1. Copyright © 2010 R. R. Dickerson1 LECTURE 20 Atmospheric Odd Hydrogen, HOx AOSC 637 Spring 2010 Atmospheric Chemistry Russell R. Dickerson

2. Odd Hydrogen: Outline Importance Chemistry Sources Sinks Reservoirs and Ratios Detection Techniques Fluorescence FAGE DOAS Chem Amplification Global Budget Calculations Remaining Challenges Bibliography

3. Copyright © 2010 R. R. Dickerson3 Odd Hydrogen Importance: Ozone destruction in both the stratosphere and trospsphere. Removal of NOx, ClOx, CO, VOC’s, SOx, HCFC’s The most important species for transformations. Many pollutants have no other sink. Chemistry Sources (remember what HCO and H do)

4. Copyright © 2010 R. R. Dickerson4 HYDROCARBRONS REACTIVITY FOR URBAN SMOG (OZONE) FORMATION HYDROCARBONk(O) k(O ₃ ) k(OH) (All units: cm³s ⁻ ¹) Methane, CH ₄ 1.1x10 ⁻ ¹ ⁷ SLOW 7.9x10 ⁻ ¹ ⁵ Ethane, C ₂ H ₆ 9.6x10 ⁻ ¹ ⁶ SLOW 2.7x10 ⁻ ¹³ Propane, C ₃ H ₈ 1.5x10 ⁻ ¹ ⁴ SLOW 1.2x10 ⁻ ¹² Butane, C ₄ H ₁₀ 3.1x10 ⁻ ¹ ⁴ SLOW 2.3x10 ⁻ ¹² Hexane, C ₆ H ₁₄ 9.5x10 ⁻ ¹ ⁴ SLOW 5.7x10 ⁻ ¹² 2,3 Dimethyl butane (C ₆ H ₁₄ ) 2.1x10 ⁻ ¹³ SLOW 6.3x10 ⁻ ¹² Ethene, C ₂ H ₄ 8.4x10 ⁻ ¹³1.8x10 ⁻ ¹ ⁸ 8.0x10 ⁻ ¹² Propene, C ₃ H ₆ 3.6x10 ⁻ ¹²1.1x10 ⁻ ¹ ⁷ 2.5x10 ⁻ ¹¹ Benzene, C ₆ H ₆ 1.6x10 ⁻ ¹ ⁴ SLOW 1.2x10 ⁻ ¹² Toluene, C ₇ H ₈ 5.9x10 ⁻ ¹ ⁴ SLOW 6.4x10 ⁻ ¹²

5. Copyright © 2010 R. R. Dickerson5 Odd Hydrogen To calculate OH we need: j(O 3 ), [O 3 ], [H 2 O] Sinks

6. Copyright © 2010 R. R. Dickerson6 Reservoir species, control ratios of OH/HO 2

7. Copyright © 2010 R. R. Dickerson7 RONO 2

8. Copyright © 2010 R. R. Dickerson8 Steady State Approximations: From Logan, JGR (1981)

9. Copyright © 2010 R. R. Dickerson9 Calculated mean OH (x10 -6 cm -3 ) in a CH 4, CO, O 3 atmosphere, from Crutzen’s model at MPI. Why does het max occur in the LFT in the tropics?

10. Copyright © 2010 R. R. Dickerson10 Potential Energy Curves for OH Chem. Phys. 237(1-2), 123-138 (1998) DOI:10.1016/S0301-0104(98)00219-5

11. Copyright © 2010 R. R. Dickerson11 Solar flux induced fluorescence was successful for the detection of OH in the Stratosphere (Anderson JGR, 7820, 1971). Anderson put a scanning spectrometer on the nose of a rocket and measured the emission at 308 nm due to solar excitation. In situ resonance fluorescence with a microwave discharge lamp worked in the strat (Anderson GRL, 1976), but not in the trop.

12. Copyright © 2010 R. R. Dickerson12 FAGE, Fluorescence Assay by Gas Expansion, is essentially laser-induced fluorescence at low pressure.

13. Copyright © 2010 R. R. Dickerson13 Radiation at 308 nm photolyzes O 3 to O( 1 D). Early attempts to measure OH via fluorescence failed – why?

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16. Copyright © 2010 R. R. Dickerson16 Diurnal variation of OH measured using LIF (o) and DOAS () during POPCORN (Adapted from Hofzumahaus et al., 1998).

17. Copyright © 2010 R. R. Dickerson17 Correlation plot of all LIF OH data versus the photolysis frequency of ozone, j(O 1 D). (Adapted from Holland et al., 1998).

18. Copyright © 2010 R. R. Dickerson18

19. Copyright © 2010 R. R. Dickerson19 Atmospheric absorption spectra measured using DOAS as a function of time of day (UT). Solid lines are reference absorption spectra of OH radicals fitted to the measurements (Adapted from Dorn et al., 1996).

20. Copyright © 2010 R. R. Dickerson20

21. Copyright © 2010 R. R. Dickerson21 Dependence of the measured OH concentration on NO 2 during the POPCORN field campaign. To make this behavior visible, the OH data were first normalized with respect to j(O 1 D) and then plotted versus equal log(NO 2 )-intervals of 0.1. Full curve corresponds to the model-calculated dependence. [Adapted from Ehhalt, 1999].

22. Copyright © 2010 R. R. Dickerson22 Comparison of observed and calculated OH concentrations versus NO X during the 1993 Idaho Hill experiment (THOPE). The different model calculations account for different amounts of unmeasured biogenic hydrocarbons [Adapted from McKeen et al., 1997].

23. Copyright © 2010 R. R. Dickerson23 Altitude profiles of measured (open circles) and modeled OH for 10 May 1996 during SUCCESS. Measurements and models are averaged into 0.5 km altitude bins. Models with (dash- dot line) and without (dashed line) acetone are compared. (Adapted from Brune et al., 1998). CH 3 C(O)CH 3 + hv → 2CH 3 + CO

24. Copyright © 2010 R. R. Dickerson24 Calculated mean OH (x10 -6 cm -3 ) in a CH 4, CO, O 3 atmosphere, from Crutzen’s model at MPI. Why does het max occur in the LFT in the tropics?

25. Copyright © 2010 R. R. Dickerson25 Horowitz et al., JGR 2003.

26. Copyright © 2010 R. R. Dickerson26 Prinn et al., Science, 1995. Global mean OH can be calculated from the rate of loss of methyl chloroform, CCl 3 CH 3

27. Copyright © 2010 R. R. Dickerson27 Concentrations of CCl 3 CH 3 continue to fall.

28. Copyright © 2010 R. R. Dickerson28 From NOAA Earth System Research Laboratory http://www.esrl.noaa.gov/gmd/ odgi/

29. Copyright © 2010 R. R. Dickerson29 Calculating Global Mean OH from CH 3 CCl 3 Concentrations Because OH is so hard to measure, we would like to get at the concentration another way [ Prinn et al., 1987; Prinn et al., 1995]. Let’s designing a good experiment – a good OH tracer must have: 1.Only one sink – reaction with OH 2.A lifetime » inter-hemispheric mixing » seasonal variations in OH 1.Well known atmospheric burden 2.Well known production rate 3.Well known rate const, k OH 4.A reliable, precise measurement technique.

30. Copyright © 2010 R. R. Dickerson30 In the original work, Prinn performed a simple global burden calculation of mean [OH]. 1.Assume steady state, i.e., production = loss. 2.Measured or calculated production rate. 3.Loss (= production) = k OH [CH 3 CCl 3 ][OH] 4.Assume [CH 3 CCl 3 ] is constant in time and space (we’ll revisit this later).

31. Copyright © 2010 R. R. Dickerson31 In the original work, Prinn performed a simple global burden calculation of mean [OH]. 1.Assume stready state, i.e., production = loss. 2.Measured or calculated production rate. 3.Loss (= production) = k OH [CH 3 CCl 3 ][OH] 4.Assume [CH 3 CCl 3 ] is constant in time and space. First order estimate (box model) CH 3 CCl 3 + OH → H 2 O + CH 2 CCl 3 k OH = 1.64x10 -12 e (-1520/T) Mean middle trop temp ~ 255 K; k 255 = 4.2x10 -15 cm 3 s -1

32. Copyright © 2010 R. R. Dickerson32 What was the mean [CH 3 CCl 3 ]? LatutudeMixing Ratio (in 1981) 52°N169 (ppt) 45°N163 13°N147 14°S122 41°S117 Lat weighted mean144 ±25 ppt Total tropospheric burden = mass of atmosphere x mean mixing ratio x ratio of molecular weights. 4.0x10 21 g x 1.44x10 -10 x 133.5/29 = 2.65x10 12 g From Prinn et al., (1983).

33. Copyright © 2010 R. R. Dickerson33 What was the mean [CH 3 CCl 3 ]? The global production rate in 1981 was ~5.0x10 11 (±0.5) g yr -1 We don’t know for sure that release = production.

34. Copyright © 2010 R. R. Dickerson34 What was the uncertainty mean [OH] of 1.4x10 6 cm -3 ? 1.Rate Constant ±15% 2.Absolute concentration ± 20% 3.Production rate ± 10% 4.Mean global conc ± 25% 5.Annual variation ± 30% RMS ±50% Using a 12-box model, the global mean OH was estimated to be 9.7 ± 0.5x10 5 cm -3 in 1994 with little temporal change by Prinn et al., 1995. They also derived a residence timje for methylchloroform of 4.8 yr.

35. Copyright © 2010 R. R. Dickerson35 Remaining challenges I.What are the controlling factors in the upper trop.? II.Is the mean OH derived from methyl chloroform correct in light of recent discoveries about Cl chemistry in the trop? III.How will changes in the composition and climate impact the atmosphere’s oxidizing capacity and what unintended consequences await us?

36. Copyright © 2010 R. R. Dickerson36 Bibliography Anderson, J.G., The absolute concentration of OH (X 2 P) in the earth's stratosphere, Geophys. Res. Lett. 3, 165-168, 1976. Brune, W.H. et al., Airborne in-situ OH and HO 2 observations in the cloud-free troposphere and lower stratosphere during SUCCESS, Geophys. Res. Lett., 25, 1701-1704, 1998. Ehhalt, D.H., Photooxidation of trace gases in the troposphere, Phys. Chem. Chem. Phys., 1, 5401-5408, 1999. McKeen et al., Photochemical modeling of hydroxyl and its relationship to other species during the Tropospheric OH Photochemistry Experiment, J. Geophys. Res., 102, 6467-6493, 1997. Dorn, H. P., U. Brandenburger, T. Brauers, M. Hausmann, and D. H. Ehhalt, In-situ detection of tropospheric OH radicals by folded long- path laser absorption. Results from the POPCORN field campaign in August 1994., Geophys. Res. Lett., 23, 2537-2540, 1996. Hard, T.M., L. A. George, and R. J. O'Brien, FAGE Determination of Tropospheric HO and HO 2, J. Atmos. Sciences, 52, 3354-3372, 1995. Hausmann, M., U. Brandenburger, T. Brauers, and H.-P. Dorn, Detection of tropospheric OH radicals by long-path differential-optical-absorption spectroscopy: Experimental setup, accuracy, and precision, J. Geophys. Res., 102, 16011-16022, 1997. Logan, J. A., M. J. Prather, S. C. Wofsy, and M. B. McElroy (1981), Tropospheric chemistry: A global perspecvtive, J. Geophys. Res., 86, 7210-7254. Prinn, R., D. Cunnold, R. Rasmussen, P. Simmonds, F. Alyea, A. Crawford, P. Fraser, and R. Rosen (1987), Atmospheric trends in methylchloroform and the global average for the hydroxyl radical, Science, 238, 945-950. Prinn, R. G., R. F. Weiss, B. R. Miller, J. Huang, F. N. Alyea, D. M. Cunnold, P. J. Fraser, D. E. Hartley, and P. G. Simmonds (1995), Atmospheric Trends and Lifetime of CH 3 CCl 3 and Global OH Concentrations, Science, 269, 187-192.