Control regimes of nitrate air pollution reconciled by multiphase dissociation equilibrium.

Particulate nitrate is a major component of haze pollution, yet substantial debate exists regarding its effective control policies. Here, we develop a multiphase dissociation equilibrium equation, based on which the influence of meteorological conditions and chemical profiles can be decoupled, and the analytical expression of nitrate sensitivity to different species can be derived. With this framework, four nitrate control regimes are identified, namely the nitric-acid-control, ammonia-control, transition, and meteorology regimes. The framework also explains the trade-offs between sulfate and nitrate, and elucidates the influence of sulfate and non-volatile cations on the regime transitions. Across most Northern Hemisphere continental regions, and nearly all of China, particulate nitrate is generally more sensitive to nitric acid than to ammonia due to the universal NH3 excess. The framework provides transparent criteria for interpreting nitrate sensitivity and control regime classifications, serving as an important scientific basis for nitrate control policies and reactive nitrogen interactions.