Abstract
Reduced visibilities usually are due to particles in the atmosphere which cause the decrease in contrast of distant objects. Once the observed contrast is less than the contrast threshold of the observer's eye, the object is invisible. The distance of visibility is reached when the observed object is seen with a contrast equal to the threshold. The contrast of an object seen through an aerosol can then be calculated, when the extinction coefficient and the intrinsic brightness of the object are known. In the simplest case of a black object and a homogeneous atmosphere, the contrast is a negative exponential function of the extinction coefficient. Since the extinction coefficient of the atmospheric aerosol shows a wavelength dependence such that a higher extinction occurs in the blue than in the red part of the spectrum, the contrast of an object seen through the atmosphere is minimal in the blue and medium in the red. Therefore a distant object should be best visible in the red (medium contrast), to some extent visible in the green (low contrast), and practically invisible in the blue (very low contrast). On the other hand the eye is very sensitive in the green, where a low contrast occurs and is less sensitive in the red, where a medium contrast occurs; obviously vision takes place at an intermediate wavelength, which can be calculated using the perception function. This wavelength depends on the type of aerosol present in the atmosphere and is around 580 to 600 nm. When non-black objects are observed through the atmosphere their intrinsic contrast has to be taken into account and they are usually less visible than black objects, especially for colors that show a high reflectivity around the wavelength of maximum perception. Simulation experiments performed in a hydrosol verified this. Since the most relevant parameter for the visibility determination is the contrast, a telephotometer was built using an astronomic telescope which measures the contrast of distant targets and thus permits a quick in situ measurement of the extinction coefficient and an accurate determination of the visibility. Several measurements performed both in polluted and unpolluted areas have shown that the extinction coefficient of atmospheric aerosol shows large variations. Normally the extinction coefficient is higher in the blue than in the red. The extinction coefficient can be related to the mass concentration of the atmospheric particulate pollution, permitting a quick estimate of the mass of suspended particulates. Since the telephotometer measures the average extinction coefficient, it has been used to determine the spatial distribution of atmospheric pollution over Vienna. A determination of the spatial distribution requires approximately 30 minutes and can be performed from one location, thus being much faster and cheaper than the traditional methods.