Effect of formic acid on O + OH˙CHOH → HCOOH + HO reaction under tropospheric condition: kinetics of and isomers.

Formic acid (HCOOH) is one of the highly abundant acids in the troposphere. It is important in the formation of atmospheric aerosols and impacts the acidity of rainwater. In the present scenario, the model chemistry of HCOOH(FA) sources and sinks is poorly understood. In this work, we apply quantum chemical methods coupled with advanced statistical rate theories to understand the production of FA and its catalytic behavior under tropospheric conditions. The potential energy surfaces (PES) for O + OH˙CHOH and O + OH˙CHOH(+FA) reactions were constructed using the CCSD(T)/6-311++G(3df,3pd)//M06-2X/6-311++G(3df,3pd) level and rate constants for the production of FA were predicted using Rice-Ramsperger-Kassel-Marcus (RRKM)/master equation (ME) simulation with Eckart tunnelling and canonical variational transition state theory (CVT) with small curvature tunnelling (SCT) between the temperature range of 200-320 K and pressure range of 0.001 to 10 bar. The reaction follows the formation of and intermediates followed by spontaneous decomposition concerted HO elimination to form stable post intermediates, which then leads to the straight formation of and formic acid. The current study also helps understand the role of and contribution to the total rate constants. The results show that O + OH˙CHOH is dominated by both and isomers; however, the isomer plays a more important role in the catalytic reaction. This result is due to the formation of a strong hydrogen-bonded complex in the isomer, which is dominated by the enthalpy factor rather than the entropy factor. The results predict that the catalytic effect of FA on the O + OH˙CHOH reaction is important when the concentration of FA is not included in the calculations; however, it has no effect under tropospheric conditions, when the FA concentration is included in the calculation. As a result, the total effective reaction rate constants are smaller. This work provides experimental/theoretical confirmation of Franco who predicted that methanediol is the precursor for the FA formation, resolving an open problem in the kinetics of the gas phase reaction. O + OH˙CHOH and O + OH˙CHOH(+FA) probably explain other diol systems, which can help explain the formation of other atmospheric acids that affect aerosol formation and cloud evolution.