Understanding the factors that influence the airborne survival of viruses such as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in aerosols is important for identifying routes of transmission and the value of various mitigation strategies for preventing transmission. We present measurements of the stability of SARS-CoV-2 in aerosol droplets (∼5 to 10 µm equilibrated radius) over timescales spanning 5 s to 20 min using an instrument to probe survival in a small population of droplets (typically 5 to 10) containing ∼1 virus/droplet. Measurements of airborne infectivity change are coupled with a detailed physicochemical analysis of the airborne droplets containing the virus. A decrease in infectivity to ∼10% of the starting value was observable for SARS-CoV-2 over 20 min, with a large proportion of the loss occurring within the first 5 min after aerosolization. The initial rate of infectivity loss was found to correlate with physical transformation of the equilibrating droplet; salts within the droplets crystallize at relative humidities (RHs) below 50%, leading to a near-instant loss of infectivity in 50 to 60% of the virus. However, at 90% RH, the droplet remains homogenous and aqueous, and the viral stability is sustained for the first 2 min, beyond which it decays to only 10% remaining infectious after 10 min. The loss of infectivity at high RH is consistent with an elevation in the pH of the droplets, caused by volatilization of CO 2 from bicarbonate buffer within the droplet. Four different variants of SARS-CoV-2 were compared and found to have a similar degree of airborne stability at both high and low RH.

Understanding the factors that influence the airborne survival of viruses such as SARSCoV2 in aerosols is important for identifying routes of transmission and the value of various mitigation strategies for preventing transmission. We present measurements of the stability of SARSCoV2 in aerosol droplets (5 to 10 micrometres equilibrated radius) over timescales spanning from 5 seconds to 20 minutes using a novel instrument to probe survival in a small population of droplets (typically 5-10) containing ~1 virus/droplet. Measurements of airborne infectivity change are coupled with a detailed physicochemical analysis of the airborne droplets containing the virus. A decrease in infectivity to 10 % of the starting value was observable for SARS-CoV-2 over 20 minutes, with a large proportion of the loss occurring within the first 5 minutes after aerosolisation. The initial rate of infectivity loss was found to correlate with physical transformation of the equilibrating droplet; salts within the droplets crystallise at RHs below 50% leading to a near instant loss of infectivity in 50 to 60% of the virus. However, at 90% RH the droplet remains homogenous and aqueous, and the viral stability is sustained for the first 2 minutes, beyond which it decays to only 10% remaining infectious after 10 minutes. The loss of infectivity at high RH is consistent with an elevation in the pH of the droplets, caused by volatilisation of CO2 from bicarbonate buffer within the droplet. Three different variants of SARS-CoV-2 were compared and found to have a similar degree of airborne stability at both high and low RH.

Atmospheric water-soluble organic gases such as pyruvic acid are produced in large quantities by 13 photochemical oxidation of biogenic and anthropogenic emissions and undergo water-mediated 14 reactions in aerosols and hydrometeors. These reactions can contribute to aerosol mass by forming 15 less volatile compounds. While progress is being made in understanding the relevant aqueous 16 chemistry, little is known about the chemistry that takes place during droplet evaporation. Here we 17 examine the evaporation of aqueous pyruvic acid droplets using both the Vibrating Orifice Aerosol 18 Generator (VOAG) and an electrodynamic balance (EDB). In some cases pyruvic acid was first 19 oxidized by OH radicals. The evaporation behavior of oxidized mixtures was consistent with 20 expectations based on known volatilities of reaction products. However, independent VOAG and 21 EDB evaporation experiments conducted without oxidation also resulted in stable residual 22 particles; the estimated volume yield was 10-30% of the initial pyruvic acid. Yields varied with 23 temperature and pyruvic acid concentration across cloud, fog, and aerosol-relevant concentrations. 24The formation of low volatility products, likely cyclic dimers, suggests that pyruvic acid accretion 25 reactions occurring during droplet evaporation could contribute to the gas-to-particle conversion 26 of carbonyls in the atmosphere. 27 mass. [4][5][6] However, the contribution of aqueous reactions to SOA mass remains uncertain due in 35 part to a limited understanding of precursors and limited laboratory results to parameterize 36 models. 1,[7][8][9][10][11] Quantifying the impacts of aqueous and multiphase chemistry on aerosol mass 37 remains challenging, and a more detailed understanding of product volatility is needed.A significant fraction of low molecular weight acids, aldehydes and carbonyls dissolve into 39 cloud or fog droplets. In the absence of additional reactions, these WSOGs largely evaporate 40 during water evaporation; the trace amounts that remain in the aerosol phase are determined by 41 their partial pressure in the gas phase and activity in the aerosol matrix. However, multiphase 42 reactions can generate low-volatility products that are retained in the equilibrated aerosol. Several 43 important criteria determine whether aqueous processing can appreciably increase SOA mass: (1) 44 the precursor must be abundant, (2) it must have a high vapor pressure before aqueous reactions, 45(3) it must have a high Henry's law coefficient and thus strongly partition into water, and (4) it 46 must react in the aqueous phase to form less volatile products. 47

A refined numerical model for the evaporation and transport of droplets of binary solutions is introduced. Benchmarking is performed against other models found in the literature and experimental measurements of both electrodynamically trapped and freefalling droplets. The model presented represents the microphysical behavior of solutions droplets in the continuum and transition regimes, accounting for the unique hygroscopic behavior of different solutions, including the Fuchs–Sutugin and Cunningham slip correction factors, and accounting for the Kelvin effect. Simulations of pure water evaporation are experimentally validated for temperatures between 290 K and 298 K and between relative humidity values of approximately 0% and 85%. Measurements and simulations of the spatial trajectories and evaporative behavior of aqueous sodium chloride droplets are compared for relative humidity values between 0 and 40%. Simulations are shown to represent experimental data within experimental uncertainty in initial conditions. Calculations of a time-dependent Péclet number, including the temperature dependence of solute diffusion, are related to morphologies of sodium chloride particles dried at different rates. For sodium chloride solutions, dried particles are composed of collections of reproducibly shaped crystals, with higher evaporation rates resulting in higher numbers of crystals, which are smaller.

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