Ground-based Solar Absorption Studies for the Carbon Cycle Science by Fourier Transform Spectroscopy (CC-FTS) Mission Dejian Fu, K. Sung, C.D. Boone, K. Walker and P.F. Bernath June 19 th, 2007

Introduction - Previous Observations Keeling Curve About the Keeling Curve The concentrations of CO 2 were obtained by in situ surface measurements. It is the first evidence of atmospheric CO 2 increase (Keeling C.D., Tellus, 1960). It shows a persistent year-to-year increase. About the annual cycle of CO 2 Biospheric respiration in winter produces CO 2 Photosynthesis in summer consumes CO 2 a Two plots were drawn using the data downloaded from the link: ftp://ftp.cmdl.noaa.gov/ccg/co2/in-situ/

Introduction - Previous Observations Washenfelder et al. GRL 2003 Dufour et al. JGR 2003 a VMR: Volume Mixing Ratio b FTS: Fourier Transform Spectrometer  Location: Kitt Peak, Arizona, USA  Instrument: 1 meter McMath-Pierce FTS b  Time: CO 2 CH 4 VMR a time series CH 4 VMR a annual cycle CO 2

Satellites are carrying out pioneering studies on global carbon budgets. Orbiting Carbon Observatory, OCO (NASA, 2008)  Three near infrared grating spectrometers  Spectral resolution ~ 0.3 cm -1  Measuring CO 2 only at high (1 km) spatial resolution Greenhouse Gases Observing Satellite, GOSAT (JAXA, Japan, 2008)  Fourier Transform Spectrometer  Low spatial resolution ~ 10 km  Single pixel detector OCO Satellite Missions for Greenhouse Gas Monitoring  Satellite images were downloaded from the homepages of each mission. GOSAT SCIAMACHY on ENVISAT AIRS on AQUA TES on AURA IASI on METOP-A MOPITT on AURA

CC-FTS Mission Next generation satellite mission for carbon cycle science  Heritage from the successful SCISAT-1 mission  Orbits the earth at the tail of ‘A train’  Monitors many species using a satellite- borne FTS: CO 2, CH 4, CO, N 2 O, (four gases listed in IPCC a 2007 report) and O 2  High spatial resolution ~ 1 km  Precision goals under most favorable situation: ~ 0.3% for CO 2 ; ~ 1% for other species  Global coverage  To characterize distribution and variability of sources and sinks on a regional scale Advanced study for the mission on the  Spectral resolution?  Spectral range?  Spectroscopic parameters? … A: CCFTS orbiting the earth and primary modes B: Coverage strategy using a detector array with 8x8 elements and cross-track scanning C: An area 8 km square per FOV covers the area of Kitchener-Waterloo, Canadian twin city with a population of 300,000 D: Footprint on University of Waterloo campus Nadir Glint a Intergovernmental Panel on Climate Change (IPCC)

Ground-based Atmospheric Absorption Spectra In order to determine optimum values for a greenhouse gas mission, advanced studies were accomplished for the mission on  Spectral resolution?  Spectral range?  Spectroscopic parameters?  … Measurement sites  Preliminary comparisons of spectral resolution effects using spectra from Kiruna (67.84ºN, 20.41ºE, and 419 m above sea level), Sweden on 1 st Apr a. Spectral region: 3950 cm -1 to 7140 cm -1 ; Instrumentation: Bruker IFS 120 HR FTS.  Further investigations using spectra from Kitt Peak, Arizona (31.9ºN, 111.6ºW and 2.1 km above sea level) and Waterloo, Ontario (43.5ºN, 80.6ºW and 0.3 km above sea level) a Meier A, et al., Spectroscopic atlas of atmospheric microwindows in the middle infrared (2nd edition). IRF Technical Report No.48, ISSN , Kiruna, Apr 2004.

Atmospheric Absorption Spectra at Kiruna, Sweden  Spectral resolution of a FTS = 1/ (2 x Maximum Optical Path Difference) cm -1 (MOPD 120 cm), the typical resolution setting used by NDACC a FTSs cm -1 (MOPD 50 cm), the resolution setting close to that used by the FTSs in TCCON b cm -1 (MOPD 5/3 cm), the resolution setting used in OCO mission cm -1 (MOPD 5 cm), the resolution setting candidate for the CC-FTS mission CO 2 a The Network for the Detection of Atmospheric Composition Change: b Total Carbon Column Observing Network, Washenfelder R.A., et al. J. Geophys. Res CO

Ground-based Solar Absorption Studies Using Fourier Transform Spectrometers at Two Observatories  Further comparisons of spectral resolution of 0.01 cm -1 and 0.1 cm -1 settings using spectra from two sites. The McMath-Pierce Fourier Transform Spectrometer  Located National Solar Observatory at Kitt Peak, Arizona  A folded cat’s-eye Michelson interferometer housed in a vacuum vessel (Maximum Optical Path Difference: 100 cm; Spectral Range: 550 to 45,000 cm -1 ) and more detail available in the link: The WAO DA8 Fourier Transform Spectrometer  Located at Waterloo Atmospheric Observatory, Waterloo, Ontario  A Michelson interferometer using ABB Bomem’s dynamical alignment techniques (Maximum Optical Path Difference: 25 cm; Spectral Range: 400 to 55,000 cm -1 )

Observations at National Solar Observatory at Kitt Peak, Arizona (31.9ºN, 111.6ºW and 2.1 km above sea level)  Date: July 25 th, 2005  Region: 1700–15000 cm -1  Resolution: 0.01 cm -1  Detector: InSb  Scan Time: ~30 minutes  Filters: RG715  Number of Scans: 2

Observations at Waterloo Atmospheric Observatory at Waterloo, Ontario (43.5ºN, 80.6ºW and 0.3 km above sea level) Sun Tracker WAO DA8 Spectrometer  Date: November 22 nd, 2006  Region: 2000–15000 cm -1  Resolution: 0.1 cm -1  Detector: InSb and Si  Scan Time: ~16 minutes  Filters: RG715  Number of Scans: 20

Spectral Overview of the Atmospheric Absorption Spectra Recorded on 22 nd November 2006 Using WAO DA8 Spectrometer  Spectral resolution 0.1 cm -1  InSb and Si detectors InSb: 2000 to cm -1 Si: 8500 to cm -1  Solar Zenith Angles (SZA)  InSb band  Si band  Number of Scans 20  Scan Time ~16 minutes WAO

Data Analysis – from spectra to retrieved columns Retrieval Code: SFIT2 (version 3.91)  Widely used for the analysis of ground-based infrared solar spectra  Jointly developed at the NASA-Langley Research Center and the National Institute of Water and Atmospheric Research at Lauder, New Zealand.  Employs the optimal estimation method of Rodgers (1976, 1996 and 2000).  Includes the a priori constituent profiles in the retrievals in a statistically sound manner  Allows the simultaneous retrieval of the vertical profile of the target molecule together with the total columns of interfering species Model atmosphere 1)Pressure and Temperature profiles: combining the National Centers for Environmental Prediction (NCEP) data (obtained from the Goddard Auto mailer and the Mass-Spectrometer-Incoherent-Scatter model (MSIS-2000) output. 2)In the retrievals of spectra recorded at WAO: A priori VMRs a from the HALogen Occultation Experiment (HALOE) v.19 satellite data and mid-latitude daytime 2001 Michelson Interferometer for Passive Atmospheric Sounding (MIPAS). In the retrievals of spectra recorded at Kitt Peak: A priori VMRs a from Mark IV balloon FT- IR spectra obtained in northern mid-latitudes by G. C. Toon et al. (1995). Spectroscopic parameters HITRAN 2004 data base [Rothman L.S. et al. 2004] a VMR: Volume Mixing Ratio

Fitting residuals for CO 2 at 1.57  m  Date: 25 th July 2005  Spectral Resolution: 0.01 cm -1  Solar Zenith Angle: 49.1   The fitting residuals are in the same order of Yang et al. 2002, Geophys. Res. Lett. 29(9).  Date: 22 nd November 2006  Spectral Resolution: 0.1 cm -1  Solar Zenith Angle: 66.6  NSOWAO ( , 0 = 6228 cm -1 ) transition Numerous absorption features from CO 2 with weak absorption by H 2 O, HDO and additionally CH 4 Many lines with a wide range of intensities, which provides good retrieval sensitivities in both the stratosphere and troposphere Thermal emission from the atmosphere and instrument are also negligible at these short wavelengths

Averaging Kernel a for CO 2 at 1.57  m NSO WAO  Typical averaging kernels for CO 2 retrievals in the thermal infrared at 14 microns and the near infrared at 1.6 microns (Crisp et al., 2004, Advances in Space Research) a The averaging kernel is the derivative of a derived parameter with respect to its a priori state value, i.e., when this derivative is small (nearly 0) all of the information comes from the a priori and when it is large (near 1) then the information in the retrieval comes mainly from the measured data. The required near surface to upper troposphere sensitivity (much better sampling than those observations thermal infrared region used in the previous missions) is achieved. Only minor differences are seen between spectra with resolutions of 0.01 cm -1 and 0.1 cm -1

Fitting Residuals for CH 4 at 1.68  m  The fitting residuals are of the same order as in Washenfelder et al. 2003, Geophys. Res. Lett. WAO NSO  Date: 25 th July 2005  Spectral Resolution: 0.01 cm -1  Solar Zenith Angle: 49.1   Date: 22 nd November 2006  Spectral Resolution: 0.1 cm -1  Solar Zenith Angle: 66.6  The spectral region from 5880 to 5996 cm -1 was investigated for the CH 4 retrieval.

Averaging Kernel a for CH 4 Degrees of Freedom for signal (DOFS) = 2.58Degrees of Freedom for signal (DOFS) = 2.12 WAO NSO The required near surface to upper troposphere sensitivity (much better sampling than those observations thermal infrared region used in the previous missions) is achieved. Only minor differences are seen between spectra with resolutions of 0.01 cm -1 and 0.1 cm -1 a The averaging kernel is the derivative of a derived parameter with respect to its a priori state value, i.e., when this derivative is small (nearly 0) all of the information comes from the a priori and when it is large (near 1) then the information in the retrieval comes mainly from the measured data.

Fitting residuals for O 2 at 0.76  m  The fitting residuals are in the same order as in Yang et al. 2005, JQSRT. WAO NSO  Date: 25 th July 2005  Spectral Resolution: 0.01 cm -1  Solar Zenith Angle: 49.1   Date: 22 nd November 2006  Spectral Resolution: 0.1 cm -1  Solar Zenith Angle: 66.6   b 1 Σ + g ­ X 3 Σ - g near infrared band ( 0 = cm -1 )  Provide constraints on both the surface pressure and optical path length variations associated with scattering by aerosols in the atmosphere  By taking the ratio of columns of CH 4 and CO 2 to O 2 columns, systematic errors will be reduced as long as they are measured under the same conditions  Also used for cloud detection

Features in the Fitting Residuals  The largest discrepancies between the calculated and the measured transmittances are on the order of a few percent and are observed in the vicinity of the absorption line centers.  Similar systematic fitting residual patterns in terms of positions and amplitudes also appeared in the results of previous work. They mainly arise from the spectroscopic parameters including line intensity, self- and air- broadening coefficients, and self- and air-shift coefficients.  Away from the absorption lines, the fitting residuals from spectra recorded at NSO are generally larger than those obtained using spectra recorded at WAO. This is because the WAO spectra have a higher Signal-to-Noise Ratio (SNR) mainly because of their lower spectral resolution.

 The absolute accuracy of the CO 2 retrievals obtained using spectroscopic parameters from HITRAN 2004 is expected to be limited to ~2% [Devi et al.,. JMS In Press].  Recent studies show the improvements in the CO 2 spectroscopic parameters in the spectral region of 4550 to 7000 cm -1 with a precision of 1% or better.  Very recently, Devi et al. [Devi et al., JMS In Press] made further improvements in the CO 2 spectroscopic parameters for the 6348 cm -1 band by considering line mixing and using speed-dependent Voigt line shape functions. This work by Devi et al. provides the possibility of remote sensing CO 2 with ~ 0.3% precision.  As demonstrated by Boone et al. JQSRT 2007, the use of speed-dependent Voigt line shape functions improves tropospheric remote sensing. Deficiencies in spectroscopic parameters were also found for the CH 4 and O 2 retrievals. For example, the fitting residuals show obvious difficulty in fitting the O 2 continuum (not included in our forward model) for both 1.27 μm and 0.76 μm bands. However, no recent published work has presented improvements to the spectroscopic parameters for CH 4 and O 2 over those in HITRAN Line Parameters for Greenhouse Gases

CO 2 Column Averaged Volume Mixing Ratio (VMR) CO 2 column averaged Volume Mixing Ratio = CO 2 column / total column. However, humidity can increase the total column by 0.5%, but does not change the CO 2 column. Essentially the CO 2 gets ‘diluted’ by the H 2 O. The dry-air CO 2 VMR is more directly related to source sinks and is a better tracer since it is not being influence by evaporation or condensation of H 2 O. Fortunately, the O 2 is diluted by the same amount as the CO 2 since we measure all of species at the same time (i.e. same air mass). The mole fraction of O 2 in dry air is fairly constant. O 2 column / dry air total column = Hence, we can get the CO 2 VMR using the following formula CO 2 column averaged VMR in dry air = CO 2 column / O 2 column / Similarly, CH 4 column averaged VMR in dry air = CH 4 column / O 2 column /

CO 2 Column Averaged VMR in Dry Air at WAO a The simultaneously observed total columns of O 2 at 0.76  m were used. b The precision of the observations can be estimated from one sigma standard deviation of the results of repeated measurements under similar conditions, e.g., precision = 1  of CO 2 VMR / mean value of CO 2 VMR. The precisions of the column-averaged VMRs of CO 2 in dry air b  1.00 % for the CO cm -1 band at 1.57  m;  1.07% for the CO cm -1 band at 1.57  m;  0.60% for the CO2 band at 2.06  m; CO 2  6348 cm -1 band at 1.57  m O 6228 cm -1 band at 1.57  m  2.06  m band O 2 O 1.27  m band with HITRAN 2004  1.27  m band with Goldman * 0.76  m band with HITRAN 2004 [CO 2 ]/[O 2 ]/ a  6348 cm -1 band at 1.57  m O 6228 cm -1 band at 1.57  m  2.06  m band

CH 4 Column Averaged VMR in Dry Air at Two Sites a The simultaneously observed total columns of O 2 at 0.76  m were used. b The precision of the observations can be estimated from one sigma standard deviation of the results of repeated measurements under similar conditions, e.g., precision = 1  of CH 4 VMR / mean value of CH 4 VMR. CH 4  1.68  m band from NSO * 1.68  m band from WAO [CH 4 ]/[O 2 ]/ a ∆ 1.68  m band from NSO * 1.68  m band from WAO The precisions of the column-averaged VMRs of CH 4 in dry air b  1.07 % for the CH  m band from NSO;  1.13% for the CH  m band from WAO;

Summary An advanced study was carried out for a next generation satellite mission named CCFTS, which will investigate greenhouse gas budgets, using absorption spectra recorded at three ground-based observatories. In order to obtain the absorption features of CH 4, CO 2, CO, and N 2 O together with the O 2 A-band in a single spectrum and investigate the desired spectral resolution (0.01 cm -1 and 0.1 cm -1 ), further observations over a broad spectral region from 2000 to cm -1 were taken at Kitt Peak and Waterloo at a resolution of 0.01 cm -1 and 0.1 cm -1, respectively. The vertical sampling of these observations is quantified by computing averaging kernels as defined in the Rodgers optimal estimation method. The vertical sampling of observations with a spectral resolution of 0.1 cm -1 is similar to those with a spectral resolution of 0.01 cm -1. Precision of column averaged CO 2 and CH 4 VMR are about 1% for the observations at NSO and WAO. Considering overall performances and costs of the mission, a spectral resolution of 0.1 cm -1 (MOPD = 5 cm) is recommended for the CC-FTS mission. Systematic fitting residuals are obvious in all of our retrievals and have been noted previously. These residuals are due to the deficiencies in the spectroscopic line parameters in the HITRAN 2004 database. To improve the precision of atmospheric observations, new laboratory measurements on the spectroscopic parameters are required.

Thanks to Meier A. and IRF, Sweden providing spectra recorded at Kiruna, Sweden. Funding: Canadian Space Agency, Natural Sciences and Engineering Research Council of Canada (NSERC), and other sources Acknowledgements