The isotopic characterization of methane, non-methane hydrocarbons and formaldehyde in the troposphere

Interest in the isotopic composition of trace species in the troposphere continues to increase because of the utility of isotopic information for interpreting timely concerns such as the effects of climate change, O3 production and OH depletion. With the exception of CO2, isotopic information has served, to a large extent, only an ancillary role in determining global cycles of trace species, including their chemistries. Often, isotopic information is only used to interpret isolated parts of atmospheric cycles. For example, hydrogen and carbon isotopic ratios of CH4 source emissions have been used to help quantify source contributions to the global CH4 budget. Less emphasis has been placed on isotopic information for interpreting the atmospheric cycling of CH4 and non-methane hydrocarbons (NMHCs) to formaldehyde and CO. Here, we review compositions of the stable isotopes (13C and 2H) and radioactive isotopes (l4C and 3H) for the various hydrocarbon components and formaldehyde in the tropospheric hydrocarbon---OH---CO system. Included are isotopic ratios of CH4 and NMHCs at sources and in ambient air as well as isotope effects during chemical transformation. Variations in the data are discussed with respect to measurement uncertainty and natural variation. Emphasis is on multi-isotopic signatures, for example, 13C/12C and D/H ratios and 14C activities for individual CH4 sources.