Aethalometers measure black carbon mass concentrations by monitoring light attenuation through a particle filter as it becomes laden with aerosols. As the uncertainties in the resulting measurements are not easily quantified via a bottom-up traceable approach, there is a need for inter-device comparisons to provide operationally defined uncertainties. The present work compared five micro-aethalometers to known mass concentrations of laboratory-generated soot, formed using an inverted ethylene flame and a Centrifugal Particle Mass Analyzer-Electrometer Reference Mass Standard (CERMS). Uncertainties were found to scale with mass concentration, with contributions from Gaussian, Poisson, and multiplicative components. The multiplicative errors between devices are approximately 10 % in the best case of long sampling times and/or high mass concentrations. This represents a minimum uncertainty estimate, for an aerosol of constant composition. A quantitative expression is provided for this uncertainty as a function of mass concentration, sampling interval, and flow rate. An open-source algorithm is also provided for the unsupervised reanalysis of aethalometer or other filter photometer data over varying periods to reach a specified target uncertainty.

Fresh soot is composed of chain-like lacey aggregate of primary particles that can compact when coated with liquids. The transition from a lacey to a compact structure is a key restructuring mechanism for these carbonaceous nanoparticles. In this study, fresh propane soot aggregates of mobility diameter (d(m)) between 35 nm and 800 nm were coated with oleic acid to induce compaction. The restructuring process was observed using aerosol-particle size measurements as well as fixed-angle and, for the first time, tilt-stage TEM images. Analysis of the TEM images showed that the circularity increased from similar to 0.1 to 0.4 and the sphericity increased from similar to 0.55 to 0.7 as the particles transitioned from a lacey to a fully compacted structure at volume growth factors >5. For such fully compacted aggregates, we confirmed the applicability of the universal packing factor of 0.35, and corresponding effective density of 650 +/- 20 kg/m(3) for larger aggregates (d(m) > 100 nm) from several studies, compiled in previous work (Sipkens & Corbin, 2024). That compilation identified a literature gap for the morphology of small aggregates. Here, we present measurements filling this gap. We measured negligible restructuring for smaller aggregates (d(m) < 50 nm for d(pp,100) similar to 19 nm) and corroborated this observation with TEM. Hence, the effective density of small aggregates was similar to that of fresh soot. We also quantified and parameterized the effective-density transition from this negligible-restructuring regime to the constant-packing-density regime (650 +/- 20 kg/m(3)). This parameterization estimates the effective density of fully compacted soot aggregates based on their mobility diameter and can be easily applied to other nanoparticle aggregates of known material density.

The effective density, rho eff, of an aerosol particle and the related mass-mobility exponent are useful for calculating the integrated particle mass concentration and providing fundamental insights into particle morphology. Typically, rho eff is measured using two tandem aerosol classifiers: the upstream classifier selects particles based on an equivalent property (one of mass, mobility diameter, or aerodynamic diameter), while the downstream classifiers measure the resulting distribution of a different equivalent property. Since rho eff conceptually represents a physical property of the sampled particles, it is expected to be independent of order of the classifiers. However, recent studies have reported different rho eff for different classifier arrangements, e.g. mobility-mass classification versus mass-mobility classification. In this study, we show that the root cause of the observed discrepancies lies in the fundamental properties of the underlying bivariate distribution and the calculation of effective density using a classifier setpoint and the mode of downstream distributions. The modes of conditional (i.e., classified) distributions differ depending on the conditioned property (i.e., which classifier setpoint is used in the calculation). It is only when the two properties are tightly correlated that the two fits become equal. We then provide analytical relationships that enable transformation between classifier arrangements and validate the proposed relations using soot aggregates across four classifier arrangements involving a particle mass analyzer (PMA), differential mobility analyzer (DMA), or aerodynamic aerosol classifier (AAC): PMA -* DMA, DMA -* PMA, AAC -* DMA, and DMA -* AAC. After applying the transformation, discrepancies in effective density reduce from 15 % for the APM -* DMA configuration and 30 % for the AAC -* DMA configuration effective density to 5 %. Corresponding discrepancies in integrated mass concentrations can be reduced from 20 % to 3 %, when a simple analytical correction is used.

Particulate matter (PM) from marine traffic interacts with solar radiation and clouds, ultimately influencing Earth's radiative balance. Ships operated with conventional fossil fuel oils emit light-absorbing carbonaceous PM that offsets aerosol-driven cooling and can even exert a net positive radiative forcing, i.e. warming effect. Radiative properties of PM are possibly further altered by atmospheric aging processes, the effects of which are not fully understood. We present black carbon (BC) emission factors (EF) and optical properties of fresh and photochemically aged particle emissions from a marine engine, operated using low-sulfur heavy fuel oil (LS-HFO) and marine gas oil (MGO), complying with recent maritime sulfur regulations by the International Maritime Organization (IMO). The fresh particle emissions comprised mostly BC, with average BC EFs of 144 and 43.2 mg/kWh for LS-HFO and MGO, respectively. Light absorption was mostly attributed to BC in particles from both fuels, with absorption & Aring;ngstr & ouml;m exponent (AAE, 370 to 880 nm) values 0.9-1.0 (interquartile range), and 870 nm single scattering albedo (SSA) values 0.15-0.24 during the full cycles. Fresh LS-HFO emissions exhibited lower SSA values than those of high-sulfur fuels reported in literature, primarily associated with reduced sulfate emissions. Photochemical aging led to an absorption enhancement (Eabs) of 1.2-1.5 and an increase in SSA relative to fresh emissions, although SSA remained below 0.5, and the estimated direct radiative forcing effect stayed positive. Our results show that sulfur-compliant marine fuels can emit highly absorbing particles with an atmospheric warming potential, which is mostly maintained even after photochemical aging.

Mass-concentration aerosol instruments require frequent calibration to provide precise and accurate measurements. Such calibrations for mass instruments can be achieved with the Centrifugal Particle Mass Analyzer (CPMA)-Electrometer Reference Mass Standard (CERMS). This study presents an interlaboratory comparison of CERMS and its two major components: the Faraday Cup Aerosol Electrometer (FCAE) and CPMA. The CERMS repeatability and reproducibility, defined as measurement precision under repeatable and reproducible measurement conditions, are evaluated. Our study was conducted in two phases: laboratory and field studies, and involved three independent laboratories. In the laboratory study, comparisons were made using soot and size-selected dioctyl sebacate (DOS) particles, using a transfer instrument. In the field, nebulized ammonium sulfate and soot from two turbine engine source exhausts, including a J85 turbojet engine, were used to compare the CERMS systems using three transfer instruments. Results indicated that the FCAEs exhibited excellent repeatability and reproducibility (<2%), while the CPMAs showed excellent repeatability (<3%) but poorer reproducibility (about 10%) due to instrument biases. In the laboratory study, the entire CERMS system demonstrated low uncertainty under repeatable conditions (3%) but higher uncertainty under reproducible conditions (similar to 11%). Field study uncertainties for CERMS were larger than in the laboratory (repeatability similar to 8%, reproducibility similar to 11%), likely due to the combined uncertainties from the transfer instruments, particle sources, CERMS components, and the less-controlled environment. Since biases between CPMAs were the major contributor to overall CERMS reproducibility, CPMA calibration could provide a significant improvement to CERMS reproducibility. [GRAPHICS] .

An accurate assessment of black carbon (BC) climate and health impacts requires knowledge of its mass absorption cross-section (MACBC) – a parameter linking optical and mass measurements. The mean MACBC for freshly emitted soot typically spans a narrow range of 8 ± 1 m2 g⁻1 at 550 nm1,2 but is modified by subsequent atmospheric aging. Determination of MACBC requires simultaneous measurements of aerosol light-absorption coefficient (βabs) and BC mass. Here, we compile 230 measured MACBC values from 80 atmospheric studies and explore the effects of sampling location, study duration, instrumentation, and measurement wavelength. The compiled data set shows a broad variability in MACBC values (a factor of about 200%). We conclude that this variability is attributable to a combination of the above-mentioned effects with additional instrumental uncertainties (e.g., cross-sensitivities and/or inadequate instrument calibration). The current state of knowledge does not support the use of simplistic generalizations or assumptions about MACBC in the atmosphere, motivating a recommendation to further improve and standardize measurement practices.

Photothermal spectroscopy, and photothermal interferometry (PTI) in particular, has the potential to provide reference measurements of aerosol absorption coefficients beta abn. Such beta abn measurements represent a fundamental property of aerosols such as black carbon and brown carbon, the accurate knowledge of which is essential for understanding their climate forcing: beta abn can be used in calculating the imaginary refractive index, absorption function, or mass absorption cross-section of a sample. Furthermore, beta abn is commonly expressed in units of equivalent black carbon mass (eBC) in the many photoacoustic or filter-based instruments that are used for air-quality monitoring and emissions testing of on-road, airborne, and marine transportation. Here, we propose the use of tunable-laser PTI (TL-PTI) as a self-calibrating reference technique for quantifying beta abn. In TL-PTI, a wavelength-tunable laser is used to quantify the narrow absorption lines of a gas via direct absorption spectroscopy, providing a reference beta abn,ref in units of Mm-1. The A-band absorption lines of ambient oxygen at about 760 nm provide a convenient reference. The PTI signal is then calibrated to this beta abn,ref. This gas-PTI signal can then be "switched off" by tuning the laser wavelength away from the absorption lines, so that only particulate PTI signals remain in the background-subtracted signal. The PTI signal is directly related to light absorption for both aerosols and gases, since the PTI pump-laser modulation is slower than the thermalization timescales of both. We demonstrate the accuracy of our PTI prototype by retrieving the refractive index of nigrosine dye.

Particle filtration efficiency (PFE) is a critical property of face masks, with the most common test methods using sodium chloride as a challenge aerosol. In the absence of bottom-up uncertainty budgets for PFE, interlaboratory comparisons provide an alternative route to robustly quantify the precision and bias of the method. This work presents the results of several interlaboratory comparisons of particle filtration efficiency performed across a network of laboratories. Using log-penetration as a surrogate for PFE, it is shown that expanded reproducibility intervals were consistent across most samples, at around 26% of the nominal value of log-penetration. Between-laboratory contributions to this reproducibility were significant, nearly doubling the lab-reported uncertainties in most instances and emphasizing the need for ongoing interlaboratory studies to be performed for particle filtration. More work is required to identify the causes of these between-laboratory differences, requiring dedicated testing. Alongside uncertainty quantification, testing materials across a range of variables (such as the number of layers, amount of charge on the material, and basis weight) affirm that constant quality is a good approximation when layering or changing the basis weight on an otherwise identical material.

Aerosol classifiers allow particle populations to be described in terms of mass, mobility diameter, or aerodynamic diameter distributions. When this classification is combined with a second layer of classification, a bidimensional distribution can be retrieved that provides additional insights into the distribution of aerosol properties. Bidimensional distributions are often transformed from extensive quantities related to the measurement (e.g., particle mass) to intensive ones that provide more intuitive insights of particle morphology (e.g., effective density or black-carbon mass fraction). Further, most extensive properties are highly correlated with one another (e.g., particle mass and mobility diameter). This complicates inversion, resulting in retrieved distributions that are considerably broader than the true distribution. In this work, we show that these problems can be solved using a single analysis step to compute distributions-of-interest, phrased in terms of intensive properties. This yields a direct inversion scheme that (1) avoids the need for postprocessing to retrieve common distributions-of-interest; (2) reduces the correlation between the aerosol properties for which the bidimensional distribution is defined; (3) makes regularization easier and more objective; and (4) improves the minimum resolvable distribution width by up to 96 %. The approach is demonstrated using both simulated distributions (phantoms) and experimental data.

Aerosols have a wide-ranging impact on the climate, air quality, human health, and agriculture. Despite the ongoing advances in aerosol measurement science and technology, the uncertainties in quantifying aerosol physical properties remain significant in many applications. The accurate characterization of airborne particles - including number and mass concentration, size distribution and light absorption - is critical for understanding their behavior in the atmosphere and environmental fate. We delve into the physical characterization of aerosols, highlighting the measurement and documentary standards that underpin measurement traceability and enable comparison of data collected by instruments based on measurement principles at different times or locations. In particle metrology, recent advances have led to sophisticated primary measurement standards, with relative expanded measurement uncertainties down to 1.1 % (coverage factor k = 2; 95 % confidence interval). These standards enable time- and cost-effective instrument calibration to support research, industry, and legislation. We discuss documentary standards and regulations related to air quality and control of particle emissions from vehicles, aviation, shipping, and stationary sources, with the aim to increase awareness of these documents and underline differences in measurement protocols in different sub-fields of aerosol sciences. Importantly, we emphasize the need for further harmonization of measurement procedures, providing specific examples and making suggestions towards this goal. This review, with its comprehensive coverage of aerosol measurement and documentary standards across different sub-disciplines, can serve as a reliable guide for scientists and regulators interested in improving the accuracy of their measurements.

Ultrafine soot particles emitted from combustion devices and biomass burning are a major particulate pollutant for human health and a major climate forcer. Unprecedented efforts have been made to understand the mechanism of soot formation and the physical, chemical, and optical properties of soot particles at different stages of maturity. Pulsed laser-induced incandescence (pLII) has become a powerful tool for in-situ measurements of soot volume fraction and primary particle size and to investigate the effects of pulsed laser irradiation on soot absorption properties. Experimental studies have confirmed that a high-power laser pulse can enhance the absorption of young soot particles through laser-induced annealing. Previous studies have ascribed the observed changes in soot absorption by pulsed laser irradiation to thermal annealing. In this study, a numerical study was conducted to model the effect of pulsed laser irradiation on the absorption efficiency of soot of different maturities to reproduce the results of a recent double-pulse pLII experiment. The numerical results based on thermal annealing models proposed in the literature failed to capture the enhanced peak LII signals of laser-heated young soot compared to those of un-preheated soot. By assuming the laser-induced annealing of soot particle is attributed to both thermal and photon mechanisms, the modified LII model can reproduce the experimentally observed enhancement in the peak LII signal of laser irradiated soot of different maturities. The findings of this study serve as indirect evidence to support the conjecture that the photon mechanism plays an important role in laser-induced annealing of young soot.

Graphene oxide (GO) particles have a wide and growing range of applications. They may also be converted to reduced graphene oxide (rGO) particles, which are increasingly used in energy storage devices like batteries and supercapacitors. However, the downstream functionality of GO and rGO particles depends strongly on their morphology, which is highly variable depending on the synthesis process. Here, we report morphological and optical properties for GO particles in the aerosol phase. These include the mobility diameter, effective density (0.93 +/- 0.06 g/cm3), mass-mobility exponent (2.97 +/- 0.06), Angstrom absorption exponent (2.48 between 370 nm and 950 nm), mass absorption cross-section (0.99 +/- 0.22 m2 g- 1), and mass scattering crosssection. Although this study reports measurements on re-aerosolized GO powder, the demonstrated techniques and measured properties serve as a foundation for in-reactor optical diagnostics that may allow for online control of the synthesis processes enabling the ability to control and characterize the functionality of downstream particles

Black carbon (BC) is a strongly absorbing component of atmospheric aerosols that has a significant warming effect. BC particles are emitted from combustion sources as open-structured fractal aggregates. After emission, BC is often compacted due to capillary condensation of semivolatile vapors to form coatings. The addition of coatings influences the size and radiative properties of BC, but representing these details in radiative transfer models is computationally difficult and often neglected. Laboratory studies have measured BC restructuring during coating but rarely provide information on changes in particle shape. Here, we combine laboratory measurements of BC compaction with detailed restructuring models to develop a framework for predicting the size and shape of BC as a function of coating volume ratio, a property already tracked in large-scale atmospheric models. The framework predicts the mobility diameter and fractal dimension of BC particles as a function of coating volume throughout compaction with root-mean-squared error (RMSE) values less than 6.8 and 4.3%, respectively. These properties are predicted for both the coated particle and the BC core. Our proposed framework will enable a more complete representation of the evolving size and shape of BC throughout its atmospheric lifetime, thereby improving model accuracy at a low computational cost.

Sodium chloride (NaCl) plays an important role both in the laboratory as a calibration aerosol and in nature as a component of sea-spray particles. Despite their ubiquity, NaCl particles show significant variation in their shape depending on the drying rate. This work builds on the current literature by establishing the influence of drying rate on the distribution of effective density and dynamic shape factor of laboratory generated NaCl particles. A calibration factor is first established using a spherical aerosol composed of Santovac (polyphenyl ether), to account for instrumental uncertainties. A total of three different drying rates were tested: -97 RH/s (slow), -260 RH/s (intermediate), and -506 RH/s (fast). The effective density and dynamic shape factor results (in the transition regime) shows that slow dried particles attained a distinct cube-like shape (rho eff approximate to 2200-1600 kg/m(3), chi approximate to 1-1.15), intermediate particles achieved much rounded corners while still showing some cube-like features (rho eff approximate to 2200-1800 kg/m(3), chi approximate to 1.00-1.08), and fast dried particles retained its spherical morphology (rho eff approximate to 2200-2000 kg/m(3), chi approximate to 1.00-1.02). The range of shapes observed is also influenced by particle size; smaller particles (<50 nm) were more spherical regardless of the drying rate. Additionally, bidimensional effective density analysis revealed shape variability within particles of the same size, suggesting a distribution of morphologies, especially for the slow drying case. These results are validated using TEM images at three approximately different mobility sizes - 100, 200 and 400 nm.Copyright (c) 2025 American Association for Aerosol Research

The average number of charges on non-spherical particles after charging is much higher than for spherical particles of the same mobility equivalent diameter. This makes it difficult to interpret measurements from instruments employing chargers like the electrical low-pressure impactor (ELPI+), which is often used to measure aerosols of varying shapes. Previous efforts to determine this difference have used non-spherical particles classified by a differential mobility analyzer (DMA). However, the presence of multiple charged particles introduces uncertainty, in part given that the multiple charged fractions from the classifier itself are impacted by particle morphology. To largely eliminate this issue, we classify the particles first with an Aerodynamic Aerosol Classifier (AAC) prior to classification with a DMA. Three different aerosols - polyethylene glycol (PEG - spherical), sodium chloride (NaCl - cube-like), and soot aggregates were tested. PEG and NaCl were generated using an atomizer while soot was produced from a laminar inverted flame with either ethylene or propane as the fuel. Both NaCl and soot particles showed a higher average charge than PEG. The charge carried by these particles is a function of the details of the charger, including penetration and ion concentration time, which is minimized by using charging equivalent diameter (dqe). We found this relation to be dqe(NaCl) = 1.106 dm1.014 and dqe(Soot) = 1.489 dm1.088 (where dm is mobility diameter in mu m).Copyright (c) 2025 American Association for Aerosol Research

The optical properties of soot are crucial in estimating its climate impact through direct radiative forcing. Soot light absorption is typically quantified by the mass absorption cross-section (MACλ) or the absorption function E(mλ), which are wavelength dependent. Light absorbed by soot can be predicted from its MACλ using mass-concentration measurements, or from its E(mλ) using material density and an optical model accounting for soot-aggregate morphology. Recent work has shown that the soot MACλ shows a size dependency, due to a size-dependent degree of graphitization. We therefore hypothesized here that a similar size dependency may be observed for E(mλ), which we quantify here. To test this hypothesis, we present a novel approach to obtain size-resolved MACλ and E(mλ) of soot from a gas turbine engine by combining pulsed laser-induced incandescence signals with total mass-concentration measurements. E(mλ) was found to vary with soot-particle size, with values ranging between 0.23 to 0.31 for the smallest (≈ 0.13 fg) and largest (≈ 3 fg) particles measured. To our knowledge, these measurements are the first to demonstrate that E(mλ) not only varies between soot samples, but also within a population of soot particles, which impacts the interpretation of optical diagnostics and prediction of the radiative properties of soot.

Non-contact methods are useful to improve the quality control of particle filtration media. The purpose of this paper is to investigate the correlation between the filtration efficiency of a porous sheet and its ultrasonic properties obtained using a non-contact technique. An air-coupled ultrasonic technique is used to obtain rapid measurements without affecting the integrity of the material. High frequencies (from 0.1 to 2.5 MHz) are used to improve technique sensitivity, and transmitted waves are measured to probe the internal properties of the material. Measurements of transmission coefficient spectra (amplitude and phase) and the corresponding ultrasound velocity and attenuation coefficient at different frequencies are obtained for a set of filtration media with well-characterized properties. Results show that the ultrasonic properties of filtration media vary as a function of basis weight, and therefore filtration efficiency, for a given charge state. However, the effect of electrostatic charge on ultrasonic propagation is almost negligible, as expected. We conclude that ultrasonic transmission may provide a valuable tool for the continuous online monitoring of material quality during fabrication and as a method to tease apart mechanical and electrostatic contributions to particle filtration.