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 (CHO) has minimal direct involvement in sulfuric acid (HSO) and water nucleation, it remains unclear whether its atmospheric aldol condensation product, hydroxyacetaldehyde (CHO), one of the simplest bifunctional oxygenated volatile organic compounds (OVOCs), plays a role in NPF. This study investigates both the aldol condensation of CHO and its role in NPF involving HSO and CHO through quantum chemical calculations and atmospheric cluster dynamics modeling. Kinetic calculations indicate that the reaction rate of CHO aldol condensation catalyzed by HSO 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 CHO are thermodynamically favorable. Furthermore, CHO, with its hydroxyl group, stabilizes HSO clusters more effectively than CHO, 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 HSO 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.