A comparison of sun photometer derivations of total column water vapor and ozone to standard measures of same at the Southern Great Plains Atmospheric Radiation Measurement site

Total column water vapor measurements made over a 1-year period by colocated instruments at the Southern Great Plains Atmospheric Radiation Measurement program site in northern Oklahoma are compared. Microwave radiometer measurements were validated in an earlier program with extensive comparisons to balloon sonde measurements. Sun photometry, using the multifilter rotating shadowband radiometer (MFRSR), was the second technique used to derive water vapor using the 940-nm water band when no clouds were between the radiometer and the sun. Bruegge et al.'s implementation of Reagan et al.'s modified Langley technique was applied in this case. This approach is noteworthy in that it does not rely on a comparison to a standard for calibration. The comparison of 101 one-half-hour to two-hour averages produced a root-mean-square difference of 0.18 cm compared to a mean value of 1.49 cm, or about 12%, with the MFRSR showing an overall bias of +0.03 cm relative to the microwave radiometer data. This is a somewhat higher rms error than other results using the modified Langley technique, but the comparison was made over the entire range of water vapor columns experienced in a year of observations. As a by-product of the water vapor derivation procedure for the MFRSR, ozone column abundance was estimated. Seasonally averaged, these data compare favorably with Dobson measurements from Nashville, Tennessee, which is near the same latitude. A more definitive conclusion regarding the Sun photometer ozone derivation technique requires colocated ozone measurements using an accepted standard approach, however, the technique appears to provide a useful ozone estimate for correcting aerosol optical depth derivations.