# **1.  Introduction**

Air pollution from urban traffic is a major source of health damage in developing countries. In a global study, Dora et al. (2011) find that urban road transport often accounts for more than 50% of dangerous air pollutants (e.g., fine particulates, nitrogen oxides, carbon monoxide, ozone, sulfur dioxide). The World Health Organization (2016) attributes about 3.3 million annual premature deaths to outdoor air pollution in low- and middle-income countries: 72% from heart disease and strokes, 14% from chronic obstructive pulmonary disease, and 14% from lung cancer. The World Bank (2016) estimates welfare losses from ambient fine particulate and ozone pollution at 4.8% of GDP equivalent in East Asia, 3.5% in South Asia, 2.1% in the Middle East and North Africa, 1.6% in Latin America and 1.5% in Sub-Saharan Africa.

Urban traffic and pollution problems are essentially geographic, because their incidence and impacts depend on the spatial distributions of economic activities, households, and transport links. Although urban planners, researchers and policy makers are sensitive to this spatial component, incomplete information has hindered their attempts to identify and implement cost-effective strategies. Local traffic and pollution are often poorly measured because monitoring systems are sparse, costly and difficult to maintain. Since this problem is pervasive in developing countries, rigorous empirical analyses of alternative strategies are rare. As a consequence, urban planners and policy makers have little guidance on appropriate spatial measurement techniques, spatiallyoriented methodologies for benefit-cost assessment of policy alternatives, and transferable empirical models from other urban areas. In practice, they have frequently been forced to estimate city-wide traffic volume from observations on a few traffic arteries, and air pollution from aggregate fuel consumption data, coupled with air pollutant emission factors for gasoline and diesel vehicles that may have been generated from small samples in other places. As a result, educated guesswork has often guided the placement of subway lines, road corridor improvements and other spatially-targeted interventions for reducing traffic and pollution.

Recent developments in satellite monitoring offer a potential escape from this information trap. High-resolution imagery now permits frequently-updated observations of polluting aerosols and traffic in multiple zones within urban areas. Appropriately mobilized and interpreted, data from these new sources can elevate the state of the art in urban traffic/pollution research, policy analysis and implementation for developing countries.

Satellite coverage is global, so the new approach can enable cities in all regions to benefit from the rigorous analyses that have traditionally been limited to well-endowed cities in developed countries. This paper is a pilot exercise for Dar es Salaam that incorporates spatially and temporally non-uniform distributions of vehicle traffic and air pollution into an investigation of impacts on vulnerable households. We highlight potential health damage in areas of Dar es Salaam with high poverty incidence and large numbers of vulnerable children and elderly adults.

The remainder of the paper is organized as follows. Section 2 reviews prior research on the health impacts of NO2 pollution, the links between traffic and air pollution, and the socioeconomic dimensions of air pollution impact. In Section 3, we introduce the new data sources for our analysis. Section 4 uses econometric modeling to investigate the spatial dynamics of NO2 pollution in the metro region, with a particular focus on critical weather factors that drive the locus 2