1. Quantifying Photolysis Rates in the Troposphere and Stratosphere (An Overview) William H. Swartz Department of Chemistry and Biochemistry Friday, November 1, 2002

2. Important Chemical Processes in the Troposphere and Stratosphere Tropospheric Ozone: j-values are critical P : j NO2 (polluted) L : j O3 (remote)

3. Important Chemical Processes in the Stratosphere (continued) Stratospheric Ozone: HCl + ClONO 2  HNO 3(s) + Cl 2(g) Cl 2  2Cl PSC h j-values are critical P : j O2 (tropics) L : j ClOOCl (polar vortex)

4. “j-Values”: Definition NO 2 + h  NO + O ( < 424 nm) “actinic” flux (photons cm -2 s -1 nm -1 ) absorption cross section (cm 2 ) photolysis quantum yield (photons -1 )

5. Components of the Radiation Field Actinic Flux F = A (direct attenuated flux) + B (scattered flux) + C (reflection of direct) + D (reflection of scattered) (Adapted from Meier et al. [1982])

6. Factors Affecting Actinic Flux solar zenith angle observer altitude ozone profile/amount other absorbers/scatterers (O 2, air) surface reflectivity (albedo) surface altitude aerosol morphology/optical properties cloud morphology/optical properties (including polar stratospheric clouds) atmospheric refraction

7. Sensitivity: Surface Albedo/Height [Swartz et al., 1999]

8. Sensitivity: Ozone Profile [Swartz et al., 1999]

9. Determining j-Values Photolysis Rate Coefficient Chemical Actinometry Radiative Transfer Modeling Radiometry Irradiance Actinic Flux Filter Radiometer Spectroradiometer Eppley Radiometer Why measurements? Why modeling? (measure chemical change) (measure solar flux) (model solar flux)

10. APL Radiative Transfer Model developed over 20+ years, for the calculation of j-values in the stratosphere and troposphere [Anderson and Meier, 1979; Meier et al., 1982; Anderson, 1983; Anderson and Lloyd, 1990; Anderson et al., 1995; DeMajistre et al., 1995; Swartz et al., 1999] direct solar deposition and reflection from Lambertian surface calculated in a spherical, refracting atmosphere multiple scattering using a plane-parallel approximation integral solution to radiative transfer parameterization of solar transmission through O 2 Schumann–Runge bands (175–204 nm) developed by R. DeMajistre, based on work of K. Minschwaner wavelength range: 175–850 nm 75 altitude layers, 0–120 km

11. Objectives How do various factors affect j-values important to the ozone balance of the troposphere and stratosphere? How well can we measure/model j-values? How well can we model j-values with the APL model, over a range of wavelengths, altitudes, and solar zenith angles? Can we use stellar occultation remote sensing to measure polar stratospheric ozone loss rates? How can j-value measurement and modeling help elucidate factors influencing photochemical ozone loss within the polar vortex? 1 2 j-Values Polar Ozone Loss

12. POLARIS 1997 IPMMI 1998 SOLVE 1999/2000 surface; low SZA lower strat; moderate SZA lower strat; high SZA

13. Is j NO2 Known Accurately Enough? The State of the Art?! [Lantz et al., 1996]

14. International Photolysis Frequency Measurement and Modeling Intercomparison (IPMMI) NCAR Marshall Field Site, 39°N 105°W, elevation: 1.8 km; June 15–19, 1998 Objectives: j [NO 2  NO + O], j [O 3  O 2 + O( 1 D)], spectral actinic flux. Measurements by 21 researchers (US, UK, Germany, New Zealand). Modeling by 18 researchers (US, UK, Canada, Germany, Austria, Netherlands, France, Norway).

15. 1.Measure j NO2 at the surface and compare with other measurements 2.Model j NO2 and j O3 at the surface with APL model 3.Evaluate model by comparing modeled j-values with measurements 4.Evaluate model by comparing modeled j-values with other models My Objectives

16. IPMMI: Measurements and Modeling Photolysis Rate Coefficient Chemical Actinometry Radiative Transfer Modeling Radiometry Irradiance Actinic Flux Filter Radiometer Spectroradiometer Eppley Radiometer (measure chemical change) (measure solar flux) (model solar flux)

17. IPMMI Measurement Site Photo by Chris Cantrell (NCAR)

18. UMD j NO2 Actinometer Schematic NO 2 + h  NO + O

19. Trailer #2 UMD Actinometer

20. inside on top quartz photolysis tube

21. UMD j NO2 Actinometer Data

22. June 15–19, Overlaid High day-to-day precision in clear-sky periods.

23. UMD vs. NCAR Actinometers June 16 June 19 NCAR actinometer failed

24. j NO2 Measurement Comparison vs. Composite Actinometer JPL97 Harder et al. 97 Larger NO 2 absorption cross sections lead to better spectroradiometer–actinometer agreement.

25. IPMMI June 19 Model Specifications aerosol optical depth: aerosol single-scattering albedo: fraction of photons scattered aerosol asymmetry factor: 1 = completely forward-scattering, 0 = isotropic scattering, -1 = completely backward-scattering aerosol Ångström parameter:  AOD dependence (APL*) (APL)

26. Model vs. Measurement: Effects of Aerosol Optical Depth Though optically thin, aerosols did have a measurable impact on j NO2.

27. j NO2 Model Comparison (June 19) Excellent overall agreement with TUV and model consensus. Larger NO 2 absorption cross sections lead to better model–actinometer agreement. (ACD  TUV) Good high-SZA behavior. “composite” actinometer + 

28. j O3 Model Comparison (June 19) Excellent overall agreement with TUV and model consensus, when IPMMI aerosol specification and ATLAS extraterrestrial solar flux are used.

29. IPMMI: Summary & Conclusions first “blind,” international intercomparison of many j-value measurement and modeling techniques UMD chemical actinometer measured j NO2 with excellent precision, and in good agreement with NCAR actinometer APL model calculated j NO2 in excellent agreement with spectroradiometers (<1–2% on average) APL model calculated j O3 in excellent agreement with actinometer and spectroradiometers (<1–2% on average) spectroradiometers and models underestimated actinometer j NO2 by a significant amount (APL model –14%; though within combined uncertainties) larger NO 2 absorption cross sections (e.g., Harder et al. [1997]) lead to better agreement—We need to re-evaluate laboratory measurements! aerosol parameters must be accurately determined in order to reach model–measurement agreements of <~5% ATLAS extraterrestrial irradiance gives best j-value agreement (esp. j O3 )

30. Arctic Ozone Depletion [Newman et al., 1997]

31. Summertime Arctic Ozone Loss [Lloyd et al., 1999]

32. Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) Based in Fairbanks, Alaska, 65°N 148°W; April–September 1997 Objectives: evaluate (measure and model) naturally occurring summertime ozone loss at high northern latitudes, to determine contributions from chemical loss cycles and transport. NASA ER-2 high-altitude aircraft, balloons, ground-based, and space-based observations.

33. 1.Model j-value sensitivity in the lower stratosphere 2.Model j NO2 and j O3 along ER-2 flight tracks (20 km) with APL model 3.Evaluate model by comparing modeled j-values with measurements, particularly in light of modeled sensitivities New Challenges: Characterizing aircraft geophysical environment My Objectives

34. Sensitivity: Ozone Profile [Swartz et al., 1999]

35. POLARIS: Measurements and Modeling Photolysis Rate Coefficient Chemical Actinometry Radiative Transfer Modeling Radiometry Irradiance Actinic Flux Filter Radiometer Spectroradiometer Eppley Radiometer (measure chemical change) (measure solar flux) (model solar flux)

36. CPFM CPFM Spectroradiometer (Environment Canada) surface albedo overhead ozone column j-values

37. j NO2 along June 29, 1997 Flight Track [Swartz et al., 1999] APL CPFM, APL TOMS vs. CPFM

38. POLARIS: Summary & Conclusions modeled sensitivity of j NO2 and j O3 to surface albedo, surface altitude, total ozone, ozone and temperature profiles, and refraction, in the context of the POLARIS mission j NO2 : albedo > surface altitude » total ozone (at 20 km) j O3 : total ozone » albedo > surface altitude (at 20 km) j NO2 : APL CPFM > CPFM by 6%; APL TOMS > CPFM by 9% (average) j O3 : APL CPFM > CPFM by 7%; APL TOMS > CPFM by 1% (average) model–measurement agreement has improved to the point where variability along flight tracks can be attributed to geophysical variability

39. SAGE III Ozone Loss and Validation Experiment (SOLVE) Based in Kiruna, Sweden, 68°N 20°E; November 1999–March 2000 Objectives: study the development of the polar vortex and PSCs, quantify chlorine activation, and measure and model ozone loss. NASA ER-2 high-altitude and DC-8 aircraft, balloons, ground-based, and space-based observations.

40. 1.Add new geophysical inputs to the APL model 2.Model j NO2 and j O3 along ER-2 flight tracks (20 km) with APL model 3.Model j NO2 and j O3 along DC-8 flight tracks (11 km) with APL model 4.Evaluate model by comparing modeled j-values with measurements New Challenges: Twilight conditions (wintertime); fewer direct ancillary measurements My Objectives

41. APL Model Input Data ModeAlbedoOzone APL clim climatologyclimatology APL TOMS TOMSTOMS (total ozone) APL POAM TOMSPOAM III (O 3 –PV reconstruction) APL CPFM CPFMCPFM (overhead ozone, TOMS total) APL clim*, APL TOMS*, APL POAM*, and APL CPFM* also use in situ ozone.

42. SOLVE: Measurements and Modeling Photolysis Rate Coefficient Chemical Actinometry Radiative Transfer Modeling Radiometry Irradiance Actinic Flux Filter Radiometer Spectroradiometer Eppley Radiometer (measure chemical change) (measure solar flux) (model solar flux)

43. SAFS Spectroradiometer (DC-8) (NCAR) (downwelling) (upwelling)

44. Model–SAFS Agreement (DC-8) j NO2 j O3 TOMS albedo and POAM III O 3 reconstructions, as well as in situ O 3, lead to the best agreements with SAFS.

45. Attenuation of (Measured) j NO2 Outlying points (from PSC flights) indicate attenuated actinic flux, relative to clear-sky model calculations.

46. Polar Stratospheric Clouds (PSCs)

47. SOLVE: Summary & Conclusions unique set of measured j-values at high SZAs in the wintertime Arctic new temperature/pressure/ozone/albedo climatologies, POAM III O 3 –PV reconstructions, and in situ O 3 constraints added to model measured O 3 (POAM, in situ) and albedo (TOMS) were superior to climatologies for calculating j-values in nearly all cases j NO2 : model–SAFS agreement: 2–4% ( 85°) (average) j O3 : model–SAFS agreement: 0–13% ( 85°) (average) attenuation of j NO2 up to 75% (attributed to PSCs)

48. Objectives (revisited) How do various factors affect j-values important to the ozone balance of the troposphere and stratosphere? How well can we measure/model j-values? How well can we model j-values with the APL model, over a range of wavelengths, altitudes, and solar zenith angles? Can we use stellar occultation remote sensing to measure polar stratospheric ozone loss rates? How can j-value measurement and modeling help elucidate factors influencing photochemical ozone loss within the polar vortex? 1 2 j-Values Polar Ozone Loss

49. Photochemical Ozone Loss using MSX/UVISI Stellar Occultation (during SOLVE) MSX UVISI

50. Extinction: Refraction:

51. Observed Stellar Spectra Minimum Ray Height (km) Wavelength (nm) Star Equivalent Brightness (R/nm)

52. Sampling of the Polar Region during SOLVE 25 in-vortex occultations, Jan 23–Mar 4

53. MSX–POAM III Ozone Comparison POAM III ozone based on ozone–PV reconstruction. [Swartz et al., 2002]

54. Air Parcel Trajectories Jan 15–Mar 31 Diabatic forward and back trajectories of air parcels sampled with the January 23 occultation.

55. Ozone Change since Jan 23 [Swartz et al., 2002]

56. Ozone Loss using Individual Trajectories Average ozone loss rates on 3 surfaces derived from occultation measurements and related by individual diabatic trajectories. [Swartz et al., 2002]

57. SOLVE Ozone Loss Profiles [Swartz et al., 2002]

58. Stellar Occultation Summary & Conclusions first science application of space-based stellar occultation 25 profiles within the polar vortex during SOLVE good temperature agreement with UKMO analysis good ozone agreement with POAM III ozone–PV reconstructions analysis using diabatic descent trajectory calculations to derive photochemical ozone loss rates in the Arctic during SOLVE: up to ~24 ppbv/day (average) at 400–500 K over 1/23/2000 to 3/4/2000, or about 1 ppmv, consistent with other analyses demonstrates the utility of extinctive–refractive stellar occultation for ozone monitoring, having several advantages over other techniques

59. Objectives (revisited) How do various factors affect j-values important to the ozone balance of the troposphere and stratosphere? How well can we measure/model j-values? How well can we model j-values with the APL model, over a range of wavelengths, altitudes, and solar zenith angles? Can we use stellar occultation remote sensing to measure polar stratospheric ozone loss rates? How can j-value measurement and modeling help elucidate factors influencing photochemical ozone loss within the polar vortex? 1 2 j-Values Polar Ozone Loss

60. What Are the (Optical) Effects of PSCs on Photolysis and Ozone Loss? PSCs, over Kiruna, Sweden, January 2000 Photo by Jim Ross (NASA/Dryden)

61. Identification of PSC Effects Modeled/measured j NO2 > 1.18 considered PSC-attenuated.

62. Temperature Dependence March 8, 2000 all flights: PSC attenuation coincides with cold temperatures (13–25 km) relative to the saturation point of nitric acid trihydrate (NAT).

63. j-Value f direct Dependence j NO2 a b c PSC attenuation as a function of j direct /j total (f direct ).

64. Slant Path (SZA) Dependence SZA dependence follows Beer–Lambert relationship as a function of the slant path (through 13–25 km).

65. 2-D Fit PSC effect as functions of j-value direct fraction and slant path only.

66. ClOOCl Loss Cycle and j ClOOCl Source: P. A. Newman (NASA/Goddard) PSC-affected vs. clear-sky j ClOOCl.

67. PSC Probability UKMO meteorological temperature fields January 25, 2000; 68.1 mb (18 km)

68. Diurnal PSC j ClOOCl Effect × P PSC January 25, 2000

69. Diurnally Integrated j ClOOCl Effect January 25, 2000; 68.1 mb (18 km) polar night vortex edge Photolysis affected within the cold vortex, when the Sun is present. Integrated photolysis:

78. Vortex-Averaged j ClOOCl Effect

79. SOLVE: Summary & Conclusions attenuation of j NO2 up to 75% (attributed to PSCs) attenuation correlated with cold temperatures along solar line of sight attenuation also related to the slant path through PSC layer putative PSCs have up to 10% effect on daily ClOOCl photolysis (  ozone loss) within the Arctic polar vortex, during SOLVE we are ready and in a unique position to accurately model j-values during SOLVE-2, even in the presence of PSCs….

80. SOLVE-2 (2003): Modeling is All There Is Photolysis Rate Coefficient Chemical Actinometry Radiative Transfer Modeling Radiometry Irradiance Actinic Flux Filter Radiometer Spectroradiometer Eppley Radiometer (measure chemical change) (measure solar flux) (model solar flux)

81. Final Remarks if you want to get modeled j-values right, you need to know: altitude, solar zenith angle, day of year (Earth–Sun distance), ozone profile, pressure/temperature profile, surface altitude, spectral surface albedo, spectral aerosol properties (optical depth, single-scattering albedo, scattering phase function)…in cloud-free skies we need to learn how to better handle clouds, including PSCs we need to measure the optical effects of PSCs throughout the stratosphere and model their impact in chemistry–transport models we need to consider using stellar occultation as a means of monitoring long-term trends in ozone

82. Acknowledgments COLLABORATORS: Measurements: Russ Dickerson, Jeff Stehr, Shobha Kondragunta (UM) Modeling: Steve Lloyd, Don Anderson, Tom Kusterer (APL) Data Analysis: Sam Yee, Ron Vervack (APL) Paul Newman (Goddard) IPMMI: Rick Shetter, Sasha Madronich (NCAR) POLARIS: Tom McElroy, Clive Midwinter (Environment Canada) SOLVE: Rick Shetter (NCAR) Karl Hoppel (NRL), Cora Randall (LASP) Stacey Hollandsworth Frith, Gordon Labow (Goddard) $$$: NASA OES, C 4 (NSF), APL, …. T = –30°C

83. Acknowledgments ADVISOR: Russ Dickerson (UM) MENTORS: Steve Lloyd (APL) Don Anderson (APL  NASA/HQ) T = –30°C