This study investigates observations of coarse particles during a Saharan dust event using low-cost optical particle counters (OPC) mounted on uncrewed aircraft systems (UAS). Measurements from an AlphaSense OPC-N3 and a Universal Cloud and Aerosol Sounding System (UCASS) are compared, and the impact of refractive index (R.I.) correction on OPC observations is examined. On 6 April 2022, a dust layer existing from 1250 to 2500 m a.g.l. advected over Orounda, Cyprus. Two UAS performed vertical profiles to examine meteorological and particle dynamics. Refractive index (R.I) corrections were applied to AlphaSense OPC-N3 size distributions to assess effects on aerosol particle load estimates. The correction substantially altered apparent number and volume concentrations, reducing discrepancies to an additional UAS (UCASS). Whereas varying the real part of the R.I. had a minor influence on the apparent distribution, sensitivity to the imaginary component was more pronounced. Assuming a R.I. of 1.53 + 0.0015i improved the fit of derived quantities with lidar and a TEOM observations; however, 1.53 + 0.0024i provided closer cross-instrument alignment, suggesting a higher imaginary component for the observed dust. The correction highlights long-range transport of super-coarse aerosol particles, and suggests that the OPC-N3's ability may be extended to detect particles with geometric mean diameter up to three times larger than nominal bin classifications during Saharan dust events. Observations from a 2025 dust event further support these results. R.I. corrections improve interpretation of OPC-N3 data and enhance its ability to characterize coarse, super-coarse, and giant particles, ultimately supporting aerosol particle transport dynamic studies and reducing persistent knowledge gaps.
The size characterization of aerosols emitted from pressurized metered dose inhalers (pMDIs) is complicated by propellant evaporation and potential hygroscopic size changes in the presence of ambient humidity. This study used laser diffraction (LD) for in situ measurements of pMDI particle size distributions downstream of the Alberta Idealized Throat (AIT) and Alberta Idealized Child Throat (AICT) in low and high ambient humidity. In low humidity, agreement between LD-derived measurements and cascade impactor size distributions varied across inhalers. Extrathoracic deposition was higher in the AICT than the AIT. In high humidity, LD-derived particle sizes downstream of the model throats increased for one solution inhaler, but decreased for the remaining (suspension) inhalers. Experiments performed with a blank pMDI containing only propellant recorded non-zero size distributions downstream of the extrathoracic region in high humidity, suggesting that drug-free droplets may contribute to size distributions measured by LD for suspension pMDIs. Further experiments conducted using an extension tube downstream of the extrathoracic models to prolong exposure to high humidity demonstrated that hygroscopic size changes progressed slowly downstream of the extrathoracic airways. Differing drug deposition and particle sizes measured between the AIT and AICT underscore the importance of using age-appropriate extrathoracic models during size characterization of particles entering the lungs of adults and children. The use of LD for the measurement of pMDI aerosols should be considered on a product-specific basis. Size measurements in high humidity require careful interpretation, particularly for suspension pMDIs, as LD estimates could be impacted by the measurement of API-free droplets.Copyright (c) 2026 American Association for Aerosol Research
We compared aerosol light absorption measurements by a photothermal interferometer (PTI) and by extinction minus scattering (EMS) in a laboratory setting using test aerosols with single scattering albedo (SSA) in the range from about 0.1 to 1. The EMS method reported higher light absorption coefficients, b abs, by 25% up to a factor of 2 compared to PTI, with the deviation increasing when coating fresh soot with secondary organic matter. In a second step, the attenuation measured by numerous filter-based absorption photometers and in-situ-measuring instruments was calibrated against PTI at the wavelengths of 450 and 808 nm to determine reliable calibration factors for each instrument. The attenuation coefficient measured by the filter-based instruments, b atn, was larger by a factor of 4 - 9 than the reference b abs measurements of PTI depending on the instrument model and filter tape. We determined similar multiple-scattering correction factors C for the aethalometer AE33 to those reported by recent studies using PTI as a reference method, but almost two times higher than those reported by older studies referenced to the Multi-Angle Absorption Photometer (MAAP). In-situ-measuring instruments showed a reasonable agreement with PTI despite the low aerosol absorption coefficients which were close to the limit of detection for some of the photoacoustic instruments. In view of the revised Air Quality Directive of the European Union, our study highlights the need for traceable aerosol light absorption measurements to harmonize calibration procedures across Europe.
Prior work found that switching the analysis of organic carbon (OC) and elemental carbon (EC) from the IMPROVE_A protocol on a DRI Model 2015 multiwavelength instrument to a Sunset Labs instrument in the US Environmental Protection Agency's Chemical Speciation Network (CSN) resulted in substantial shifts in the OC4 and EC2 thermal carbon fractions. These fractions permitted the separation of source-specific PM2.5 contributions from spark-ignition (GAS) and compression-ignition (DIE) vehicle emissions. A subsequent effort was made to adapt the Sunset IMPROVE_A protocol implementation to permit the development of long-term time series of gasoline (GAS) and diesel (DIE) engine exhaust PM2.5 source contributions using CSN data. Extending the OC4 step to a full 580 s increased OC4 and decreased EC2, leading to Positive Matrix Factorization (PMF) source contribution estimates that were similar to those previously obtained using the DRI Model 2015 implementation. The validity of this modification was evaluated using CSN quartz filters collected from January 2020 to July 2024 at the Queens College 2 site in New York City that were analyzed using DRI and Sunset implementations. Although there were some notable differences in the actual OC and EC fractions between the Sunset and DRI analyses, the PMF results demonstrated sufficient agreement after the modified Sunset protocol was incorporated into the CSN carbon analysis. Future CSN measurements will include this modification.
Lithium-ion batteries (LIBs) undergoing thermal runaway vent a complex mixture of particles alongside condensable and non-condensable gases. These emissions may ignite or remain unignited, potentially producing a broader range of species. This study presents experimental data and physical insights into the coupled behavior of vented gases and aerosol particles emitted from cylindrical 18650-format LIBs. Thermal abuse experiments were first conducted with near-source measurements of gases and aerosol particles, providing a benchmark for condensable gas characterization. The near-source sampling configuration featured short residence times and a high air change rate. The cell thermal abuse test was then conducted in a poorly ventilated, reduced-scale room, resulting in longer gas and particle residence times. These conditions more closely represent enclosed environments such as residences or aircraft cabins and enable investigation of aerosol particle growth dynamics under confinement. In both configurations, aerosol particles smaller than 0.5 & micro;m dominate number concentrations. Following thermal runaway, the aerosol particle number size distribution broadens, with increases in geometric mean and mode diameters. The reduced-scale room experiments reveal aerosol particle growth driven by electrolyte solvent condensation and coagulation. Notably, an increase in aerosol particle number concentration after direct cell emissions ceased is observed only in the reduced-scale room configuration. To interpret these observations, a simplified moment-based general dynamic equation model for aerosol evolution is applied to experimental data. By linking observed size distribution changes to condensation, coagulation, and sedimentation processes, this work connects fundamental battery venting mechanisms to exposure, safety, and mitigation considerations in enclosed spaces.Copyright (c) 2026 American Association for Aerosol Research
In recent years, Electronic Nicotine Delivery Systems (ENDS) have gained popularity as an alternative to combustible cigarettes (CC). However, their increased use raises concerns regarding exposure to harmful and potentially harmful constituents (HPHCs), especially metals. While several methods have been published for collecting metals from ENDS aerosol, they are often hindered by background interference and inefficient aerosol collection. To address these challenges, a new aerosol collection method was developed and validated utilizing a modified electrostatic precipitation (EP) system connected to a Cerulean SM-450e puffing machine. Analysis of collected aerosol using an Agilent 7800 single quadrupole inductively coupled plasma mass spectrometer (ICP-MS) showed the new approach consistently provided a low background for chromium (Cr), nickel (Ni), copper (Cu), cadmium (Cd), and lead (Pb) in aerosol blanks, ensuring low method limits of detection (LOD). The unique design of the EP system effectively precipitated uncharged aerosol particles enabling collection of approximately 3.5 g of aerosol that minimized the average number of collections to two, required for complete sampling of ENDS products in this study. This method was used to collect aerosol samples from commercially purchased ENDS including pods and illicit disposables, many of which were found to contain quantifiable amounts of Cr (<10.1-1046.44 ng/g), Ni (<6.13-7659.7 ng/g), Cu (<7.53-25171.1 ng/g), Cd (<5.11-75.8 ng/g) and Pb (<5.18-1536.1 ng/g). This work shows the utility and advantages of electrostatic precipitation as a well-suited collection method for the analysis of low-level metals in ENDS aerosol.
Do-it-yourself (DIY) air cleaners can be a temporary, low-cost ventilation strategy for improving indoor air quality by reducing airborne particulate contaminants. However, to our knowledge, no comprehensive assessment of multiple low-cost sensor systems (LCSs) for evaluating DIY air cleaner performance has yet been conducted. In this study, we investigated seven LCSs when used to evaluate the air cleaning performance of DIY air cleaners. Three portable high efficiency particulate air (HEPA) air cleaners (HACs) and three different DIY air cleaners, namely the single filter, Ford Scrappy, and Corsi-Rosenthal (CR) Box designs, were evaluated. All air cleaners were challenged with a NaCl aerosol, and their clean air delivery rate (CADR) was calculated from PM2.5 mass concentrations measured by the LCSs and a reference particulate matter (PM) monitor. The published Dust CADR ratings of the HACs and the CADRs calculated from the reference PM monitor were used to determine the mean absolute percent error (MAPE) in the CADRs calculated from the LCSs. The LCSs' MAPE in CADR among all air cleaners tested ranged from 1.6-44.2%. Three of the seven LCSs had a MAPE <= 10%. No correlations between LCS performance in our study and their published performance evaluations were identified. However, we found that the correction factor of PM sensors from one manufacturer, found in four of the LCSs evaluated, impacted performance. We also suspect the transient test conditions created by operating mixing fans and air cleaners during experiments, the LCS housing design, and the calibration of the LCS may be driving factors impacting LCS performance in our study. Overall, we demonstrated that certain LCSs can be a viable option for evaluating the air cleaning performance of DIY air cleaners.
New particle formation and growth (NPF&G) is a significant contributor to the number of cloud condensation nuclei in the atmosphere, which influence Earth's radiative budget by modulating cloud properties. While our understanding of NPF&G in certain environments has advanced, the processes that govern NPF&G in some environments are still poorly understood. As part of the Tracking Aerosol Convection Interactions Experiment (TRACER) Ultrafine Aerosol Formation and Impacts (UFI; together TRACER-UFI) campaign, the University of California, Riverside's Captive Aerosol Growth and Evolution (CAGE) chamber operated in La Porte, Texas. The CAGE chamber filters out ambient particles, allowing only ambient vapors to enter, thus providing the opportunity to probe NPF&G processes under pseudo-ambient low-condensation sink conditions. On most days during TRACER-UFI, NPF&G events occurred in the chamber, however, only three such NPF&G events occurred in the chamber during the time period with concurrent measurements of ambient vapors relevant to NPF&G. In this work, we use the SOM-TOMAS chemistry and aerosol-microphysics model to represent the CAGE chamber during the three NPF&G events. We calculate that aerosol mass in the chamber is primarily derived from toluene, trimethylbenzene, styrene, and monoterpene oxidation products during these events. Aerosol measurements corroborate the contribution of anthropogenic species to aerosol particles in the chamber; however, the measurements indicate higher sulfur contribution to aerosol than is produced by the model.Copyright (c) 2026 American Association for Aerosol Research
Low-cost optical particle sensors (LCS) can improve the spatio-temporal coverage of particle matter (PM) air pollution monitoring. These sensors require calibration to ensure accuracy across diverse aerosol and environmental conditions. Using a global dataset (January 2023-March 2025) of 1,013 PurpleAir sensors co-located (<= 500 m) with reference grade monitors on six continents, we evaluated three widely used U.S. correction equations and developed global and regional calibration equations. Performance was assessed for 24-h and 1-h averages using R2, RMSE, nRMSE, precision (CV) and Air Quality Index (AQI) category correctness (24-h and NowCast EPA AQI). Raw PurpleAir readings overestimate PM2.5 in most regions and do not meet the EPA sensor performance targets. Previously suggested calibrations based on sensors located in the USA improve the accuracy but exhibit limited transferability outside North America. A data-driven global quadratic model improves results in some regions, such as Australia and New Zealand, but underperforms in other places. However, regional calibrations consistently reach the best agreement, meeting EPA's 24-h performance thresholds in multiple regions and increasing EPA's 24-h AQI correctness to >= 77%. These findings indicate that, given the strong RH sensitivity and aerosol composition variability, region-specific corrections should be preferred where co-locations exist. We provide the derived equations and policy-relevant suggestions to support more robust use of LCS in air-quality research, management, and public communication.Copyright (c) 2026 American Association for Aerosol Research
Airborne particulate matter (PM), particularly black carbon from vehicle exhaust, poses significant risks to urban air quality and human health. While urban vegetation is known to reduce PM through deposition on leaf surfaces, the resuspension of deposited particles under wind conditions remains insufficiently understood. In this study, the resuspension behavior of soot particles deposited on four tree species commonly planted in South Korea (Metasequoia glyptostroboides, Euonymus japonicus, Pinus densiflora, and Ginkgo biloba) was experimentally investigated using a custom-designed wind tunnel. Soot aerosols generated by a diffusion flame burner exhibited a log-normal size distribution with a peak at 125-130 nm, consistent with diesel exhaust particles. Trees were exposed to soot aerosol for 5 h, followed by clean-air flows at 1, 3, 5, and 7 m/s to simulate urban wind conditions. Results indicated that particle resuspension was most pronounced during the first 20-30 min of clean-air supply, after which concentrations decreased rapidly. Exponential decay functions accurately described the resuspension trends, with decay coefficients increasing with airflow velocity. Broadleaf species (Euonymus japonicus and Ginkgo biloba) exhibited higher decay coefficients than conifers (Metasequoia glyptostroboides and Pinus densiflora), reflecting weaker adhesion on smoother leaf surfaces and stronger leaf fluttering under airflow. Conversely, coniferous species with rough surfaces and dense leaf structures exhibited slower resuspension. These findings confirm that leaf structural traits significantly influence resuspension processes. It is therefore suggested that, in urban greening strategies, the potential for particle resuspension should be considered together with deposition capacity to maximize the air-quality benefits of vegetation. [GRAPHICS] .
Electrically charged droplets play a crucial, yet often unquantified, role in numerous natural and laboratory-based processes. Bridging this critical knowledge gap is challenging, primarily due to the current lack of experimental methods capable of quantifying the charge state of individual droplets with high precision under controlled conditions. Here, we present a novel optical electrometer that enables simultaneous, in-situ determination of electric charge and mass of single optically trapped droplets within seconds. The charged droplet is immobilized in air between two ring capacitor plates using counter-propagating optical tweezers. An oscillating electric field between the two ring plates sets the droplet into oscillatory motion, which is precisely tracked. A lock-in amplifier controls the applied voltage and records the frequency-dependent droplet response from which charge and mass are deduced. This versatile method extends our established single-droplet mass measurement technique to include simultaneous charge retrieval, providing a powerful platform for probing charge-dependent phenomena of individual droplets by readily combining it with complementary spectroscopic techniques.
Organic components often contribute 50% or more of the submicron aerosol mass in coastal urban environments, but their partitioning between the gas and particle phases is controlled by a complex mixture of unidentified organic compounds that are poorly constrained by observations. This study compares daily filter organic functional groups (OFG) with online organic mass fragments from La Jolla, California, as part of the Eastern Pacific Cloud Aerosol Precipitation Experiment (EPCAPE), quantifying the contributions of four types of nonvolatile (NV) organic emission sources to the submicron composition. Daily filters retained only 0.79-0.98 & micro;g/m3 NV submicron organic mass concentration, even though 1.8-1.9 & micro;g/m3 non-refractory (NR) submicron organic mass concentration was measured online. The 62-64% of measured NR submicron organic mass concentration that exceeded what remained on the filters after 23-h sampling is interpreted as semi-volatile, consistent with the moderate correlation of the NR OM-NV OM difference to NR ammonium, NR nitrate, and biomass burning-related NV and NR organic factors. The association between semi-volatile organic components and ammonium nitrate likely results from both co-emission and co-evaporation. Size-resolved filter analysis showed that NV organic mass concentration accounts for 68% of NR organic mass concentration for 0.5-1 & micro;m dry diameter, but account for 9.0% for 0.18-0.5 & micro;m dry diameter, showing the differences in volatility between particle modes. Importantly, the volatility of organic components was size-dependent and associated with ammonium nitrate and biomass burning, providing guidance for constraining atmospheric aerosol properties in global models.
This study investigates the key parameters influencing tandem measurements of effective density for low-volatility spherical particles and examines the impact of instrument resolution on the operational range of different tandem configurations. Three tandem setups were evaluated using a differential mobility analyzer (DMA), a Centrifugal Particle Mass Analyzer (CPMA), and an Aerodynamic Aerosol Classifier (AAC). To enable fast measurements, the downstream classifier was used as a scanning instrument, either a Scanning Mobility Particle Sizer (SMPS) or a Scanning Aerodynamic Size Spectrometer (SASS). The effects of instrument resolution on the operational range and measurement accuracy were assessed using three resolution settings. Data were collected for mass set points ranging from 0.05 fg to 5 fg and aerodynamic diameter set points from 50 nm to 500 nm with aerosol flow rates of 0.3 L/min (low-flow mode) and 1.5 L/min (high-flow mode). The results indicate that the characteristic scan time (CST) required for accurate measurements depends on the flow mode and the downstream classifier, with longer CST required in low-flow mode and for the SASS compared to the TSI 3082 SMPS. Additionally, the medium-resolution setup provides the best balance between accuracy and range for effective density measurements. Furthermore, calibrating the effective density of Santovac particles using the measured DOS effective density improves measurement accuracy. The conclusions apply to spherical, low-volatility particles (50-500 nm) measured with the instruments in this study, while the framework and procedures are broadly applicable to other tandem configurations and aerosol types.
