Gas-phase aldol condensation of formaldehyde to produce hydroxyacetaldehyde and its implication to new particle formation: a theoretical study

Abstract

Aldehydes have been proposed as important precursor species in new particle formation (NPF). Although formaldehyde (CH₂O) has minimal direct involvement in sulfuric acid (H₂SO₄) and water nucleation, it remains unclear whether its atmospheric aldol condensation product, hydroxyacetaldehyde (C₂H₄O₂), one of the simplest bifunctional oxygenated volatile organic compounds (OVOCs), plays a role in NPF. This study investigates both the aldol condensation of CH₂O and its role in NPF involving H₂SO₄ and C₂H₄O₂ through quantum chemical calculations and atmospheric cluster dynamics modeling. Kinetic calculations indicate that the reaction rate of CH₂O aldol condensation catalyzed by H₂SO₄ is 8 to 16 orders of magnitude higher than that of the uncatalyzed pathway at 200–298 K. Based on molecular structures and formation Gibbs free energies, interactions between sulfuric acid/its polymers and C₂H₄O₂ are thermodynamically favorable. Furthermore, C₂H₄O₂, with its hydroxyl group, stabilizes H₂SO₄ clusters more effectively than CH₂O, thereby enhancing nucleation. Additional cluster kinetic modeling suggests that particle formation rates in this system exceed those in the sulfuric acid–water binary system under conditions of low ambient H₂SO₄ concentrations and low relative humidity. However, cluster growth remains limited due to weak formation of larger clusters, indicating that other stabilizing vapors are needed for sustained cluster growth and stable particle formation.