Aerosol absorption is a major uncertainty in addressing radiative balance in modeling climate on regional scales. Of particular concern are carbonaceous aerosols as they are a potentially important light absorbing species in the troposphere that may be significantly changing radiative forcing. Incomplete combustion leads to the formation of soots or “black carbon” that have been well known as a significant if not the major absorbing aerosol species in the troposphere. Fossil fuel combustion and in particular diesel engines have gotten significant attention as black carbon sources. Use of radioactive carbon content in aerosols to examine the strengths of carbonaceous aerosol sources from biomass as well as fossil fuels has found biomass sources to be significant and in many cases at a level equal to or greater than fossil fuel sources. Our measurements taken in Mexico City along with literature data clearly indicate that there are significant amounts of absorbing carbonaceous aerosols from both primary and secondary sources and that these sources are strong absorbers in UV-VIS-NIR and IR regions relevant to radiative forcing in the troposphere. This data will be briefly overviewed. The potential of biomass burning from agricultural burning practices, as well as forest and grass fires will be discussed in light of changing climate and energy demand. Proposed is a future field experiment in the Southwestern U.S. that would focus on agricultural burning (sugar cane, strawgrass, and other agricultural field wastes) and the use of C-isotopes to evaluate source strengths of black carbon as well as humic-like substances or HULIS. This work is supported by the Office of Science (BER) U.S. Department of Energy under Grant No. DE-FG02-07ER6428, as part of the DOE Atmospheric Science Program.