Analysis of the performance characteristics of the five-channel Microtops II Sun photometer for measuring aerosol optical thickness and precipitable water vapor

[1] Five Microtops II Sun photometers were studied in detail at the NASA Goddard Space Flight Center (GSFC) to determine their performance in measuring aerosol optical thickness (AOT or tau(alambda)) and precipitable column water vapor (W). Each derives tau(alambda) from measured signals at four wavelengths lambda (340, 440, 675, and 870 nm), and W from the 936 nm signal measurements. Accuracy of tau(alambda) and W determination depends on the reliability of the relevant channel calibration coefficient (V-0). Relative calibration by transfer of parameters from a more accurate Sun photometer (such as the Mauna-Loa-calibrated AERONET master Sun photometer at GSFC) is more reliable than Langley calibration performed at GSFC. It was found that the factory-determined value of the instrument constant for the 936 nm filter (k = 0.7847) used in the Microtops' internal algorithm is unrealistic, causing large errors in V-0(936), tau(a936), and W. Thus, when applied for transfer calibration at GSFC, whereas the random variation of V-0 at 340 to 870 nm is quite small, with coefficients of variation (CV) in the range of 0 to 2.4%, at 936 nm the CV goes up to 19%. Also, the systematic temporal variation of V-0 at 340 to 870 nm is very slow, while at 936 nm it is large and exhibits a very high dependence on W. The algorithm also computes tau(a936) as 0.91 tau(a870), which is highly simplistic. Therefore, it is recommended to determine tau(a936) by logarithmic extrapolation from tau(a675) and tau(a870). From the operational standpoint of the Microtops, apart from errors that may result from unperceived cloud contamination, the main sources of error include inaccurate pointing to the Sun, neglecting to clean the front quartz window, and neglecting to calibrate correctly. If these three issues are adequately taken care of, the Microtops can be quite accurate and stable, with root-mean-square (rms) differences between corresponding retrievals from clean calibrated Microtops and the AERONET Sun photometer being about +/-0.02 at 340 nm, decreasing down to about +/-0.01 at 870 nm.