Using Long-Term Global Ethane Measurements to Understand Global Methane Variations: Insights Based on a Modified Version of the TM5 Model Used by CarbonTracker

Atmospheric methane (CH4) is a key radiative forcing agent and an important ozone precursor. Atmospheric ethane (C2H6) is the most abundant nonmethane hydrocarbon in the remote atmosphere and it influences the atmosphere’s oxidizing capacity. In addition to natural sources such as wetlands, CH4 has numerous anthropogenic sources, two of which are shared with C2H6 (biomass burning and fossil fuel). In addition to many other trace gas compounds, UC-Irvine has directly monitored and reported global CH4 and C2H6 mixing ratios since 1978 and 1984, respectively. Our measurements show a long-term slow-down in the growth rate of CH4 and a remarkable long-term decline in the global abundance of C2H6, from 790 ± 20 pptv in 1986 to 610 ± 10 pptv in 2008 (see graph). The global C2H6 record is unique to our network and it provides us with a powerful new constraint on the causes of global CH4 variations. Currently NOAA/ESRL uses its CarbonTracker model to optimize estimates of CH4 emissions based on global observations, a global atmospheric transport model, and the best available a priori estimates of CH4 sources and sinks. Here we have adapted the TM5 transport model that underlies the CarbonTracker model to better understand the long-term variations in global C2H6 levels. A total of 4942 C2H6 observations collected from September 1984 to December 2008 were input into the model. The C2H6 measurements were collected by UC-Irvine every 3 months at surface sites in the remote Pacific Basin between latitudes of 89N and 90S, with the majority of samples collected between 71N and 47S. The results from this modeling analysis and its implications for our understanding of global CH4 trends will be presented and discussed.