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

To investigate spatial variations of CH₄ concentration, δ¹³CH₄, and δD-CH₄ over Alaska, aircraft observations were conducted during the summer of 2006. CH₄ concentrations elevated above the background level were observed over areas with wetlands and wildfires, important sources of CH₄. Several flights showed elevated CH₄ values, with corresponding δ ¹³CH₄ and δD-CH₄ signatures of -63.4‰ ± 3.0‰ and -424‰ ± 79‰, respectively, which are based on the relationship between δ ¹³CH₄ (or δD-CH₄) and CH₄ concentration (single mixing relation), an indication of wetland source. It was also noted that both wetlands and wildfires influenced the CH₄ concentrations observed over the wildfire area. Assuming certain emission ratios of CH₄ to CO (ER_CH4_/CO) for the wildfire and certain values of δ¹³CH₄ and δD-CH₄ for wetland CH₄, we derived δ¹³CH₄ and δD-CH₄ of CH₄ 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_CH4_/CO value was plausible and that δ¹³CH₄ and δD-CH ₄ obtained by the bonfire experiments agreed with the estimates by the aircraft observations. We also found that δ¹³CH₄ and δD-CH₄ values became enriched with increasing combustion efficiency. By using the relationship between δD-CH₄ for biomass burning and δD of precipitation, global average of δD-CH₄ emitted from biomass burning was estimated to be -204‰ ± 11‰.