Inadvertant modification of visibility in the vicinity of a major metropolitan area

A visibility reduction amounting to 50% of prevailing regional upwind visibilities has been found to occur at transport times of 2-3 hours downwind of the St. Louis Metropolitan area. This visibility minimum corresponds in space and time to increased values of CCN active at 0.5% supersaturation. Aerosol distributions, derived from the CCN spectra and in situ optical counting devices, indicate that increases in the number of particles in the 0.l-2.5um diameter range are responsible for the reduction in visibility. Furthermore, a comparison of the evolution of the aerosol distribution for size, surface area and volume with those derived from laboratory studies suggest that most of the light scattering aerosols are of secondary origin, produced by heterogeneous gas to particle conversions. The secondary conversion products tend to accumulate in the particle size range between 0.I-2.5um, maximizing around 0.8-I.0um within 2-3 hours because of some form of the growth law and the shape of the primary aerosol size distribution. This elevated size range may be explained by the fact that St. Louis is located in a region of the country in which large SO2 sources (mainly power plants, non-ferrous metal smelters and oil refTneries) are spaced from a few tens to a few hundreds of kilometers apart in virtually all directions extending for several hundred kilometers. Since the typical removal distance for SO2 and its oxidation products is probably on the order of a thousand kilometers or more, any site in the region will constantly be under the influence of the SO2 oxidation products from a large number and a variety of sources, thus providing a unique regional background aerosol distribution.