Can cloud-processing of HCHO explain the missing formic acid?

2023-2024
Previous studies revealed a large, unidentified source of formic acid in the Earth's atmosphere. Liquid cloud processing of formaldehyde was recently proposed as a major contributor. However, a new international study using theoretical and modelling tools demonstrated that this process accounts for less than 10% of the missing source.

Model simulations performed at BIRA-IASB showed that the mechanism is especially inefficient over vegetated areas, which is precisely where the missing source is expected to be largest.

Theoretical assessment of key processes

Using advanced theoretical tools, the gas-phase reaction of methanediol (CH₂(OH)₂) with OH was found to be much slower (by a factor of 3) than assumed previously on the basis of laboratory experiments. The discrepancy is explained by the presence of other compounds (oligomers of methanediol) contributing to the formation of formic acid (HCOOH) in the experiments, but not in the real atmosphere. 

Next, high-level theoretical assessment of the equilibrium between gas-phase formaldehyde (CH2O) and methanediol in clouds shows that gas-phase methanediol is present in only tiny amounts (about 2% of the formaldehyde present), independently of the liquid water content of the cloud.

Simulating the impact of in-cloud processing of formaldehyde on formic acid

Model simulations performed at BIRA-IASB quantify how aqueous methanediol transitions to the gas phase as clouds evaporate. The results were then used to parameterize the overall cloud-processing mechanism in the global chemistry-transport model MAGRITTE, which was then used to quantify the production of formic acid in the atmosphere through this process.

Great attention was paid to account for major sources of uncertainties, most notably the average time that air parcels spend in clouds. 

Other processes affecting methanediol – such as deposition on the Earth’s surface and re-incorporation in clouds – were also found to play an important role. As shown on Fig. 2, cloud-processing has the greatest impact on formic acid over oceans (up to +50%), where clouds are abundant and other sources of formic acid are scarce. 

However, its influence is lowest over forests, where the missing source of formid acid is expected to be largest.

Despite these insights, the missing source of HCOOH remains decidedly elusive! More research - and a good dose of scientific creativity - will be needed to explore possible ways to resolve this long-standing discrepancy. 

Reference

  • Nguyen, T.L .; Peeters, J .; Müller, J.-F .; Perera, A .; Bross, D.H .; Ruscic, B .; Stanton, J.F. Methanediol from cloud-processed formaldehyde is only a minor source of atmospheric formic acid. Proceedings of the National Academy of Sciences, Vol. 120, Issue 48, e2304650120, DOI: 10.1073/pnas.2304650120, 2023.
Schematic representation of in-cloud processes
Schematic representation of in-cloud processes
Figure 1. Schematic representation of in-cloud processes relating gas-phase HCHO and MD (methanediol), and atmospheric fate of the released methanediol. The rates (black) and the relative molar loads (red) are estimated for typical cloud conditions.
Model-calculated impact of in-cloud HCHO processing
Figure 2. Model-calculated impact of in-cloud HCHO processing on the vertical column of HCOOH (in %) for January (left) and July (right).