[hal-00734141] Biogenic Methanol and Formic Acid Constrained from IASI Measurements

Methanol and formic acid, the simplest alcohol and carboxylic acid in the atmosphere, are directly emitted from anthropogenic activities, fire events and vegetation, as well as from photochemical sources. They are removed from the atmosphere through OH oxidation, and wet and dry deposition, and their global lifetime is estimated to 6 days for methanol and to 4 days for formic acid. According to state-of-art estimates, the ocean and terrestrial biosphere account for two thirds of the total methanol source, whereas the contributions from human activities and biomass burning are minor. Plant emissions represent the major part of the methanol flux from terrestrial ecosystems, but the uncertainty regarding the magnitude and localization of this source is very large, ranging between 100 and 300 Tg on an annual basis. Formic acid is a ubiquitous trace gas in air and precipitation and a major contributor to rain acidity in remote environments. Its sources, however, are far from being fully understood, as testify severe underpredictions of observed formic acid concentrations in earlier global modelling studies, spotting the existence of unaccounted sources. New insights into our understanding of methanol and formic acid atmospheric cycle are brought forward by newly acquired measurements of tropospheric columns retrieved from the IASI thermal infrared satellite sounder. In this communication, we report findings from two source inversion studies of methanol and formic acid emissions performed using the IMAGESv2 CTM constrained by one complete year of IASI column measurements. The optimized global methanol source is estimated to about 100 Tg on a yearly basis, with significant emission decreases, compared to the bottom-up inventory, inferred over tropical rainforests and unexpectedly high flux strengths in arid environments. The IASI column data suggest an annual formic acid source about three times higher than estimated from known sources, with a biogenic contribution of about 90%, mostly from tropical and boreal forests. We investigate the implications of the large formic source on precipitation acidity on the global scale. The resulting fluxes are extensively evaluated against previous modelling work, available aircraft and in situ independent measurements. Variabilities of Chemical Species Over the Mediterranean Basin: Measurements



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