Examining The Causes And Effects Of DownwardUltraviolet Irradiance Reductions In Los Angeles

Abstract

In 1973 and 1987, downward ultraviolet (UV) irradiance measurements within the boundary layer in Los Angeles were as much as 50% less than those above the boundary layer. Downward total solar irradiances reductions were no more than 14% in both cases. Standard paniculate absorbers and scatterers can account for only part of the observed UV reductions. It is hypothesized that absorption by organic material, most likely nitrated aromatics, benzaldehydes, benzoic acids, aromatic polycarboxylic acids, phenols, and polycyclic aromatic hydrocarbons, and absorption by nitrated inorganics, caused much of the additional reductions. This finding uncovers previously ignored sources of UV light absorption. In Los Angeles, the finding is important since UV reductions were estimated to cause a 5-8% decrease in ozone mixing ratios in August, 1987. UV and ozone reductions in smog require high particle loadings; thus, such reductions cannot be considered health benefits of smog.