Uptake of nitric acid on NaCl single crystals measured by backscattering spectrometry

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Abstract

The uptake of nitric acid in sea salt aerosols via the reaction HNO3 (g) + NaCl (c)  NaNO3 (c) + HCl (g) is a main contributor to the chlorine balance in the troposphere, especially in polluted, coastal regions. Backscattering spectrometry was used to measure oxygen concentration profiles on the surface of NaCl single crystals (1 0 0) after exposure to a nitric acid vapor pressure of 10−3 torr and different relative humidities at room temperature. Comparison with the chlorine signal leads to the conclusion that the replacement of chlorine by nitrate is the only significant chemical reaction that occurs. The relative humidity is revealed to be a crucial parameter, because it determines the kinetics of the uptake. At 65% RH, the nitrate concentration increases almost linearly with time and shows no saturation at all. At 20% RH, it shows square root dependence of time implying diffusion control.

Introduction

The tropospheric halogen budget is strongly influenced by halogens released from sea salt aerosols [1]. As a matter of fact, sea salt aerosols in polluted regions of the marine troposphere have been often found to display a depletion of chlorine compared to the original composition of seawater. This was generally attributed to the displacement of alkali halides by strong inorganic acids, such as nitric acid [2]. The reaction HNO3 (g) + NaCl (c)  NaNO3 (c) + HCl (g) as a main contributor to chlorine depletion in the condensed phase (c) leads to formation of NaNO3 on the aerosol and the release of the relatively unreactive HCl into the gas phase (g). HCl is photochemically inert and highly soluble. The importance of this reaction stems mainly from its potential to compete with other reactions of nitric oxides and NaCl producing ClNO, ClNO2 and Cl2, which can undergo photolysis and form highly reactive chlorine radicals [3].

Measured chlorine depletions of up to 80% in sea salt aerosols [4] can only be explained in an environment with sufficiently high humidity allowing the nitric acid uptake to proceed into a depth of microns. However, such depth information is still missing up to now, as only indirect information is available from surface monolayer, crystal bulk or gas phase measurements.

Section snippets

Experimental

Backscattering spectrometry in this study was performed on a Dynamitron accelerator in the Ion Beam Laboratory at SUNY-Albany. We used Rutherford backscattering spectrometry [5], [6] (RBS) to measure the Cl depth profile and the resonant cross section of O at approximately 3.05 MeV [5], [6] to measure the O depth profile. A beam of 4He+ ions was used with energies between 3.00 and 3.26 MeV and typical beam currents below 5 nA. To avoid radiation damages, a beam spot with an area of about 1 mm2 was

Results and discussion

For example, the measured concentration profiles of O (given by squares) and Cl (thin black line) are shown in Fig. 1 for a sample exposed for 99 h to 20% RH and a nitric acid vapor pressure of 10−3 torr. Clearly, the two profiles are mirror inverted. The profile given by circles is a prediction of the Cl concentration profile from the measured O profile assuming that the replacement of Cl by nitrate due to the reaction HNO3 (g) + NaCl (c)  NaNO3 (c) + HCl (g) is the only significant chemical reaction

Conclusions

Exposure of a NaCl surface to an atmosphere with nitric acid leads to the reaction HNO3 (g) + NaCl (c)  NaNO3 (c) + HCl (g). The uptake of nitric acid increases with increasing HNO3 vapor pressure. At 20% RH, the uptake is restricted to the near surface region. Since the composition of the surface approaches that of pure NaNO3, this reaction becomes limited by solid state diffusion. At 65% RH, the uptake proceeds into a depth of several micrometers. The uptake rate is linear in time, indicating a

Acknowledgments

We gratefully thank Art Haberl and Wayne Skala from the Ion Beam Laboratory, SUNY at Albany for the excellent support concerning all questions around the SUNY particle accelerator. Also, we thank Kurt Barmettler and Ruben Kretschmar for performing the bulk analysis measurements. This research has been funded by ETH Zurich under Grant TH-2202-2.

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