JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 89, NO. D4, PAGES 5227-5238, JUNE 30, 1984 Intercomparison of NIMBUS 7 Solar Backscattered Ultraviolet Ozone Profiles With Rocket, Balloon, and Umkehr Profiles P. K. BHARTIA, • K. F. KLENK, • A. J. FLEIG, 2 C. G. WELLEMEYER, • AND D. GORDON • A global ozone data set covering the two years from November 1978 until October 1980, with an average of 1200 profiles per day, has been I•roduced and archived from the solar backscattered ultraviolet (SBUV) instrument flown on the NIMBUS 7 spacecraft. The SBUV ozone profiles are compared with measurements from chemical and optical sensors launchedon balloonsand rockets and with ozone profiles obtained from the ground-based Dobson spectrophotometers using the Umkehr method. The biasesbetween the SBUV results and the balloon and Umkehr results are generally less than 10%. These biases are functionsof layer heightand latitude and are believed to be largely due to inconsistencies in the ozone absorptioncrosssections used' for the various measurementsystems.The precision of the SBUV measurements is found to be better than 8% for pressuresbetween 1 and 64 mbar and better than 15% from 64 to 253 mbar. 1. INTRODUCTION The solar backscattered ultraviolet (SBUV) instrument aboard the NIMBUS 7 satellite is designed to measure total columnar ozone and its vertical distribution in the earth's atmosphere. The instrument is operated 3 out of every 4 days and provides an average of 1200 sets of measurements per day (325,000 observations per year). The observations cover the entire daylit portion of the globe and, except for the polar regions, are made close to local noon. Data for the period from October 31, 1978, until November 2, 1980, have been processed, validated, and archived. At the time of submission of this paper the instrumentwas still operating well. Data processing is continuing, and the remaining data will also be archived. The SBUV instrument for NIMBUS 7 is conceptually similar to the backscattered ultraviolet (BUV) instrumentflown on NIMBUS 4. However, improve- ments were made in both the instrument and the processing algorithms used to reduce the data. The results presented herein are only applicable to the SBUV data. Great care shouldbe exercised in comparingresults from NIMBUS 7 SBUV with resultsfrom NIMBUS 4 BUV (see, for example, D. F. Heath and B. M. Schlesinger, unpublished manuscript, 1984). The NIMBUS 7 satelliteis in a retrograde,near-polar, sun- synchronous orbit and crosses the equator moving north- ward at local noon every 104 min. Consecutive orbits are 26 ø apart in longitude. The SBUV instrument aboard this satel- lite contains an ultraviolet monochromator and a scene photometerthat are describedby Heath et al. [1975]. In its normal ozone-measuring mode the instrument looks at 200 x 200 km spotson the earth in the nadir direction and measures the intensity of solar radiation backscattered by the earth and its atmosphere in 12 discrete 1-nm-wide wavelength bands centered at 339.9, 331.3, 317.6, 312.6, 305.6, 302.0, 297.6, 292.3,287.7, 283.1,273.6, and 255.7 nm. It takes the instrument32 s to step through these bands, during which • Systems andApplied Sciences Corporation. 2 NASA/Goddard SpaceFlight Center Laboratory for Atmo- spheric Science. This paper is not subject to U.S. copyright. Published in 1984 by the American Geophysical Union. Paper number 3C1227. time the satellite footprint moves by about 200 km. To account for the changes in scene in the satellite IFOV (instantaneous field of view), a 5-nm bandpass filter photom- eter centered at 343 nm makes 12 measurements concurrent to the monochromator. SBUV also makes daily measure- ments of the solar flux at 0.2-nm intervals in the 160-400 nm range of the solar ultraviolet spectrum by deploying an aluminum diffuserthat reflectsthe incident sunlight into the slit. Ozone information is derived from the ratio of the backscattered intensity to the incoming solar flux. Since both the solar irradiance and terrestrial radiance measure- ments are made with the same instrument, many instrument- relatederrors and drifts cancelout. The diffuser plate is the only component in the instrument not shared by both the radiance and irradiance measurements, and therefore its reflectivity does enter into the computation of ozone. The SBUV ozone profile inversion technique has been developed from an algorithmoriginally proposed by Mateer [1977]. This technique is sometimes referred to as the "optimum statistical" technique, for one uses the full error covariance matricesassociated with the a priori climatology and the satellite measurements to combinethese two types of information. The mathematical method describingthis procedureis given in a review paper by Rodgers [ 1976] and its application to the ozone problem by Schneider et al. The SBUV ozone retrieval algorithm obtains an ozone profilein a two-stepprocess. In the first stepthe four longest wavelength bands of the instrument are used to obtain a columnartotal ozone amount via an algorithmbased on the NIMBUS 4 BUV algorithmdescribed by Klenk et al. [1982]. Comparisonof the total ozone derived by the SBUV tech- nique with those measured by the ground-based Dobson and M83 network is given in a companion paper of this journal [Bhartia et al., this issue]. The derived total ozone amount is used in the next step of the algorithm in two ways: (1) to estimatethe multiply scatteredand reflectedcomponents of the backscatteredradiation at longer profile wavelengths that require an accurateknowledgeof total ozone [Taylor et al., 1980] and (2) as a constraint on the retrieval of ozone profile. The algorithmiteratively perturbsan a priori ozone profile until the radiances and integrated ozone calculated from the profile agree with the measuredradiances and total ozone to within a prescribed uncertainty. The error covariance matrix associated with the a priori profile provides the algorithm an estimate of the confidence 5227