Role of Ammonia on the Feedback Between AWC and Inorganic Aerosol Formation During Heavy Pollution in the North China Plain

Atmospheric NH₃ plays a vital role not only in the environmental ecosystem but also in atmosphere chemistry. To further understand the effects of NH₃ on the formation of haze pollution in Beijing, ambient NH₃ and related species were measured and simulated at high resolutions during the wintertime Air Pollution and Human Health-Beijing (APHH-Beijing) campaign in 2016. We found that the total NH_x (gaseous NH₃+particle NH₄ ⁺) was mostly in excess of the SO₄ ²⁻-NO₃ ⁻-NH₄ ⁺-water equilibrium system during our campaign. This NH_x excess made medium aerosol acidity, with the median pH value being 3.6 and 4.5 for polluted and nonpolluted conditions, respectively, and enhanced the formation of particle phase nitrate. Our analysis suggests that NH₄NO₃ is the most important factor driving the increasing of aerosol water content with NO₃ ⁻ controlling the prior pollution stage and NH₄ ⁺ the most polluted stage. Increased formation of NH₄NO₃ under excess NH_x, especially during the nighttime, may trigger the decreasing of aerosol deliquescence relative humidity even down to less than 50% and hence lead to hygroscopic growth even under RH conditions lower than 50% and the wet aerosol particles become better medium for rapid heterogeneous reactions. A further increase of RH promotes the positive feedback “aerosol water content-heterogeneous reactions” and ultimately leads to the formation of severe haze. Modeling results by Nested Air Quality Prediction Monitor System (NAQPMS) show the control of 20% NH₃ emission may affect 5–11% of particulate matter PM_2.5 formation under current emissions conditions in the North China Plain.