Soot black carbon concentration and isotopic composition in soils from an arid urban ecosystem

Black carbon (BC) is a poorly understood type of organic carbon but it is present in almost all environmental systems (i.e., atmosphere, soil and water). This work focuses on soot BC in desert soils and, in particular, urban soils from the Phoenix, Arizona metropolitan area. Soot BC is that fraction of black carbon formed from the condensation of gas phase molecules produced during burning. Soot BC in Phoenix area soils exhibits a range in both concentration and isotopic composition. Soot BC concentration in 52 soils (desert, agricultural and urban) ranges from 0.02–0.54 wt% and comprises from < 1 to as much as 89% of the soil organic carbon (OC). Soot BC concentrations are higher in urban soils than in desert or agricultural soils. The average isotopic composition of soot BC is −18‰ ± 3‰; this is an enrichment of 5.5‰ relative to bulk soil organic carbon. The distribution in concentration and variation in isotopic composition across the study area suggests soot BC in this arid-land city has multiple sources, including a significant fossil fuel component.