Global atmospheric methanol emissions inferred from IASI satellite measurements and aircraft data

Abstract. We employ an updated retrieval of space-based methanol (CH₃OH) column measurements from the Infrared Atmospheric Sounding Interferometer (IASI) and an emission optimisation framework built on the MAGRITTE chemical transport model to assess terrestrial emissions of methanol to the atmosphere between 2008 and 2019. We first carry out a IASI CH₃OH validation study based on concentration measurements from three airborne campaigns, using the model and the IASI averaging kernels to compute aircraft-based columns directly comparable to IASI data. IASI is found to underestimate high columns in the considered region. A linear regression gives Ω_IASI = 0.46 Ω_airc + 10.6·10¹⁵ molec.cm^-2, with Ω_IASI and Ω_airc the IASI and aircraft-derived columns, respectively. Inverse modelling of terrestrial methanol emissions using MAGRITTE and bias-corrected IASI columns leads to much-improved overall agreement against in situ measurement campaigns and column data at eight FTIR stations. The optimised global biogenic methanol emissions (~160 Tg yr^-1) are 22–60 % higher than previous top-down estimates, due to (1) column enhancements caused by the IASI bias-correction and (2) higher dry deposition velocities over land, compared to previous model studies, based on a parametrisation constrained by extensive campaign data. The inversion results are less reliable over boreal forests due to shortcomings of both the bias-correction and the dry deposition scheme over these regions. The optimisation suggests large changes in the seasonality of emissions. Over tropical ecosystems, radiation and temperature appear to exert a stronger control on biogenic emissions than is currently accounted for in the MEGAN model.