Fig. 2: Schematic of the multiphase production of formic acid.
From: Ubiquitous atmospheric production of organic acids mediated by cloud droplets
The common assumption in global atmospheric chemistry models is illustrated in black: aqueous-phase methanediol (HOCH2OH) is neglected and aqueous-phase formic acid (HCOOH) is assumed to form directly from formaldehyde (HCHO) on reaction with OH. The implementation of HOCH2OH multiphase equilibria is illustrated in red: the explicit representation of the slow dehydration of aqueous-phase HOCH2OH, of its fast outgassing from cloud droplets and of its OH-initiated oxidation in the gas phase leads to a pervasive production of gaseous HCOOH. Under typical daytime conditions with average [OH](g) = 1 × 106 molecules cm−3 and [OH](aq) = 1 × 10−13 mol l−1, the lifetimes of HOCH2OH against OH are about 1 × 105 s and 3 × 104 s, respectively. Under typical midday conditions with [OH](g) = 5 × 106 molecules cm−3, the gas-phase sink is five times stronger. Thus, gas-phase oxidation sustains the chemical gradient that drives HOCH2OH from the aqueous to the gas phase.
