Critical Inflection Points Govern PM2.5 Decline Dynamics in the Guangdong–Hong Kong–Macao Region

The Guangdong–Hong Kong–Macao (GHM) region (especially the Greater Bay Area), a low-lying economic hub in southern China, faces complex particulate matter (PM2.5) pollution dynamics under the combined influence of monsoonal systems and global warming. While long-term PM2.5 reductions are documented, phase-specific trends remain obscured. Here, we analyze high-resolution ChinaHighPM2.5 dataset observations (2000–2023) using moving averages and piecewise regression to quantify abrupt shifts in interannual and seasonal PM2.5 trends across the region. We identify 2014 and 2016 as critical breakpoints for annual PM2.5 concentration (Mean-5y-Year) and its linear acceleration rate (k-5y-Year), respectively. Critical breakpoints delineate phases where declines persisted but decelerated. Prior to 2014, the PM2.5 levels exhibited an upward trend (+0.203 µg·m−3·a−1, p > 0.05), which reversed sharply post-2014 (−2.046 μg·m−3·a−1, p < 0.01). Spatially, breakpoints clustered post-2014 for concentrations, while acceleration rate shifts reveal a latitudinal divergence near 23° N (23.873°~22.812° N); southern areas transitioned earlier (2010–2011) versus post-2014 in the north. Post-inflection declines are strongest toward the GBA urban core, with winter and autumn driving seasonal improvements (winter: steepest decline −2.646 μg·m−3·a−1; autumn: largest trend reversal Δ−3.961 μg·m−3·a−1), while improvement rates narrowed post-2016 (Δk = +0.527 µg·m−3·a−2). This study establishes that apparent regional PM2.5 reductions mask significant spatiotemporal heterogeneity, underscoring the necessity of phase-specific analysis for effective pollution control in climatically vulnerable megaregions.