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Introduction
Ambient or outdoor air pollution has significant impacts on public health around the globe. According to the World Health Organization, an estimated 4.2 million1 people die every year worldwide from exposure to tiny solid and liquid particles— referred to as aerosols or fine particulate matter with an aerodynamic diameter less than or equal to 2.5 microns (PM2.5)—suspended in outdoor air. Reducing these impacts through air pollution controls is thus a major goal of environmental efforts to improve public health. However, quantifying and reducing the health impacts of PM2.5 is especially challenging in low- and middle-income countries (LMICs) because these countries tend to have limited air-quality (AQ) monitoring infrastructure, have insufficient quality assurance and quality control of their AQ monitoring data, and have limited local technical expertise in AQ monitoring, modeling, and planning. These shortcomings related to data and expertise can undermine the effectiveness of the design and implementation of policies to improve AQ in LMICs. Earth-orbiting satellites can detect aerosols using observations of the sunlight scattered by the aerosols (figure 2.1; Bernard and others 2011; Levy and others 2013). This report examines whether the extensive spatial and temporal coverage of these satellites (figure 2.2) can potentially complement the sparse networks of ground-level monitors that typically exist in LMICs, thereby improving AQ monitoring and enforcement actions in these countries. However, satellites are not able to measure ambient ground-level concentrations of PM2.5 directly. Instead, satellites use the observations of the sunlight scattered by aerosols to estimate a parameter called aerosol optical depth (AOD), which represents the extinction (absorption and scattering) of light by all of the aerosols in a column of air from the Earth’s surface up to the top of the atmosphere (for example, Levy and others 2013). As shown in figure 2.3, the reflected sunlight measured by the satellite depends on the intensity of sunlight reaching the Earth (Io), the fraction of the sunlight reflected by the ground (α, also called the “surface reflectance” or “albedo”), the AOD, and the aerosol light scattering and absorbing properties (also called aerosol optical properties, here represented by a single factor ω). Although the intensity of the incoming sunlight reaching Earth is well known, the other three properties are more uncertain. 3
