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 2008–2019. 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 ΩIASI=0.46Ωairc+10.6×1015molec.cm-2, with ΩIASI and Ωairc 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 (∼160Tgyr-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 in the model 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 distribution and 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.