Aerosol Optical Depth measurements at 340 nm with a Brewer spectrophotometer and comparison with Cimel sunphotometer observations at Uccle, Belgium

摘要

The Langley Plot Method (LPM) is adapted for the retrieval of Aerosol OpticalDepth (AOD) values at 340 nm from Brewer#178 sun scan measurements between335 and 345 nm (convoluted with the band pass function of the Cimelsunphotometer filter at 340 nm) performed in Uccle, Belgium. The use of sunscans instead of direct sun measurements simplifies the comparison of the AODvalues with quasi-simultaneous Cimel sunphotometer values. Also, theirradiance at 340 nm is larger than the one at 320.1 nm due to lower ozoneabsorption, thus improving the signal to noise ratio. For the selection ofthe cloudless days (from now on referred to as calibration quality cleardays), a new set of criteria is proposed. With the adapted method, individualclear sky AOD values, for which the selection criteria are also presented inthis article, are calculated for a period from September 2006 until the endof August 2010. These values are then compared to quasi-simultaneous Cimelsunphotometer measurements, showing a very good agreement (the correlationcoefficient, the slope and the intercept of the regression line arerespectively 0.974, 0.968 and 0.011), which proves that good qualityobservations can be obtained from Brewer sun scan measurements at 340 nm.The analysis of the monthly and seasonal Brewer AODs at Uccle is consistentwith studies at other sites reporting on the seasonal variation of AODs inEurope. The highest values can be observed in summer and spring, whereas morethan 50% of the winter AODs are lower than 0.3. On a monthly scale, thelowest AOD are observed in December and the highest values occur in June andApril. No clear weekly cycle is observed for Uccle. The currentcloud-screening algorithm is still an issue, which means that some AOD valuescan still be influenced by scattered clouds. This effect can be seen whencomparing the calculated monthly mean values of the Brewer with the AERONETmeasurements.