Atmosphere, optically thick

As Chylek et al. (2003) noted, the maximum permissible error in outgoing radiation flux determination from satellite data Afr = 0.5Wm-2 determines the necessity to retrieve the atmospheric optical thickness, r, with an error not more than At = 0.015 on land and 0.010 over the oceans. However, this level of error has still not been achieved. Using AVHRR data, the MSD of r values varies within 0.06-0.15, whereas in the case of MODIS data over land, At = 0.05-0.2t, which corresponds to the interval of At values from 0.07 to 0.21, with t varying from 0.1 to 0.8. The use of the extra-nadir data of multi-spectral thermal video-radiometer MTI for intermediate angles of scattering provides the level of error At = 0.03. 

As part of some international and national projects, many measurements of atmospheric optical thickness were carried out. By using passive satellite sensors, estimates can be averaged vertically over a surface pixel. Therefore, to get a deeper understanding of the optical thickness of atmospheric layers, aircraft measurements are made which give the vertical distributions both of tropospheric aerosols and other characteristics of the atmosphere. Among successful airborne experiments we should highlight the ITOP, SHADE, and SAMUM experiments. These experiments made it possible to study the transformation of aerosols during the distant transport of smoke and desert dust. 

Tegen, I., and I. Fung, Modeling of Mineral Dust in the Atmosphere Sources, Transport, and Optical Thickness, J. Geophys. Res., 99, 22897-22914 (1994). 

In the UV, the atmospheric factors to be considered are total ozone amount, aerosol optical thickness and the atmospheric pressure. 

Sensor Performance. The intensity of electromagnetic radiation transmitted through an optical path of length L of an atmosphere that is not optically thick can be described by ... 

Accomplishment of the complex observational experiment LACE-98 made it possible to obtain extensive information about atmospheric aerosol (aircraft measurements of the size distribution and number density of fine aerosols, coefficients of aerosol absorption, backscattering and depolarization, chemical composition of aerosol, as well as surface observations of the spectral optical thickness of the atmosphere, coefficients of extinction and backscattering). Fiebig et al. (2002) compared the observational data on optical parameters obtained from the results of numerical modeling for total H2S04 aerosol near the tropopause as well as for the ammonium sulfate/soot mixture in the remainder of the air column (Osborne et al., 2004). 

The indirect climatic impact of aerosol at the ABL is determined by numerous interactions between aerosol and the dynamics of the microphysical and optical properties of clouds. The input to the atmosphere of anthropogenic aerosol particles functioning as CCN favors an increase in cloud droplet number density. As mentioned above, the related increase in the optical thickness and albedo of clouds, with their constant water content, was called the first indirect effect , which characterizes the climatic impact of aerosol. 

Brenguier et al. (2003) discussed the results of eight series of aircraft measurements of the microphysical characteristics of marine stratocumulus clouds in a broad range of observation conditions (different physico-chemical properties of aerosol, number density values in the interval 50 cm 3-25 cm-3, etc.)- The unique complex of synchronous observations of the microphysical and radiative characteristics of cloud cover obtained can be used to assess the indirect impact of aerosol on clouds and climate based on analysis of the ratio between the cloud optical thickness and effective radius of cloud droplets. Correlation between these values is usually negative, but in a heavily polluted atmosphere it can be positive. From the observational data obtained during ACE-2, the polluted systems of clouds turned out to be somewhat drier and therefore thinner, resulting in the positive correlation between the indirect impact of aerosol on climate and the effective radius of droplets. 

The optical thickness of the fog used in the above-described laboratory experiments is typical for clouds like cumulus or stratocumulus [76]. To extend those results to atmospheric scales, experiments have been performed with... 

Figure 9.1 Examples of spectral energy distributions from young Sun-like stars with circumstellar dust disks. Optically thick dust disks (solid line) have excess emission relative to the stellar photosphere over a broad wavelength range, from near-infrared to millimeter wavelengths. Transition disks (dashed line) lack near-infrared excess emission, but have large mid- and far-infrared emission. Debris disks (dotted line) have small excess emission starting at wavelengths typically longer than 10 pm. Primordial and transition disks often show a prominent 10 pm silicate emission feature from warm dust grains in the disk atmosphere.

Fig. 1. Model calculations of the temporal dependence of the vertical optical thickness of a nuclear dust cloud, averaged over the Northern Hemisphere (X = 550 nm). The mass concentration of dust particles in the cloud corresponds to the injection to the atmosphere of 3.3 X 10 (1), 1 X 10 (2), and 1 X 10 (3) tons of particulate matter per Mt of an equivalent charge. Particles size distribution is log-normal and parameterized by the power law (see the text) 4,5 — calculations on the assumption of the prevailing content in the cloud of large (r , = 1.0 pm) and small (r , = 0.1 pm) particles.

Since F(0,A,)/FTOa( ) is effectively zero for each of these bands, which extend up to 242 nm, we see that atmospheric 02 completely removes all radiation below X = 242 nm from reaching the Earth s surface. [In such a case, the species is called optically thick in the region conversely, for a species for which exp(-x) = 1, the species is optically thin.]... 

This result is known as the Beer Lambert law of extinction. When x is measured vertically in the atmosphere the optical thickness is called the optical depth. 

For planets with optically thick atmospheres, the first measurements in the visual spectral region were performed by Lyot [36] in 1922 1925. These observations revealed a series of new effects. Polarization measurements of details of the Jovian disk showed the following 1) The polarization plane makes a 0 angle with the equatorial plane of intensity at the western and... 

The dust-comet halo represents an extreme case of an extended and sparse planetaiy atmosphere. Its optical thickness is veiy small. Therefore, the role of multiple scattering is small too. 

The Martian atmosphere is characterized by a small value of the optical thickness. Therefore polarization properties of the reflected light are formed both by the underlying surface and the gas-aerosol atmosphere. One fortunate exception occurs at phase angles near the inversion angle orf of the surface, where the resulting polarization P a-) is formed only by the Martian atmosphere. In modehng the first-order scattering, the expression for the observed Q value becomes... 

A polarization decrease was detected in the innermost coma in comets Halley [6,67] and Hale-Bopp [28,29] at distances < 1000 km from the nucleus, which led to the hypothesis that this is due to fresh dust recently ejected from nucleus [6], However, we believe that a decrease of the polarization in the vicinity of the nucleus also may be due to multiple scattering. According to Dollfus and Suchail [6] the optical thickness of comet Halley s atmosphere was about 0.9 at the cometocentric distance 500 km. Ferndndez [68] also indicated a significant optical thickness >1 within roughly 100 km of the nucleus s center of comet Hale-Bopp. 

The requirements for very sharp and well-defined laser frequencies when using the DIAL teclmique can be relaxed by using a gas correlation lidar technique [10.108], where the back-scattered radiation from a broadband transmitter is split up in two channels one supplied with a cell filled with optically thick gas which only passes the off-resonance components for the signal, and one channel where the total signal (on and off) is detected. If the cell gas is present in the atmosphere, a differential lidar effect is obtained using many absorption lines simultaneously (compare Fig. 6.77). This technique... 

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