Respiratory viruses, including influenza virus and SARS-CoV-2, are transmitted by the airborne route. Air filtration and ventilation mechanically reduce the concentration of airborne viruses and are necessary tools for disease mitigation. However, they ignore the potential impact of the chemical environment surrounding aerosolized viruses, which determines the aerosol pH. Atmospheric aerosol gravitates toward acidic pH, and enveloped viruses are prone to inactivation at strong acidity levels. Yet, the acidity of expiratory aerosol particles and its effect on airborne virus persistence have not been examined. Here, we combine pHdependent inactivation rates of influenza A virus (IAV) and SARS-CoV-2 with microphysical properties of respiratory fluids using a biophysical aerosol model. We find that particles exhaled into indoor air (with relative humidity ≥ 50%) become mildly acidic (pH ∼ 4), rapidly inactivating IAV within minutes, whereas SARS-CoV-2 requires days. If indoor air is enriched with nonhazardous 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 s. Conversely, unintentional removal of volatile acids from indoor air may elevate pH and prolong airborne virus persistence. The overlooked role of aerosol acidity has profound implications for virus transmission and mitigation strategies.

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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.

Multiple respiratory viruses including Influenza A virus (IAV) can be transmitted via expiratory aerosol particles, and many studies have established that indoor environmental conditions can affect viral infectivity during this transmission. Aerosol pH was recently identified as a major factor influencing the infectivity of aerosol-borne IAV and SARS-CoV-2, and for indoor room air, modelling indicates that small exhaled aerosols will undergo rapid acidification (pH below 5.5). However, there is a fundamental lack of understanding as to the mechanisms leading to viral inactivation within an acidic aerosol micro-environment. Here, we identified that transient exposure to acidic conditions impacted the early stages of the IAV infection cycle, which was primarily attributed to loss of binding function of the viral protein haemagglutinin. Viral capsid integrity was also partially affected by transient acidic exposures. The structural changes associated with loss of viral infectivity were then characterized using whole-virus hydrogen-deuterium exchange coupled to mass spectrometry (HDX-MS), and we observed discrete regions of unfolding in the external viral protein haemagglutinin and the internal matrix protein 1. Viral nucleoprotein structure appeared to be unaffected by exposure to acidic conditions. Protein analyses were complemented by genome and lipid envelope characterizations, and no acid-mediated changes were detected using our whole-virus methods. Improved understanding of the fate of respiratory viruses within exhaled aerosols constitutes a global public health priority, and information gained here could aid the development of novel strategies or therapeutics to control the airborne persistence of seasonal and/or pandemic influenza in the future. This study also provides proof-of-concept that HDX-MS is a highly effective method for characterization of both internal and external proteins for whole enveloped viruses such as IAV.

To mitigate the spread of a viral disease, it is crucial to understand the factors that influence airborne virus transmission. However, the micro-environment to which the virus is exposed in expiratory aerosol particles is highly complex. The relative humidity, the aerosol particle size and composition, and the air composition affect virus infectivity by modulating the salt and organic concentrations within the particle, as well as the phase state. A parameter that has been overlooked is the aerosol pH. Several viruses are sensitive to acidic pH; for example, the inactivation of influenza A virus becomes very fast at pH 5.5 and below, a threshold that is quickly reached in an expiratory aerosol particle exhaled in a typical indoor environment. Therefore, aerosol acidity plays a significant role in controlling the persistence of airborne, acid-sensitive viruses such as influenza virus, and aerosol pH control could be applied to limit the risk of airborne virus transmission.