The current pandemic has added to the growing evidence that respiratory viruses can be transmitted by the airborne route. Non-pharmaceutical interventions, such as mask wearing or ventilation, aim to reduce the public health burden of respiratory diseases by physically preventing airborne virus transmission. Alternatively, a reduction in transmission may be achieved by inactivation viruses in air. This latter approach, however, is hampered by an incomplete understanding of the parameters that modulate airborne virus infectivity. In this study, we investigate the role of aerosol pH in airborne virus inactivation. Aerosol particles in the natural environment are known to be acidic, and the persistence of some respiratory viruses is known to be reduced by acidic pH. However, the pH of expiratory aerosol particles and its effect on virus transmission remain unknown. Here, we measured the inactivation kinetics of influenza virus and two coronaviruses (SARS-CoV-2 and hCoV-229E) over a pH range of 2.1 to 7.4 in surrogate lung fluid and in nasal mucus. The physicochemical properties of micron-sized SLF and mucus droplets, such as the water diffusion coefficient, were determined by injecting a fluid droplet into an electrodynamic balance and measuring the relative changes in mass and volume upon changes in relative humidity. Finally, the physicochemical and virological data were integrated into a biophysical aerosol model, to determine virus inactivation as a function of relative humidity, air composition and aerosol particle size. We found that influenza virus is readily inactivated at pH < 5, leading to airborne inactivation over the course of hours to minutes, depending on the size of the carrier particles. The inactivation coincides with structural changes in viral proteins, including those involved in host attachment. In contrast, coronaviruses are more stable and can remain infectious within aerosol particles for days. However, acidification of indoor air, and hence of aerosol pH, causes a dramatic decrease in inactivation times for influenza virus and SARS-CoV-2, and to a lesser extend for hCoV-229E. Consequently, pH control of indoor air, e.g., through the addition of nitric acid to the air, is a promising tool for reducing virus transmission. Interestingly, our biophysical aerosol model predicts that air acidification is a more effective tool for reducing the risk of virus transmission than ventilation or filtration.