As previously stated, the path length of the stratosphere is much longer than that of the troposphere, but the higher density of the troposphere and its increasing scattering capabilities are believed to enhance the absorptive powers of this region (Bruhl and Crutzen, 1989). Atmospheric pressure increases dramatically toward the earth's surface and as a result, the mean free path for molecular interaction is greatly reduced from its value in the stratosphere. From 40 km (the approximate center of the stratosphere) to sea level, the pressure increases almost 1,000 times, and the mean free path changes by at least a factor of 106. This, combined with the pollution "soup" that is present in the troposphere and especially concentrated in the planetary boundary layer (PBL), make for an interesting array of interaction cases for each UV photon impacting this portion of the atmosphere.
The planetary boundary layer is approximately 1 km thick and extends from the surface to roughly that height. During the morning hours, it can be as small as 400 m - 800 m and can rise to 1,500 m - 2,000 m during the afternoon with the heating of the layer. This area contains the most significant concentration of air pollutants, especially, aerosols. Land use and surface topography can also cause the PBL to have spatial variations in the layer depth and structure. The spatial variations can also be caused by large-scale meteorological variables. Thermal inversions can compress the PBL, and low pressure systems may increase its thickness.
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