Raman spectroscopy measurements of Polar Stratospheric Cloud (PSC) mimics

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Eugene Greer
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1 Raman spectroscopy measurements of Polar Stratospheric Cloud (PSC) mimics Eoin Riordan and John R Sodeau CRAC - Department of Chemistry & Environment Research Institute, University College Cork, Ireland.

2 Background In the stratosphere (25km) species such as NO x can lock active chlorine (Cl, ClO) as inactive forms (ClONO 2, HCl) that cannot destroy O 3. Discovery of Antarctic ozone hole (17km) blamed on catalysed release of photolytically active chlorine from reservoir species. ClONO 2 + HCl Cl + O 3 ClO + ClO HNO 3 + Cl 2 ClO + O 2 Cl 2 +O 2 (1) (2) (3)

3 Reaction Mechanism Reaction (1) is very slow in the gas phase and so the idea of heterogeneous processing on PSC surfaces to release the active chlorine via ionic intermediates (fast) was proposed.

4 Polar Stratospheric Clouds over Kiruna, Sweden.

5 Composition Theories Early (laboratory-based) theories favoured three-phase concept for PSC composition: Sulfuric acid/sulfate core Condensation of NO x as nitric acid trihydate (HNO 3 3H 2 O called NAT) Condensation of water onto the particle (~189K)

6 Solids, liquids or slushes? Recent work has centred on the existence of liquid particles Theoretical models favour stability of liquid ternary solutions under stratospheric conditions Lidar measurements suggest (some) liquid droplets exist Measurements of Schreiner et al. show H 2 O:HNO 3 to be above 10:1 in PSC particles (balloon-borne particle analysers) The state of the PSC would be expected to exert a key control on the chemical pathways and their efficiency

7 Formation Mechanism Stratospheric sulfate aerosol exists (Junge layer) It deliquesces water and nitric acid vapour (liquid-solid equilibrium) A rapid uptake and size increase occurs from K Result is an NSW ternary solution. Whether it exists as a solid, liquid or slush will depend upon temperature and concentrations

8 UCC Project Aim of this work is to measure solution/solid/slush equilibria in PSC-mimics (e.g. N:S:W 1:1:10) as a function of ionic composition and temperature Raman Spectroscopy used as a probe of the ionic speciation: HNO 3 + H 2 O NO 3- + H 3 O + H 2 SO 4 + H 2 O HSO 3- + H 3 O + SO H 3 O +

very strong IR absorber but a poor Raman scatterer IR absorption Hence

9 The Raman Experiment Need qualitative and quantitative analysis of ions such as nitrate in aqueous solutions and water-ice Water is a very strong IR absorber but a poor Raman scatterer IR absorption Hence use Raman Spectroscopy to distinguish guest ion from solvent Raman scattering

10 UCC Methodology Ocean Optics R2000 Raman Spectrometer 500mW 785nm (red) diode laser 1200mm -1 blazed grating 2048 element CCD array 400nm diameter silica fibreoptic probe lends flexibility for in situ analysis Cheap!

11 Experimental Setup

12 Nitric Acid solutions NO 3 - HNO 3 Increase in solute:solvent ratios (label) Equilbrium data (ion:molecule) can be obtained Non-ionic form prevails as solvent becomes scarce

13 Temperature effect NO 3 - HNO K 241 K 1:3 HNO 3 - H 2 O solution Dissociation is exothermic aided by cooling When ionisation is total THEN freezing occurs 227 K

14 Sulfuric acid solutions SO 4 2- HSO 4 - Similar effects to Nitric acid Speciation change from acid to bisulfate to sulfate ions Freezes after total disappearance of bisulfate ions

15 Ternary Solutions (NSW) HSO 4 - HNO 3 NO3-1:1:10 10:1:100 Bisulfate ion always present even in dilute solutions As [H 2 SO 4 ] decreases, Nitric acid equilibrium shifts toward ionic species 100:1:1000

16 Ternary Freezing HSO 4 - HNO 3 303K 244 K 1:1:10 NSW solution Freezing begins at 233K after ionisation of sulfate system (1:10 NW freezes at 248K) 233K

17 Conclusions Associated forms (HNO 3, HSO 4- ) prevail as concentration is increased Decreasing temperature drives equilibrium toward ionic species Solid phase forms only after complete ionisation has occurred HNO 3 ionises before H 2 SO 4, i.e. trace amounts of H 2 SO 4 hinder formation of solid phase in HNO 3 solutions Cooling of stratosphere due to greenhouse effect may aid formation of solid phases

ClO + O -> Cl + O 2 Net: O 3 + O -> O 2 + O 2

ClO + O -> Cl + O 2 Net: O 3 + O -> O 2 + O 2 Lecture 36. Stratospheric ozone chemistry. Part2: Threats against ozone. Objectives: 1. Chlorine chemistry. 2. Volcanic stratospheric aerosols. 3. Polar stratospheric clouds (PSCs). Readings: Turco: p.