Fine particulate air pollution (PM₂.₅) has wide ranging influence on global climate (through radiative scattering, cloud formation, etc.) and human health (through the increased incidence of respiratory illness, etc.). Studies have shown that a major fraction of global PM₂.₅ (also called fine aerosol) is formed dynamically in the atmosphere from volatile gas-phase precursors that are emitted by both anthropogenic and biogenic sources. This class of aerosol is called secondary aerosol. Due to the numerous uncertainties associated in simulating their atmospheric formation and fates, earth science models have historically struggled to accurately represent secondary aerosols, and continue to demonstrate significant bias when compared to observational datasets. The goal of this doctoral thesis is to better constrain the sources and atmospheric fates of a few key secondary particulate species, with the intention of improving the model representation of these aerosols. With these overarching objectives in mind, this thesis spans a series of four projects that use chemically-speciated observational constraints and targeted model development to conduct (1) a comparative study of global organic aerosol schemes using airborne observations; (2) an exploration of atmospheric ammonia oxidation as a source of secondary aerosol and nitrous oxide; (3) an investigation of compositional constraints from surface, aircraft and satellite measurements to improve PM₂.₅ source-attribution over India; (4) a model evaluation of global PM₂.₅ exposure guidelines that highlights the importance of non-anthropogenic sources and proposes a chemically-speciated paradigm for PM₂.₅ measurement and source-apportionment. In aggregate, these projects contribute to a body of scientific literature that can be leveraged to inform air quality management efforts around the world.Ph.D.