Understanding Model Representation of Aerosol Composition: From Secondary Inorganic Aerosols to Phosphorus

Abstract

Aerosols in our atmosphere impact climate, human health, air quality, and biogeochemical cycling. Models can be a valuable tool for predicting aerosol abundance depending on how well they describe the processes controlling aerosols (e.g., emissions, thermodynamic partitioning, chemical production and loss, deposition). This thesis aims to improve our understanding of two aerosol components, secondary inorganic aerosols and phosphorus, through model development, evaluation, and exploration. In the second chapter of this thesis, we focus on secondary inorganic aerosols, which dominate fine particulate matter (PM2.5) in many regions of the world. We evaluate how well the trends and magnitude of secondary inorganic aerosols (sulfate, nitrate, and ammonium) are captured in a global chemical transport model using a suite of aircraft campaigns and identify high biases in ammonium nitrate. We also explore how remaining uncertainties in certain processes (emissions, deposition) may contribute to these biases and how others (thermodynamic partitioning, several chemical processes) are less likely to be the cause of current model bias. In the third and fourth chapters, we focus on phosphorus, which is an essential nutrient that can control ecosystem productivity. In the third chapter, we develop a comprehensive description of atmospheric phosphorus in a global atmospheric chemistry model. Leveraging a wide range of observational data, we evaluate our current understanding of the processes that control phosphorus (emissions, chemical aging) and highlight two improvements to our phosphorus description that result in less phosphorus from two sources (dust and combustion). In the fourth chapter of this thesis, we apply the atmospheric phosphorus scheme we developed to investigate how atmospheric phosphorus will evolve under projected anthropogenic land use changes. Looking at two scenarios for the future (SSP1 and SSP3) that have the most significant (and opposing) changes in land use compared to the present day, we find that global phosphorus deposition increases or decreases minimally (<±3%), but with substantial regional changes (>±50%). In aggregate, this thesis advances our understanding of aerosol composition by identifying key gaps in how aerosol composition (e.g., secondary inorganic aerosols and phosphorus) is represented and provides insight into how atmospheric composition responds to changing loss and production processes.