Carbon and hydrogen stable isotopic ratios of methane emitted from wetlands and wildfires in Alaska: Aircraft observations and bonfire experiments

Abstract

To investigate spatial variations of CH 4 concentration, δ 13 CH 4, and δD-CH 4 over Alaska, aircraft observations were conducted during the summer of 2006. CH 4 concentrations elevated above the background level were observed over areas with wetlands and wildfires, important sources of CH 4. Several flights showed elevated CH 4 values, with corresponding δ 13 CH 4 and δD-CH 4 signatures of -63.4‰ ± 3.0‰ and -424‰ ± 79‰, respectively, which are based on the relationship between δ 13 CH 4 (or δD-CH 4) and CH 4 concentration (single mixing relation), an indication of wetland source. It was also noted that both wetlands and wildfires influenced the CH 4 concentrations observed over the wildfire area. Assuming certain emission ratios of CH 4 to CO (ER CH 4 /CO) for the wildfire and certain values of δ 13 CH 4 and δD-CH 4 for wetland CH 4, we derived δ 13 CH 4 and δD-CH 4 of CH 4 emitted from the wildfire to be -27.5‰ ± 2.0‰ and -285‰ ± 111‰, respectively, which agreed relatively well with, but was slightly lower than, those obtained by previous studies at lower latitudes. To verify these estimates, bonfire experiments were conducted in the interior of Alaska using the same biomass material burned in the wildfire observed by the aircraft. The result showed that the previously reported ER CH 4 /CO value was plausible and that δ 13 CH 4 and δD-CH 4 obtained by the bonfire experiments agreed with the estimates by the aircraft observations. We also found that δ 13 CH 4 and δD-CH 4 values became enriched with increasing combustion efficiency. By using the relationship between δD-CH 4 for biomass burning and δD of precipitation, global average of δD-CH 4 emitted from biomass burning was estimated to be -204‰ ± 11‰. Copyright 2011 by the American Geophysical Union.