As one adds an absorbing component to the aerosol, it can lead to a heating of the earth-atmosphere system if the reflectivity of the underlying surface is sufficiently high. Early studies of the effects of aerosols on atmospheric radiative transfer on a global scale assumed a dominantly scattering aerosol with only a small absorbing component, since this appears to be a good representation of naturally occurring aerosols. However, as discussed above, urban aerosols have a large black carbon component and if these particles can be transported on a global scale then one would expect a heating effect over surfaces with a high surface albedo like snow or ice. Furthermore, if these particles are deposited in the snow an additional heating effect would occur due to reductions in the surface albedo. Measuring and modeling spatial distribution Levels of Black carbon are most often determined based on the modification of the optical properties of a fiber filter by deposited particles. Either filter transmittance, filter reflectance or a combination of transmittance and reflectance is measured. Aethalometers are frequently used devices that optically detect the changing absorption of light transmitted through a filter ticket. The USEPA Environmental Technology Verification program evaluated both the aethalometer and also the Sunset Laboratory thermal-optical analyzer. A multiangle absorption photometer takes into account both transmitted and reflected light. Alternative methods rely on satellite based measurements of optical depth for large areas or more recently on spectral noise analysis for very local concentrations. In the late 1970s and early 1980s surprisingly large ground level concentrations of black carbon were observed throughout the western Arctic. Modeling studies indicated that they could lead to heating over polar ice. One of the major uncertainties in modeling the effects of the Arctic haze on the solar radiation balance was limited knowledge of the vertical distributions of black carbon. During 1983 and 1984 as part of the NOAA AGASP program, the first measurements of such distributions in the Arctic atmosphere were obtained with an aethalometer which had the capability of measuring black carbon on a real-time basis. These measurements showed substantial concentrations of black carbon found throughout the western Arctic troposphere including the North Pole. The vertical profiles showed either a strongly layered structure or an almost uniform distribution up to eight kilometers with concentrations within layers as large as those found at ground level in typical mid-latitude urban areas in the United States. The