Atmospheric particulate matter (aerosol) plays a crucial role in global climate via impacts on radiative forcing and in human health as a regional-scale pollutant. Following the recent implementation of increased secondary organic aerosol (OA) sources, particularly from isoprene, atmospheric models tend to overestimate OA burdens. They also tend to treat such particles as inert, with deposition as their only major loss. At the same time, models frequently underestimate atmospheric sources of small organic acids, which set the acidity of aerosols and rainwater, and sinks of reactive nitrogen, which plays a crucial role in the formation of ozone and other secondary pollutants. We perform a series of atmospheric chamber experiments and bulk-phase aqueous experiments to constrain the rates and mechanisms of various reactions of organic compounds within atmospheric aerosols, and show that reactions can be competitive with deposition and revolatilization on atmospherically relevant timescales. Using a variety of spectroscopic techniques, we constrain the mechanisms of these reactions, and show that they can constitute substantial losses of organic aerosol and reactive nitrogen from the atmosphere, and can be important sources of small organic acids. We further constrain the impacts of these processes with atmospheric models, showing that oxidation reactions within atmospheric aerosols can help resolve model-measurement discrepancies in the atmospheric budgets of organic aerosol, reactive nitrogen, and organic acids.