A shortcut from broadband to spectral aerosol optical depth

Abstract

The concept behind the shortcut idea is a close correlation between column broadband aerosol optical depth (BAOD) and aerosol optical depth at 500 nm (AOD500). The method uses only two input parameters: (a) the Bouguer broadband coefficient of column transparency for optical mass m = 2 (solar elevation about 30°) and (b) integrated column precipitable water vapour which can be roughly estimated using surface water vapour pressure. In creating the method, a large database, including almost 20 000 complex, spectral and broadband direct solar beam observations at Tõravere, Estonia, during all seasons of a 8-year period, 2002-2009, was used. The AOD500 observations were performed by the NASA project AERONET and the broadband direct beam ones by the Estonian Meteorological and Hydrological Institute. Analysis of this database revealed a high correlation between BAOD and AOD500 which enabled transition from broadband to spectral AOD. Almost 82% of the observations in the database belonged to lower turbidities when AOD500 < 0.2. The root mean square deviation (RMSD) for AOD500 prediction in this range was 0.022. For AOD500 = 0.2-0.4, the RMSD was 0.035, for 0.4-0.6, the RMSD was 0.042. Relative RMSD for these ranges was about 22%, 12%, and 9%, respectively. For AOD500 > 0.6, relative RMSD remained 9%. For comparison, the same database was used to test Gueymard's broadband parameterization based on his SMARTS2 classic model. The last one, apparently due to problems with circumsolar radiation, slightly but systematically underestimated the AOD500. However, there was a close correlation between our shortcut results and Gueymard's broadband parameterization.

Spectral aerosol optical depth prediction by some broadband models. Validation with AERONET observations

“…It was estimated from surface conditions using correlation with vapour pressure, 0 e (Okulov et al, 2002). At the level of monthly means the used correlation performs quite well (Section 6), but for single observations the coefficient of determination, 2 0.83, R = indicates a moderate scatter of 0 ( ) W e around (AERONET) W (Kannel et al, 2012). Overestimation of W leads to underestimation of AODλ, even to a physically unrealistic negative AODλ.…”

“…W These parameters allow calculating for 2 m = a specific quantity, the column broadband aerosol optical depth (BAOD2). According to Kannel et al (2012), the two optical depth parameters, AOD500 and BAOD2, are strongly correlated 2 ( 0.96) R = through a second-degree polynomial: 2 AOD500 1.7(BAOD2) 1.3(BAOD2), = +…”

“…Note that the calculation of BAOD2 is not sensitive to uncertainties in precipitable water ( ) W estimations, thus W was estimated from the station surface water vapour pressure. Details of data processing are given in (Kannel et al, 2012). The quality of the W estimation is discussed in Section 6.…”

“…Besides, nitrogen dioxide would only be of concern over polluted areas. Therefore, in the extensive runs of Gueymard's model, we used the given fixed input values, typical for the Baltic Sea region: O 3 = 0.35 atm cm, NO 2 (stratospheric) = 0.00012 atm cm, NO 2 (tropospheric) = 0.00004 atm cm, p = 1013.25 hPa (Kannel et al, 2012).…”

“…This parameterization was developed from clear-sky radio soundings in Tallinn (Okulov et al, 2002). For Tõravere, Kannel et al (2012) where the latter were considered as reference. Use of almost 20 000 parallel observations from 2002-2009 showed that the prediction overestimates the reference as an average only by 3%.…”