Dominic F. Ferretti

Shipboard determinations of the distribution of 13C in atmospheric methane in the Pacific

Measurements of the mixing ratio and δ 13 C in methane (δ 13 CH 4 ) are reported from large, clean air samples collected every 2.5° to 5° of latitude on four voyages across the Pacific between New Zealand and the West Coast of the United States in 1996 and 1997. The data show that the interhemispheric gradient for δ 13 CH 4 was highly dependent on season and varied from 0.5‰ in November 1996 with an estimated annual mean of 0.2-0.3‰. The seasonal cycles in δ 13 CH 4 reveal three distinct latitude bands differentiated by phase. Maxima occur in January-February for the extratropical Southern Hemisphere, in September-October for the tropics, and in June-July for the extratropical Northern Hemisphere. The data are compared with results from a three-dimensional transport and atmospheric chemistry model that simulates the observed latitudinal structure of either δ 13 CH 4 or the methane mixing ratio well, but not both simultaneously. The requirement that a methane source-sink budget be consistent with both types of data clearly imposes stricter constraints than arise from either mixing ratio or isotopic data alone. The seasonal δ 13 CH 4 data in the extratropical Southern Hemisphere are used to estimate a value for the net fractionation in the CH 4 sink of 12-15‰, which is larger than can be explained by current laboratory measurements of a kinetic isotope effect for the OH + CH 4 reaction and soil sink processes. The hypothesis that the discrepancy is caused by competitive reaction of active chlorine with methane in the marine boundary layer is discussed.

A new gas chromatograph‐isotope ratio mass spectrometry technique for high‐precision, N2O‐free analysis of δ13C and δ18O in atmospheric CO2 from small air samples

A new gas Chromatograph-isotope ratio mass spectrometry (GC-IRMS) technique for the first N2O-free, high-precision (<0.05‰) isotopic analysis of δ13C and δ18O in atmospheric CO2 from small air samples has been developed. On-line GC separation of CO2 and N2O from a whole air sample is combined with IRMS under elevated ion source pressures. A specialized open split interface is an integral part of the inlet system and ensures a continuous flow of either sample gas or pure helium to the IRMS. The analysis, including all flushing, uses a total amount of 45 mL of an air sample collected at ambient pressure. Of this, three 0.5 mL aliquots are injected onto the GC column, each providing ∼0.8 nmol CO2 in the IRMS source. At this sample size, δ13C precision obtained is at the theoretical shot noise limit. For typical ambient air samples collected in the Southern Hemisphere, demonstrated precisions for δ13C, δ18O, and the CO2 mixing ratio (all measured simultaneously) are 0.02‰, 0.04‰, and 0.4 ppm, parts per million (ppm) respectively. Since these data are achieved from small air samples without contamination by atmospheric N2O or the use of cryogen, the technique will be a valuable tool in global carbon cycle research.

The Global Methane Budget over the Last 2000 Years: CH413 Reveals Hidden Information

Publisher Summary Methane is arguably the most dynamic greenhouse gas in the atmosphere. With human population increase and industrialization, pCH 4 is now about 250% higher than it was in the early 1800s, and it has shown a significantly larger percentages increase than that of carbon dioxide, the gas which is most often the focus of greenhouse gas mitigation and adaptation strategies. Surprisingly, in the past decade, pCH 4 has stabilized at a global average of ∼1750 ppb. Human impacts on methane are also important. The methane emissions from rice cultivation, cattle farming, and biomass burning clearly impact the atmospheric methane budget today and may have been important long before the industrial era. The baseline against which early human impacts on methane so far, have been measured is pCH 4 levels in the past obtained from air trapped in ice cores. The goal in this chapter is to use measurements of the stable carbon isotopes of methane ( δ 13 CH 4 ) to help in source partitioning and thereby enhance the understanding of the LPIH methane budget. Carbon isotopes of methane are useful because the sources can be loosely binned into categories with distinct δ 13 CH 4 values. Due to the small amount of air trapped in ice core bubbles and typical sample size requirements for δ 13 CH 4 analyses, such measurements from ice cores have been scarce to date. This chapter reviews 2000-year records of pCH 4 and δ 13 CH 4 at high temporal resolution and precision from the Law Dome ice cores in Antarctica. This record reveals that contrary to expectations, δ 13 CH 4 , unlike pCH 4 , was not slowly varying in the pre-industrial period, and that its value was significantly different from that expected from previous estimates. These isotope analyses not only reveal the presence of a heavier human hand on the methane cycle than previously thought but also that methane history has been linked significantly to human history over the last two millennia.