Formation of HONO from the NH ₃ -promoted hydrolysis of NO ₂ dimers in the atmosphere

Significance As the primary source of “detergent” OH radicals, nitrous acid (HONO) plays an essential role in the chemistry of the atmosphere. Despite extensive studies, the source of HONO is still elusive. Although recent studies have shown the importance of reactive nitrogen compounds during aerosol formation, mechanistic insight into how these compounds react is still missing. Herein, based on Born–Oppenheimer molecular-dynamics simulations and free-energy sampling, we identified a formation mechanism for HONO via the NH 3 -promoted hydrolysis of NO 2 dimer (ONONO 2 ) on water clusters/droplets. The near-spontaneous formation of HONO at the water–air interface sheds light on the catalytic role of water droplets in atmospheric chemistry. This finding provides not only a missing HONO source but also insight into HONO chemistry.