The thick solid line depicts the theoretical ideal MAC frontier, which is the lowest annual cost per µg/m³ of PM_2.5 reduction at each pollution level (cleaner air to the left). This frontier is not static; it can shift downwards (approximated by thin dashed lines) over time, driven by technological advancements, learning effects from accumulated experience, and crucially, institutional and policy evolution. Effective governance, improved regulatory design, and well-functioning market mechanisms can significantly lower societal abatement costs. In practice, however, regions often diverge from this ideal: some may initially incur higher costs (e.g., thick dashed line) due to technical, informational, or institutional constraints before costs decline, while others, through more efficient policy and quicker innovation adoption (e.g., thick dotted line), may approach optimal cost pathways more rapidly.