Investigating Urban Ammonium Nitrate Aerosol Formation over Asian Cities with In-Situ Measurements and Thermodynamic Modelling

Wintertime aerosol loading is a prevalent public health issue in Asian megacities. A combination of meteorological effects, local emissions, and biomass burning contributes to high aerosol loading in some of the most densely populated regions of the globe. Local controls of this pollution have been effective at reducing urban aerosol loading, but the effectiveness of controls of nitrogen oxides (NO_x) versus ammonia (NH₃) remains to be seen. Ammonium nitrate (AN) formation is especially relevant during the cold season due to increased thermodynamic favorability at lower temperatures. Understanding these chemical controls remains difficult due to a lack of comprehensive measurements that incorporate the precursors and products of aerosol formation spatially in both vertical and horizontal transport. Using in-situ measurements from the NASA DC-8 taken during the 2024 ASIA-AQ campaign, the relative contributions of NH₃ and NO_x to inorganic AN formation is explored over and near Asian megacities in the Philippines, South Korea, Taiwan, and Thailand. Missed approaches at airports in and near these cities provide insight into the vertical distribution of the relevant gas-phase precursors and their corresponding aerosol products across both tropical and wintertime urban environments.

By quantifying the thermodynamic AN dissociation constant, we calculate that over Taiwan 25% of vertical profiles near urban centers have conditions where NH₃ and nitric acid (HNO₃) are abundant enough for thermodynamically favorable AN formation. Preliminary results show this is generally NO_x limited and more favorable aloft at or above the boundary layer due to lower temperatures. This estimation will be further constrained against the ISORROPIA-II aerosol thermodynamic model and expanded across missed approaches over all the sampled countries. To further understand the impact of urban emissions on this aerosol formation, tracers such as carbon monoxide, methane, and nitrous oxide are used to determine the relevant contributions of urban versus agricultural emissions to the relevant precursors. Results will be compared to relevant policy on emissions regulations to evaluate the effectiveness of currently implemented controls.