1. BrO Retrievals for UV-Visible Ground-Based Measurements Cristen Adams 1, Annemarie Fraser 1, Kimberly Strong 1, Robyn Schofield 2 1 Department of Physics, University of Toronto 2 Alfred Wegener Institute, Potsdam ARC-IONS Workshop Jan 7, 2009

2. Outline Measurement site & the instruments Differential slant column density (DSCD) retrievals Vertical column density (VCD) retrieval techniques 1. Langley plot 2. Optimal estimation profile retrieval Future Work: Tropospheric BrO

3. Measurement site M. Okraszewski Polar Environment Atmospheric Research Laboratory (PEARL) Run by the Canadian Network for the Detection of Atmospheric Change (CANDAC)

4. The two nearly identical instruments UT-GBS University of Toronto ground- based spectrometer Involved in spring field campaigns since 1999 Currently in Eureka PEARL-GBS Installed permanently at PEARL in August, 2006 Seeking NDACC certification Photo by Annemarie Fraser

5. CCD Diffraction Gratings Focusing System Crossed Czerny-Turner Sunlight Lens The GBSs UV-visible Triple-grating spectrometers Cooled CCD detector Automated operation (Labview) TO CCD PEARL-GBS UT-GBS

6. Differential Slant Column Densities Analyzed using WinDOAS developed at BIRA/IASB Analysis performed in 345- 360 nm range Daily reference spectrum at smallest solar zenith angle Fit cross-sections for: BrO, NO 2, OClO, Ring, ozone at 241 K, ozone at 221 K, O 4, and offset (accounts for dark current and stray light in the instrument) Differential slant column density (DSCD): the amount of an absorber present in the path that the light follows to the instrument at twilight minus that at noon. Zenith-sky measurements give mostly stratospheric information, but we retrieve the total column

7. Day 80Day 90Day 100 Day 110 Day 120 DOD SZA = 80 DOD SZA = 85 DOD SZA = 90 Wavelength (nm) ref=79.8 o ref=75.9 o ref=72.1 o ref=68.6 o ref=65.3 o BrO fits improve as noon-time sun gets higher BrO fits improve toward twilight

8. Available Measurements Due to poor signal to noise require  High BrO concentrations  Low sun DSCDs retrieved for  Spring 2005-2007 using lower resolution grating  Spring 2007 and 2008 using higher resolution grating Below detection limit during summer

9. Retrieval of Vertical Column Densities (VCDs) Using Langley Plots

10. Langley Plots Air Mass Factors (AMF): -McLinden et al., 2002 -Calculated from radiative transfer model over diurnally varying input BrO profile - Trop BrO comes mostly from AMF values of 0.5ppt - Very dependent on input ozone profile! DSCD = VCD * AMF - RCD BrO Noontime Profile used in AMF Using DSCDs from UT-GBS zenith-sky only!

11. 2008 BrO VCD’s at Various SZA Still have to assess uncertainties VCD at SZA=90 were previously estimated to be 18.2% Testing underway for retrievals at SZA=85, SZA=80

12. Retrieval of VCDs Using Profile Code

13. Profiling Code R. Schofield et al., JQRST, 2004 Input:  DSCDs, a priori, ozone, T, P, aerosol Output:  profiles at various SZA A priori:  Set of BrO profiles at different SZA  Used 50% uncertainty  Box model for stratosphere (McLinden et al., JGR, 2000) for 75 o N  Peak in troposphere of 1x10 7 mol/cm 2 Still have to run tests to determine sensitivities of retrievals to a priori, uncertainties, etc… (ie: this is a very preliminary 1 st try run!)

14. Retrieved Total Columns for High Sun Averaging kernels for full 80.9 o profile Good sensitivity to stratosphere 1.3 degrees of freedom Tropospheric partial columns mostly derived from a priori.

15. Comparison of Retrieval Techniques Profile AM and PM values at SZA = 80.9 Langley AM and PM values at SZA = 80 OMI data retrieved for midday OMI data provided by K. Chance, T. Kurosu, R. Salawitch

16. Future work Determine best way to retrieve VCDs from zenith-sky measurements Compare zenith-sky VCDs with satellite data Retrieve direct-sun DSCDs Combine direct-sun and zenith-sky measurements to retrieve tropospheric partial column Plan to take test MAX-DOAS measurements in Spring 2009 This suntracker, installed in Feb 2008, allows us to get direct-sun measurements.

17. Acknowledgements We gratefully acknowledge support from the following groups:  PEARL/CANDAC: CFCAS, CFI, EC, MRI, MSC, NSERC, NSIRT, OIT, PCSP, SEARCH  ACE Arctic Campaign: CSA, EC, NSERC, NSTP, CGCS Logistical and operational support at Eureka:  Team at the EC Weather Station  CANDAC/PEARL PI J. Drummond  CANDAC operators Special thanks to the 2007 and 2008 campaign teams, including:  K. Walker for making the ACE validation campaigns happen  R. Batchelor for taking great care of the suntracker for the second part of the campaign  C. Midwinter for designing the suntracker and helping with its installation and calibration at Eureka And to:  C. Fayt and M. Van Roozendael at IASB-BIRA for WinDOAS software  K. Chance, T. Kurosu, and R. Salawitch for OMI data

19. Averaging Kernels for Total Columns Day 100 Afternoon