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