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On the formation of formic acid from formaldehyde processing in liquid clouds
  • Jean-Francois Muller,
  • Jozef Peeters,
  • Trisevgeni Stavrakou
Jean-Francois Muller
Royal Belgian Institute for Space Aeronomy

Corresponding Author:[email protected]

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Jozef Peeters
University of Leuven
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Trisevgeni Stavrakou
Royal Belgian Institute for Space Aeronomy
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It was recently proposed (Franco et al., Nature 2021) that methanediol (MD, HOCH2OH ) formed by hydration of formaldehyde in liquid cloud droplets is outgassed to a larger extent than previously estimated, and reacts in the gas phase with the hydroxyl radical (OH), leading to formic acid (HCOOH). Whereas the resulting global production of formic acid is greatly dependent on poorly constrained parameters, such as the Henry’s law constant (HLC) of methanediol and the rate constant of its reaction with OH, Franco et al. suggest, based on global model calculations and on newly conducted chamber experiments (for the rate constant) and on statistical prediction methods (for the HLC), that this mechanism explains the large “missing source” of HCOOH in the atmosphere (e.g. Stavrakou et al. 2012). If true, this finding would be of tremendous importance for our understanding of the biogeochemical cycling of oxygenated organic compounds. For this reason, it is of utmost importance to double check the validity of thehypotheses and parameterizations behind this assessment. Here we examine two critical aspects of this determination: the HLC (taken equal to either 10^4 or 10^6 M atm^-1 in model simulations by Franco et al.) and the rate of the MD+OH reaction (taken equal to 7.5 × 10^12 cm^3 s^-1 ). The representation of chemical processing in liquid clouds in global models is also briefly discussed. Plausible ranges for those parameters are proposed , and causes of uncertainty are discussed . The potential consequences for the resulting production of formic acid are briefly explored.