Photo-oxidation of soot black carbon in central Arizona

Soot black carbon (BC) is the product of incomplete combustion of fossil fuels and biomass. Little is known about soot BC in a terrestrial urban setting; however, soot BC is a significant portion of the organic carbon in central Arizona soils (31%). Soot BC could, therefore, play a major role in organic biogeochemical processes in this area. Since the soot BC global budget is unbalanced with respect to sources and sinks, we hypothesize that soot BC is undergoing bio-, chemical-, or photo-degradation in soil. Photo-degradation of black and organic carbon in a desert and urban soil was monitored under a mercury lamp. Mercury lamp experiments showed a statistical decrease in organic and soot black carbon for both desert and urban soils. We performed Fourier-transform infrared spectroscopy (FTIR) on all soil samples pre- and post-photo-oxidation. This technique detects various carbon containing chemical functional groups in soils. Differences in carbon chemical functionality were found when comparing pre- and post-photo-oxidation in both urban and desert soil samples. Pre-photo-oxidation soil samples contained mostly hydrocarbon ring (aromatic) and straight chain (alkane) structures, which are commonly associated with soot BC. Post-photo-oxidation samples contained oxygenated functional groups such as aldehydes and ketones, which are not commonly associated with soot BC and are generally considered to be more reactive than hydrocarbons. We suggest that prolonged exposure to UV radiation can alter the chemical functionality of soot BC and, therefore, can alter soot BC chemical reactivity. These results indicate that soot BC can be photo-oxidized and that soot BC in a desert/urban ecosystem might be more reactive than previously thought.