Methodological Reconstruction of Ammonium-Salt Haze Triggering Mechanism: Critical Threshold Effect of Ammonia Emissions from Municipal Wastewater Treatment Plants

Abstract Current haze causation studies exhibit significant methodological biases: utilizing annual average data from non-haze periods to analyze heavy pollution mechanisms, conflating the scientific definitions of basic emission sources and outbreak triggering factors, resulting in long-term deviation of governance from core issues. Based on the time-series data of Beijing’s wastewater treatment industry from 1954 to 2014 and the spatiotemporal distribution characteristics of haze, this study reveals through logical deduction, component correspondence, and spatiotemporal matching verification that haze is a sudden air pollution phenomenon resulting from the coupling of basic precursors, ammonium triggering factors, and extreme meteorological conditions. Research indicates that motor vehicles, coal combustion, and industrial emissions only provide conventional precursors such as SO₄²⁻ and NO₃⁻; NH₃ released from municipal wastewater treatment plants (WWTPs) is the key triggering factor driving rapid secondary formation of ammonium sulfate and ammonium nitrate under stable and humid meteorological conditions, inducing nonlinear jumps in PM₂.₅ concentrations. Atmospheric NH₃ concentration of approximately 8 µg/m³ serves as a universal critical threshold for PM₂.₅ outbreaks. After Beijing upgraded sludge treatment processes from open aerobic composting to enclosed anaerobic digestion in 2014, annual average boundary NH₃ concentrations decreased from 3.0–8.0 mg/m³ to 0.2–1.2 mg/m³, and heavy haze days decreased from 58 d/a to 1–4 d/a, directly confirming the causal relationship between WWTP ammonia emissions and haze outbreaks. The study also reveals that frequent haze is an external manifestation of regional water cycle imbalance and soil-water ecological degradation. Correcting methodological biases, implementing water-air collaborative governance, and reconstructing decentralized ecological water utilization patterns constitute the scientific pathway to resolving the persistent dilemma of haze control.