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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. 

Abstract. Alpha-dicarbonyl compounds are believed to form browncarbon in the atmosphere via reactions with ammonium sulfate (AS) in clouddroplets and aqueous aerosol particles. In this work, brown carbon formationin AS and other aerosol particles was quantified as a function of relativehumidity (RH) during exposure to gas-phase glyoxal (GX) in chamberexperiments. Under dry conditions (RH < 5 %), solid AS,AS–glycine, and methylammonium sulfate (MeAS) aerosol particles brown withinminutes upon exposure to GX, while sodium sulfate particles do not. When GXconcentrations decline, browning goes away, demonstrating that this drybrowning process is reversible. Declines in aerosol albedo are found to be afunction of [GX]2 and are consistent between AS and AS–glycineaerosol. Dry methylammonium sulfate aerosol browns 4 times more than dryAS aerosol, but deliquesced AS aerosol browns much less than dry AS aerosol.Optical measurements at 405, 450, and 530 nm provide an estimatedÅngstrom absorbance coefficient of -16±4. This coefficient andthe empirical relationship between GX and albedo are used to estimate anupper limit to global radiative forcing by brown carbon formed by 70 ppt GXreacting with AS (+7.6×10-5 W m−2). This quantity is< 1 % of the total radiative forcing by secondary brown carbonbut occurs almost entirely in the ultraviolet range.