While plasmas are now routinely employed to synthesize or remove nano- to micron-sized particles, the charge state (polarity and magnitude) of the particles remains relatively unknown. In this study, charging of nanoparticles was systematically characterized in low-temperature, atmospheric-pressure, flow-through plasmas previously applied for synthesis. Premade, charge-neutral nanoparticles of MgSO4, NaCl, and sea salt were introduced into the plasma to decouple other effects such as the reactive vapor precursor, and MgSO4 was selected as the focus because of its stability (i.e., no evaporation) in the plasma environment. The charge fraction and distribution of the particles was examined at the reactor outlet for different particle diameters (10–250 nm) as a function of plasma power and two types of power source, alternating current (AC) and radio frequency (RF). We found that the overall charge fraction increased with increasing plasma power and diameter for the RF plasma. A similar increasing trend was observed for the AC plasma with increasing particle diameter in the range of 50–250 nm, but the charge fraction increased with decreasing particle diameter in the range of 10–50 nm. The charge distribution was revealed to be bipolar, with particles supporting multiple charges for both the RF and AC plasmas, but the RF plasma produced a higher fraction of multiple charges. Differences in the characteristic timescales for particle charging in the AC and RF plasmas are a possible explanation of the trends observed in the experiments.

In this study, a spray flame aerosol reactor (S‐FLAR) is used to synthesize alumina nanoparticles. The as‐produced powders are then characterized by X‐ray diffraction, N2 physisorption, and transmission electron microscopy to determine the crystal phase, surface area, particle size distribution, and morphology. The effects of the precursor, dispersion oxygen, and sheath oxygen rates on the characteristics of synthesized alumina were investigated. On increasing the precursor rate, decreasing the dispersion oxygen rate or sheath oxygen rate; the alumina powder surface area decreased. With increasing precursor rates and decreasing dispersion oxygen rates, the proportion of theta alumina increased and that of eta alumina decreased. When using an S‐FLAR to synthesize alumina, the dispersion oxygen rate offers the best control of the surface area, while the precursor rate controls the crystal phase proportions. This result is useful for the design and operation of spray flame aerosol reactors to produce alumina‐based catalysts.

The highly infectious SARS-CoV-2 novel coronavirus has resulted in a global pandemic. More than a hundred million people are already impacted, with infected numbers expected to go up. Coughing, sneezing, and even talking emit respiratory droplets which can carry infectious viruses. It is important to understand how the exhaled particles move through air to an exposed person to better predict the airborne transmission impacts of SARS-CoV-2. There are many studies conducted on the airborne spread of viruses causing diseases such as SARS and measles; however, there are very limited studies that couple the transport characteristics with the aerosol dynamics of the droplets. In this study, a comprehensive model for simultaneous droplet evaporation and transport due to diffusion, convection, and gravitational settling is developed to determine the near spatial and temporal concentration of the viable virus exhaled by the infected individual. The exposure to the viable virus is estimated by calculating the respiratory deposition, and the risk of infection is determined using a dose–response model. The developed model is used to quantify the risk of short-range airborne transmission of SARS-CoV-2 from inhalation of virus-laden droplets when an infected individual is directly in front of the person exposed and the surrounding air is stagnant. The effect of different parameters, such as viral load, infectivity factor, emission sources, physical separation, exposure time, ambient air velocity, dilution, and mask usage, is determined on the risk of exposure.