Environmental contextNitrous acid (HONO) has long been recognized as an important atmospheric pollutant, with the reaction of HOSO+NO2 being a source of HONO. We explore the effects of an additional water or ammonia molecule on this reaction. Calculations show that the ammonia molecule has a more effective role than the water molecule in assisting the reaction. AbstractDepending on different ways that NO2 approaches the HOSO radical, the main reactant complexes HOS(O)NO2 and HOS(O)ONO–L (lowest energy structure of the isomer) were revealed by Lesar et al. (J. Phys. Chem. A 2011, 115, 11008), and the reaction of HOSO+NO2 is a source of trans (t)-HONO and SO2. In the present work, the water molecule in the hydrolysis reaction of HOSO+NO2 not only acts as a catalyst giving the products of t-HONO+SO2, but also as a reactant giving the products of t-HONO+H2SO3, c-HONO+H2SO3 and HNO3+t-S(OH)2. For the reaction of HOSO+NO2+H2O, the main reaction paths 2, 7, and 9 are further investigated with an additional water or ammonia molecule. The CBS-QB3 calculation result shows that the process of HOS(O)NO2–H2O → t-HONO–SO2–H2O is favourable with a barrier of 0.1kcal mol–1. Although the following process of t-HONO–SO2–H2O → t-HONO–H2SO3 is unfavourable with a barrier 33.6kcal mol–1, the barrier is reduced by 17.3 or 26.3kcal mol–1 with an additional water or ammonia molecule. Starting with HOS(O)ONO–L–H2O, the energy barriers of path 7 and path 9 are reduced by 8.9 and 8.5kcal mol–1 with an additional water molecule and by 9.9 and 9.2kcal mol–1 with an additional ammonia molecule. Ammonia is more beneficial than water for assisting the HOSO+NO2+H2O reaction. Three t-HONO–H2SO3 isomers which contain double intermolecular hydrogen bonds are studied by frequency and natural bond orbital calculations. Frequency calculations show that all hydrogen bonds exhibit an obvious red shift. The larger second-order stabilisation energies are consistent with the shorter hydrogen bonds. H2SO3 can promote the process of t-HONO → HNO2, and reduce the barrier by 45.2kcal mol–1. The product NH3–H2SO3 can further form a larger cluster (NH3–H2SO3)n (n=2, 4) including NH4+HSO3– ion pairs.