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α: calibrated so average coauthorship-adjusted count equals average raw count
We discuss two approaches to estimating the air quality impacts of public transit projects, focusing on Metro projects in the context of developing countries: air quality modeling and reduced-form econometric methods. As we illustrate, pollution reductions due to Metro projects implied by pollutant chemistry, vehicle emissions factors, and modal shifts may differ from econometric estimates of the impact of transit projects on ambient pollution concentrations. We discuss both approaches and illustrate how economics researchers can use estimated emissions reductions associated with a transit project and pollutant chemistry as a check on their estimates of changes in ambient concentrations.