Isolating nitrated and aromatic aerosols and nitrated aromatic gases as sources of ultraviolet light absorption

Measurements in 1973 and 1987 showed that downward ultraviolet (UV) irradia nces within the boundary layer in Los Angeles were up to 50% less than thos e above the boundary layer. Downward total solar irradiances were reduced b y less than 14% in both studies. It is estimated that standard gas and part iculate absorbers and scatterers accounted for only about 52-62% of the obs erved UV reductions at Claremont and Riverside. It is hypothesized that abs orption by nitrated and aromatic aerosol components and nitrated aromatic g ases caused at least 25-30% of the reductions (with aerosols accounting for about 4/5 of this]percent). The remaining reductions are still unaccounted for. Absorbing aerosol components include nitrated aromatics, benzaldehyde s, benzoic acids, aromatic polycarboxylic acids, phenols, polycyclic aromat ic hydrocarbons, and nitrated inorganics. Many of these species have been o bserved to date in atmospheric particles, and absorption coefficient data i ndicate many are strong absorbers at long UV wavelengths. Since aerosols co ntaining nitrated or aromatic aerosols have been observed widely in many ar eas aside from Los Angeles the finding may account for a portion of UV exti nction in those regions as well. In Los Angeles, the finding may be importa nt for predicting smog evolution, since UV reductions associated with high aerosol loadings were estimated to cause a 5-8% decrease in ozone mixing ra tios in August 1987. Further laboratory and field studies are needed to qua ntify better the extent of UV absorption due to nitrated and aromatic aeros ols and nitrated aromatic gases.