Introduction

Instrumentation

Results

Conclusion

References

Acknowledgements

Contact Information

Joseph Mendonca

60 St. George St.,

Toronto, Ontario,

M5S 1A7, Canada

Tel: 416-946-0869

Email: joseph.mendonca@utoronto.ca

The Bruker 125HR Fourier Transform Infrared

Spectrometer (which will be called the PEARL

FTS) is located at the Polar Environmental

Atmospheric Research Laboratory (PEARL) in

Eureka, Nunavut.

The PEARL FTS measures absorption spectra from

direct solar radiation in both the mid-infrared region

(2006-present) and the near-infrared region (Sept

2009-present).

Total Columns of CO2 are retrieved from spectra

recorded in the near-infrared region.

The optical layout of the instrument is based on a

modified Michelson interferometer.

As of 2010, the concentration of carbon dioxide (CO2)

in the atmosphere has risen by 109 ppm (WMO GHG

report, 2011) from its pre-industrial concentration of

280 ppm (Jacob, 1999). Atmospheric CO2 is

responsible for ~85% increase in the radiative forcing,

over the past decade (WMO GHG report, 2011). For

this reason it is important to monitor the amount of

CO2 in the atmosphere.

One of the goals of the Total Carbon Column

Observing Network (TCCON) is to make precise and

accurate measurements of CO2, in order to identify

regional sources and sinks of CO2 (Wunch et al., 2011).

In order to do so, measurements of the column-

averaged abundance of CO2 (XCO2) need to be made

with a precision of 0.3% (Hartmann et al., 2009).

The work done by Hartmann et al. (2009) shows that

one has to take into account line-mixing in the forward

spectral model if one wants to improve the precision of

XCO2 retrieved from atmospheric spectra. Line-mixing

occurs at pressures where lines are broadened enough

to overlap, altering the spectral shape calculated by the

forward spectral model (Boulet., 2004). Excess CO2 is

retrieved from absorption in the troughs between lines

where line-mixing has its largest effects (Hartman et

al., 2009). The effect of line-mixing becomes more

prevalent as one measures through larger amounts of

air mass. In this study I will use the line-mixing

software of Lamouroux et al. (2010), as implemented

in GGG/GFIT, to retrieve XCO2 from solar absorption

spectra recorded by the PEARL FTS for different

amounts of air mass (which is proportional to the solar

zenith angle (SZA) that the measurement is made at).

Voigt Line-mixing

Spectral Fits at SZA=86o

Figure 1. Shows the spectral fits of the 6220 cm-1 band (top 2

plots) and 6339 cm-1 band (bottom 2 plots) using a Voigt line

profile and Line-mixing for a spectrum recorded at SZA of 86o.

Including line-mixing in the spectral fitting process

improved the agreement between the measured and

the calculated spectrum.

However, one can see from Figure 1 that

significant features in the residuals still remain.

Comparison of XCO2 Retrieved With and Without Line-mixing for SZA>85o

Figure 2. The plot shows the amount of XCO2 retrieved using

the standard Voigt line profile and line-mixing. The bottom plot

shows the difference in XCO2 retrieved.

As seen in Figure 2, more XCO2 is retrieved as SZA

increases when the standard Voigt line profile is

used in the spectral fitting process (XCO2 increases

by about 6 ppm).

When line-mixing is taken into account less XCO2

is retrieved but XCO2 is still increasing as a function

of SZA.

Figure 3. Shows the spectral fits of the 6220 cm-1 band (top 2

plots) and 6339 cm-1 band (bottom 2 plots) using a Voigt line

profile and line-mixing for a spectrum recorded at SZA of 69o.

Voigt Line-mixing

Spectral Fit at SZA=69o

Including line mixing in the spectral fitting process

when the SZA=69o does not improve the spectral fits

as seen in Figure 3.

Figure 4. The plot shows the amount of XCO2 retrieved using

the standard Voigt line profile and line-mixing. The bottom plot

shows the difference in XCO2 retrieved.

Comparison of XCO2 Retrieved With and Without Line-mixing for SZA between 68o and

71.5o

As seen in Figure 4, including line-mixing slightly

decreases the amount of XCO2 retrieved.

The amount of XCO2 retrieved as a function of

SZA in Figure 4 remains almost constant compared

to the increase seen in Figure 2.

At SZA<71.5 the effect of line-mixing on the

spectral fits diminishes.

Retrievals of XCO2 from Eureka spectra are

influenced by line-mixing at SZA 85-87 and to a

lesser extent SZA 68-71.5.

The influence of line-mixing on the spectral fits

increases as the amount of air mass increases.

Using the line-mixing software improves the

spectral fits but systematic features in the residuals

still remain.

The investigation of the impact of line-mixing must

be done at other TCCON sites for various SZA (air

mass measured through).

Boulet, C. (2004): Collisional effects on spectral line-shape. Comptes Rendus Physique 5, (2),

Pages: 201-214.

Hartmann, J.-M. , Tran, H. and Toon, G. C. (2009): Influence of line mixing on the retrievals of

atmospheric CO2 from spectra in the 1.6 and 2.1 μm regions, Atmos. Chem. Phys., 9, 7303–7312.

Lamouroux, J., Tran, H., Laraia, A.L., Gamache, R.R., Rothman, L.S., Gordon, I.E., Hartmann, J.-

M.: Updated database plus software for line-mixing in CO2 infrared spectra and their test using

laboratory spectra in the 1.5–2.3 μm region, J. Quant. Spectrosc. Radiat. Transfer, DOI:

10.1016/j.jqsrt.2010.03.006, 2010.

Rodgers, C. D. (2000): Inverse Methods for Atmospheric Sounding: Theory and Practice, Volume 2

of Series on Atmospheric, Oceanic and Planetary Physics, World Scientific Co. Pte. Ltd.

WMO Secretariat (2011): WMO GREENHOUSE GAS BULLETIN: The State of Greenhouse

Gases in the Atmosphere Based on Global Observations through 2010, : http://www.wmo.int/gaw,

November 2011

Wunch, D., G.C. Toon, J.-F.L. Blavier, R.A. Washenfelder, J. Notholt, B.J. Connor, D.W.T.

Griffith, V. Sherlock, P.O. Wennberg. The Total Carbon Column Observing Network. Phil. Trans.

R. Soc. A, 369, 2011 .

CANDAC and PEARL are supported by: ARIF, AIF/NSRIT, CFCAS, CFI, CSA, EC, GOC-IPY, NSERC, OIT, ORF, INAC, and PCSP Logistical and operational support at Eureka: CANDAC operators, the team at Environment Canada’s Weather Station, CANDAC/PEARL PI J.R. Drummond, CANDAC data manager Yan Tsehtik, and PEARL site manager Pierre Fogal. Canadian Arctic ACE Validation Campaigns (led by PI Kaley Walker): CSA, EC, NSERC, and NSTP GOSAT Validation: CSA, JAXA, NIES, and MOE. Special thanks to David Griffith, Ron Macatangay, Nick Deutscher, Debra Wunch, Geoff Toon, and Vanessa Sherlock for all the help they provided.