Early Morning UV-B During the 1994-1995 Record Low Ozone at Mauna Loa

The Smithsonian Environmental Research Center (SERC) has been monitoring surface spectral ultraviolet-B (UV-B) irradiance at Mauna Loa (MLO) since fall 1984. The instrument is similar to a radiometer in operation in Edgewater, Maryland [Correll et al., 1992]. The instrument measures UV-B irradiance in a series of eight, 5-nm band pass channels (290-325 nm) and records 1-minute averages. Operation is continuous except for an annual break of about 1 month when the instrument is returned to Maryland for calibration. Our primary objective is to monitor long-term changes in incident solar UV-B irradiance. Records of absolute calibration of the MLO instrument are under review, as are data from intercomparisons of the Edgewater and MLO instruments at Edgewater. We expect the results from these tests will be the subject of a future report.

While absolute irradiance is presently not available, ratios of signal intensity between instrument channels provide information on relative changes in the solar UV-B spectrum. There are several ways such data may be useful. Previously, we used the data to estimate atmospheric optical depth in the UV-B at MLO [Neale et al., 1994]. Recently, there was evidence of a record low total column ozone at MLO during the winter of 1994-1995 [Hofmann et al., 1996]. In this latter report, we presented UV irradiance measured in the 295-nm, 300-nm and 305-nm channels, relative to the 325-nm channel for clear days when the secant of the solar zenith angle was equal to 1.5 (about 48°). Here we present additional UV data for the winter of 1994-1995 as well as other ancillary information concerning instrument operation during this period.

The instrument was calibrated at SERC and sent to MLO in July 1994 and remained in operation until April 1995. In June 1994, transmission spectra were measured for all eight interference filters in the instrument. The center wavelengths (wavelength midpoint between the upper and lower wavelengths at which transmission is 50% of maximum) calculated from these spectra for the 295-, 300-, 305- and 325-nm channels were 294.9, 300.4, 304.6 and 324.8 nm. After return of the instrument to SERC in April 1995, filter transmission spectra were again measured and these channels had center wavelengths of 295.1, 300.7, 304.9, and 325.4 nm. Apart from the 325-nm channel (0.6 nm shift), the shift in filter center wavelength was 0.3 nm or less.

Figure 1 shows measurements of 295-, 300-, and 305 nm irradiance during the winter of 1994-1995 on the clear sky mornings when the secant of solar zenith angle (sec _s) was equal to 2.5 (about 66°), i.e., between 0814 and 0857 LST in October and December, respectively. These are given as ratios of the irradiance in each channel to the irradiance in the 325-nm channel. The time series of Dobson spectrometer ozone measurements at MLO is shown for comparison. Over the period of decreasing ozone, November to December, there is a steady climb in the irradiance ratio in each channel. The increase from minimum to maximum irradiance was a factor of 3.0, 1.9, and 1.6, respectively, in the 295-, 300- and 305-nm channels. There is a strong correlation between ozone and the irradiance ratio in each channel. A linear regression of the 305 nm/ 325 nm ratio on total ozone (interpolating the ozone series as necessary for missing dates) had a coefficient of determination (R²) of 0.86. The R² for the 295- and 300-nm ratios were 0.85 and 0.86, respectively.

Fig. 1. Time series of total ozone and UV-B irradiance ratios at MLO during winter 1994-1995. Early morning (secant 2.5) irradiance on clear-sky days in three bands (5-nm full bandwidth at half height) with nominal center wavelengths of 295 nm (upper panel), 300 nm (middle panel) and 305 nm (lower panel) relative to 325 nm. Note different ratio scales in each panel.

These results reinforce the conclusion of Hofmann et al. [1996] that incident solar UV-B was significantly elevated during the record low total-column ozone event at MLO (December-January 1994). Measurements during both early morning (sec _s= 2.5) and midmorning (sec _s = 1.5) show specific increases in irradiance during this period that are highly correlated with the decrease in total column ozone. Since the mean optical path length of solar irradiance through the stratosphere is longer at sec _s = 2.5 compared to 1.5, a given decrease in ozone should result in a larger relative increase in irradiance at higher secant. This was indeed the case for the 300- and 305-nm channels, for which there was less of an increase from minimum to maximum (factor 1.5 and 1.8, respectively) at sec _s= 1.5 [Hofmann et al., 1996]. However, for the 295-nm channel, the factor increase was actually higher at the shorter secant (3.2). For this channel, early morning and midmorning measurements may not be strictly comparable. This is because the effective center wave-length, i.e., the filter center wavelength adjusted for the greater proportion of longer-wavelength solar irradiance within the pass band [Correll et al., 1992], is likely to be at a significantly longer wavelength in the early morning versus midmorning.

Correll, D. L., C. O. Clark, B. Goldberg, V. R. Goodrich, D. R. Hayes Jr., W. H. Klein and W. D. Schecher, Spectral ultraviolet-B radiation fluxes at the earth's surface: long-term variations at 39°N, 77°W, J. Geophys. Res., 97, 7579-7591, 1992.

Hofmann, D.J., S.J. Oltmans, B.A. Bodhaine, G.L. Koenig, J.M. Harris, J.A. Lathrop, R.C. Schnell, J. Barnes, J. Chin, D. Kuniyuki, S. Ryan, R. Uchida, A. Yoshinaga, P.J. Neale, D.R. Hayes, Jr., V.R. Goodrich, W.D. Komhyr, R.D. Evans, B.J. Johnson, D.M. Quincy, and M. Clark, Record low ozone at Mauna Loa Observatory during winter 1994-1995: A consequence of chemical and dynamical synergism?, Geophys. Res. Lett., 23(12), 1533-1536, 1996.

Neale, P.J., D.L. Correll, V.R. Goodrich, and D.R. Hayes Jr., UV-B optical depths at Mauna Loa: Relative contribution of ozone and aerosols, in Summary Report 1993, edited by J.T. Peterson, and R.M. Rosson, NOAA Environmental Research Laboratories, Boulder, CO, pp. 132-134, 1994.

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