The hydrolysis of formaldehyde [HCHO + H 2 O → H 2 C-(OH) 2 ] assisted by bimolecular acidic catalyst of X (X = H 2 SO 4 •••H 2 O and (H 2 SO 4 ) 2 ) under different concentrations of H 2 SO 4 and H 2 O was performed by quantum chemical calculations of CCSD(T)-F12a/cc-pVDZ-F12//M06-2X/6-311++G(3df,3pd) and the master equation method. The calculated results show that the hydrolysis reaction of HCHO catalyzed by X can occur through both HCHO•••H 2 O + X (one-step reaction) and H 2 O•••X + HCHO (stepwise reaction) routes. The stepwise reaction assisted by H 2 SO 4 •••H 2 O is the most favorable reaction among all the hydrolysis reactions assisted by X, with its effective rate constant larger by at least 2 orders of magnitude. In comparison with the hydrolysis of HCHO without X, the favorable route for the hydrolysis of HCHO with H 2 SO 4 •••H 2 O can reduce the energy barrier by 32.0 kcal•mol −1 . Meanwhile, the energy barrier of this reaction is also lower by another 0.8−17.6 kcal•mol −1 than those of the hydrolysis reaction of HCHO assisted by H 2 SO 4 , HCOOH, HNO 3 , CH 3 COOH, H 2 O, and (H 2 O) 2 . From the viewpoint of the calculated effective rate constant k′, the hydrolysis of HCHO with H 2 SO 4 •••H 2 O {[H 2 SO 4 ] = 10 6 molecules•cm −3 and [H 2 O] at 40−100% relative humidity (RH) along with [H 2 SO 4 ] = 10 7 −10 8 molecules•cm −3 and [H 2 O] at 20−100% RH} is more effective than the hydrolysis reaction assisted by H 2 O, (H 2 O) 2 , HCOOH (0.01−10 ppbv), HNO 3 (10 9 −10 11 molecules•cm −3 ), and CH 3 COOH (1−5 ppbv) within the temperature range of 280−320 K. Under the same condition, the hydrolysis of HCHO with H 2 SO 4 •••H 2 O can compete well with the hydrolysis of HCHO assisted by H 2 SO 4 (10 6 −10 8 molecules•cm −3 ). Therefore, this work predicts that H 2 SO 4 •••H 2 O can play a significant role in the hydrolysis of HCHO in the condition of wet areas with relatively high H 2 SO 4 pollution.

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The hydrolysis of SO3 plays an important role in atmospheric sulfuric acid formation. It has been found that the neutral (H2O and (H2O)2), basic (NH3), and acidic (HNO3, HCOOH, H2SO4, and C2H2O4) atmospheric species can be involved in and facilitate this reaction. However, the hydrolysis reaction of SO3 assisted by H2SO4···H2O and (H2SO4)2 has not been reported, which will limit the accurate understanding of the H2SO4 formation from the hydrolysis reaction of SO3 in the H2SO4 pollution areas. Herein, the hydrolysis reaction mechanism of SO3 assisted by H2SO4···H2O and (H2SO4)2 has been investigated by using CCSD(T)-F12/cc-pVDZ-F12//M06-2X/6-311+G(2df,2pd) methods and the reaction rate constants were evaluated by the Master Equation/Rice–Ramsperger–Kassel–Marcus model. The results show that the hydrolysis reactions with H2SO4···H2O and (H2SO4)2 are barrierless or nearly barrierless processes and lower the energy barrier at least by 21.6 kcal·mol–1. As compared with the hydrolysis reaction assisted by (H2SO4)2 within the temperature range of 280–320 K, H2SO4···H2O ([H2SO4] in 107 to 108 molecules·cm–3 and [H2O] at 20–100% RH) assisted reaction is more favorable with its effective rate constant larger by 8–10 orders of magnitude. Though H2SO4···H2O assisted reaction cannot compete with the reaction assisted by a neutral H2O (20–100% RH) catalyst due to its relatively lower concentration, it can compete well with the reaction in the presence of NH3 (109 to 1011 molecules·cm–3), HNO3 (109 to 1011 molecules·cm–3), HCOOH (108 to 1011 molecules·cm–3), H2SO4 (106 to 108 molecules·cm–3), and C2H2O4 (107 to 109 molecules·cm–3) within the temperature range of 280–320 K. As H2SO4···H2O is abundant in atmospheric aerosols, this work may suggest that the hydrolysis reaction of SO3 is promoted significantly by pre-existing aerosols, ultimately leading to aerosol growth and particle formation.

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