ABSTRACT
Enveloped viruses are prone to inactivation when exposed to strong acidity levels characteristic of atmospheric aerosol. Yet, the acidity of expiratory aerosol particles and its effect on airborne virus persistence has not been examined. Here, we combine pH-dependent inactivation rates of influenza A virus and SARS-CoV-2 with microphysical properties of respiratory fluids under indoor conditions using a biophysical aerosol model. We find that particles exhaled into indoor air become mildly acidic (pH $\approx$ 4), rapidly inactivating influenza A virus within minutes, whereas SARS-CoV-2 requires days. If indoor air is enriched with non-hazardous levels of nitric acid, aerosol pH drops by up to 2 units, decreasing 99\%-inactivation times for both viruses in small aerosol particles to below 30 seconds. Conversely, unintentional removal of volatile acids from indoor air by filtration may elevate pH and prolong airborne virus persistence. The overlooked role of aerosol pH has profound implications for virus transmission and mitigation strategies.