principal data sources to provide a novel comparison of the data sources in terms of policy evaluation. In addition to accurate information on air pollution levels, which documents the extent of the problem, policy makers need accurate estimates of the impacts of policies on air pollution to design effective solutions. We evaluate the performance of these different data sources in a rigorous analysis of a set of policies that have an important environmental externality.

Third, we contribute policy recommendations for designing air quality monitoring systems in LMICs. We illustrate the value of operating at least one regulatory-grade monitor in a network of low-cost monitors. Co-locating a regulatory-grade monitor with a low-cost monitor allows for context specific calibration of the data from the low-cost monitor network that improves the accuracy of the data relative to using an off-the-shelf calibration formula. We also show that, in the absence of a regulatory-grade monitor, satellite PM2.5 estimates can be used to calibrate low-cost monitors to the local context. For a given budget, a combination of data sources including low-cost monitors can capture the spatial variation of pollution across an urban area and achieve better accuracy than a single data source alone.

The next section describes the context, and section 3 describes the three data sources in detail. Section 4 lays out the methods, section 5 provides the results, and section 6 concludes.

# **2 Context**

Dakar is the capital and largest city of Senegal, which is located on the coast of West Africa. Similar to many other cities in LMICs, Dakar has experienced rapid population growth and urbanization. The Dakar metro area has a population of 3.9 million and a population density of 14,000 per km<sup>2</sup> which is in line with cities such as New York City (Agence Nationale de la Statistique et de la Démographie, 2023).

Dakar currently has 7 regulatory-grade air quality monitors or approximately one monitor per 580,000 residents, which is on the higher side of the typical range (one monitor per 100,000 to 600,000 residents) for urban areas of Europe and North America (Pinder _et al._ , 2019).<sup>3</sup> However, for long stretches of time only one or two regulatory-grade monitors, and for a period of approximately a year no regulatory-grade monitors, were reporting PM2.5 data (see Figure 2 for a visualization of all available data from the regulatory monitors from 2012 to 2024). In practice, this puts Dakar on par with other African urban areas, which typically have one monitor per 4.5 million residents (Pinder _et al._ , 2019).

Dakar experiences high levels of ambient air pollution, typically exceeding World Health Organization standards. The main sources of particulate matter in Dakar are traffic (49%), mineral dust (16%–25%), sea salts (15%–20%), and industries (10%–11%) (Dou). While a large share of particulate matter is generated by anthropogenic sources such as traffic (vehicular, two-wheel two-strokes and resuspended road particles), biomass (wood, charcoal and fuel) combustion, waste burning, and industries, Saharan dust storms are another major source of particulate matter (Dou). Particularly during the first few months of the year, Saharan dust storms can lead to extremely high levels of particulate matter (Dou). Relative to other African cities, the air pollution in Senegal is reported to be moderate-to-severe. Of 24

> 3The 2023 population of the Dakar region was reported at 3,896,563 according to the Agence Nationale de la Statistique et de la Démographie (2023).

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