Methanediol from cloud-processed formaldehyde is only a minor source of atmospheric formic acid

Area of Science:

• Atmospheric Chemistry
• Chemical Kinetics
• Environmental Science

Background:

• Atmospheric formic acid (HCOOH) concentrations are significantly underpredicted by current models.
• A previous study suggested abundant HCOOH production from the reaction of hydroxyl radical (OH) with methanediol (HOCH2OH), a formaldehyde (HCHO) derivative.
• This proposed mechanism relied on a high experimental rate constant for the OH + HOCH2OH reaction.

Purpose of the Study:

• To accurately determine the rate constant for the reaction between OH and HOCH2OH.
• To re-evaluate the contribution of this reaction to atmospheric formic acid production.
• To investigate the role of aqueous phase processes and cloud dynamics on HCOOH formation.

Main Methods:

• High-level accuracy coupled cluster calculations were performed.
• E,J-resolved two-dimensional master equation analyses were employed.
• Global atmospheric modeling and numerical simulations were used to assess reaction impacts.

Main Results:

• The calculated rate constant for OH + HOCH2OH (k1) is (2.4 ± 0.5) × 10^-12 cm^3 s^-1, significantly lower than the previously reported 7.5 × 10^-12 cm^3 s^-1.
• Aqueous phase equilibria limit the HOCH2OH to HCHO ratio to approximately 2%, reducing its availability for reaction with OH.
• Global HCOOH production from this pathway is estimated at 3.3 Tg/y, about 30 times lower than recent estimates.

Conclusions:

• The discrepancy in atmospheric formic acid modeling is not resolved by the proposed OH + HOCH2OH pathway due to a lower rate constant and limited HOCH2OH availability.
• Re-evaluation of this HCOOH source indicates a substantially smaller contribution to the global formic acid budget.
• The study also provides an estimated Henry's law constant for methanediol.