Horizontal Flux Results

This section presents results from the spectral fitting of the horizontal flux using a high resolution extra-terrestrial solar spectrum as the reference. After some initial trials, it was apparent that there were small wavelength shifts (most notably near 325 nm) between the reference and the measurements. This, combined with small path enhancements, made it difficult to obtain accurate columns based on matching of the fine absorption structure. However, for ozone, the rapid decrease in cross-section longward of 300 nm can be utilized to improve the fitted values. Essentially, the slowly varying cross-section curvature can be fitted along with the detailed absorption structure. This requires some modifications to the DOAS fitting process as the use of a second order polynomial in determining the differential optical depths removed much of this curvature. Instead, a first order polynomial is used and the DOAS fitting window is expanded to 310-360 nm. In addition, the Rayleigh component must be removed before the fitting since the $\lambda^{-4}$ dependence is similar to the envelope of the ozone cross-sections. The optical depth was redefined as,

$$D(\lambda) = \ln{ \left[ \frac{I_{\rm {xt}}(\lambda)}{I_{\rm ... ... \right] } - {\rm C_{air}} \, \sigma_R(\lambda) \sec{\theta_o}$$ (11.33)

where $I_{\rm xt}$ is the extra-terrestrial flux, $I_{\rm hf}$ is the horizontal flux, and C $_{\rm air}$ is the vertical column density of air above the aircraft and can be calculated based on the local pressure, a suitably chosen scale height, and a correction for air mass. The retrieved VCDs were found to be quite insensitive to the estimated value of C $_{\rm air}$ and scale height. Finally, to take full advantage of the slowly varying curvature of the spectra, the DOAS fitting window is expanded to 310-370 nm. The only other modification is that Ring was excluded from the DOAS fits as the horizontal flux is composed almost entirely of direct sunlight.

  

Comparisons can be made against VCDs above the aircraft calculated from horizontal flux measurements using the Brewer method, as outlined in section 3.3. Originally a high resolution (0.01 nm) extra-terrestrial spectrum was to be used. However, the DOAS VCDs were quite sensitive to small wavelength shifts. Further work is necessary to properly diagnose this problem. Instead, an extra-terrestrial spectrum of moderate resolution (0.2-0.4 nm) (Arvesen et al., 1969) was used.

Results of the analysis of the 21 September 1997 flight are shown in Figure 6.25. The rapidly decreasing ACDs from panel (a) is mainly the result of the decreasing solar zenith angle from a maximum of 74.8$^{\circ }$ at the beginning of the flight to 35.2$^{\circ }$ towards the end as the ER-2 flew from Fairbanks to Hawaii. The VCDs are calculated by applying the $\cos{\theta_o}$ air mass factor. From panel (b) the VCDs are roughly constant, decreasing only by 20 DU over the course of the flight, as a result of the competing effects of a smaller total column but a larger fraction of the total column above the aircraft as the ER-2 flew from the low Arctic to the tropics. There was fair agreement with the much noisier CPFM-Brewer VCDs with a maximum difference of about 10 DU. However, this slight decline in VCDs was not observed at all in the CPFM-Brewer VCDs.

Due mainly to the much smaller absorption signals by NO₂ and BrO, it was not possible to obtain VCDs of these species above the aircraft.