Influence of Ammonia and Water on the Fate of Sulfur Trioxide in the Troposphere: Theoretical Investigation of Sulfamic Acid and Sulfuric Acid Formation Pathways

Abstract

Reaction of ammonia with SO3 as a potential source of sulfamic acid in the troposphere has been investigated by means of electronic structure and chemical kinetic calculations. Besides, the hydrolysis reaction, which is known to be a major atmospheric decay channel of SO3, has also been investigated. The catalytic effects of ammonia and water on both the reactions have been studied. Rate coefficients for all the studied reaction channels were calculated using the transition state theory employing pre-equilibrium approximation. Calculated rate coefficients for a number of catalyzed hydrolysis and ammonolysis processes were found to be much higher (by ∼105 to ∼109 times) than the gas kinetic limit at ambient temperature. With decrease in temperature because of negative temperature dependence of rate coefficients, that difference became even larger (up to ∼1016 times). Therefore, in order to remove the discrepancies, rate coefficients for all the studied reaction channels were calculated by means of the master equation. The results showed marked improvements, with only one channel showing a slightly higher rate coefficient above the gas kinetic limit. The rate coefficients for catalyzed channels obtained from the master equation also showed negative temperature dependence, albeit to a much smaller extent. The uncatalyzed ammonolysis reaction, similar to the corresponding hydrolysis, was found to be too slow to have any practical atmospheric implication. For both reactions, ammonia-catalyzed pathways have higher rate coefficients than water-catalyzed ones. Between hydrolysis and ammonolysis, the latter showed a higher rate coefficient. In spite of that, ammonolysis is expected to have negligible contribution in the tropospheric loss process of SO3 because of large difference in concentration values between water and ammonia in the troposphere.

Effect of ammonia, ammonia‐water, and sulfuric acid on the HO2 + HO2 → H2O2 + 3O2 reaction in troposphere: Competition between stepwise and one‐step mechanisms

“…Assuming that the intermediate IM was in equilibrium with the corresponding reactants (HO 2 and HO 2 ··· X ) and was at steady state, [ 52 ] the high‐pressure limit rate constant k b between HO 2 ··· X and HO 2 has been performed as Equation (). This method has been used in many atmospheric reactions [ 53–58 ] catalyzed by catalyst X , and the predicted rate constants are reasonably consistent with the corresponding experimental values. …”

Effect of NH3 and HCOOH on the H2O2 + HO → HO2 + H2O reaction in the troposphere: competition between the one-step and stepwise mechanisms

“…Thus, H 2 O 2 /HO complex can be neglected and it is not taken into account here. Such investigations have been studied in similar investigations, 52,[66][67][68][69][70] where the dimer between the two reactants has not been involved in X assisted reactions. Above step is followed since it is very necessary to rst identify the stable binary complexes of H 2 O 2 /X and HO/X.…”

“…where K eq1a , K eq1d , and K eq1f respectively denote the equilibrium constant for the formation of H 55,64,65,70,75 The relative rates for v R_AM1 /v R_WM and v R_FA1 /v R_WM within the temperature range of 280-320 K are given in Table 3. As seen in Table 3, when the concentrations of H 2 O at 100% (2.60 Â 10 17 molecule per cm 3 to 2.30 Â 10 18 molecule per cm 3 ) 76 RH and NH 3 at 10 ppbv (2.60 Â 10 11 molecule per cm 3 to 2.30 Â 10 11 molecule per cm 3 ), [22][23][24] the calculated v R_AM1 /v R_WM value is below 2.47 Â 10 À3 to 9.40 Â 10 À5 within the temperature range…”

Path sampling for atmospheric reactions: formic acid catalysed conversion of SO3 + H2O to H2SO4

“…The SO 3 • H 2 O complex can form readily, however early investigations of the hydrolysis of SO 3 to form H 2 SO 4 found the uncatalysed rate to be too low to explain the concentration of atmospheric H 2 SO 4 (Wang et al, 1988;Hofmann & Von Ragué Schleyer, 1994;Loerting & Liedl, 2000). Further studies revealed that the formation of H 2 SO 4 is catalysed by collisions with other atmospheric molecules (Akhmatskaya et al, 1997;Hazra & Sinha, 2011;Long et al, 2012;Torrent-Sucarrat, Francisco & Anglada, 2012;Bandyopadhyay, Kumar & Biswas, 2017;Sarkar, Oram & Bandyopadhyay, 2019), where the common denominator is the facilitation of a double proton transfer via the catalysing molecule (Kumar, Sinha & Francisco, 2016). The relative importance of collisions with different molecules is determined by both the atmospheric concentration of the colliding molecule and the species-dependent reaction rate.…”