Light absorbing components of aerosols, often called black carbon (BC), are emitted from combustion sources and are believed to play a considerable role in direct atmospheric radiative forcing by a number of climate scientists. In addition, it has been shown that BC is associated with adverse health effects in a number of epidemiological studies. Although the optical properties (both absorbing and scattering) of combustion aerosols are needed in order to accurately assess the impact of emissions on radiative forcing, many models use radiative properties of diesel particulate matter that were determined over two decades ago. In response to concerns of the human health impacts of particulate matter (PM), regulatory bodies around the world have significantly tightened PM emission limits for diesel engines. These requirements have resulted in considerable changes in engine technology requiring updated BC measurements from modern engines equipped with aftertreatment systems. In this study, a variety of common ambient monitoring techniques were used to characterize the light absorbing properties of diesel aerosol. Aerosol optical properties were directly measured with an Aethalometer and Photoacoustic Extinctionmeter and compared to filter based analysis. The results showed excellent correlation (R2 = 0.95) between aerosol light absorption at the short IR wavelength with elemental carbon (EC) concentration from a thermal optical reflectance, NIOSH 5040 method. Resulting EC mass absorption cross-section efficiencies differed by 25 to 30% from manufacturer published values indicating the optical properties used by the instrument may not be representative of modern diesel engine emissions.