by two types of monitors, the Met One BAM-1020 and the Envea MP101M, which both use a beta ray attenuation measurement technique.

We are able to access data at the hourly or daily level (depending on the station) from 2012 to 2024. The PM2.5 data availability is sparse (Figure 2), both because some air quality stations do not collect PM2.5 data and because of challenges maintaining the regulatory-grade monitors. Beginning in 2012, there is relatively consistent coverage with at least one monitor active, except for 2021 and 2022, which have almost no data. This is important because the data from a regulatory-grade monitor can be used to calibrate other data sources. Additionally, when more than one monitor is reporting data concurrently we can use readings from multiple monitors to validate the data. At the times when monitors overlap, the values are closely aligned. Importantly, on days when the values seem to be improbably high, readings from different active monitors both have very high values, which is reassuring that these represent days of extremely high pollution rather than an issue with the measurement instruments. The data was pre-processed by the CGQA for extreme values. In addition, we removed PM2.5 values greater than 500 _μm_ /m<sup>3</sup> .<sup>6</sup> To analyze the data, we take the values from whichever monitor is active, and in cases where there are multiple monitors, we use data from all active monitors.

# **Low-Cost Monitors**

In the last decade there has been significant advancement in the technical specifications of low-cost monitors and substantial growth in the number of low-cost monitors on the market, leading to an explosion in their use (Kumar _et al._ , 2015; Demanega _et al._ , 2021). In addition to their use by individuals and businesses, the recognition that air pollution in urban areas is hyperlocal led to initiatives in highincome settings to complement networks of regulatory-grade monitors with low-cost monitors to provide air pollution data at a fine spatial scale. For instance, London set up a dense network of low-cost monitors across the city and several cities in California mapped pollution street by street using sensors attached to Google Street View cars (Gil-Alana _et al._ , 2020; Mittal, 2020).

The ability to capture variation in pollution at a granular spatial level is crucial for low-income settings where the prevalence of pollution is high and where the technology for capturing the origin of the pollution is limited (Hajat _et al._ , 2015; Kamigauti _et al._ , 2024). Therefore, the team set up a network of low-cost monitors in Dakar to measure pollution at a higher spatial granularity. We selected to use the Purple Air PA-II-SD, which, at the time the study began in 2019, had one of the highest correlations with regulatory monitors (R-squared of 0.93 to 0.97) (South Coast Air Quality Management District, 2024).<sup>7</sup> Additionally, its accuracy has been tested under atmospheric conditions when exposed to a variety of pollutants and PM2.5 concentrations and, unlike monitors from other providers with a similar technology, PM2.5 measures captured by PA-II monitors are not found to be affected by changes in temperature and relative humidity, making them more reliable (Ardon-Dryer _et al._ , 2020). From February 2020 until 2023, the team progressively deployed PA-II monitors across 28 locations in the Dakar region (Figure 1). The team also used data from an additional PA-II monitor set up by the Ecole

> 6Only 0.17% of PM2.5 values were above 500.

> 7At the time when the decision was made, in 2019, only PA-II classic monitors were available on the market.

7