BIAS IN DOBSON TOTAL OZONE MEASUREMENTS AT HIGH LATITUDES...
1
BIAS IN DOBSON TOTAL OZONE MEASUREMENTS AT HIGH LATITUDES DUE TO APPROXIMATIONS IN CALCULATIONS OF OZONE ABSORPTION COEFFICIENTS AND AIRMASS G. Bernhard 1 , R. D. Evans 2 , G. J. Labow 3 , and S. J. Oltmans 4 1 Biospherical Instruments Inc., San Diego, CA; [email protected] 2 NOAA Climate Monitoring and Diagnostics Laboratory, Boulder, CO; [email protected] 3 NASA Goddard Space Flight Center, Greenbelt, MD; [email protected] 4 NOAA Climate Monitoring and Diagnostics Laboratory, Boulder, CO; [email protected] This poster is a summary of the paper: G. Bernhard et al., J. Geophys. Res., doi:10.1029/2004JD005559, 2005, in press. Acknowledgements. We acknowledge Dr. Bryan Johnson from CMDL for his help with the ozonesondes data. We especially appreciate the invaluable efforts of the many CMDL technical staff members and NOAA Corps Officers who operate the facility year round at the South Pole to obtain the data used in this report. The U.S. National Science Foundation’s Office of Polar Programs (NSF/OPP) supplied the logistics support for the facility at the South Pole. SUV-100 measurements at the South Pole were supported by NSF/OPP via a subcontract to Biospherical Instruments Inc. from Raytheon Polar Services Company. The new TOMS Version 8 ozone retrieval was created and implemented by the Ozone Processing Team at NASA Goddard Space Flight Center, particularly P.K. Bhartia, R.D. McPeters and C. Wellemeyer. Total column ozone measurements from Dobson spectrophotometers No. 80 and 82 operated by CMDL at the South Pole were compared with total ozone data calculated from UV measurements of a collocated SUV-100 spectroradiometer. This instrument is part of the National Science Foundation’s UV monitoring network for polar regions and operated by Biospherical Instrument Inc. Data measured between 1991 and 2003 were used. For total ozone larger than 220 DU, the ratio of concurrent Dobson and SUV-100 measurements is 0.986±0.022 (±1σ). For lower ozone values or at large airmass, Dobson data are biased low compared to the SUV-100 data set as indicated in the figure. The objective of this study is to determine whether this bias is caused by approximations applied in the standard Dobson data reduction method. Motivation 1 – The set of ozone absorption coefficients applied in the Dobson network was originally calculated by Komhyr et al. [1993]. We document an error in these calculations. 2 – The same set of ozone absorption coefficients are applied at all locations of the Dobson network without site- specific corrections for the temperature dependence of the ozone absorption cross sections. The temperature of the ozone layer is assumed to be 226.85 K (-46.3 °C). The figure indicates that actual temperatures at the South Pole vary between 190 and 255 K. 3 – The Standard Method assumes that the ozone layer is infinitesimal thin and located at a fixed height. This approximation leads to errors in airmass calculations, which are particularly relevant at high latitudes where ozone measurements are performed at large solar zenith angles. Sources of Error in the Standard Dobson Data Reduction Method Effective temperature at the South Pole calculated from ozone profiles measured by CMDL. The red line indicates periods when Dobson measurements are available. The broken green line indicates the standard temperature of 226.85 K. Corrections to Dobson Measurements 1 – Recalculation of ozone absorption coefficients for standard conditions We recalculated the set of ozone absorption coefficients for all Dobson wavelengths and for the same standard conditions as used by Komhyr et al. [1993]. The values by Komhyr et al. [1993] agree with our calculations to within ±2.3%, except for the coefficient at 339.9 nm, where our value is 0.010 and the value by Komhyr et al. [1993] is 0.017 (66% difference). Further analysis indicates that the value by Komhyr et al. [1993] is clearly to high. However, this error has little effect because an empirical adjustment was applied to the original calculations by Komhyr et al. [1993] before the coefficients were adopted by the World Meteorological Organisation (WMO) and the Dobson network. The values of the three sets of coefficients are summarized below. Effective ozone absorption coefficient in (atm-cm) -1 Wavelength or Dobson Pair [Komhyr et al., 1993] WMO Our calculation 339.9 nm 0.017 0.010 A 1.806 1.806 1.805 C 0.833 0.833 0.832 D 0.367 0.374 0.377 AD 1.439 1.432 1.428 CD 0.466 0.459 0.455 2 – Temperature dependence of ozone absorption coefficients For each Dobson measurement performed at the South Pole, we calculated a separate set of ozone absorption coefficients based on the actual ozone and temperature profiles at the time of the measurement. These profiles were taken from ozonesonde observations performed by CMDL at the South Pole. Balloons were launched approximately every third day during the austral spring. The figure shows the ratio of effective ozone absorption coefficients α for the Dobson AD and CD wavelength pairs calculated by us to the associated standard values of 1.432 and 0.459, respectively. Our values deviate from standard values by up to ±4%. 3 – Airmass calculation Relative optical airmass (i.e. the ratio of the slant and vertical path length) was calculated by taking the actual ozone and temperature profiles into account. In contrast to the standard Dobson method, our calculations also consider the pressure-dependence of the air’s refraction index. The figure shows a comparison between the exact airmass μ, calculated by us, to the approximated airmass μ approx , calculated with the standard Dobson method. The figure indicates that μ is lower than μ approx by 1-4% . The overestimate of the airmass by the standard method leads to an underestimate of ozone. The difference increases with SZA and is largest for periods affected by the ozone hole. Dobson measurements were corrected for the temperature dependence of the ozone absorption coefficients and the approximations in airmass calculations. The corrected data set was again compared with SUV-100 data. The ratio of Dobson and SUV-100 data is 0.984±0.024 (±1σ). There is no obvious dependence on either total ozone or airmass. A similar comparison between NASA TOMS Version 8 and SUV-100 data results in a ratio of 0.983 ± 0.038. Results and Conclusions References Bernhard, G., C.R. Booth, and R.D. McPeters, Calculation of total column ozone from global UV spectra at high latitudes, J. Geophys. Res., 108(D17), 4532, doi:10.1029/2003JD003450, 2003. Komhyr, W.D., Mateer, C.L. and R.D. Hudson, Effective Bass-Paur 1985 ozone absorption coefficients for use with Dobson ozone spectrophotometers, J. Geophys. Res., 98(D11), 20,451-20,465, 1993.
Transcript of BIAS IN DOBSON TOTAL OZONE MEASUREMENTS AT HIGH LATITUDES...
BIAS IN DOBSON TOTAL OZONE MEASUREMENTS AT HIGH LATITUDES DUE TO APPROXIMATIONS IN CALCULATIONS OF OZONE ABSORPTION COEFFICIENTS AND AIRMASS G. Bernhard1, R. D. Evans2, G. J. Labow3, and S. J. Oltmans4
1Biospherical Instruments Inc., San Diego, CA; [email protected] Climate Monitoring and Diagnostics Laboratory, Boulder, CO; [email protected] Goddard Space Flight Center, Greenbelt, MD; [email protected] Climate Monitoring and Diagnostics Laboratory, Boulder, CO; [email protected]
This poster is a summary of the paper: G. Bernhard et al., J. Geophys. Res., doi:10.1029/2004JD005559, 2005, in press.
Acknowledgements. We acknowledge Dr. Bryan Johnson from CMDL for his help with theozonesondes data. We especially appreciate the invaluable efforts of the many CMDL technical staff members and NOAA Corps Officers who operate the facility year round at the South Pole to obtain the data used in this report. The U.S. National Science Foundation’s Office of Polar Programs (NSF/OPP) supplied the logistics support for the facility at the South Pole. SUV-100 measurements at the South Pole were supported by NSF/OPP via a subcontract to Biospherical Instruments Inc. from Raytheon Polar Services Company. The new TOMS Version 8 ozone retrieval was created and implemented by the Ozone Processing Team at NASA Goddard Space Flight Center, particularly P.K. Bhartia, R.D. McPeters and C. Wellemeyer.
Total column ozone measurements from Dobson spectrophotometers No. 80 and 82 operated by CMDL at the South Pole were compared with total ozone data calculated from UV measurements of a collocated SUV-100 spectroradiometer. This instrument is part of the National Science Foundation’s UV monitoring network for polar regions and operated by Biospherical Instrument Inc. Data measured between 1991 and 2003 were used.
For total ozone larger than 220 DU, the ratio of concurrent Dobson and SUV-100 measurements is 0.986±0.022 (±1σ). For lower ozone values or at large airmass, Dobson data are biased low compared to the SUV-100 data set as indicated in the figure. The objective of this study is to determine whether this bias is caused by approximations applied in the standard Dobson data reduction method.
Motivation
1 – The set of ozone absorption coefficients applied in the Dobson network was originally calculated by Komhyr et al. [1993]. We document an error in these calculations.
2 – The same set of ozone absorption coefficients are applied at all locations of the Dobson network without site-specific corrections for the temperature dependence of the ozone absorption cross sections. The temperature of the ozone layer is assumed to be 226.85 K (-46.3 °C). The figure indicates that actual temperatures at the South Pole vary between 190 and 255 K.
3 – The Standard Method assumes that the ozone layer is infinitesimal thin and located at a fixed height. This approximation leads to errors in airmass calculations, which are particularly relevant at high latitudes where ozone measurements are performed at large solar zenith angles.
Sources of Error in the Standard Dobson Data Reduction Method
Effective temperature at the South Pole calculated from ozone profiles measured by CMDL. The red line indicates periods when Dobson measurements are available. The broken green line indicates the standard temperature of 226.85 K.
Corrections to Dobson Measurements
Effective ozone absorption coefficient in (atm-cm)-1 Wavelength or Dobson Pair [Komhyr et al., 1993] WMO Our calculation
339.9 nm 0.017 0.010
A 1.806 1.806 1.805
C 0.833 0.833 0.832
D 0.367 0.374 0.377
AD 1.439 1.432 1.428
CD 0.466 0.459 0.455
1 – Recalculation of ozone absorption coefficientsfor standard conditions
We recalculated the set of ozone absorption coefficients for all Dobson wavelengths and for the same standard conditions as used by Komhyr et al. [1993]. The values by Komhyr et al. [1993] agree with our calculations to within ±2.3%, except for the coefficient at 339.9 nm, where our value is 0.010 and the value by Komhyr et al. [1993] is 0.017 (66% difference). Further analysis indicates that the value by Komhyr et al. [1993] is clearly to high. However, this error has little effect because an empirical adjustment was applied to the original calculations by Komhyr et al. [1993] before the coefficients were adopted by the World Meteorological Organisation (WMO) and the Dobson network. The values of the three sets of coefficients are summarized below.
Effective ozone absorption coefficient in (atm-cm)-1 Wavelength or Dobson Pair [Komhyr et al., 1993] WMO Our calculation
339.9 nm 0.017 0.010
A 1.806 1.806 1.805
C 0.833 0.833 0.832
D 0.367 0.374 0.377
AD 1.439 1.432 1.428
CD 0.466 0.459 0.455
Effective ozone absorption coefficient in (atm-cm)-1 Wavelength or Dobson Pair [Komhyr et al., 1993] WMO Our calculation
339.9 nm 0.017 0.010
A 1.806 1.806 1.805
C 0.833 0.833 0.832
D 0.367 0.374 0.377
AD 1.439 1.432 1.428
CD 0.466 0.459 0.455
2 – Temperature dependence of ozone absorptioncoefficients
For each Dobson measurement performed at the South Pole, we calculated a separate set of ozone absorption coefficients based on the actual ozone and temperature profiles at the time of the measurement. These profiles were taken from ozonesonde observations performed by CMDL at the South Pole. Balloons were launched approximately every third day during the austral spring. The figure shows the ratio of effective ozone absorption coefficients α for the Dobson AD and CD wavelength pairs calculated by us to the associated standard values of 1.432 and 0.459, respectively. Our valuesdeviate from standard values by up to ±4%.
Effective ozone absorption coefficient in (atm-cm)-1 Wavelength or Dobson Pair [Komhyr et al., 1993] WMO Our calculation
339.9 nm 0.017 0.010
A 1.806 1.806 1.805
C 0.833 0.833 0.832
D 0.367 0.374 0.377
AD 1.439 1.432 1.428
CD 0.466 0.459 0.455
3 – Airmass calculation
Relative optical airmass (i.e. the ratio of the slant and vertical path length) was calculated by taking the actual ozone and temperature profiles into account. In contrast to the standard Dobson method, our calculations also consider the pressure-dependence of the air’s refraction index. The figure shows a comparison between the exact airmass µ, calculated by us, to the approximated airmass µapprox, calculated with the standard Dobson method. The figure indicates that µ is lower than µapprox by 1-4% . The overestimate of the airmass by the standard method leads to an underestimate of ozone. The difference increases with SZA and is largest for periods affected by the ozone hole.
Dobson measurements were corrected for the temperature dependence of the ozone absorption coefficients and the approximations in airmass calculations. The corrected data set was again compared with SUV-100 data. The ratio of Dobson and SUV-100 data is 0.984±0.024 (±1σ). There is no obvious dependence on either total ozone or airmass. A similar comparison between NASA TOMS Version 8 and SUV-100 data results in a ratio of 0.983 ± 0.038.
Results and Conclusions ReferencesBernhard, G., C.R. Booth, and R.D. McPeters, Calculation of total column ozone from
global UV spectra at high latitudes, J. Geophys. Res., 108(D17), 4532,doi:10.1029/2003JD003450, 2003.
Komhyr, W.D., Mateer, C.L. and R.D. Hudson, Effective Bass-Paur 1985 ozoneabsorption coefficients for use with Dobson ozone spectrophotometers, J. Geophys. Res., 98(D11), 20,451-20,465, 1993.
