Tropical Tropospheric Ozone (TTO)
Maps from Nimbus 7/TOMS by the
Modified-Residual Method: 1979-1992

An apparent wave-one pattern seen in the total ozone data from TOMS in the tropics has been used to develop a new remote- sensing method for tropical tropospheric ozone (TTO). The so- called modified-residual method for deriving time-averaged TTO from TOMS was developed for a two-year period, 1991-1992 (Hudson and Thompson, 1998; cf Kim et al. [1996]). This period was selected because of the availability of a unique sonde record at tropical Atlantic and near-Atlantic sites: Natal, Brazil (6S, 35W); Ascension Island (8S, 15W); Brazzaville (4S, 15E).

The modified-residual (MR) method uses the wave-like pattern with version 7 Nimbus 7/TOMS high-density total ozone to estimate stratospheric ozone column and a background tropospheric ozone column, which is defined as the amount of ozone expected in the absence of chemical pollution. Stratospheric column ozone is derived for two reasons. First, different footprint, observing times and operational lifetime of various sensors used in standard residual methods are factors that complicate the derivation of TTO, and usually limit maps to climatological averages, eg those published by Fishman and coworkers [Fishman et al, 1990, 1991, 1996; Fishman and Brackett, 1997]. Second, when sensors change, consistent calibration is difficult to assure, compromising precision and the potential for deriving trends. Thus, the MR method, like the CCD (Convective Cloud Differencing) technique of Ziemke et al [1998] is preferred as a TOMS-only based calculation of TTO.

In Hudson and Thompson [1998] parameters derived from the modified-residual method were evaluated with independent data from the 1991-1992 period to determine the quality of the TTO. The precision of the method is +/- 5 DU, owing to the normalization to 2-week averaged sonde data. Derived tropical stratospheric column ozone for 1991-92 agrees very well with the corresponding UARS MLS and SAGE records (to within 5 DU for the latter).

In Section 2 below, the Modified-Residual (MR) method is summarized. The maps shown on this homepage (Section 4) extend the original published record to the remainder of the Nimbus 7/TOMS record, for the years 1979-1990. The resulting tropospheric ozone column amounts, as 13-15-day averages, are compared to a reasonably dense Atlantic sonde record for 1990 and to Natal (1979-1989) for the first 11 years (Section 3, Validation).

Excellent agreement between the MR method and the sondes assures that we have a consistent, validated technique for TTO for the 14 complete calendar years of the Nimbus 7/TOMS period: 1979-1992. Users of these data will find them applicable to process studies, studies of interannual variability, modeling and interpretation of tropical field experiments.

The relationship of stratospheric ozone, tropospheric ozone and the tropical wave-one is shown in Figure 1.


Fig. 1

The steps used in the modified-residual method are as follows:

(1)

13-16 day averages of Level 2 version 7 TOMS data (cloud-free, reflectivity <= 0.15) are used to obtain total ozone. See McPeters and Beach [1996] and Hsu et al [1997]. Averaging minimizes scan angle effects and aerosol interferences, except for extreme cases. The first 15 days of each month are averaged and the remaining 13-16 days are averaged, to give two maps per month.

(2)

Fourier analysis is used to delineate the latitudinal limits of the wave one and to compute the magnitude. The method is only applied to the appropriate latitude range for each 13-15 day period, so the coverage is not uniform from one map to the next. Figure 2a shows a set of total ozone cross-sections for the first 15 days of April 1992.


Fig. 2a

(3)

Total ozone at 180 longitude is normalized to a single value and the underlying wave introduced as shown in Figure 2b. The wave is attributed to the underlying background tropospheric ozone. Stratospheric ozone is assumed to be longitudinally uniform. Thus,

O3)total = O3)tropo, background + O3)excess + O3)strat     Eq 1.

where O3)tropo, background and O3)excess vary with longitude. Note that two of these parameters are known with high accuracy: O3)total from TOMS and O3)excess from the magnitude of the wave-one.


Fig. 2b

(4)

For 1991-1992, ozonesonde data at Ascension Island (8S, 15W), Brazzaville (4S, 15E) and Natal, Brazil (6S, 35W) are available for most of the twice/month periods. These are used to determine O3)tropo, background near the wave maximum:

          O3)sonde = O3)tropo, background + O3)excess

In practice, O3)*tropo, background for 0 degrees longitude is obtained by averaging O3)tropo, background over the three sites. A seasonally varying pattern for O3)tropo, background (Figure 3) is actually used in Eq. 1.


Fig. 3

(5)

With all terms in Eq. 1 determined, O3)strat is obtained. Note that if ozonesonde data were available from sites near 180 degrees longitude, Eq 1 could be used to obtain O3)strat and O3)tropo, background , there as well. In 1991-1992 no sondes were launched at equatorial Pacific sites.

(6)

Eq. 1 is used with O3)strat, wave amplitude, and O3)*tropo, background to determine a best-estimate for TTO, "O3)tropo, best" at each latitude and longitude. Correction for retrieval efficiency of tropospheric ozone over low-reflectivity surfaces below 500 mb must be used in computing O3)tropo, best [Hudson et al, 1995; Kim et al, 1996]. Figure 2c shows O3)tropo, best for 1-15 April 1992, with the corresponding averaged ozonesonde data (*) for comparison.


Fig. 2c

(7)

O3)tropo, best is used with O3)tropo, background at each tropospheric longitude to calculate the best estimate of TTO and O3)excess (Figure 2d for 1-15 April 1992).


Fig. 2d

Figure 4 shows the map for 1-15 April 1992. The background tropospheric ozone function derived from the 1991-1992 Atlantic basin sounding sites is the basis for deriving O3)tropo, best from 1979-1990.

Fig. 4

The background tropospheric ozone function derived from the 1991- 1992 Atlantic basin sounding sites is the basis for deriving O3)tropo, best from 1979-1990.

3. Validation of the Modified Residual Method

The precision of the method is +/- 5 DU, owing to the normalization to 2-week averaged sonde data [Hudson and Thompson, 1998]. Validation for 1991-1992 for O3)strat, as derived from the MR method, was made through comparison with the available SAGE data, which was valid for the first 5.5 months of 1991, prior to contamination by the eruption of Mt Pinatubo. Agreement with SAGE stratospheric ozone is within 5 DU.

TABLE 1, SAGE II Comparison (early 1991, pre-Mt Pinatubo eruption) with stratospheric ozone derived from modified-residual (MR) method.

Julian Day	No. Profiles	Mean Strat. O3 (DU)	Derived O3) strat (DU) (Modified-Residual)
5	11	226.2	225 (1-15 Jan. 1991)
6	15	225.0
7	15	223.3
8	14	222.9
9	3	218.5
40	13	224.6	228 (1-15 Feb. 1991)
77	15	235.1	239 (16-31 Mar. 1991)
78	15	234.9
79	7	230.2
114	13	236.1	239 (16-30 April 1991)
115	15	234.4
116	15	235.8
117	7	232.4

From September 1991-December 1992, comparisons of O3)strat with stratospheric ozone from the UARS/MLS (Microwave Limb Sounder) average between 5-10 DU [Fig. 11 in Hudson and Thompson, 1998]. Two of the three sites for which sondes are used to derive background tropospheric ozone, Ascension and Brazzaville [Nganga et al, 1996], were not operational prior to 1990. Of tropical sites, only Natal has an ozonesonde record from the beginning of Nimbus 7/TOMS [Kirchhoff et al, 1996]. Comparisons of the 1990- 1992 Ascension, Brazzaville and Natal records appear in Figure 5. Comparison of TTO for October 1992 with ozonesondes and aircraft data appear in Kim et al [1996] and Thompson et al [1996].

Fig. 5a

Fig. 5b

Fig. 5c

Comparison with Natal sondes is also excellent from 1979-1990 (Figure 6); note the seasonal cycle.

Fig. 6

Over the Pacific, ozonesondes at Samoa were launched from 1986-1990 and resumed in late 1995 [Komhyr et al, 1989]. The typically low (10-20 DU) values shown in the TTO maps are consistent with the Samoan record and agree well with the low equatorial Pacific ozone measured from ship in 1993 [Kley et al, 1996].

The maps on this homepage are the based on O3)tropo, best, our best estimate for TTO. They have been calculated on a 1 degree latitude by 2 degrees longitude grid. NOTE THAT THESE MAPS ARE PRELIMINARY. They will be further evaluated by comparison with independent ozone measurements and they will be examined for interferences from absorbing aerosols (as during intense smoke episodes and volcanic eruptions, [Herman et al, 1997]).

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