The global budget of atmospheric methanol: new constraints on secondary, oceanic, and terrestrial sources

Abstract:

Methanol is the second-most abundant organic gas in the remote atmosphere after methane, but its sources are poorly understood. Here, we report a global budget of methanol constrained by observations from the Atmospheric Tomography (ATom) aircraft campaign as implemented in the GEOS-Chem global atmospheric chemistry model. ATom observations under background marine conditions can be fit in the model with a surface ocean methanol concentration of 61 nM and a methanol yield of 13% from the newly implemented CH₃O₂ + OH reaction. While terrestrial biogenic emissions dominate the global atmospheric methanol budget, secondary production from CH₃O₂ + OH and CH₃O₂ + CH₃O₂ accounts for 29% of the total methanol source, and contributes the majority of methanol in the background marine atmosphere sampled by ATom. Emission from the ocean is minor in comparison, particularly because of rapid deposition from the marine boundary layer. Aged anthropogenic and pyrogenic plumes sampled in ATom featured large methanol enhancements to constrain the corresponding sources. Methanol enhancements in pyrogenic plumes did not decay with age, implying in-plume secondary production. The atmospheric lifetime of methanol is only 5.3 days, reflecting losses of comparable magnitude from photooxidation and deposition. GEOS-Chem model results indicate that methanol photochemistry contributes 5%, 4%, and 1.5% of the tropospheric burdens of formaldehyde, CO, and ozone respectively, with particularly pronounced effects in the tropical upper troposphere. Them CH₃O₂ + OH reaction has substantial impacts on radical budgets throughout the troposphere and should be included in global atmospheric chemistry models.