Quantifying Photolysis Rates in the Troposphere and Stratosphere (An Overview)

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: P : jNO2(polluted) L : jO3(remote) j-values are critical

3. Important Chemical Processes in the Stratosphere (continued) PSC HCl + ClONO2 HNO3(s) + Cl2(g) h Cl2  2Cl Stratospheric Ozone: P : jO2(tropics) L : jClOOCl(polar vortex) j-values are critical

4. “j-Values”: Definition NO2 + h NO + O ( < 424 nm) “actinic” flux (photons cm-2 s-1 nm-1) absorption cross section (cm2) 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 (O2, 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 Why modeling? Why measurements? Radiative Transfer Modeling Chemical Actinometry (measure chemical change) (model solar flux) Photolysis Rate Coefficient Radiometry (measure solar flux) Actinic Flux Irradiance Spectroradiometer Eppley Radiometer Spectroradiometer Filter Radiometer

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 O2 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 1 j-Values 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? 2 Polar Ozone Loss

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

13. Is jNO2 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 [NO2 NO + O], j [O3  O2 + O(1D)], 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. My Objectives • Measure jNO2 at the surface and compare with other measurements • Model jNO2 and jO3 at the surface with APL model • Evaluate model by comparing modeled j-values with measurements • Evaluate model by comparing modeled j-values with other models

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

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

18. UMD jNO2Actinometer Schematic NO2 + h NO + O

19. Trailer #2 UMD Actinometer

20. UMD Actinometer on top inside quartz photolysis tube

21. UMD jNO2Actinometer Data

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

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

24. jNO2 Measurement Comparisonvs. Composite Actinometer JPL97 Harder et al. 97 JPL97 Harder et al. 97 Larger NO2 absorption cross sections lead to better spectroradiometer–actinometer agreement.

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

26. Model vs. Measurement:Effects of Aerosol Optical Depth Though optically thin, aerosols did have a measurable impact on jNO2.

27. “composite” actinometer Good high-SZA behavior. jNO2 Model Comparison (June 19) +  (ACDTUV) Excellent overall agreement with TUV and model consensus. Larger NO2 absorption cross sections lead to better model–actinometer agreement.

28. jO3 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 jNO2 with excellent precision, and in good agreement with NCAR actinometer • APL model calculated jNO2 in excellent agreement with spectroradiometers (<1–2% on average) • APL model calculated jO3 in excellent agreement with actinometer and spectroradiometers (<1–2% on average) • spectroradiometers and models underestimated actinometer jNO2 by a significant amount (APL model –14%; though within combined uncertainties) • larger NO2 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. jO3)

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. My Objectives • Model j-value sensitivity in the lower stratosphere • Model jNO2 and jO3 along ER-2 flight tracks (20 km) with APL model • Evaluate model by comparing modeled j-values with measurements, particularly in light of modeled sensitivities • New Challenges: • Characterizing aircraft geophysical environment

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

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

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

37. jNO2 alongJune 29, 1997 Flight Track APLCPFM, APLTOMS vs. CPFM [Swartz et al., 1999]

38. POLARIS: Summary & Conclusions • modeled sensitivity of jNO2 and jO3 to surface albedo, surface altitude, total ozone, ozone and temperature profiles, and refraction, in the context of the POLARIS mission • jNO2: albedo > surface altitude » total ozone (at 20 km) • jO3: total ozone » albedo > surface altitude (at 20 km) • jNO2: APLCPFM > CPFM by 6%; APLTOMS > CPFM by 9% (average) • jO3: APLCPFM > CPFM by 7%; APLTOMS > 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. My Objectives • Add new geophysical inputs to the APL model • Model jNO2 and jO3 along ER-2 flight tracks (20 km) with APL model • Model jNO2 and jO3 along DC-8 flight tracks (11 km) with APL model • Evaluate model by comparing modeled j-values with measurements • New Challenges: • Twilight conditions (wintertime); fewer direct ancillary measurements

41. APL Model Input Data ModeAlbedoOzone APLclim climatology climatology APLTOMS TOMS TOMS (total ozone) APLPOAM TOMS POAM III (O3–PV reconstruction) APLCPFM CPFM CPFM (overhead ozone, TOMS total) APLclim*,APLTOMS*,APLPOAM*, and APLCPFM* also use in situ ozone.

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

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

44. Model–SAFS Agreement (DC-8) jNO2 jO3 TOMS albedo and POAM III O3 reconstructions, as well as in situ O3, lead to the best agreements with SAFS.

45. Attenuation of (Measured) jNO2 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 O3–PV reconstructions, and in situ O3 constraints added to model • measured O3 (POAM, in situ) and albedo (TOMS) were superior to climatologies for calculating j-values in nearly all cases • jNO2: model–SAFS agreement: 2–4% (<85°), 4–6% (>85°) (average) • jO3: model–SAFS agreement: 0–13% (<85°), 3–15% (>85°) (average) • attenuation of jNO2 up to 75% (attributed to PSCs)

48. Objectives (revisited) 1 j-Values  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?   2 Polar Ozone Loss

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

50. Extinction: Refraction: