Why Emission Reductions Do Not Yield Proportional Air-Quality Improvements: Atmospheric Nonlinearities and Implications for Sustainable Pollution Control

Emission reduction remains the foundation of air-pollution control, yet the relationship between reduced emissions and improved ambient air quality is frequently non-proportional. This mismatch is not an exception to atmospheric behavior but a consequence of coupled chemical, meteorological, transport, and removal processes. Here, we critically synthesize global evidence for nonlinear air-quality responses to emission controls, with particular attention to fine particulate matter (PM2.5) and ozone (O3). We distinguish five response forms that are directly relevant to policy: near-linear, sublinear, superlinear, threshold, and sign-reversal behavior. Ozone provides the clearest example because the response to nitrogen oxides (NOx) and volatile organic compounds (VOCs) depends on the prevailing photochemical regime and can change as emissions decline. PM2.5 responses are likewise nonlinear because precursor controls alter atmospheric oxidation capacity, gas-particle partitioning, aerosol water, and interactions among nitrate, sulfate, ammonium, and secondary organic aerosol. Aerosol reductions can further modify photolysis and boundary-layer processes, linking PM2.5 control to O3 production. Regional transport and background concentrations attenuate or redistribute the benefits of local controls, while meteorological variability changes both chemical sensitivity and the realized concentration response. These mechanisms imply that sustainable air-quality management cannot be evaluated solely by tonnes of emissions avoided. Instead, policy performance should be assessed along the complete pathway from emission reduction to ambient concentration, exposure, health, climate, ecosystem, equity, and economic outcomes. We propose a sustainability-oriented framework in which control strategies are evaluated for atmospheric effectiveness, multipollutant coherence, spatial equity, climate compatibility, and robustness across changing chemical and meteorological regimes. The evidence supports adaptive, coordinated, and regionally integrated control portfolios rather than fixed single-pollutant reduction ratios.