1. Permanently Located Spectrophotometer

          A correctly calibrated spectrophotometer that is permanently located at a station has associated with it a Table of Settings of Q. In this table are listed Q lever settings (in degrees of arc) for A, B, C, D, and Hg-3129 wavelengths vs. instrument temperature (in degree Celsius). To ensure that ozone observations are being made at all times on correct wavelengths, routine mercury lamp tests are performed and the data obtained are compared with reference data presented in the Table of Settings of Q. Experimental and reference Q1values for the Hg-3129 wavelengths should agree to within 0.3 degree.

          If there should appear, a persistent difference greater than about 0.3 degree in the experimental and reference Q1 values, all the wavelength settings of A, B, C, and D wavelengths should be corrected in the following proportions:

Hg = 1.00 A = 0.67 C = 0.99 B = 0.87 D = 0.97.
For example, if the mercury lamp reading is consistently +0.7 above the value given in the Table of Settings of Q, the following amounts should be added to the values of Q1:

A + 0.45 B + 0.60 C + 0.70 D + 0.70.

2. Relocated Spectrophotometer

          A properly adjusted spectrophotometer located at sea level will yield seemingly erroneous Q values when mercury lamp tests are performed on it after it has been transported to a higher altitude station. The reason for this is that the refractive index of quartz in air varies with air pressure. It is found necessary to decrease the Q values by about 0.5 degree for every 100 mb decrease in pressure. At the higher altitude station, therefore, the exact difference between experimental and reference Q1 values for the Hg-3129 wavelength should be determined and corrections to Q1 settings for A, B, C, and D wavelengths computed and applied according to instructions given in Section 1 above.

          The spectrophotometer optics are adjusted so that at sea level pressure (~1012 mb) and at a temperature of 15C, Q1 ~ Q2 = 84.0 0.5 when the Hg-3129 wavelength passes through slit S2; also Q1 ~ Q2 = 84.0 0.5 when the Hg-3129 wavelength passes through slit S3. Should it be necessary to make an adjustment to the optics of a spectrophotometer located high above mean sea level, the Q1 and Q2 lever setting at 15C for Hg-3129 and Hg-3342 wavelengths should be as follows:

Mean Station Pressure Q1 ~ Q2
1,000 mb 84.0 0.5 degrees 900 83.5 0.5 800 83.0 0.5 700 82.5 0.5 600 82.0 0.5

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Forward to Appendix C. Calibration of the Dobson Spectrophotometer Optical Wedge