Verifying Estimates of Methane Emissions with Background Measurements

Quantitative understanding of methane sources and sinks is necessary to develop a reasonable strategy to mitigate its potential influence on climate and background air quality. At NOAA CMDL, we have measured atmospheric CH₄ from a globally-distributed network of background air sampling sites since 1983. The observations are used to constrain the global methane budget, including the global burden (4840 Tg CH₄ in 2001) and global emissions (about 550 Tg CH₄ yr^-1 based on a lifetime of 9 years). So far, there are too few measurements to estimate CH₄ emissions on regional scales. CH₄ column abundances determined as part of the NACP will help verify estimates of US CH₄ emissions. Using EPA estimates (about 30 Tg CH₄ yr^-1 for anthropogenic sources), we expect about 7 ppb increase in the column abundance during transit of air parcels across the North American continent, excluding CH₄ lost because of reaction with OH. Using CH₄ vertical profiles from Carr, CO as a guide, if about 100 profiles are measured each year, we would be able to determine annual mean CH₄ column abundances to about +/-1 ppb, allowing verification of emissions estimates to +/-10%.