# Presentation: 223364

## Constraining the ClO / ClOOCl Equilibrium Constant from Aura MLS Measurements of Nighttime ClO

**M.L. Santee, **L. Froidevaux,** **N.J. Livesey, S.P. Sander, G.L. Manney
- Jet Propulsion Laboratory, California Institute of Technology
- Aura Science Team Meeting29 October 2008

- National Aeronautics and Space Administration

## Motivation for Studying ClO / ClOOCl Equilibrium Constant

**Motivation for Studying ClO / ClOOCl Equilibrium Constant**

- The primary ozone loss process in the cold polar lower stratosphere involves ClO and its dimer, ClOOCl:
- ClO + ClO + M  ClOOCl + M
- According to current theory, the partitioning between ClO and ClOOCl in darkness is governed by the thermal equilibrium constant:
*K*eq = *k*f/*k*b = [ClOOCl]/[ClO]2
*K*eq depends inversely on temperature: ClOOCl decomposes at high T
- All laboratory determinations of *K*eq to date have been conducted at temperatures considerably above those relevant for the lower stratospheric winter polar vortices
- The current recommendation, JPL06, extrapolates experimental values to lower stratospheric conditions
- Several recent studies based on in situ and satellite measurements of nighttime ClO have inferred lower values of *K*eq than in JPL06
- We use Aura MLS measurements of enhanced nighttime abundances of ClO to constrain the value of *K*eq

## MLS Nighttime ClO Enhancement: 520 K

**MLS Nighttime ClO Enhancement: 520 K**

- 520 K (~46 hPa, ~19 km) is near the ClO profile peak
- Nighttime enhancement in ClO is observed poleward of 60–65° EqL in all winters
- Antarctic: from late May through September
- Arctic: from December through February
- Nighttime values approach 1 ppbv in the Arctic
- Temperature / activation conditions do not favor such large enhancements in darkness in the Antarctic

## Temperature Dependence of MLS Nighttime ClO

**Temperature Dependence of MLS Nighttime ClO**

- MLS nighttime ClO inside the winter polar vortices generally increases with temperature, as expected
- Here the v2.2 MLS ClO data are corrected for a known negative bias at the lowest retrieval levels
- Focus on points in the vortex core (>70° EqL)
- MLS ClO is compared to equilibrium ClO calculated using selected values of *K*eq from the literature
- ClOx = ClO + 2Cl2O2 taken from SLIMCAT, sampled at MLS locations / times

- EqL

## MLS vs Equilibrium ClO Amounts Based on Selected Keq Values

**MLS vs Equilibrium ClO Amounts Based on Selected *****K*****eq**** Values**

- Focus further on period of peak chlorine activation (ClOx/Cly > 0.9):
- 1 Jan – 15 Feb in Arctic
- 15 Jun – 1 Sep in Antarctic
- Best agreement with MLS data is provided by different models of *K*eq at different times and temperatures
- SLIMCAT substantially overestimates ClOx during the peak activation  period, especially in the Arctic [Santee et al., JGR 113, 2008]
- Reducing assumed ClOx by 50% significantly influences results

## MLS vs Equilibrium ClO Amounts: Seasonal Variations

**MLS vs Equilibrium ClO Amounts: Seasonal Variations**

- Some day-to-day variability seen in agreement between MLS ClO and the equilibrium ClO values
- Differences from MLS are larger at higher temperatures
- The von Hobe et al. [2005] *K*eq uniformly overestimates MLS ClO
- The Plenge et al. [2005] *K*eq typically overestimates MLS ClO, whereas JPL06 typically underestimates it
- Neither consistently provides the best match to MLS data
- No significant trend over the peak chlorine activation season

## Altitudinal, Interannual, and Interhemispheric Variations

**Altitudinal, Interannual, and Interhemispheric Variations**

- Different degrees of agreement seen at different pressure levels
- Plenge et al. [2005] produces slightly better agreement with MLS ClO at 32 hPa
- JPL06 consistently matches MLS ClO better at 68 hPa
- Similar variations with temperature, pressure, and time seen in other Arctic winters
- Essentially similar variations seen in the Antarctic as well
- Temperature range sampled during peak activation period typically much narrower than in the Arctic

## Summary & Future Work

**Summary & Future Work**

- MLS observes substantial enhancements (as much as ~1 ppbv) in ClO in darkness inside the winter polar vortices in both hemispheres
- Preliminary results indicate that the temperature dependence of the MLS nighttime ClO measurements is generally in line with that of the ClO / ClOOCl equilibrium relationship
- However, these results may not ultimately rule out alternative exchange processes with other nighttime chlorine reservoirs having similar temperature dependence
- Contrary to previous studies, using a lower ClO / ClOOCl equilibrium constant, *K*eq, than currently recommended [JPL06] does not consistently improve agreement with MLS nighttime ClO data
- Next: estimate *K*eq directly from MLS nighttime ClO measurements
- Standard expression is:* K*eq = A × exp(B/T)
- We can constrain A, then solve for B as a function of ClOx