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Abstract An accepted murine analogue for the environmental behavior of human SARS coronaviruses was aerosolized in microdroplets of its culture media and saliva to observe the decay of its airborne infectious potential under relative humidity (RH) conditions relevant to conditioned indoor air. Contained in a dark, 10 m3 chamber maintained at 22°C, murine hepatitis virus (MHV) was entrained in artificial saliva particles that were aerosolized in size distributions that mimic SARS-CoV-2 virus expelled from infected humans’ respiration. As judged by quantitative PCR, more than 95% of the airborne MHV aerosolized was recovered from microdroplets with mean aerodynamic diameters between 0.56 and 5.6 μm. As judged by its half-life, calculated from the median tissue culture infectious dose (TCID50), saliva was protective of airborne murine coronavirus through a RH range recommended for conditioned indoor air (60% < RH < 40%; average half-life = 60 minutes). However, its average half-life doubled to 120 minutes when RH was maintained at 25%. Saliva microaerosol was dominated by carbohydrates, which presented hallmarks of vitrification without efflorescence at low RH. These results suggest that dehydrating carbohydrates can affect the infectious potential coronaviruses exhibit while airborne, significantly extending their persistence under the drier humidity conditions encountered indoors. 

Internally-mixed aerosol particles containing organic molecules and inorganic salts are prevalent in the atmosphere, arising from direct emission ( e.g., from the ocean) or indirect production by condensation of organic vapors onto existing inorganic particle seeds. Aerosol particles co-exist with water vapor and, under humid conditions, will exist as dilute aqueous solution particles that can be well described by thermodynamic models. Under low humidity conditions, the increase in solute concentrations leads to molecular interactions and significant non-ideality effects that drive changes in important physical properties, such as viscosity and phase state, that are not predicted using simple models. Here, we explore a model system containing ammonium sulfate (AS) and citric acid (CA). We measure the hygroscopicity, viscosity, and rate of water diffusion in particles across a range of RH conditions and organic fractions to better understand the influence of organic–inorganic mixtures on particle properties. We report the RH dependence of these properties and explore the applicability of commonly used methods that connect them together, such as the Stokes–Einstein relationship and thermodynamic modelling methods. We show that at low RH, the addition of AS to CA leads to a reduction in the amount of water as indicated by the radial growth factor at a fixed RH, while observing an increase in the viscosity over several orders of magnitude. Contrary to the viscosity, only minor changes in water diffusion were measured, and analysis with the fractional Stokes–Einstein relationship indicates that changes in the molecular matrix due to the presence of AS could explain the observed phenomena. This work reveals that small additions of electrolytes can drive large changes in particle properties, with implications for chemical reactivity, lifetime, and particle phase that will influence the environmental impacts and chemistry of aerosol particles.