Public health authorities have mandated wearing face coverings indoors as a tool for preventingthe spread of COVID-19. Since then, people have worn various face coverings, including masks and respirators, when such mandates have been in effect. While masks and respirators reduce respiratory particle emissions, various studies indicate that the efficiency of face coverings can vary substantially. Here, we characterized the outward particle emissions from a real person's speaking activity while wearing various cloth masks, surgical masks, respirators, bandanas, and neck gaiters in different wearing styles. In addition, we evaluated adding extra filters to cloth masks, crossed ear loops for surgical masks, and double masking (cloth over surgical). Cloth masks, surgical masks, respirators, and double masking reduce the outward particle emission rates on average by 43.8%, 79.7%, 88.4%, and 86.4%, respectively, for speaking. For all three types of surgical masks, crossing the ear loop did not yield a significant particle reduction compared to the standard wearing style. In contrast, the particle emission rates from two types of cloth face coverings—a bandana and two different neck gaiters—were greater than that for wearing no masks, suggesting that some face coverings shed non-respiratory particles. We also characterized the particle emission rate and size distribution from rubbing the masks against themselves and against human skin to quantify the non-respiratory particle emission from friction during speaking. Our results show that cotton made face coverings tend to emit more non-respiratory particles than noncotton made face coverings during speaking. The skin particles emission rates were similar to the non-cotton made face coverings, with particle size typically smaller than 0.58 μm. Furthermore, we characterized the non-respiratory particle emission by flowing particle-free air through the masks. The non-respiratory emission from the hand rubbing experiments and clean-air flow experiments had a positive linear correlation, suggesting cloth masks tend to emit more nonrespiratory particles by themselves due to the friction and air flow than respirators and surgical masks. The non-respiratory particle emission level is strongly correlated with the mask material. Additionally, the hand rubbing experiments had higher particle emission in the range of 2-15 μm compared to the speaking activity, which primarily produced particles in the range of 0.5-2 μm, indicating the non-respiratory particles were smaller on average than respiratory particles. Overall, our results provide further evidence that wearing masks can reduce the outward particle emissions, but with the efficacy dependent on the mask type.

Vacuum Ultraviolet (VUV) photoionization mass spectrometry has been used to measure the evolution of chemical composition for two distinct organic aerosol types as they are passed through a thermodenuder at different temperatures. The two organic aerosol types considered are primary lubricating oil (LO) aerosol and secondary aerosol from the alpha-pinene + O3 reaction (alphaP). The evolution of the VUV mass spectra for the two aerosol types with temperature are observed to differ dramatically. For LO particles, the spectra exhibit distinct changes with temperature in which the lower m/z peaks, corresponding to compounds with higher vapor pressures, disappear more rapidly than the high m/z peaks. In contrast, the alphaP aerosol spectrum is essentially unchanged by temperature even though the particles experience significant mass loss due to evaporation. The variations in the LO spectra are found to be quantitatively in agreement with expectations from absorptive partitioning theory whereas the alphaP spectra suggest that the evaporation of alphaP derived aerosol appears to not be governed by partitioning theory. We postulate that this difference arises from the alphaP particles existing as in a glassy state instead of having the expected liquid-like behavior. To reconcile these observations with decades of aerosol growth measurements, which indicate that OA formation is described by equilibrium partitioning, we present a conceptual model wherein the secondary OA is formed and then rapidly converted from an absorbing form to a non-absorbing form. The results suggest that although OA growth may be describable by equilibrium partitioning theory, the properties of organic aerosol once formed may differ significantly from the properties determined in the equilibrium framework.

Measurements of the total ion yield (TIY) x-ray absorption spectrum (XAS) of liquid water by Wilson et al (2002 J. Phys.: Condens. Matter 14 L221 and 2001 J. Phys. Chem. B 105 3346) have been revisited in light of new experimental and theoretical efforts by our group. Previously, the TIY spectrum was interpreted as a distinct measure of the electronic structure of the liquid water surface. However, our new results indicate that the previously obtained spectrum may have suffered from as yet unidentified experimental artifacts. Although computational results indicate that the liquid water surface should exhibit a TIY-XAS that is fundamentally distinguishable from the bulk liquid XAS, the new experimental results suggest that the observable TIY-XAS is actually nearly identical in appearance to the total electron yield (TEY-)XAS, which is a bulk probe. This surprising similarity between the observed TIY-XAS and TEY-XAS likely results from large contributions from x-ray induced electron stimulated desorption of ions, and does not necessarily indicate that the electronic structure of the bulk liquid and liquid surface are identical.