Formic acid contributes significantly to acid rain in remote environments1,2. Direct sources of formic acid include human activities, biomass burning and plant leaves. Aside from these direct sources, sunlight-induced oxidation of non-methane hydrocarbons (largely of biogenic origin) is probably the largest source3,4. However, model simulations substantially underpredict atmospheric formic acid levels5,6,7, indicating that not all sources have been included in the models. Here, we use satellite measurements of formic acid concentrations to constrain model simulations of the global formic acid budget. According to our simulations, 100–120 Tg of formic acid is produced annually, which is two to three times more than that estimated from known sources. We show that 90% of the formic acid produced is biogenic in origin, and largely sourced from tropical and boreal forests. We suggest that terpenoids—volatile organic compounds released by plants—are the predominant precursors. Model comparisons with independent observations of formic acid strengthen our conclusions, and provide indirect validation for the satellite measurements. Finally, we show that the larger formic acid emissions have a substantial impact on rainwater acidity, especially over boreal forests in the summer, where formic acid reduces pH by 0.25–0.5.
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This study has been supported by the projects PRODEX A3C of the European Space Agency funded by the Belgian Science Policy Office, and the IBOOT, BIOSOA, AGACC and AGACC-II projects within the ‘Science for a Sustainable Development’ research programme funded by the Belgian Science Policy Office. Financial support by the ‘Actions de Recherche Concertées’ (Communauté Française de Belgique) is also acknowledged. IASI has been developed and built under the responsibility of the Centre National d’Etudes Spatiales (CNES, France). It is flown onboard the Metop satellites as part of the EUMETSAT Polar System. The IASI L1 data are received through the EUMETCast near-real-time data distribution service. L.C. and P-F.C. are respectively Postdoctoral Researcher and Research Associate with F.R.S.-FNRS. The Australian Research Council (Grant DP110101948) is gratefully acknowledged for their funding of the Wollongong HCOOH measurements. C.V. and M.D.M. are grateful to the BIRA and LACY team members who support the FTIR observations at Reunion Island.
The authors declare no competing financial interests.
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Stavrakou, T., Müller, JF., Peeters, J. et al. Satellite evidence for a large source of formic acid from boreal and tropical forests. Nature Geosci 5, 26–30 (2012). https://doi.org/10.1038/ngeo1354
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