Aerosol Measurement in the Australian Outback: Intercomparison of Sun Photometers

Abstract

The low background aerosol loadings prevailing over much of the Australian continent necessitate careful attention to the calibration of sun photometers. The validity of such calibrations can only be assessed objectively by intercomparison of independent systems operating side by side. This paper documents two intercomparisons: the first between three dissimilar photometers collocated at Alice Springs using independent calibration methods, and the second between identical photometers sited at Tinga Tingana in the Strzelecki Desert of South Australia. The intercomparison of total optical depth derived from two Cimel CE318 systems at Tinga Tingana shows negligible biases (<0.0004) at all four wavelengths. Instantaneous differences in total optical depth are used to infer 95% uncertainty intervals, which range from 0.003 at 670 nm to 0.005 at 870 nm. The Alice Springs intercomparison shows negligible bias between the Carter?Scott SPO1A and Cimel CE318 at 500 nm, while a bias of 0.004 between the two at 868 nm is identified as sideband leakage in one of the filters. The 95% uncertainty interval for each instrument is <0.007 at both 500 and 868 nm. The multifilter rotating shadowband radiometer (MFRSR) shows a consistent positive bias of 0.012?0.014 at the three wavelengths studied, most probably related to issues of alignment and angular response characterization. The 95% uncertainty interval is greater than 0.02, comparable with the typical background midvisible aerosol optical depth at these sites. Hence this instrument is unsuitable for the measurement of background aerosol under Australian conditions without careful characterization. The impact of uncertainties in surface pressure and ozone on aerosol optical depth is shown to be negligible for the case where the surface pressure is measured on site, and the ozone amount is taken from monthly mean data from stations of commensurate latitude to the observing site. Comparison with previous work suggests that calibration of collimated sun photometers at remote inland sea level Australian sites yields accuracy exceeding that obtained from techniques presently in use in the Northern Hemisphere involving calibration at high altitude sites.