3.2.4. MLO UV Spectroradiometer
A UV spectroradiometer was installed at MLO in July 1995. The first 3 months of data were described by Bodhaine et al. , and the first complete year of data was described by Bodhaine et al. . The UV irradiances measured at MLO are much more intense than at low-altitude midlatitude locations. A brief introduction to the program and description of the instrument were presented in section 3.2.4 of CMDL Summary Report No. 23. The spectroradiometer was located on a small concrete pad just upslope from the solar radiation building for almost 2 full years (July 1995-June 1997). Here we present the 2-year data set selected for clear mornings only. Clear mornings occur at MLO approximately 60% of the time providing an excellent site for solar radiation measurements and especially Langley calibrations. All processed spectral data are available from CMDL data archives.
The UV spectroradiometer was described by McKenzie et al.  and Bodhaine et al. . Briefly, a diffuser designed to minimize cosine error is mounted as a horizontal incidence receptor to view the whole sky. A quartz dome protects the diffuser from the weather. A shading disk can be mounted on the instrument in order to separate the diffuse and direct radiative components. Stepper-motor driven gratings cover the spectral range of 290-450 nm with a bandpass of about 1 nm. A complete scan requires about 200 seconds. The computer control and data logging system are located in the solar radiation building.
Absolute calibration of the spectroradiometer is performed at approximately 6-month intervals using a National Institute of Standards and Technology (NIST)-traceable 1000-W FEL lamp. Weekly stability calibrations are performed using a mercury lamp and a 45-W standard lamp. The expected long-term accuracy of the spectroradiometer system is better than ±5%. A detailed error analysis for this instrument was given by McKenzie et al.  and was also discussed by Bodhaine et al. .
For the following analysis, UV spectroradiometer data for 45o solar zenith angle (SZA) were chosen for clear mornings at MLO during the July 1995-June 1997 time period. This comprises nearly 2 full years of data, amounting to 230 data points, and includes ozone values in the range 212-309 Dobson Units (DU).
Clear mornings at MLO were determined in the same manner as in the previous studies, that is, a day was accepted as a clear day if the sky was cloudless from dawn through the time of the 45o scan and if Dobson ozone data were available for that morning. Figure 3.17 shows 1-nm means for selected wavelengths for a SZA of 45°. At the shortest wavelengths the variations in UV irradiance are caused primarily by variations in ozone. Variations at the longer wavelengths are less influenced by ozone but may be caused by other atmospheric constituents such as Asian desert dust. Note that the large decrease evident at 325 nm in February 1997 amounts to only about 20%, whereas the corresponding decrease at 195 nm is more than 50% and is caused primarily by an increase in ozone. Missing ozone data caused the unfortunate gap in March-April 1997. Erythemal radiation data were obtained from the spectroradiometer data by applying the erythemal weighting function of McKinlay and Diffey  and integrating over wavelength for each scan as discussed by Bodhaine et al. . Figure 3.18 shows the time series of erythema data and ozone data to illustrate the direct correspondence for the same 2-year period.
Fig. 3.17. Spectral irradiance (1-nm averages) on 230 clear sky mornings at MLO for selected wavelengths at SZA 45° over the time period July 1995-June 1997.
Fig. 3.18. Erythemal irradiance at SZA 45° (bottom) and total ozone (top) on 230 clear sky mornings at MLO over the time period July 1995-June 1997.
The radiative amplification factor (RAF), defined as the percent change of UV (erythemal) irradiance divided by the percent change of total ozone, for the 2-year period is shown in Figure 3.19. Here the RAF was calculated using the power-law formulation of Madronich : RAF = ‑Dln(I)/Dln(O3), where I is UV irradiance. The RAF is simply the slope of the fit on a log-log plot, in this case 1.22. It should be noted that this value is somewhat less than the value of 1.33 found by Bodhaine et al.  for the first year of data.
Fig. 3.19. Power law regression between erythemal irradiance at SZA 45° and Dobson total ozone for 230 clear sky mornings at MLO over the time period July 1995-June 1997. The graph is plotted on a linear scale to facilitate reading the units. The coefficient of the linear term gives the power law RAF.
In conclusion, erythema irradiance calculated from the spectroradiometer is strongly correlated (inversely) with Dobson total ozone. Spectral UV irradiance variations are strongly correlated with total ozone variations, with the highest correlations at the shortest wavelengths. The erythema RAF measured at MLO for the 2-year period is about 1.22, somewhat less than the 1.33 value reported for the first year, and no significant trend in UV irradiance may yet be inferred because of the limited time period.
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