A Hidden Source of Atmospheric HONO: The Hydrolysis of t -ONONO2 Catalyzed by Abundant Aerosol Amino Acids

Abstract Nitrous acid (HONO) is a key precursor of atmospheric hydroxyl radicals. Current models generally underestimate the concentration of atmospheric HONO. The NO2 conversion process involving amino acids may be an important theoretical pathway that has not yet been fully developed. This study employs the DLPNO–CCSD(T)/aug-cc-pVTZ//B3LYP/6–311G(d,p) quantum chemical method, combined with thermodynamics, kinetics, and NBO electronic analysis, to systematically investigate the microscopic mechanism of six typical atmospheric amino acids (alanine, glycine, valine, aspartic acid, serine, and cysteine) catalyzing the hydrolysis of t-ONONO2 to produce HONO. The results show that amino acids build hydrogen-bond networks through amino groups, carboxyl groups, and polar side-chain groups to lower the reaction energy barrier. Monofunctional amino acids mainly rely on the carboxyl channel, while multifunctional amino acids with double hydroxyl/double carboxyl groups have stronger multisite catalytic performance. Cysteine exhibits special activity due to the spatial configuration and electronic effects of its thiol group. The amino-hydroxyl channel of serine catalyzes the reaction optimally, with a rate that is 8 orders of magnitude higher than the bare reaction at 320 K. An increase in water molecules in the system competitively weakens the catalytic activity of serine, but the catalytic ability is retained. Notably, in the absence of amino acids, water clusters themselves catalyze the hydrolysis and thus constitute an atmospherically relevant competing pathway; the relative contribution of the amino acid and water-cluster routes is determined by the composition of the reaction cluster. Serine can drive the trans-HONO isomerization to the cis-HONO, and an increase in water molecules inhibits this isomerization process. This study provides theoretical support for explaining the observed-simulated deviation of atmospheric HONO measurements.