Br), which were added to our regular analysis in 1991, are the primary tropospheric compounds supporting increasing stratospheric bromine concentrations. Methyl bromide alone represents half of the stratospheric bromine burden, but its atmospheric chemistry is particularly complicated because of its multiple reported sources (oceanic, biomass burning, anthropogenic) and sinks (hydroxyl radical (HO), oceanic, soil), about which relatively little quantitative data are available. Numerous nonmethane hydrocarbons (NMHCs) have also been measured since the mid-1980s.
The shorter-lived gas, perchloroethene (C2Cl4), has a strong gradient of decreasing concentrations from the northern to southern hemisphere, with very low concentrations in the southern hemisphere throughout the year, consistent with its predominant input from the northern hemisphere. The late-winter maximum and late-summer minimum in the northern hemisphere are strongly coupled to the atmospheric abundance of HO, the only important species responsible for C2Cl4 removal. Using our measurements of the global atmospheric burden along with estimates of its emissions, the lifetime of C2Cl4 was calculated to be about 5 months [Wang et al., 1994]. This is in good agreement with the 3.3 month estimate obtained by comparison of C2Cl4 and CH3CCl3 reaction rate constants, assuming an atmospheric lifetime for CH3CCl3 of 4.8 years. The fact that these two methods agree well suggests current global average HO concentrations obtained from the CH3CCl3 lifetime calculations are reasonably accurate.
Emissions of the three primary chlorofluorocarbons have slowed markedly during the 1990s in response to the regulatory restrictions of the Montreal Protocol. September 1995 ambient levels of CFC-11 and CFC-113 were decreasing. Although yearly emissions of CFC-12 estimated from our measurements have decreased by more than 60% from 1988 to 1995, the remaining emissions were still large enough to sustain a growth rate of about 4 pptv yr-1 at the end of 1995.
The latitudinal distribution of CH4 has been part of this research since January 1978 [Blake and Rowland, 1988]. Since 1990, there has been considerable short-term variation in the CH4 growth rate, particularly in the northern hemisphere. The global growth rate for late 1994 and early 1995 was approximately 5 ppbv yr-1 but with indications of further oscillations. The reason for these short- and long-term changes has not been conclusively identified but changing natural gas emissions leakage has been reported as a partial cause.
The latitudinal and seasonal NMHC data collected for this work furnish unique information that provides an excellent record of the effects from seasonal fluctuations in HO concentrations [Blake and Rowland 1986]. When combined with the vertical NMHC distributions measured during the various NASA and NSF aircraft field missions that we have taken part in since 1988, we can obtain good estimates of the atmospheric burden of these gases [Blake et al., 1992, 1994, 1996; N.J. Blake 1996, 1997]. For example, average annual burdens have been calculated as 640 ± 60 pptv for ethane; 120 ± 30 pptv for ethyne; and 150 ± 40 pptv for propane from March 1994 through March 1995. This information allows source strengths and source latitudinal distributions to be derived, which in turn can provide useful information about sources such as biomass burning, which has a significant southern hemisphere component.
References
Blake, D.R., and F.S. Rowland, Global atmospheric concentrations and source strength of ethane, Nature, 321, 231-233, 1986.
Blake, D.R., and F.S. Rowland, Continuing world-wide increase in tropospheric methane, 1978 to 1987, Science, 239, 1129-1131, 1988.
Blake, D.R., D.F. Hurst, T.W. Smith, Jr., W.J. Whipple, T-Y. Chen, N.J. Blake, and F.S. Rowland, Summertime measurements of selected nonmethane hydrocarbons in the arctic and subarctic during the 1988 Arctic Boundary Layer Experiment [ABLE-3A], J. Geophys. Res., 97, 16,559-16,588, 1992.
Blake, D.R., T.W. Smith, Jr., T.-Y. Chen, W.J. Whipple, and F.S. Rowland, Effects of biomass burning on summertime nonmethane hydrocarbon concentrations in the Canadian wetlands, J. Geophys. Res., 99, 1699-1719, 1994.
Blake, D.R., T.-Y. Chen, T.W. Smith Jr., C.J.-L. Wang, O.W. Wingenter, N.J. Blake, F. S. Rowland, and E.W. Mayer, Three-dimensional distribution of nonmethane hydrocarbons and halocarbons over the northwestern Pacific during the 1991 Pacific Exploratory Mission [PEM-West A], J. Geophys. Res., 101, 1763-1778, 1996.
Blake, N.J., D.R. Blake, B.C. Sive, T.-Y. Chen, F.S. Rowland, J.E. Collins Jr., G.W. Sachse, and B.E. Anderson, Biomass burning emissions and vertical distribution of atmospheric methyl halides and other reduced carbon gases in the South Atlantic region, J. Geophys. Res., 101, 24,151-24,164, 1996.
Blake, N.J., D. R. Blake, T.-Y. Chen, J.E. Collins Jr., G.W. Sachse, B.E. Anderson, and F.S. Rowland, Distribution and seasonality of selected hydrocarbons and halocarbons over the Western Pacific Basin during PEM-West A and PEM-West B, J. Geophys. Res., 102, 28,315-28,332, 1997.
Wang, C.J.-L., D. R. Blake, and F.S. Rowland, Seasonal variations in the atmospheric distribution in remote surface locations of a reactive chlorine compound, tetrachloroethylene [CCl2=CCl2], Geophys. Res. Let., 22, 1097-1100, 1994.