Relationship between stations and PM_2.5 areal concentration: <i>y_i</i> denotes PM₁₀ concentration reported by station <i>i</i>, and <i>X</i> is areal PM_2.5 concentration for Beijing; St. US denotes the U.S. Embassy station at which daily PM_2.5 concentration x₀ is observed; <i>X</i> is estimated by <i>x</i>₀ using the SPA technique, based on observed PM_2.5 data at the embassy station, and their correlation with PM₁₀ concentrations observed at the 18 (evenly distributed) stations operated by BJ-EPB.

<p>Relationship between stations and PM_2.5 areal concentration: <i>y_i</i> denotes PM₁₀ concentration reported by station <i>i</i>, and <i>X</i> is areal PM_2.5 concentration for Beijing; St. US denotes the U.S. Embassy station at which daily PM_2.5 concentration x₀ is observed; <i>X</i> is estimated by <i>x</i>₀ using the SPA technique, based on observed PM_2.5 data at the embassy station, and their correlation with PM₁₀ concentrations observed at the 18 (evenly distributed) stations operated by BJ-EPB.