The effect of cations on NO2 production from the photolysis of aqueous thin water films of nitrate salts.

Nicole K Richards-Henderson,Cort Anastasio,Crisand Anderson,Barbara J Finlayson-Pitts

doi:10.1039/c5cp05325k

Abstract

The photochemistry of nitrate ions in bulk aqueous solution is well known, yet recent evidence suggests that the photolysis of nitrate may be more efficient at the air-water interface. Whether and how this surface enhancement is altered by the presence of different cations is not known. In the present studies, thin aqueous films of nitrate salts with different cations were deposited on the walls of a Teflon chamber and irradiated with 311 nm light at 298 K. The films were generated by nebulizing aqueous 0.5 M solutions of the nitrate salts and the generation of gas-phase NO2 was monitored with time. The nitrate salts fall into three groups based on their observed rate of NO2 formation (R(NO2)): (1) RbNO3 and KNO3, which readily produce NO2 (R(NO2) > 3 ppb min(-1)), (2) Ca(NO3)2, which produces NO2 more slowly (R(NO2) < 1 ppb min(-1)), and (3) Mg(NO3)2 and NaNO3, which lie between the other two groups. Neither differences in the UV-visible spectra of the nitrate salt solutions nor the results of bulk-phase photolysis studies could explain the differences in the rates of NO2 production between these three groups. These experimental results, combined with some insights from previous molecular dynamic simulations and vibrational sum frequency generation studies, show that cations may impact the concentration of nitrate ions in the interface region, thereby directly impacting the effective quantum yields for nitrate ions.

Highlights

The conventional view of simple salt solutions has been that ions are repelled from the air–water interface and prefer bulk solvation.[1]
All experiments with the exception of KNO3 were conducted at 5% above the deliquescence relative humidity (DRH, Table 1) so that the salts were aqueous solutions, which was confirmed by visual inspection
To gain added insight into how this might affect the rate of NO2 production, photolysis of NaNO3 was studied as a function of relative humidity above and below the deliquescence point (Fig. 2)

Summary

Introduction

The conventional view of simple salt solutions has been that ions are repelled from the air–water interface and prefer bulk solvation.[1]. Computational modeling and X-ray photoelectron spectroscopy show that cations (Na+ and Rb+) have a strong impact on BrÀ surface enhancement, affecting the ion’s partitioning of halide ions at the interface.[32] Though NO3À prefers bulk solvation compared to halide ions, NO3À has similar size and molecular polarizability as BrÀ,[45] and its interfacial concentration and photochemistry could be affected by the nature of the counter cation. Recent vibrational sum frequency generation measurements (VSFG) on aqueous nitrate salts determined that cations are less surface active relative to the NO3À ion and that the surface propensity of NO3À is dependent on the magnitude of the electric field between the cation and NO3À.88,95 The present paper is directed to elucidating the impact cations have on nitrate ion photolysis in thin films of aqueous aerosols deposited on a Teflon substrate, by monitoring NO2 production

Thin film photolysis studies

Bulk photolysis studies

Chemicals

Results and discussion

Conclusions