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

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.