## Catalytic effect of (H_{2}O)_{n} (*n* = 1–3) clusters on the reaction of HNO_{2} + HO → H_{2}O + NO_{2} under tropospheric conditions

_{n}

The effect of (H_{2}O)* _{n}* (

*n*= 1–3) on the HNO

_{2}+ HO → H

_{2}O + NO

_{2}reaction has been investigated theoretically at the CCSD(T)/CBS//B3LYP/6-311 + G(3

*df*,2

*pd*) level of theory, coupled with rate constant calculations by using variational transition state theory. Our results show that, when (H

_{2}O)

*(*

_{n}*n*= 1–3) was introduced into HNO

_{2}+ HO → H

_{2}O + NO

_{2}reaction, the product of the reaction did not change, but the potential energy surface became quite complex, yielding two kinds of reactions, namely HNO

_{2}···(H

_{2}O)

*(*

_{n}*n*= 1–3) + HO and HO···(H

_{2}O)

*(*

_{n}*n*= 1–3) + HNO

_{2}. In all catalysed reactions with (H

_{2}O)

*(*

_{n}*n*= 1–3), the former reaction type is favourable than the latter one with its effective rate constant respectively larger by 6–1 orders of magnitude than that of latter one. Within the temperature range of 240–320 K, the relative impacts on water monomer are much more obvious than dimer and trimer. However, the effective rate constant with water is larger by 658%–17% times of magnitude, showing that the positive water effect is obvious under atmospheric conditions.