Uptake of N2O5 by aqueous aerosol unveiled using chemically accurate many-body potentials

Abstract

The reactive uptake of N2O5 to aqueous aerosol is a major loss channel for nitrogen oxides in the troposphere. Despite its importance, a quantitative picture of the uptake mechanism is missing. Here we use molecular dynamics simulations with a data-driven many-body model of coupled-cluster accuracy to quantify thermodynamics and kinetics of solvation and adsorption of N2O5 in water. The free energy profile highlights that N2O5 is selectively adsorbed to the liquid–vapor interface and weakly solvated. Accommodation into bulk water occurs slowly, competing with evaporation upon adsorption from gas phase. Leveraging the quantitative accuracy of the model, we parameterize and solve a reaction–diffusion equation to determine hydrolysis rates consistent with experimental observations. We find a short reaction–diffusion length, indicating that the uptake is dominated by interfacial features. The parameters deduced here, including solubility, accommodation coefficient, and hydrolysis rate, afford a foundation for which to consider the reactive loss of N2O5 in more complex solutions.

Temperature Dependence of the Relative Rates of Chlorination and Hydrolysis of N2O5 in NaCl-Water Solutions.

“…Under dilute solution conditions, the diffusion constants for N 2 O 5 and Clare approximately 𝐷𝐷 N2O5 = 1.6× 10 −5 cm 2 /s and 𝐷𝐷 Cl − = 1.7× 10 −5 cm 2 /s at 25 °C (scaled from their values in H2O to D2O by multiplying by 0.82.) 27,43 Assuming a capture radius of 𝜆𝜆 = 3.1 Å, taken from the peak in the first solvation shell of the two solutes in Figure 6, the diffusion-limited rate constant can be estimated as 𝑘𝑘 Cl − diff = 4𝜋𝜋(𝐷𝐷 N2O5 + 𝐷𝐷 Cl − )𝜆𝜆 = 8×10 9 /M/s, roughly an order of magnitude faster than the estimated rate constant. This calculation is both temperature and concentration dependent: at 5 °C the Cldiffusion constant is 58% of the value at 25 °C, while in a 2.4 M NaCl solution the Cldiffusion constant is 77% of the infinite dilution value.…”

“…While there is not a direct measurement of either the absolute chlorination or hydrolysis rates, recent theoretical work combining molecular simulation and observed gaseous uptake measurements has narrowed a likely value of the hydrolysis rate constant in pure water to be 𝑘𝑘 𝑤𝑤 = 7× 10 5 /M/s. 27 Because this simulation neglects nuclear quantum effects, it should be more applicable to the D2O solutions investigated here than to H2O solutions. Taking the branching ratio of 1139 from our 25 °C low concentration measurements (averaged between 0.0054 and 0.21 M), the chlorination rate constant is estimated to be 𝑘𝑘 Cl − = 8×10 8 /M/s.…”

“…rationalized their proposed temperature dependance on the ClNO2 yield via this mechanism by invoking a barrierless reaction between the oppositely charged NO2 + and Clreactants versus a more significant barrier for the ion-neutral reaction of NO2 + with H2O. 11 However, recent theoretical [23][24][25][26][27][28] and laboratory 9,18 studies have called into question the existence of isolated aqueous NO2 + ions. Bianco and Hynes first proposed concerted hydrolysis involving nucleophilic OH δattack from H2O on molecular NO2 δ+ NO3 δthat has incipient ion-pair character.…”

“…Generally, such computational studies indicate that hydrolysis proceeds from a hydrophobically solvated N2O5 molecule with weak hydrogen bonding but significant and fluctuating ion-pair character, NO2 δ+ NO3 δ-. [25][26][27] Hydrolysis can proceed by nucleophilic attack of OH δon NO2 δ+ or electrophilic attack of H δ+ on NO3 δ-. In both cases, solvent water molecules may rearrange into more structured configurations to stabilize a transition state consisting of ion pair-like NO2 δ+ NO3 δand adjacent H δ+ OH δ-.…”

Weak Temperature Dependence of the Relative Rates of Chlorination and Hydrolysis of N2O5 in NaCl–Water Solutions

“…However, recent theoretical − and laboratory , studies have called into question the existence of isolated aqueous NO 2 + ions. Bianco and Hynes first proposed concerted hydrolysis involving nucleophilic OH δ− attack from H 2 O on molecular NO 2 δ+ NO 3 δ− having incipient ion-pair character .…”