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.

Abstract. Iodine and carbonate species are important components in marine and dust aerosols, respectively. The non-ideal interactions between these species and other inorganic and organic compounds within aqueous particle phases affect hygroscopicity, acidity, and gas–particle partitioning of semivolatile components. In this work, we present an extended version of the Aerosol Inorganic–Organic Mixtures Functional groups Activity Coefficients (AIOMFAC) model by incorporating the ions I−, IO3-, HCO3-, CO32-, OH−, and CO2(aq) as new species. First, AIOMFAC ion interaction parameters for aqueous solutions were determined based on available thermodynamic data, such as water activity, mean molal activity coefficients, solubility, and vapor–liquid equilibrium measurements. Second, the interaction parameters for the new ions and various organic functional groups were optimized based on experimental data or, where data are scarce, alternative estimation methods such as multiple linear regression or a simple substitution by analogy approach. Additional bulk water activity and electrodynamic balance measurements were carried out to augment the database for the AIOMFAC parameter fit. While not optimal, we show that the use of alternative parameter estimation methods enables physically sound predictions and offers the benefit of a more broadly applicable model. Our implementation of the aqueous carbonate–bicarbonate–CO2(aq) system accounts for the associated temperature-dependent dissociation equilibria explicitly and enables closed- or open-system computations with respect to carbon dioxide equilibration with the gas phase. We discuss different numerical approaches for solving the coupled equilibrium conditions and highlight critical considerations when extremely acidic or basic mixtures are encountered. The fitted AIOMFAC model performance for inorganic aqueous systems is considered excellent over the whole range of mixture compositions where reference data are available. Moreover, the model provides physically meaningful predictions of water activity under highly concentrated conditions. For organic–inorganic mixtures involving new species, the model–measurement agreement is found to be good in most cases, especially at equilibrium relative humidities above ∼ 70 %; reasons for deviations are discussed. Several applications of the extended model are shown and discussed, including the effects of ignoring the auto-dissociation of water in carbonate systems, the effects of mixing bisulfate and bicarbonate compounds in closed- or open-system scenarios on pH and solution speciation, and the prediction of critical cloud condensation nucleus activation of NaI or Na2CO3 particles mixed with suberic acid.

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.