Population exposure to coal PM2.5 was reduced through various actions taken on individual coal EGUs across the study period, including reduced operations, emissions controls (“scrubbers”; control technologies identified by the following labels in the AMPD data set: Dry Lime FGD, Dry Sorbent Injection, Dual Alkali, Fluidized Bed Limestone Injection, Magnesium Oxide, Sodium Based, Wet Lime FGD, Wet Limestone, and Other), and retirements. Using PWE from HyADS and data from EPA AMPD, we calculated PWE contributed by operational facilities and PWE avoided through each of these three interventions.
We used dates of unit retirements and scrubber installations listed in the AMPD data set to designate each unit’s operational or emissions control status. Additionally, we employ each unit’s annual heat input—also available in the AMPD data set—to characterize units as operating at high capacity (annual heat input above each unit’s median annual heat input reported in operational years from 1999 to 2020) or low capacity (annual heat input below median heat input). This characterization of high vs. low operational capacity allows for the quantification of exposure avoided by reduced operations. Using this information, we characterize each unit into one of six categories: a) operating at high capacity without a scrubber, b) operating at low capacity without a scrubber, c) operating at high capacity with a scrubber, d) operating at low capacity with a scrubber, e) retired without previously installing a scrubber, and f) retired after operating with a scrubber. These six operational/control categories led to seven contributed and avoided exposure designations that could be calculated using modeled PWE across subsets of years for each unit (Table 1).
We calculated each quantity listed in Table 1 for each unit in years that met the corresponding criteria and presented the sum of each exposure class across units. We did not include the years of scrubber installation or retirement in the PWE averaging to avoid transition years. Each unit’s potential PWE designation among these five categories remained constant across any given range of years for which its scrubber and operational status did not change. We presented the annual results as a percentage of total potential exposure in each year.
The approach was designed to explore trends across years, and the calculated values were somewhat sensitive to the criteria listed in Table 1. Therefore, the results were not precise enough to diagnose a given year’s exposure distribution across the seven categories, and we focused on overarching trends in the discussion. Sensitivity of the results to the selection of the heat input value cutoff used to define high/low operating capacity is presented in Figure S7.