Abstract:Air pollution at many types of intersections and other roadside "hot spots" is not accurately characterized by state-of-the-practice models. In this study, data were collected on traffic flows, second-by-second CO and NO₂ ambient concentrations in Shanghai, China. The sampled data were compared with CAL3QHC modeling results. We found that: (1) intersection hot spot emission concentrations were explained primarily by queuing activities of motor vehicles; (2) air quality concentrations are difficult to predict because of complex dispersion processes near high-rise buildings; and (3) screening models such as CAL3QHC are prone to large errors in dense cities with mixed traffic and high-rising buildings. Suggestions are made for improved models relevant to dense developing cities.