TRANSIMS is a simulation system for the analysis of transportation options in metropolitan areas. Its major functional components are: (1) a population disaggregation module, (2) a travel planning module, (3) a regional microsimulation module, and (4) an environmental module. In addition to the major functional components, it includes a strong underpinning of simulation science and an analyst`s toolbox. The purpose of the environmental module is to translate traveler behavior into consequent air quality. The environmental module uses information from the TRANSIMS planner and the microsimulation and it supports the analyst`s toolbox. Transportation systems play a significant role in urban air quality, energy consumption and carbon-dioxide emissions. Recently, it has been found that current systems for estimating emissions of pollutants from transportation devices lead to significant inaccuracies. Most of the existing emission modules use very aggregate representations of traveler behavior and attempt to estimate emissions on typical driving cycles. However, recent data suggest that typical driving cycles produce relatively low emissions with most emissions coming from off-cycle driving, cold-starts, malfunctioning vehicles, and evaporative emissions. Furthermore, some portions of the off-cycle driving such as climbing steep grades are apt to be correlated with major meteorological features such as downslope winds. These linkages are important, but they are not systematically treated in the current modeling systems. The TRANSIMS system holds the promise of a more complete description of the role of heterogeneity in transportation in emission estimation. The TRANSIMS micro-simulation produces second by second vehicle positions defined by 7.5 meter cell locations. An approach has been used to convert average cell populations and average transitions between cells into fine-grained distributions of speeds and accelerations. This paper describes the approach and compares the emissions that result from: (1) actual ...