Optical, depth, aerosol

FIGURE 14-32 Typical contributions of aerosol liquid water, carbonaceous compounds, and sulfate in the lower troposphere of the east coast of the United States to the total aerosol optical depth. The contribution of light absorption is also shown. The different bars represent different sets of measurements during different flights (adapted from Hegg et al., 1997). 

Jayaraman et al. (1998) measured the aerosol optical depth, aerosol size distribution, and the solar flux close to the coast of India, over the Arabian Sea, and then... 

During the period with intensive Etesian flow, aerosol optical depth was around 0.2. During periods with not very intensive Etesian flow, it increased up to 1. The day-to-day variability of aerosol optical depth was an order of magnitude, while the daily variability as determined from multiple measurements during one day, was up to a factor of 2.5. 

Figure 6. From figure 11 of Kylling et al. 1998. The ratio between simulated Brewer and Bentham UVB dose rates with and without aerosols as a function of the aerosol optical depth at 355 nm. Ratios of model results with aerosol single scattering albedo of (0.95 solid line), 0.87(dotted line) and 0.80 (dashed line) versus aersosol free model results are shown for solar zenith angle of 10° and an ozone column of 340 DU.

The correlation between photolysis rates and optical depths is obvious for the JN02 case in the UV-A region, it is not clear for JO( D). The model sensitivity study supports the expectation that aerosol optical depth and single scattering albedo are the two decisive parameters to describe the radiative effects of aerosols. 

The aerosol optical depth t x, f°r a given wavelength k, is expressed in terms of the optical thickness at 1 pm, p, using the Angstrom s (10) equation ... 

In order to use aerosol optical depth typical values for the Portuguese territory, monthly average values of a and p were estimated for each station and for the whole observation period. Finally, the final average value used was estimated from the average of the 4 stations. Figures 1 and 2 show the annual variation of P e a, respectively, for the 4 sites. 

Equations 11 to 15 are identical to the used by Brine and Iqbal (2) except for the factor Cx. Considering the difficulties found by Bird, in order to adjust Cx values to an analytical function of aerosol optical depth and zenith angle, it was decided to compute these values by linear interpolation from the tables given in Bird s paper. For wavelengths, aerosol optical depths and zenith angles beyond the limits of the tables Cx values were linearly extrapolated. 

The model was initially compared with LOWTRAN 7 (5) results and with Brewer spectrophotometer measurements carried out at Lisbon in 1990. LOWTRAN-MESTRad results show a reasonable good agreement over 290-400 nm range with a mean difference of about 10%. Figure 4 shows de Model/LOWTRAN ratios for direct, diffuse and global spectral irradiances computed for the same conditions of ozone (332 Dobson Units), solar zenith angle (45.2°) and aerosol optical depth (0.0). 

Figure 4. MESTRad/LOWTRAN ratios for direct, diffuse and global spectral irradiances computedfor the same conditions of ozone (332 Dobson Units), solar zenith angle (45.2 °) and aerosol optical depth at 500 nm...

UV-B spectral global irradiance and total ozone direct sun measurements carried out at Lisbon during 1990 with a MARK 11 Brewer ozone spectrophotometer and with clear-sky conditions were used for comparison at several zenith angles, total ozone and aerosol conditions. However, because aerosol optical depth measurements were not available at this time it was decided to use T5oo=0.0 for the model input. The results of the Model/Brewer ratios for global spectral irradiances are shown in Figure 5. The results are similar and consistent with the earlier LOWTRAN 7 (6) comparison, where the ratio approaches to unity as the wavelength increases,... 

The monthly mean ozone from the Dobson time series (1957-1986) of Vigna di Valle (50 km apart from Rome) and from TOMS (Total Ozone Mapping Spectrometer) satellite data (1979-1991) version 6 are assumed as climatological frames of reference for Rome and Ispra, respectively. Aerosol optical depths at 550 nm are estimated by means of sunphotometry. Data from the two meteorological stations of Rome and Milan airports are used to describe the atmospheric conditions. Standard vertical profiles of pressure, temperature, relative humidity and ozone density are selected. 

From these and other data it follows that accuracy in the estimate of radiation balance as a function of space coordinates depends on cloud distribution, their state, and atmospheric pollution, as well as on the chosen size of pixels for spatial averaging. In this connection, Henderson and Chylek (2005) used image data from the Multispectral Thermal Imager to evaluate the effect of spatial resolution on aerosol optical depth retrieval from satellite imagery. It was shown that aerosol optical depth (AOD) depends weakly on pixel size in the range 40 x 80 m2 to 2,040 x 4,080 m2 in the absence of clouds and changes monotonically with the growing size of pixels in clouds. 

Chylek P. Henderson B. and Mushchenko M. (2003). Aerosol radiative forcing and the accuracy of satellite aerosol optical depth retrieval. /. Geophys. Res., 108(D24), AAC4/1 - AAC4/8. 

Henderson B. and Chylek P. (2005). The effect of spatial resolution on satellite aerosol optical depth retrieval. IEEE Transactions on Geoscience and Remote Sensing, 43(9), 1984-1990. 

Myhre G. Stordal F. Johmsrud M. Ignatov A. Mischenko M.I. Geogdzhaev I.V. Tanre D. Denze J.-L. Goloub P. Nakajima T. Higurashi A. Torres O. and Holben B. (2004). Intercomparison of satellite retrieved aerosol optical depth over the ocean. J. Atmos. Sci., 61, 499-513. 

Stone R.S. (1998). Monitoring aerosol optical depth at Barrow, Alaska and South Pole Historical overview, recent results, and future goals. J. Geophys. Res., 103, 16565-16579. 

Arctic Oscillation Aerosol Optical Depth Aerosol Optical Thickness Arctic Precipitation Data Archive Asia-Pacific Economic Cooperation Air Pollution transport Model ARCtic System Science Arctic Research Consortium of the U.S. 

Fig. 8. Decreasing direct beam spectral irradiance as aerosol optical depth at 500 nm increases from 0.05 (top curve) to 0.04 (bottom curve).

Concentrating solar collector systems that utilize the direct beam radiation would be more likely deployed in areas with relatively low aerosol optical depth, to maximize direct beam utilization.13... 

Specification for aerosol optical depth/turbidity input (0 = AOD at 500 nm) ITURB... 

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