LIQUID STRATOSPHERIC AEROSOLS MODELING

1. LIQUID STRATOSPHERIC AEROSOLS MODELING PRESENTED BY: Yan Bo Zhu Jie Farhana Yasmin Dian Putrasahan

2. Overview • Objective • Introduction • Composition of aerosols • Gas solubilities (Henry’s Law) • Thermodynamic Models • Parameterization of Models • Results and Discussion • Summary/ Conclusion • Questions

3. OBJECTIVE • Understand the relationship between temperature and its effects on gas-liquid partitioning. • Understanding the different thermodynamic models used in liquid stratospheric aerosols

4. INTRODUCTION Background Since the 1960s, the combination of laboratory measurements, field observations and thermodynamic model calculations has led to a recognition that: Stratospheric aerosols could remain liquid to very low temperature rather than solid particles. H2SO4 concentration in the aerosols would vary from 80% to 40%(mass) with respect to temperature and different regions.

5. INTRODUCTION (cont’d) Reasons it exist in liquid form: • Many species such as HCl, HBr, HOCl, and HNO3 can partition into the sulfuric acid aerosols. • These partitioning of gases increase the aerosol size ,control their freezing properties and affect the rate of important liquid phase reactions.

6. EFFECT ON OZONE DEPLETION Why do we want to learn about liquid stratospheric composition? Impact on the stratospheric composition: • The composition, phase, and size of stratospheric aerosols can strongly affect the ozone depletion. ClONO2 + HCl Cl2 +HNO3 HOCl + HCl Cl2 +H2O

7. EFFECT ON OZONE DEPLETION Catalytic cycles that deplete ozone in both gas and liquid phase is determined by the concentrations of reactive species. These concentrations are highly affected by the partitioning into gas and liquid phase. Its solubility in liquid phase is dependent upon the temperature and composition of the liquid aerosols.

8. Soluble Gases • Hydrogen chloride and other stratospheric gases, such as HOCl, HOBr, or HBr, that dissolve into the aerosols are important because they can activate halogen species by reaction in the liquid phase. HOCl + HCl Cl2 +H2O • Nitric acid does not react in the liquid, but, its abundance means that its uptake significantly affects the composition and size of the droplets • Other gases such as ClONO2 and N2O5 and radical species such as NO2 or OH can react not only with other dissolved gases but also with water or H2SO4 in the liquid aerosols ClONO2 + H2O HOCl +HNO3 N2O5 + H2O2HNO3

9. Properties of stratospheric gases

10. Composition of Aerosol Main components of liquid aerosols are water, sulfuric acid and nitric acid. We will discuss the how various compositions of these components affect the solubilities of different solutes.

11. Composition of Aerosols Binary System: H2SO4-H2O Dotted lines: melting point of solid phase Shaded region suggest liquid phase. However, under stratospheric conditions, aerosols are supercooled, thus existing as liquid instead of solid, even below the dotted lines.

12. Water-H2SO4 system At the lower stratosphere, almost all H2SO4 will be in the condensed phase At the upper stratosphere, volatile H2SO4 will exist in both gas phase and liquid phase

13. Gas Solubility in Aerosols (1) Henry’s Law: non-dissociating species where kH is the Henry’s law constant, aN is the aqueous phase activity of species N, γN is the activity coefficient, pN is the partial pressure and mN is the molal concentration

14. Henry’s Law: weak acid where Ka is the dissociation constant for HY, γH+ and γY- are activity coefficients for aqueous ions Henry’s Law: strong acid where kH’ = kHKa is the Henry’s law constant Gas Solubility in Aerosols (2)

15. Thermodynamic Models (methods of finding activity coefficient)

16. Parameterization of Models (1)

17. Parameterization of Models (2)

18. Parameterization of Models (3) Kinetic Uptake Measurements Non-dissociating species: Weak acids: Strong acids: Conversion:

19. Results and Discussion • Gas solubilities in liquid H2SO4-H2O aerosols: • H*- effective Henry's law constant in moles per kg per atm • MH*- effective Henry's law constant in moles per dm3 per atm • γN - activity coefficient of dissolved gases controlled by the concentration of H2SO4 in the aerosol droplets kH – Henry's law constant

20. 0.0 Partitioning of Stratospheric Gases into Liquid Aerosols T (K) Vertical dashed line at 188 K indicates the ice frost point. Aerosols are unlikely to remain liquid at 1 or 2K below the frost point, hence shaded region indicates hypothetical properties. Above 200K, H2SO4 is predominantly in the liquid phase. Almost all other species are in gas phase.

21. Mass fraction in Liquid Phase 0.0 Principle Composition of aerosols: HNO3 (low temp) and H2SO4 (higher temp). Presence of HNO3 greatly influences the solubility of various substances (HBr, HCl, HOCl), and then significantly affect the composition and size of the aerosol droplets.

22. Aerosol Volume as a function of temperature

23. Kelvin Effect Effect of droplet radius on composition. Purpose of using Kelvin effect: Reduce amount of all volatile species in the droplet, which leads to lower droplet mass and higher H2SO4 concentration. At higher temperature, the Kelvin effect is unimportant due to the higher mass fraction of H2SO4. At lower temperature, aerosol is HNO3-rich.

24. Comparison of Measurements to Model Model and measurements show close relationship. Open symbol: HNO3-H2O Solid symbol: ternary system

25. Summary/ Conclusion • Liquid stratospheric aerosols exist particularly in the polar regions and has a significant effect on ozone depletion • Rather than being pure H2SO4-H2O, aerosols can absorb large amounts of HNO3, HCl, HBr, and HOCl • The solubilities of these gases increase with decreasing temperature owing to the combined effect of the temperature itself and the decreasing H2SO4 concentration in the aerosol • Composition of aerosol and droplet size are affected by temperature • The activity coefficient models have been used to predict equilibrium thermodynamic properties and to calculations of gas partitioning and liquid aerosol composition

26. THANK YOU

27. THAT'S ALL FOLKS!!!