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

Abstract

[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 t a l) and precipitable column water vapor (W ). Each derives t a l from measured signals at four wavelengths l (340, 440, 675, and 870 nm), and W from the 936 nm signal measurements. Accuracy of t a l 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) , t 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 t a936 as 0.91 t a870 , which is highly simplistic. Therefore, it is recommended to determine t a936 by logarithmic extrapolation from t a675 and t 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-meansquare (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.

Measurements of aerosol properties from aircraft, satellite and ground‐based remote sensing: a case‐study from the Dust and Biomass‐burning Experiment (DABEX)

“…The AERONET and Microtops sunphotometer AODs generally agree to within 0.07 and there are no obvious systematic differences between the two instruments. The differences in AOD between AERONET and Microtops are probably due to differences in the location of the instruments, since instrumental uncertainties in sunphotometer AODs are not expected to be more than 0.02 (Eck et al, 1999;Ichoku et al, 2002). Figures 5 and 6 show a fair degree of variability in AOD, which may be due to horizontal variability of the AOD since the Banizoumbou AERONET site was approximately 50 km to the northeast of Niamey where the Microtops measurements were made.…”

“…This measured AOD at wavelengths of 380, 440, 675, 936 and 1020 nm by measuring the intensity of the direct solar beam. Comparisons with AERONET sunphotometers showed Microtops II AODs to be accurate to within ±0.02 when operated correctly in cloud-free conditions (Ichoku et al, 2002). As with AERONET, the AOD at 550 nm was estimated via logarithmic interpolation.…”

Large aerosol optical depths observed at an urban location in southern India associated with rain-deficit summer monsoon season

“…Microtops-II is a 5-channel hand-held Sun photometer used to measure the instantaneous aerosol optical depth from individual measurements of direct solar flux, using a set of internal calibration constants (Morys et al, 2001;Ichoku et al, 2002). A Global Positioning System (GPS) receiver attached to the photometer provides information on the location, altitude and pressure.…”

A MODIS-based modeling scheme for the estimation of downward surface shortwave radiation under cloud-free conditions

“…Then, ω o and g, together with the MODIS green TOA reflectance and surface reflectance, were used to estimate the 1 km AOD over the Koohin and Chitgar sites at Terra satellite overpass time. The retrieved SARA AOD was then extracted and averaged for a 3 km × 3 km spatial subset region cantered on both sites (Ichoku et al 2002).…”