Rethinking Nitrate Aerosol Substitution for Sulfate in Winter Haze

Nitrate aerosol formation modulates the global nitrogen cycle and dominates fine-particle pollution in many urban areas worldwide. Over the past two decades, increasing studies have suggested a pronounced enhancement of nitrate formation by reductions in sulfate levels via shifts in aerosol thermodynamic equilibrium. However, we clarify a distinct sustained regime in winter haze that does not result in the expected nitrate enhancement, following drastic SO₂ abatement. This is evidenced by long-term observations in North China over the 2010s alongside interpretations using a full thermodynamic equilibrium model and a regional air-quality model. Our results demonstrate that low-temperature and persistent ammonia (NH₃)-rich conditions jointly drive the aerosol proportion of total nitrate (gas + aerosol) to approach 100% regardless of sulfate levels. Despite an over 80% reduction in SO₂ in this region, the resultant NH₃ increase did not shift the thermodynamic equilibrium to increase the level of nitrate aerosols. This observation-based finding contrasts with the widely proposed transition from NH₃-poor to NH₃-rich conditions in nitrate formation for this region and the eastern United States. Overlooking this persistent thermodynamic constraint under such high-NH₃ environments could have led to a significant misunderstanding of nitrate aerosol trends and clean-air measure effectiveness.