Aerosol Research Research Articles

Retrieving High-Resolution Aerosol Optical Depth from GF-4 PMS Imagery in Eastern China

Gaofen 4 (GF-4) is a geostationary satellite, with a panchromatic and multispectral sensor (PMS) onboard, and has great potential in observing atmospheric aerosols. In this study, we developed an aerosol optical depth (AOD) retrieval algorithm for the GF-4 satellite. AOD retrieval was realized based on the pre-calculated surface reflectance database and 6S radiative transfer model. We customized the unique aerosol type according to the long time series aerosol parameters provided by the Aerosol Robotic Network (AERONET) site. The solar zenith angle, relative azimuth angle, and satellite zenith angle of the GF-4 panchromatic multispectral sensor image were calculated pixel-by-pixel. Our 1 km AOD retrievals were validated against AERONET Version 3 measurements and compared with MOD04 C6 AOD products at different resolutions. The results showed that our GF-4 AOD algorithm had a good robustness in both bright urban areas and dark rural areas. A total of 71.33% of the AOD retrievals fell within the expected errors of ±(0.05% + 20%); root-mean-square error (RMSE) and mean absolute error (MAE) were 0.922 and 0.122, respectively. The accuracy of GF-4 AOD in rural areas was slightly higher than that in urban areas. In comparison with MOD04 products, the accuracy of GF-4 AOD was much higher than that of MOD04 3 km and 10 km dark target AOD, but slightly worse than that of MOD04 10 km deep blue AOD. For different values of land surface reflectance (LSR), the accuracy of GF-4 AOD gradually deteriorated with an increase in the LSR. These results have theoretical and practical significance for aerosol research and can improve retrieval algorithms using the GF-4 satellite.

Analytical and numerical calculation of the detachment of particle structures from fibers

Not much research has been done on the detachment of deposit structures from fibers in gas flow. This study presents two approaches to determine the fluid velocities uc required for detachment. The first is an analytical model based on the imbalance of drag and adhesive forces. With the help of available models in literature two curves for uc as a function of deposit diameter were calculated. The two curves represent the different conditions present in a deposited structure, consisting of a mixture of reactive and inert matter, before and after reaction. It approximates the structure by one sphere, that consists of two identical half spheres, of which one detaches due to drag. The second approach is a CFD model, in which the deposited particle structure is represented through an immersed boundary method (IBM). It calculates adhesive forces based on the van der Waals force between two half-spaces, while drag is directly evaluated from flow properties at the structure’s surface. To be comparable with the analytical approach, it also approximates the deposit with two half spheres. The models are capable of describing detachment velocity behavior as a function of deposit structure size and porosity as reported in literature.Copyright © 2021 American Association for Aerosol Research

Water uptake and optical properties of mixed organic-inorganic particles

Atmospheric aerosol particles are frequently mixtures of inorganic and organic species, both of which can contribute to aerosol water uptake and determine the particles’ ability to scatter and absorb light. While water uptake of purely inorganic aerosol is well represented in current regional and global chemical transport models, it is challenging to represent it for particles that are mixtures of organic and inorganic species. Here we quantified the growth factor for aerosols that consist of mixed organic-inorganic particles using an accurate lattice-based adsorption isotherm model (Ad-iso) as a benchmark. We then determined the error in the growth factor and resulting optical properties for simplifying assumptions that are commonly made in current chemical transport models. The systems studied here are representative of ambient atmospheric aerosols, consisting of model water-soluble inorganic-organic mixtures, with and without a core of absorbing black carbon, under conditions of relative humidity larger than 85%. The assumption of completely neglecting the water uptake by organic components, for particles with an organic mass fraction of 50%, led to errors of up to 7% in growth factor and up to 3.5% in single scattering albedo. Larger errors occurred for larger organic mass fractions. Approximating the organic water uptake with a constant hygroscopicity parameter, for organic mass fractions between 45 and 65%, the errors remained within 3% for the growth factor and 0.6% for the single scattering albedo. For organic mass fractions smaller than 45% or larger than 65%, the errors increased up to 6% for the single scattering albedo. The magnitudes of these errors underscore the importance of considering organic/inorganic mixtures for estimating direct aerosol radiative forcing.Copyright © 2021 American Association for Aerosol Research

Bioaerosols in the Amazon rain forest: temporal variations and vertical profiles of Eukarya, Bacteria, and Archaea

Abstract. The Amazon rain forest plays a major role in global hydrological cycling, and biogenic aerosols are likely to influence the formation of clouds and precipitation. Information about the sources and altitude profiles of primary biological aerosol particles, however, is sparse. We used fluorescence in situ hybridization (FISH), a molecular biological staining technique largely unexplored in aerosol research, to investigate the sources and spatiotemporal distribution of Amazonian bioaerosols on the domain level. We found wet season bioaerosol number concentrations in the range of 1–5 × 105 m−3 accounting for > 70 % of the coarse mode aerosol. Eukaryotic and bacterial particles predominated, with fractions of ∼ 56 % and ∼ 26 % of the intact airborne cells. Archaea occurred at very low concentrations. Vertical profiles exhibit a steep decrease in bioaerosol numbers from the understory to 325 m height on the Amazon Tall Tower Observatory (ATTO), with a stronger decrease in Eukarya compared to Bacteria. Considering earlier investigations, our results can be regarded as representative for near-pristine Amazonian wet season conditions. The observed concentrations and profiles provide new insights into the sources and dispersion of different types of Amazonian bioaerosols as a solid basis for model studies on biosphere–atmosphere interactions such as bioprecipitation cycling.

Seasonal Differences in Submicron Marine Aerosol Particle Organic Composition in the North Atlantic

Submicron atmospheric primary marine aerosol (aPMA) were collected during four North Atlantic Aerosol and Marine Ecosystem Study (NAAMES) research cruises between November 2015 and March 2018. The average organic functional group (OFG) composition of the aPMA samples was 72–85% hydroxyl group mass, 6–13% alkane group mass, and 5–8% amine group mass, which is similar to prior observations and to aerosol generated from Sea Sweep. The carboxylic acid group had seasonal averages that ranged from 1% for Winter, 8% for Late Spring, and 10% for Autumn. The carboxylic acid group mass concentration correlated with nitrate mass concentration and weakly with photosynthetically active radiation (PAR) above 100 W m–2, suggesting a substantial secondary organic aerosol contribution in sunnier months. The three sizes of aPMA aerosol particles (<0.18, <0.5, and <1 μm) had the same four organic functional groups (hydroxyl, alkane, amine, and carboxylic acid groups). The aPMA spectra of the three sizes showed more variability (higher standard deviations of cosine similarity) within each size than between the sizes. The ratio of organic mass (OM) to sodium (OM/Na) of submicron generated primary marine aerosol (gPMA) was larger for Autumn with project average of 0.93 ± 0.3 compared to 0.55 ± 0.27 for Winter, 0.47 ± 0.16 for Late Spring, and 0.53 ± 0.24 for Early Spring. When the gPMA samples were separated by latitude (47–60°N and 18–47°N), the median OM/Na concentration ratio for Autumn was higher than the other seasons by more than the project standard deviations for latitudes north of 47°N but not for those south of 47°N, indicating that the seasonal differences are stronger at higher latitudes. However, the high variability of day-to-day differences in aPMA and gPMA composition within each season meant that seasonal trends in organic composition were generally not statistically distinguishable.

Effect of spray properties on aerosol scavenging efficiency with water mist

In the foreseen decommissioning plans of damaged Fukushima Daiichi reactors, submicron aerosol particles will be generated when retrieving fuel debris from reactor buildings by cutting it into small pieces. A multi-nozzle spray system will be used to scavenge these aerosol particles inside the primary containment vessel. To improve the aerosol removal efficiency of the multi-nozzle spray system, it is now necessary to study the aerosol removal performance by different single spray nozzles with different spray properties (spray angle, droplet velocity, etc.). To improve the aerosol scavenging efficiency, a new method with water mist was proposed to aggregate with aerosol particles and form large-sized coagulated aerosol-mist particle clusters, which can subsequently be removed by spray with higher efficiency. In this study, the effect of spray properties on aerosol removal efficiency with preexisting water mist was investigated using three different spray nozzles. The experimental results showed that a larger droplet velocity led to stronger vortex circulation and faster gaseous entrainment from outer spray area into spray region, resulting in a smaller circulation period of gas entrainment by spray and higher aerosol removal efficiency. The numerical simulations of aerosol removal by different spray nozzles without mist were also conducted to provide more insights into the aerosol removal process. By comparing the experiment and simulation results, it is found that the accumulated spray water in the lower plenum of the vessel and the water film formed on the vessel sidewalls also contributed to aerosol removal, especially for the particles with large Stokes numbers.Copyright © 2021 American Association for Aerosol Research

A highly efficient cloth facemask design

Wearing masks and face coverings helps reduce transmission of respiratory diseases. Much prior research on mask filtration efficiency has focused on fabric type, with less consideration given to mask design and shape. Here, we present evidence that increasing the mouth-mask separation distance engenders an increase in the mask-fabric filtration efficiency toward expiratory aerosols emitted via vocalization, which we attribute to a decrease in the expiratory jet velocity and expansion of the breath-mask contact area. We further assess the performance of a new reusable cloth facemask design, originally made for, but not exclusive to, professional singers and having a large mouth-mask separation distance. With proper fitting, these masks achieve overall filtering efficiencies of >93% for both exhaled expiratory aerosols >0.5 microns in diameter from singing and speaking and for inhaled ambient aerosols, better than surgical masks and approaching N95 respirators. Air exchange between the mask deadspace and the ambient environment limits the extent of CO2 buildup and in inhaled air and maintains O2 levels near ambient. The mask design also provides for less inhibited mouth and jaw movement and lower relative humidity, suggesting it may provide a more palatable, high-efficiency alternative to medical-grade masks for the public.Copyright © 2021 American Association for Aerosol Research

The Unmanned Systems Research Laboratory (USRL): A New Facility for UAV-Based Atmospheric Observations

The Unmanned Systems Research Laboratory (USRL) of the Cyprus Institute is a new mobile exploratory platform of the EU Research Infrastructure Aerosol, Clouds and Trace Gases Research InfraStructure (ACTRIS). USRL offers exclusive Unmanned Aerial Vehicle (UAV)-sensor solutions that can be deployed anywhere in Europe and beyond, e.g., during intensive field campaigns through a transnational access scheme in compliance with the drone regulation set by the European Union Aviation Safety Agency (EASA) for the research, innovation, and training. UAV sensor systems play a growing role in the portfolio of Earth observation systems. They can provide cost-effective, spatial in-situ atmospheric observations which are complementary to stationary observation networks. They also have strong potential for calibrating and validating remote-sensing sensors and retrieval algorithms, mapping close-to-the-ground emission point sources and dispersion plumes, and evaluating the performance of atmospheric models. They can provide unique information relevant to the short- and long-range transport of gas and aerosol pollutants, radiative forcing, cloud properties, emission factors and a variety of atmospheric parameters. Since its establishment in 2015, USRL is participating in major international research projects dedicated to (1) the better understanding of aerosol-cloud interactions, (2) the profiling of aerosol optical properties in different atmospheric environments, (3) the vertical distribution of air pollutants in and above the planetary boundary layer, (4) the validation of Aeolus satellite dust products by utilizing novel UAV-balloon-sensor systems, and (5) the chemical characterization of ship and stack emissions. A comprehensive overview of the new UAV-sensor systems developed by USRL and their field deployments is presented here. This paper aims to illustrate the strong scientific potential of UAV-borne measurements in the atmospheric sciences and the need for their integration in Earth observation networks.

ICESat‐2 Atmospheric Channel Description, Data Processing and First Results

AbstractThe Advanced Topographic Laser Altimeter System (ATLAS) was launched aboard the Ice Cloud and land‐Elevation Satellite‐2 (ICESat‐2) satellite in September 2018. ATLAS is a single wavelength (532 nm) lidar system designed to acquire high resolution measurements of the earth's surface while also obtaining atmospheric backscatter from molecules, clouds, and aerosols. Because ATLAS is optimized for altimetry, the atmospheric data acquired is unique in many respects and requires non‐standard analysis techniques. For example, the high repetition rate laser limits the vertical extent of the profiles to just 14 km and causes atmospheric scattering from above 15 km to be added to the scattering in the lower 0–14 km profile. In addition, the limited vertical range of the acquired profiles renders it difficult to compute the magnitude of the solar background and hinders the application of standard calibration techniques. Despite these limitations, methods have been developed to successfully produce data products that have value to the atmospheric community for cloud and aerosol research and are currently available at the National Snow and Ice Data Center (NSIDC). In this paper we describe the ICESat‐2 atmospheric channel and the methods used to process the ATLAS raw photon count data to obtain calibrated backscatter and higher level products such as layer heights and type, blowing snow, column optical depth and apparent surface reflectance.

Experimental assessment of a micro-pulse lidar system in comparison with reference lidar measurements for aerosol optical properties retrieval

Abstract. Simultaneous observations of a polarized micro-pulse lidar (P-MPL) system and two reference European Aerosol Research Lidar Network lidars running at the Leipzig site Germany, 51.4∘ N, 12.4∘ E; 125 m a.s.l.) were performed during a comprehensive 2-month field intercomparison campaign in summer 2019. An experimental assessment regarding both the overlap (OVP) correction of the P-MPL signal profiles and the volume linear depolarization ratio (VLDR) analysis, together with its impact on the retrieval of the aerosol optical properties, is achieved; the experimental procedure used is also described. The optimal lidar-specific OVP function is experimentally determined, highlighting that the one delivered by the P-MPL manufacturer cannot be used long. Among the OVP functions examined, the averaged function between those obtained from the comparison of the P-MPL observations with those of the other two reference lidars seems to be the best proxy at both near- and far-field ranges. In addition, the impact of the OVP function on the accuracy of the retrieved profiles of the total particle backscatter coefficient (PBC) and the particle linear depolarization ratio (PLDR) is examined. The VLDR profile is obtained and compared with that derived from the reference lidar, showing that it needs to be corrected by a small offset value with good accuracy. Once P-MPL measurements are optimally (OVP, VLDR) corrected, both the PBC and PLDR profiles can be accurately derived and are in good agreement with reference aerosol retrievals. Overall, as a systematic requirement for lidar systems, an adequate OVP function determination and VLDR testing analysis needs to be performed on a regular basis to correct the P-MPL measurements in order to derive suitable aerosol products. A dust event observed in Leipzig in June 2019 is used for illustration.

Effect of air cleaner on reducing concentration of indoor-generated viruses with or without natural ventilation

Coronavirus disease (COVID-19) has infected many people worldwide. As the probability of being infected by the virus increases with an increase in the number of people gathered in an indoor space, methods for reducing its spread indoors are urgently required. In this study, a classroom with 25 students was considered as an indoor space, and an attempt was made to devise a method to reduce the concentration of the viruses generated indoors more effectively. Through numerical analysis, the age of air inside the classroom was compared and analyzed when both the air cleaner and natural ventilation were used. The age of air varied locally in the classroom depending on the position of the air cleaner, confirming its importance in effectively removing particles possibly containing viruses generated indoors. When the air cleaner flow rate was doubled, the age of air was almost halved, implying that the concentration of the particles including viruses generated indoors can be reduced efficiently by increasing the air cleaner flow rate. Natural ventilation performed by completely opening the windows and doors of the classroom reduced the age of air. It was observed that the operation of the air cleaner along with open windows and doors of the classroom reduced the concentration of the particles including viruses generated indoors more effectively. More specifically, when both natural ventilation and air cleaner were applied, the age of air decreased by almost 50% compared with the only air cleaner case and by about 22% compared with the only natural ventilation case. Copyright (c) 2021 American Association for Aerosol Research

Application of planar auto-compensating laser-induced incandescence to low-sooting turbulent flames and investigation of the detection gate width effect

Laser-induced incandescence (LII) is an optical technique that is widely used for the in situ measurement of the soot volume fraction in flames. The low intensity of LII signals in low-sooting turbulent flames means that long camera acquisition times are needed to achieve sufficiently high signal-to-noise ratios. The direct application of auto-compensating LII (AC-LII) to infer the soot volume fraction from the measured LII signals can lead to large errors due to the significant decrease in the temperature of the soot particles over the duration of the camera gate. In order to reduce the measurement errors from AC-LII, we propose an improved approach in which the cooling of soot particles during the camera acquisition of LII signals is considered. The proposed methodology is applied to planar AC-LII measurements for the determination of soot volume fraction and primary particle diameter in a low-sooting turbulent flame, with the help of the open source LII software LIISim. This study represents the first application of planar AC-LII to a turbulent flame. A sensitivity analysis is also conducted in order to determine the main factors affecting the uncertainty of the proposed approach.Copyright © 2021 American Association for Aerosol Research

Comparison of resuspended road dust concentration for identifying optimum sampling position on a mobile laboratory vehicle

The accurate measurement of resuspended road dust from driving vehicles is key to assessing its role in air pollution. In this study, we used numerical analysis to investigate the behavior of resuspended road dust and selected the optimum sampling positions for use in a mobile laboratory (ML) to measure resuspended road dust emissions. Transient simulation of vehicle driving was performed by depicting wheel rim shape and tire tread pattern in detail. The resuspended road dust distributions generated around the vehicle wheels and the flow around the vehicle were analyzed according to the vehicle speed. Based on the analysis, four sampling positions with concentrated distributions of resuspended road dust were selected. To verify the simulation results, sampling inlets were installed on a ML in the selected positions, and resuspended road dust was collected at each position using a Teflon filter at vehicle speeds of 30, 40, and 50 km/h. Mass measurements of the resuspended road dust collected at the four locations showed that the most dust was collected at the position at the bottom of the vehicle immediately behind the wheel, and the overall trend was similar to the simulation results. Therefore, when measuring resuspended road dust using a ML, sampling inlets installed at the locations derived in this study are expected to yield more effective measurements for representing the pollution level of the road.Copyright © 2021 American Association for Aerosol Research

Differential Removal of Nanoparticles on the Surface of a Thin Film Substrate.

Purposeful identification, selection, and collection of particles are of great significance in environmental research. Microscopy is the common technique used in previous studies of particle identification. However, the microscopic technique was intricate and time-consuming. To conduct an intensive analysis of targeted particles, there is a need for the development of a simple method that can differentially abandon the nontargeted particles and only retain the targeted particles on the surface of a substrate. In the study, three methods were attempted for differential removal of nontargeted nanoparticles on the surface, including air jet, nanobubble, and ultrasonic methods. Acidic particles were taken as the targeted particles, while nonacidic particles were regarded as nontargeted particles. The results showed that regardless of methods, acidic particles were retained on the surface due to the strong particle–surface interaction. As for nonacidic particles, air jet treatment and nanobubble treatment were not able to completely remove nonacidic particles from the surface with the removal efficiencies of 5.1 ± 3.4 and 89.3 ± 4.1%, respectively, while the nonacidic particles were entirely removed in the ultrasonic treatment. Ethanol rather than deionized (DI) water was the proper solution in the ultrasonic treatment to avoid contamination. In conclusion, ultrasonic by ethanol was fully efficient for differential removal of nonacidic particles on the surface. The principle of differential removal of particles is the differences in the particle–surface interaction force between nonacidic particles (i.e., physically attached particles) and acidic particles (i.e., chemically formed particles). Nonacidic particles are removed from the surface through cavitation to form bubbles in the gap between a nonacidic particle and the surface in the ultrasonic treatment. In contrast, the space between an acidic particle and the surface is filled by the reaction, and thus bubbles cannot enter the crevice to remove the acidic particle. The developed method is useful for aerosol research.

Quantifying organic matter and functional groups in particulate matter filter samples from the southeastern United States – Part 2: Spatiotemporal trends

Abstract. Organic species within atmospheric particles vary widely in molecular structure. The variety of molecules that comprise the aerosol make it rich in information about its sources and chemical life cycle but also make particulate organic matter (OM) difficult to characterize and quantify. In Part 1 of this pair of papers, we described a direct method for measuring the composition and concentrations of OM in aerosol samples that is compatible with routine monitoring of air quality. This method uses Fourier transform infrared (FT-IR) spectrometry of filter-based aerosol samples to quantify bonds, or functional groups, that represent the majority of organic composition. Summation of these functional groups gives OM. In this paper, functional group and OM concentrations are directly measured in 8 years of aerosol samples collected at two rural and two urban sites in the Southeastern Aerosol Research and Characterization (SEARCH) network. FT-IR spectrometry with a multivariate calibration is used to quantify the concentrations of aliphatic C−H (aCH), carboxylic acid (COOH), oxalate (oxOCO; representing carboxylates), non-acid and non-oxalate carbonyl (naCO), and alcohol O−H (aCOH) in approximately 3500 filter samples collected every third day from 2009 through 2016. In addition, measurements are made on samples from all days in 2016. To the best of our knowledge, this is the longest time period over which this type of analysis has been applied, and this work also demonstrates the application of a more chemically complete and less destructive method than in prior work using alternate techniques. A decline in the total OM is observed from 2011 to 2016 due to a decrease in the more oxygenated functional groups (carboxylic acid and oxalate) and is attributed to anthropogenic SO2 and/or volatile organic compound (VOC) emissions reductions. The trend in OM composition is consistent with those observed using more time- and labor-intensive analytical techniques. Concurrently, the fractional contributions of aCOH and naCO to OM increased, which might be linked to monoterpene-derived secondary OM, with plausible influences from decreasing NOx and/or increasing O3 concentrations. In addition, this work demonstrates that OM to organic carbon (OM/OC) ratios in the southeastern US (SE US) did not appreciably change over the study time period as a result of these competing functional group contributions to OM. Monthly observations support the sources suggested by these overall trends, including evidence of strong biogenic and photo-oxidation influences. Daily samples from 2016 further elucidate the consistent impact of meteorology and biomass burning events on shorter-term OM variability, including prescribed burning in the winter or spring and wildfires in the autumn, although these sources did not appear to be strong contributors to long-term OM or composition trends in the SE US. These shorter-term and spatial observations reinforce the results of the broader dataset and serve to evaluate the applicability of FT-IR spectrometry measurement to trends analysis on various timescales relevant to routine monitoring of aerosol composition.

Effect of charge on aerosol microphysics of particles emitted from a hot wire generator: Theory and experiments

The origin of ions in the lower atmosphere is attributed to sources like galactic cosmic rays, radioactive isotopes, gamma radiation, etc. These ions interact with atmospheric constituents affecting various atmospheric processes. Aerosol microphysical processes also get modified due to the presence of charge on particles. For aerosol generation, charging during generation and consequent effect on evolution dynamics is an important aspect. One of the efficient methods for the generation of nanaoparticle aerosols is via electrically heated wires i.e. hot wire generator (HWG). Although studies exist focusing on experimental insights, not much work has been performed in the direction of modeling the process for HWG systems. At present, there is a knowledge gap in the detailed understanding of charged aerosol microphysics of such high temperature systems. Models have been developed for different aspects of charged particles; but to the best of our knowledge, there are no studies which model the complete process for a hot wire generator, factoring both the effect of charge and Computational Fluid Dynamics (CFD). A CFD coupled aerosol microphysical model was developed for HWG in our earlier work. Charge has been added in the modified theoretical framework for the present work. The modified charged aerosol dynamic model has been coupled to the module for ion generation from hot sources. Thus, a newly developed CFD coupled charged aerosol microphysical model has been used for modeling the nanoparticle generation from HWG system. Results from the simulations have been compared with the experimental observations performed with a laboratory made HWG.Copyright © 2021 American Association for Aerosol Research

An indicator for sulfuric acid–amine nucleation in atmospheric environments

New particle formation (NPF) occurs frequently in various atmospheric environments and it is a major source of ultrafine particles. This study proposes an indicator, I, for the occurrence of NPF in the atmosphere based on the mechanism of H2SO4–amine nucleation. It characterizes the synergistic effects of the governing factors for H2SO4–amine nucleation, including H2SO4 concentration, amine concentrations, the stability of H2SO4–amine clusters, and aerosol surface area concentration. Long-term measurements in urban Beijing were used to validate this indicator. Good consistency was found between this indicator and the occurrence of NPF. NPF was usually observed with I > 1 for typical conditions in urban Beijing. The derivation and expressions of I also indicate a good positive association between the H2SO4 dimer concentration and NPF, as also verified by measurements. I was shown to be also applicable in urban Shanghai.Copyright © 2021 American Association for Aerosol Research

Comparison of methods of functional group analysis using results from laboratory and field aerosol measurements

A study was conducted to compare two methods for analysis of functional groups in secondary organic aerosol (SOA) generated in the laboratory and organic aerosol collected in the Southeastern Oxidants and Aerosol Study (SOAS). Although not as widely used as molecular and elemental analysis, functional group analysis of filter samples provides chemically specific information that can be used to characterize organic aerosol sources, mechanisms of formation, and physical-chemical properties. Here, we compared measurements made using Fourier Transform Infrared (FTIR) spectroscopy, the most widely used method of functional group analysis, with those made using derivatization-spectrophotometric methods. When used to analyze SOA generated from a series of environmental chamber reactions, the methods agreed quite well when results were assigned to four groups: (1) functional groups that contain an O–H bond; (2) non-acid functional groups containing a C = O moiety; (3) organonitrate groups; and (4) alkane groups. Overall, the agreement between the two methods indicates that either is adequate for measuring the functional group composition of SOA. While the derivitization-spectrophotometric methods offer more specificity, they also require more time and aerosol mass for sample analysis than the FTIR methods. It was also shown that functional group signatures obtained from SOA generated in environmental chambers can potentially be used to gain insight into oxidation and particle-phase reactions responsible for SOA formation in field samples.Copyright © 2021 American Association for Aerosol Research

Morphology and formation of crystalline leucine microparticles from a co-solvent system using multi-orifice monodisperse spray drying

The purpose of this study was twofold: to investigate leucine crystallization kinetics in co-solvent spray drying for a deeper understanding of the morphology and formation process of spray-dried crystalline particles, and also to develop multi-orifice plates using gallium focused ion beam milling in order to increase the production rate of monodisperse spray drying. A monodisperse spray drying technique using a vibrating orifice atomizer equipped with a gallium focused ion beam-manufactured dual-orifice plate was used to produce model particles in this study. The dual-orifice plate achieved an average production rate of 23.8 ± 2.0 mg/hr for the model particles, which allowed sufficient production of monodisperse powders for microscopic and spectroscopic analysis as well as fragility testing, within a reasonable spray drying time period. Leucine was dissolved in a water-ethanol co-solvent at ratios of 0.25/0.75 w/w, 0.5/0.5 w/w, and 1/0 w/w, and then spray dried at inlet temperatures of 20, 40, and 80 °C. Crystallinity of the spray-dried particles was confirmed by Raman spectroscopy. Numerical models were used to predict the crystallization and drying kinetics for the different co-solvent ratios and drying temperatures. Increasing the time available for crystallization correlated qualitatively with a larger crystal size. Changes in particle morphology affected the fragility of the particles, as illustrated in electron microscope images. This work highlights the importance of controlling crystal size and particle morphology via solvent environment and drying temperature in the design and manufacture of microparticles for desired powder properties.Copyright © 2021 American Association for Aerosol Research

Filtration efficiency of a large set of COVID-19 face masks commonly used in Brazil

The use of face masks is mandatory in public places in many countries to slow the spread of the COVID-19 pandemic. In developing countries, homemade masks with varying techniques and fabrics are used on the streets. On these fabric masks, the protection against SARS-CoV-2 varies significantly. The most common mask types are N95, surgical masks, and homemade nonwoven and cotton masks. The performance of 227 different face masks used in Brazil was evaluated to quantify their breathability and filtration efficiency (FE) for airborne particles. FE values were measured using NaCl aerosol particles sized from 60 to 300 nm and at 300 nm, minimum efficiency. The differential pressure drop over the mask and the FEmin at 300 nm was used to calculate the mask Quality Factor (QF). The N95 masks showed the highest FE60-300, around 0.98, and a QF of 13.2 KPa−1, and were considered the reference for evaluating homemade masks performance. Surgical masks have an FE60-300 of 0.89, with a good QF of 15.9 KPa−1. Nonwoven masks showed an average FE60-300 of 0.78, with an excellent QF of 24.9 KPa−1, and can be regarded as the best material for homemade masks. The most commonly used material for homemade masks, cotton fabrics, showed significant variability in FE60-300, ranging from a low 0.20–0.60, with a low QF of 1.4 KPa−1. Masks always reduce droplets and aerosols emitted by COVID-19 symptomatic and asymptomatic persons, reducing SARS-CoV-2 contamination.Copyright © 2021 American Association for Aerosol Research