Source partitioning of atmospheric methane using stable carbon isotope measurements in the Reuss Valley, Switzerland.

Measurements of methane ([Formula: see text]) mole fractions and [Formula: see text] [Formula: see text] that resolve the diel cycle in the agriculturally dominated Reuss Valley, Switzerland, were used to quantify the contributions of different [Formula: see text] sources to the atmospheric [Formula: see text] source mix. Both a nocturnal (NBL) and a diurnal convective boundary layer (CBL) approach were employed. A diel course of [Formula: see text] mole fractions was found with a daytime minimum (background around 1900 ppb) and a nocturnal maximum (up to 3500 ppb). The [Formula: see text]C value in [Formula: see text] only showed small variations during the day (9-21 hours CET, [Formula: see text]‰ [Formula: see text]) when the atmosphere was well mixed, but decreased by [Formula: see text]‰ during the night. Biogenic emissions dominated in both approaches (ranging from 60 to 94%), but non-biogenic sources were rather important (42.2% and 46.0% with CBL, 5.8% and 40% with NBL approach in 2011 and 2012, respectively, of total emissions). The [Formula: see text] sink, dominated by tropospheric OH oxidation and only to a minor extend by soil surface uptake, was quantified at roughly 4% of local emissions.