BrO Retrievals for UV-Visible Ground-Based Measurements
description
Transcript of BrO Retrievals for UV-Visible Ground-Based Measurements
BrO Retrievals for UV-Visible Ground-Based Measurements
Cristen Adams1, Annemarie Fraser1, Kimberly Strong1, Robyn Schofield2
1Department of Physics, University of Toronto2 Alfred Wegener Institute, Potsdam
ARC-IONS WorkshopJan 7, 2009
Outline
Measurement site & the instruments Differential slant column density (DSCD)
retrievals Vertical column density (VCD) retrieval
techniques1. Langley plot
2. Optimal estimation profile retrieval Future Work: Tropospheric BrO
Measurement site
M. Okraszewski
Polar Environment Atmospheric Research Laboratory (PEARL)
Run by the Canadian Network for the Detection of Atmospheric Change (CANDAC)
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 FraserPhoto by Annemarie Fraser
CCD Diffraction GratingsFocusing 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
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, NO2, OClO, Ring, ozone at 241 K, ozone at 221 K, O4, 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
Day 80 Day 90 Day 100 Day 110 Day 120
DOD SZA = 80
DOD SZA = 85
DOD SZA = 90
Wavelength (nm)
Wavelength (nm)
Wavelength (nm)
Wavelength (nm)
Wavelength (nm)
ref=79.8o ref=75.9o ref=72.1o ref=68.6o ref=65.3o
BrO fits improve as noon-time sun gets higher
BrO fits improve toward twilight
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
Retrieval of Vertical Column Densities (VCDs) Using Langley Plots
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!
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
Retrieval of VCDs Using Profile Code
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 75oN
Peak in troposphere of 1x107mol/cm2
Still have to run tests to determine sensitivities of retrievals to a priori, uncertainties, etc… (ie: this is a very preliminary 1st try run!)
Retrieved Total Columns for High Sun
Averaging kernels for full 80.9o profile
Good sensitivity to stratosphere
1.3 degrees of freedom
Tropospheric partial columns mostly derived from a priori.
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
Future workFuture 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.
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
Averaging Kernels for Total Columns
Day 100 Afternoon