Assessment of Urban Soot Emissions - The CO-Black Carbon Link

Large emissions of elemental carbon (soot) are found in urban areas as a result of combustion processes. The effects of soot on health and the environment, locally as well as globally, have raised the scientific and public consciousness and catalyzed a need for a better understanding of soot formation, evolution, and transport. Emission inventories of soot are scarce for most urban areas and yet recent studies, such as INDOEX, have demonstrated that copious amounts of soot are found downwind of major urban areas. The lack of emission inventories for soot greatly hampers modeling efforts to assess the environmental impact of urban pollutants that are transported from cities, but the current dearth of instrumentation needed to make the measurements hinders the quantification of soot concentrations. A method exists, however, by which soot can be estimated based upon measurements of carbon monoxide (CO).

Recent measurements in Mexico City show a high correlation (r² > 0.9 ) between CO and the light absorption coefficient of aerosols. This is not unexpected as both CO and soot, the principal component of absorbing particles, are the result of incomplete combustion. Since the absorption coefficient is linearly related to the mass mixing ratio of soot, CO can be used as a surrogate for soot concentrations once the physical basis that links these two atmospheric components is well understood. The amount of soot and CO produced by combustion is proportional to the type of fuel that is used and the amount of available oxygen; thus, it is likely that the soot-CO relationship varies from city to city. The ability to estimate soot concentration from CO measurements is highly promising, however, because of the ease of measuring CO compared to soot, the abundance of past CO measurements that could be analyzed with this relationship, and the potential for better monitoring of soot in the future.

The present study discusses the physics and chemistry that link CO and soot and describes an analysis approach that can provide a more accurate assessment of urban soot emissions than is available at this time