| Summary: | Exposure to particulate air pollution is a major cause of mortality and morbidity worldwide. In developing countries, the combustion of solid fuels is widely used as a source of energy, and this process can produce exposure to harmful levels of particulate matter with diameters smaller than 2.5 microns (PM<sub>2.5</sub>). However, as countries develop, solid fuel may be replaced by centralized coal combustion, and vehicles burning diesel and gasoline may become common, changing the concentration and composition of PM<sub>2.5</sub>, which ultimately changes the population health effects. Therefore, there is a continuous need for in-situ monitoring of air pollution in developing nations, both to estimate human exposure and to monitor changes in air quality. In this study, we present measurements from a 5-week field experiment in Palapye, Botswana. We used a low-cost, highly portable instrument package to measure surface-based aerosol optical depth (AOD), real-time surface PM<sub>2.5</sub> concentrations using a third-party optical sensor, and time-integrated PM<sub>2.5</sub> concentration and composition by collecting PM<sub>2.5</sub> onto Teflon filters. Furthermore, we employed other low-cost measurements of real-time black carbon and time-integrated ammonia to help interpret the observed PM<sub>2.5</sub> composition and concentration information during the field experiment. We found that the average PM<sub>2.5</sub> concentration (9.5 µg∙m<sup>−3</sup>) was below the World Health Organization (WHO) annual limit, and this concentration closely agrees with estimates from the Global Burden of Disease (GBD) report estimates for this region. Sulfate aerosol and carbonaceous aerosol, likely from coal combustion and biomass burning, respectively, were the main contributors to PM<sub>2.5</sub> by mass (33% and 27% of total PM<sub>2.5</sub> mass, respectively). While these observed concentrations were on average below WHO guidelines, we found that the measurement site experienced higher concentrations of aerosol during first half our measurement period (14.5 µg∙m<sup>−3</sup>), which is classified as “moderately unhealthy” according to the WHO standard.
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