Aerosol Classifier Research Articles

A New Method for Size‐Resolved Aerosol CCN Activity Measurement at Low Supersaturation in Pristine Atmosphere

AbstractUnderstanding aerosol‐cloud interactions under pristine atmospheric conditions is crucial for meaningful global climate change predictions. However, ability of aerosol particles to act as cloud condensation nuclei (CCN) activity of aerosols in pristine atmosphere remains limited, mainly because of the difficulty of measuring it in pristine environments. Given the extremely low CCN number concentration under low supersaturations (SSs) and the complex impact of the kinetic limitation in CCN measurements, size‐resolved CCN activity measurements at low SSs cannot be achieved with traditional methods using differential mobility analyzers and a CCN counter (CCNC) because of the low sample efficiency and the impact of particles with multiple charges. To overcome this difficulty, we propose a new method for measuring size‐resolved CCN activity at low aerosol number concentrations. The new method is based on a combined system comprising an aerodynamic aerosol classifier (AAC) and a CCNC (AAC‐CCNC system). A control program was developed to achieve high‐resolution scans of the SS and particle size within a reasonable time frame. A data inversion scheme, including corrections for the transfer function and kinetic limitation, was developed to obtain accurate size‐resolved CCN activity. In July‐August 2022, the new method was used to measure the size‐resolved CCN activity on the Tibetan Plateau. This new method can advance the size‐resolved CCN measurements, thereby enhancing our understanding of aerosol‐cloud interactions in pristine atmosphere.

A bipolar SMPS network for measuring atmospheric aerosols using natural air ions

To study atmospheric aerosols, it is essential to build networks that are continuously monitoring particle number size distribution (PNSD). PNSDs are often measured using electrical mobility spectrometers such as scanning mobility particle sizer (SMPS) which includes a neutralizer that conditions the charge state of particles to the pre-assumed distributions. During long-term atmospheric measurement, the charge state can deviate from these assumed values. Measuring both positively and negatively charged particles using a bipolar SMPS helps to capture the variations in aerosol charge state. In this study, we set up a network of bipolar SMPSs at three urban sites and three suburban sites in six Chinese cities. Removing the conventional neutralizer, the bipolar SMPS utilizes the natural aerosol charging process by atmospheric ions. Newly developed aerosol classifiers and detectors were used. Harmonized quality assurance procedures were adopted to assure the performance and the data quality, including laboratory calibrations and intercomparisons, maintenance and monitoring during field measurements, and data inversion and validation. PNSDs are effectively measured by bipolar SMPSs using natural charging process at all six sites. During six months of field measurement in these sites, ion mobility ratios fluctuate significantly while the daily averaged values are stable around 1.0. Particle number concentrations integrated from PNSDs are consistent with those directly measured by the detectors (within a variation of 10%).

Detailed Gaseous and Particulate Emissions of an Allison 250-C20B Turboshaft Engine

Abstract Aviation is known to be one of the most significant contributors to air pollutants. This includes gaseous emissions, like carbon dioxide (CO2) and nitrogen oxides (NOx), and also particulate matter (PM), especially in the form of soot. This study conducted emission measurements on an Allison 250-C20B turboshaft engine operating on Jet A-1 fuel with a focus on gaseous compounds (e.g., ozone precursors) and PM. The different engine loading points were chosen based on the percentage thrust ratios of the International Civil Aviation Organization LTO-Cycle. A standard FTIR/O2/FID system to measure general gaseous combustion compounds, e.g., CO2, carbon monoxide (CO), unburned hydrocarbons (UHC), and NOx. For the investigation of the volatile organic compounds (VOC), which are known to act as ozone precursors, a gas chromatograph was applied. Different measurement methods were used to characterize the PM emissions. For the particle size distribution (PSD), we used two types of electrical mobility analyzers and an aerodynamic aerosol classifier. All measurement systems yielded comparable PSD results, indicating reliable results. The particle measurement methods all show increasing aerosol diameter modes (electrical and aerodynamic) with increased engine loading. The aerosol diameter modes were shifting from 29 nm to 65 nm. The size and shape of different individual particles were evaluated with a scanning electron microscope. A correlation between the injection system and the particle formation was established. Gaseous turboshaft engine emissions show high CO and UHC values at Ground Idle level. NOx levels were the highest at Take-Off conditions. Acetylene and ethylene were the most significant contributors to ozone formation.

Measurement report: Size-resolved mass concentration of equivalent black carbon-containing particles larger than 700 nm and their role in radiation

Abstract. Black carbon (BC) mass size distribution (BCMSD) is crucial in both the environment and the climate system due to BC's intense size-dependent absorption of solar radiation. BC-containing particles larger than 700 nm (BC>700) could contribute to more than half of bulk BC mass concentration. Unfortunately, previous methods concentrated on BC-containing particles of less than 700 nm because of technical limitations. The contribution of BC to absorption and the radiative effect might be underestimated without consideration of BC>700. In this study, equivalent BCMSD (eBCMSD) from 150 nm up to 1.5 µm was measured at high time resolution of 1 h for the first time by an aerodynamic aerosol classifier in tandem with an aethalometer in two field campaigns over eastern China, namely Changzhou located in the Yangtze River Delta and Beijing located in the North China Plain. The results revealed that the pattern of eBCMSD in Changzhou (Beijing) was mostly bimodal (unimodal) peaking at 240 and 1249 nm (427 nm). The peak diameter of eBCMSD in Changzhou did not shift significantly with increasing pollution (240 to 289 nm). In contrast, the peak diameter of eBCMSD in Beijing shifted towards larger sizes, from 347 to 527 nm, with increasing pollution, indicating that the aging process at the urban site was different from that at the regional background site. eBCMSD in both Changzhou and Beijing had a significant diurnal cycle with a smaller (larger) value of eBCMSD during daytime (nighttime). Equivalent BC>700 (eBC>700) was ubiquitous and varied significantly with different locations and pollution levels. The campaign-averaged contribution of eBC>700 to bulk eBC mass concentration (meBC,bulk), bulk absorption coefficient (σab,bulk) and estimated direct radiative forcing of eBC (DRFeBC) in Changzhou and Beijing were 27.8 % (20.9 %–36.5 %) and 24.1 % (17.5 %–34.2 %), 19.6 % (15.8 %–24.6 %) and 25.9 % (19.6 %–33.7 %) and 20.5 % (18.4 %–22.2 %) and 21.0 % (16.3 %–26.1 %), respectively. meBC,bulk, σab,bulk and DRFeBC of eBC>700 in Changzhou (Beijing) increased by a factor of 3.6 (5.1) from 0.11 (0.07) to 0.40 (0.36) µg m−3, by a factor of 3.2 (5.5) from 0.54 (0.63) to 1.75 (3.45) Mm−1 and by a factor of 2.4 (4.7) from 0.1 (0.1) to 0.24 (0.47) W m−2, respectively, with the aggravation of pollution. The contribution of eBC>700 to meBC,bulk and σab,bulk had a significant diurnal cycle with a higher (lower) fraction during daytime (nighttime) in both Changzhou and Beijing. A case study indicated that the contribution of eBC>700 to meBC,bulk, σab,bulk and DRFeBC could reach up to 50 %, 50 % and 40 %, respectively. It was highly recommended to consider the whole size range of BC-containing particles in model estimation of the BC radiative effect.

Size-resolved effective density of ambient aerosols measured by an AAC–SMPS tandem system in Beijing

The effective density is an essential physical property of atmospheric aerosols. The study of the effective density helps characterize the aerodynamic behavior of aerosol particles, providing vital information on the chemical composition, mixing state, and aging process of the particles. Size-resolved effective density measurements based on the Aerodynamic Aerosol Classifier–Scanning Mobility Particle Sizer (AAC–SMPS) tandem system were conducted in Beijing from March 10 to 31, 2023. This study analyzed the relationship between the aerodynamic diameter and effective density and the characteristics of the effective density under different pollution conditions. During the entire observation period, the average effective densities for the 150-, 250-, 350-, 450-, and 550-nm particles were 1.48 ± 0.07, 1.59 ± 0.08, 1.69 ± 0.09, 1.73 ± 0.09, and 1.75 ± 0.13 g/cm3, respectively, i.e., the effective density was highly particle size-dependent, exhibiting higher values at larger aerodynamic diameters. The size-resolved effective density under different pollution conditions differed and was related to the air mass variations. Under dusty conditions, the effective density of particles was highest under the influence of dust air masses from the northern areas. The effective density of small particles (150–350 nm) of the northern clean air masses was lower than that of the southern polluted air masses; however, the opposite was observed for large particles (450–550 nm). The diurnal effective density variation was generally more pronounced during daytime. Under both clean and dusty conditions, there was a sudden effective density decrease at 150 nm during the evening peak traffic. Under the influence of new particle formation (NPF) events, a significant effective density decrease was observed when the accumulation mode began to grow.

Mixing state and effective density of aerosol particles during the Beijing 2022 Olympic Winter Games

Abstract. Mixing state and density are two key parameters of aerosol particles affecting their impacts on radiative forcing and human health. Here a single-particle aerosol mass spectrometer in tandem with a differential mobility analyzer and an aerodynamic aerosol classifier was deployed during the Beijing 2022 Olympic Winter Games (OWG) to investigate the impacts of emission controls on particle mixing state and density. Our results show the dominance of carbonaceous particles comprising mainly total elemental carbon (Total-EC, 13.4 %), total organic carbon (Total-OC, 10.5 %) and Total-ECOC (47.1 %). Particularly, the particles containing organic carbon and sulfate were enhanced significantly during OWG, although those from primary emissions decreased. The composition of carbonaceous particles also changed significantly which was characterized by the decreases in EC mixed with nitrate and sulfate (EC-NS), EC mixed with potassium nitrate (KEC-N), and amine-containing particles and increase in ECOC mixed with nitrate and sulfate (ECOC-NS). This result indicates that emission controls during OWG reduced the mixing of EC with inorganic aerosol species and amines yet increased the mixing of EC with organic aerosol. The average effective density (ρeff) of aerosol particles (150–300 nm) was 1.15 g cm−3 during the non-Olympic Winter Games (nOWG), with higher values during OWG (1.26 g cm−3) due to the increase in secondary particle contribution. In addition, the ρeff of most particles increased with the increases in pollution levels and relative humidity, yet they varied differently for different types of particles, highlighting the impacts of aging and formation processes on the changes of particle density and mixing state.

Tandem aerodynamic aerosol classifier – differential mobility analyzer as a single charged aerosol source for diameters up to one micrometer

This work describes a method to generate monodisperse, spherical, single-charged aerosols for instrument calibration in the size range of 32 – 1000 nm using an aerodynamic aerosol classifier (AAC) and differential mobility analyzer (DMA) in series. The AAC is used to input a monodisperse aerosol into the DMA to reduce the fraction of larger multiple charged particles exiting the DMA. The method produces test aerosols over a wide size range with less than 1% multiple charging and in concentrations between 4,000 – 10,000 particles/cm3 at more than 2 liters/minute flow rate, sufficient to be measured with the aerosol electrometer and condensation particle counter (CPC). At diameters between 32 nm and 1000 nm, the multiple charging was less than 0.1%. Operating parameters for the AAC and DMA are provided, along with CPC detection efficiencies up to 1000 nm measured with traceability to the international system of units. Use of the AAC allows for calibrations to be performed over almost the full particle diameter range of the DMA with an easily reproduced experimental setup using commercially available equipment.

Aerosol charge neutralization and its impact on particle filtration efficiency of common face mask materials

Despite the significant increase in research on mask filtration testing since the COVID-19 pandemic, there remains considerable ambiguity regarding which parameters affect particle filtration efficiency (PFE) and how differences in standard testing protocols can lead to divergent PFE values. We evaluated the PFE (and differential pressure) of several common face masks and community face mask materials including woven cotton, spunbond polypropylene, and meltblown polypropylene, testing in accordance with ASTM F2100/2299 standards for medical masks, using polystyrene latex (PSL) aerosol, as well as NIOSH standards for respirators, using NaCl aerosol. In both cases, PFE was measured with and without aerosol charge neutralization, which is used to bring the particle population to a known, equilibrium bipolar charge distribution. Aerosols of either composition that were untreated (not neutralized) led to significant increases in measured PFE, especially in the case of PSL. In contrast, effective neutralization led to lower PFE measurements that also showed little to no dependence on aerosol composition across most materials. To investigate further, the bipolar charge distributions of PSL and NaCl aerosols, both neutralized and untreated, were characterized using an aerodynamic aerosol classifier operated in tandem with a scanning mobility particle sizer (AAC-SMPS). This technique illustrated the differences in the distribution of particle charge states between PSL and NaCl aerosols of the same size, and between PSL particles of different sizes, revealing the presence of highly charged particles in many cases. Most importantly, the equilibrium charge distribution after neutralization is shown to be independent of particle composition or initial charge distribution, highlighting the crucial role of aerosol charge neutralizers in preventing overestimates of mask performance (due to electrostatic effects) and promoting consistency in standard testing procedures.

Size–resolved mixing state of ambient refractory black carbon aerosols in Beijing during the XXIV Olympic winter games

Atmospheric loading of black carbon aerosols influences air quality and visibility, and their mixing state and aerosol chemistry were sensitive to both emission scenarios and regional transport. In this study, the aerodynamic size-resolved mixing state of refractory black carbon aerosols (rBC) was investigated using a novel tandem system consisting of an aerodynamic aerosol classifier (AAC), a single particle soot photometer (SP2), and single-particle-aerosol mass spectrometry (SPA-MS) during the XXIV Olympic Winter Games (OWG) at an urban site in Beijing. Particles with aerodynamic diameter (Dae) of 200 nm and 300 nm were selected by AAC to focus on the effect of relative differences in morphology on the coating thickness, coating compositions, and optical properties of rBC particles. We found that rBC-containing particles with Dae of 300 nm were characterized by a rBC core with a count median diameter (CMD) of 108 ± 4 nm that typically have regular morphology (χ:mean∼1), higher relative coating thickness (RCT: mean ∼1.9), and strong light absorption enhancement (Eabs: mean ∼1.5). The rBC-containing particles with Dae = 200 nm normally had irregular shapes (χ:mean∼1.4) and weak Eabs (Eabs: mean ∼1.37). Classification based on air mass clustering and air pollution level revealed that the coating thickness of rBC particles only exhibited unimodal patterns under clean air conditions and distinct bimodal distributions under heavy pollution conditions, implying multiple origins of rBC particles, even during OWG periods. The coating thickness and Eabs of rBC particles were greater when influenced by the higher RH air from east. Furthermore, the chemical compositions of the rBC coating quantitatively determined by SPA-MS were used to estimate the possible origins of the Dae = 300 nm, featured by a thicker coating thickness and stronger light absorption enhancement. Our results showed that rBC particles were primarily mixed with organics, nitrate, and sulfate during the XXIV OWG period. The organic components have a limited role in increasing rBC coatings under polluted conditions, while the alkali metal ions (K and Na) associated with traffic emissions and the secondary inorganic species favor the formation of thick coatings. Higher RH makes limited contributions to rBC mixed with sulfate and organics only (BCOC*_S) and pure BC. The relatively fresh BC could directly mix with organics and sulfate at low RH levels while evolving to mix with nitrate at high RH conditions. In addition to high concentrations of locally emitted precursors and high RH that tend to form thick coatings of nitrate-related rBC aerosols, secondary transport from the west and southwest also contributes to rBC aerosols mixed with nitrate with relatively thick coatings.

Characterisation of the Aerodynamic Aerosol Classifier Transfer Function for Particle Sizes up to 5 Micrometres

Characterisation of the Aerodynamic Aerosol Classifier Transfer Function for Particle Sizes up to 5 Micrometres

New method to determine black carbon mass size distribution

Abstract. Black carbon (BC) is an important atmospheric component with strong light absorption. Many attempts have been made to measure BC mass size distribution (BCMSD) for its significant impact on climate and public health. Larger-coverage BCMSD, ranging from upper submicron particles sizes to larger than 1 µm, contributes to a substantial proportion of total BC mass and absorption. However, the current time resolution of larger-coverage BCMSD measurement was limited to 1 d, which was insufficient to characterize variation of larger-coverage BCMSD. In this study, a new method to determine equivalent BCMSD (eBCMSD) was proposed from size-resolved absorption coefficient measured by an aerodynamic aerosol classifier in tandem with an aethalometer. The proposed method could measure larger-coverage eBCMSD with a time resolution as high as 1 h and was validated by comparing the measurement results with refractory BCMSD (rBCMSD) measured by a differential mobility analyzer in tandem with a single-particle soot photometer (DMA–SP2) for particle sizes larger than 300 nm during a field measurement in the Yangtze River Delta. Bulk refractory BC mass concentration (mrBC,bulk) by DMA–SP2 was underestimated by 33 % compared to bulk equivalent BC mass concentration (meBC,bulk) by this method because of the limited size range of measurement for DMA–SP2. Uncertainty analysis of this method was performed with respect to mass absorption cross-section (MAC), transfer function inversion, number fraction of BC-containing particle and instrumental noise. The results indicated that MAC was the main uncertainty source, leading to meBC,bulk values that varied from −20 % to 28 %. With the advantage of a wide size coverage up to 1.5 µm, high time resolution, easy operation and low cost, this method is expected to have wide applications in field measurement for better estimating the radiative properties and climate effects of BC.

Characterization of tandem aerosol classifiers for selecting particles: implication for eliminating the multiple charging effect

Abstract. Accurate particle classification plays a vital role in aerosol studies. Differential mobility analyzers (DMAs), centrifugal particle mass analyzers (CPMAs) and aerodynamic aerosol classifiers (AACs) are commonly used to select particles with a specific mobility diameter, aerodynamic diameter or mass, respectively. However, multiple charging effects cannot be entirely avoided when using either individual techniques or tandem systems such as DMA–CPMA, especially when selecting soot particles with fractal structures. In this study, we calculate the transfer functions of the DMA–CPMA and DMA–AAC in static configurations for flame-generated soot particles. We propose an equation that constrains the resolutions of the DMA and CPMA to eliminate the multiple charging effect when selecting particles with a certain mass–mobility relationship using the DMA–CPMA system. The equation for the DMA–AAC system is also derived. For DMA–CPMA in a static configuration, our results show that the ability to remove multiply charged particles mainly depends on the particle morphology and resolution settings of the DMA and CPMA. Using measurements from soot experiments and literature data, a general trend in the appearance of the multiple charging effect with decreasing size when selecting aspherical particles is observed. As for DMA–AAC in a static configuration, the ability to eliminate particles with multiple charges is mainly related to the resolutions of the classifiers. In most cases, the DMA–AAC in a static configuration can eliminate the multiple charging effect regardless of the particle morphology, but multiply charged particles will be selected when decreasing the resolution of the DMA or AAC. We propose that the potential influence of the multiple charging effect should be considered when using the DMA–CPMA or DMA–AAC systems in estimating size- and mass-resolved optical properties in field and lab experiments.

Cloud condensation nuclei (CCN) activity analysis of low-hygroscopicity aerosols using the aerodynamic aerosol classifier (AAC)

Abstract. The aerodynamic aerosol classifier (AAC) is a novel instrument that size-selects aerosol particles based on their mechanical mobility. So far, the application of an AAC for cloud condensation nuclei (CCN) activity analysis of aerosols has yet to be explored. Traditionally, a differential mobility analyzer (DMA) is used for aerosol classification in a CCN experimental setup. A DMA classifies particles based on their electrical mobility. Substituting the DMA with an AAC can eliminate multiple-charging artifacts as classification using an AAC does not require particle charging. In this work, we describe an AAC-based CCN experimental setup and CCN analysis method. We also discuss and develop equations to quantify the uncertainties associated with aerosol particle sizing. To do so, we extend the AAC transfer function analysis and calculate the measurement uncertainties of the aerodynamic diameter from the resolution of the AAC. The analysis framework has been packaged into a Python-based CCN Analysis Tool (PyCAT 1.0) open-source code, which is available on GitHub for public use. Results show that the AAC size-selects robustly (AAC resolution is 10.1, diffusion losses are minimal, and particle transmission is high) at larger aerodynamic diameters (≥∼ 85 nm). The size-resolved activation ratio is ideally sigmoidal since no charge corrections are required. Moreover, the uncertainties in the critical particle aerodynamic diameter at a given supersaturation can propagate through droplet activation, and the subsequent uncertainties with respect to the single-hygroscopicity parameter (κ) are reported. For a known aerosol such as sucrose, the κ derived from the critical dry aerodynamic diameter can be up to ∼ 50 % different from the theoretical κ. In this work, we do additional measurements to obtain dynamic shape factor information and convert the sucrose aerodynamic to volume equivalent diameter. The volume equivalent diameter applied to κ-Köhler theory improves the agreement between measured and theoretical κ. Given the limitations of the coupled AAC–CCN experimental setup, this setup is best used for low-hygroscopicity aerosol (κ≤0.2) CCN measurements.

A comparative study on effective density, shape factor, and volatile mixing of non-spherical particles using tandem aerodynamic diameter, mobility diameter, and mass measurements

Combustion-generated particles are typically non-spherical (soot aggregates) and sometimes mixed with organic compounds (e.g. in vehicle emissions). The effective density, dynamic shape factor, and volatile mixing of particles are widely studied using aerosol instruments that measure the particle mobility diameter, aerodynamic diameter, and mass. In theory, any of these three physical properties can be obtained from a combination of the other two. In the present study, a tandem arrangement of aerodynamic aerosol classifier (AAC; measuring aerodynamic diameter), differential mobility analyzer (DMA; measuring mobility diameter), optional catalytic stripper (CS), and centrifugal particle mass analyzer (CPMA; measuring particle mass) was used to study the effective density, dynamic shape factor, and volatile mixing of non-spherical non-homogenous particles. In terms of mass, the vast majority of the particles were purely semi-volatile mixed with soot with and without semi-volatile coating. The effective density of polydisperse non-stripped particles was relatively constant (indicating nearly spherical particles), while that of polydisperse stripped particles decreased from ∼1200 to ∼800 kg/m3 as the particle size increased (indicating a compact structure). The effective density of monodisperse particles, measured by DMA-CPMA, AAC-DMA, and AAC-CPMA methods, was consistent within the measurement uncertainty; however, the latter method had larger discrepancy with the other two methods, particularly for non-spherical particles. The dynamic shape factor, measured by AAC-CPMA and DMA-CPMA methods, increased with the mobility diameter, a trend also supported by electron micrographs. The volatile mass fraction of particles decreased as their mobility diameter increased, with smaller particles having volatile mass fraction of ∼20%. This result was further confirmed by chemical characterization of size-selected particles, proving the robustness of online aerosol measurements.

Physical and chemical properties of black carbon and organic matter from different combustion and photochemical sources using aerodynamic aerosol classification

Abstract. The physical and chemical properties of black carbon (BC) and organic aerosols are important for predicting their radiative forcing in the atmosphere. During the Soot Aerodynamic Size Selection for Optical properties (SASSO) project and a EUROCHAMP-2020 transnational access project, different types of light-absorbing carbon were studied, including BC from catalytically stripped diesel exhaust, an inverted flame burner, a colloidal graphite standard (Aquadag) and controlled flaming wood combustion. Brown carbon (BrC) was also investigated in the form of organic aerosol emissions from wood burning (pyrolysis and smouldering) and from the nitration of secondary organic aerosol (SOA) proxies produced in a photochemical reaction chamber. Here we present insights into the physical and chemical properties of the aerosols, with optical properties presented in subsequent publications. The dynamic shape factor (χ) of BC particles and material density (ρm) of organic aerosols was investigated by coupling a charging-free Aerodynamic Aerosol Classifier (AAC) with a Centrifugal Particle Mass Analyzer (CPMA) and a Scanning Mobility Particle Sizer (SMPS). The morphology of BC particles was captured by transmission electron microscopy (TEM). For BC particles from the diesel engine and flame burner emissions, the primary spherule sizes were similar, around 20 nm. With increasing particle size, BC particles adopted more collapsed/compacted morphologies for the former source but tended to show more aggregated morphologies for the latter source. For particles emitted from the combustion of dry wood samples, the χ of BC particles and the ρm of organic aerosols were observed in the ranges 1.8–2.17 and 1.22–1.32 g cm−3, respectively. Similarly, for wet wood samples, the χ and ρm ranges were 1.2–1.85 and 1.44–1.60 g cm−3, respectively. Aerosol mass spectrometry measurements show no clear difference in mass spectra of the organic aerosols in individual burn phases (pyrolysis or smouldering phase) with the moisture content of the wood samples. This suggests that the effect moisture has on the organic chemical profile of wood burning emissions is through changing the durations of the different phases of the burn cycle, not through the chemical modification of the individual phases. In this study, the incandescence signal of a Single Particle Soot Photometer (SP2) was calibrated with three different types of BC particles and compared with that from an Aquadag standard that is commonly used to calibrate SP2 incandescence to a BC mass. A correction factor is defined as the ratio of the incandescence signal from an alternative BC source to that from the Aquadag standard and took values of 0.821 ± 0.002 (or 0.794 ± 0.005), 0.879 ± 0.003 and 0.843 ± 0.028 to 0.913 ± 0.009 for the BC particles emitted from the diesel engine running under hot (or cold idle) conditions, the flame burner and wood combustion, respectively. These correction factors account for differences in instrument response to BC from different sources compared to the standardised Aquadag calibration and are more appropriate than the common value of 0.75 recommended by Laborde et al. (2012b) when deriving the mass concentration of BC emitted from diesel engines. Quantifying the correction factor for many types of BC particles found commonly in the atmosphere may enable better constraints to be placed on this factor depending on the BC source being sampled and thus improve the accuracy of future SP2 measurements of BC mass concentrations.

Online shape and density measurement of single aerosol particles

Aerosols play crucial roles in affecting air quality, climate and human health. Particle morphology and density would provide critical information on the chemical composition, mixing state and aging process of particles. Many investigations used dynamic shape factor (χ) to quantify particle shape. However, the measurement for χ often requires the knowledge of particle density, which is usually not readily available for atmospheric aerosols. Besides, χ is not a pure geometric descriptor, as it is also dependent on Knudsen number (Kn). Here, a method, consisting of an aerodynamic aerosol classifier (AAC, aerodynamic particle diameter, Da, measurement), a differential mobility analyzer (DMA, electrical mobility particle diameter, Dm, measurement) and a single particle aerosol mass spectrometer (SPAMS, vacuum aerodynamic particle diameter, Dva, measurement), was developed to investigate two geometric descriptors that are solely dependent on particle geometry: the orientationally averaged projected area (AP) and the hydrodynamic radius (RH). The shape indicator, S, was defined as the square root of the ratio AP/πRH2. To test the performance of the AAC-DMA-SPAMS system, the S and χ of 9 types of laboratory-generated particles with known density were measured with this system, and the quantitative relationship between χ (1<χ<1.05) and S (1<S<1.5) was established at room temperature (T≈300K) and ambient condition. Finally, this method was applied to investigate the S of ambient aerosols in an urban area. A significant fraction of near-spherical aerosols was observed. Thus, their densities can be calculated. Generally, the AAC-DMA-SPAMS system can provide shape, density and the mass spectral information of ambient particle, which can help elucidate their mixing state and aging processes.

The nano-scanning electrical mobility spectrometer (nSEMS) and its application to size distribution measurements of 1.5–25 nm particles

Abstract. Particle size measurement in the low nanometer regime is of great importance to the study of cloud condensation nuclei formation and to better understand aerosol–cloud interactions. Here we present the design, modeling, and experimental characterization of the nano-scanning electrical mobility spectrometer (nSEMS), a recently developed instrument that probes particle physical properties in the 1.5–25 nm range. The nSEMS consists of a novel differential mobility analyzer and a two-stage condensation particle counter (CPC). The mobility analyzer, a radial opposed-migration ion and aerosol classifier (ROMIAC), can classify nanometer-sized particles with minimal degradation of its resolution and diffusional losses. The ROMIAC operates on a dual high-voltage supply with fast polarity-switching capability to minimize sensitivity to variations in the chemical nature of the ions used to charge the aerosol. Particles transmitted through the mobility analyzer are measured using a two-stage CPC. They are first activated in a fast-mixing diethylene glycol (DEG) stage before being counted by a second detection stage, an ADI MAGIC™ water-based CPC. The transfer function of the integrated instrument is derived from both finite-element modeling and experimental characterization. The nSEMS performance has been evaluated during measurement of transient nucleation and growth events in the CLOUD atmospheric chamber at CERN. We show that the nSEMS can provide high-time- and size-resolution measurement of nanoparticles and can capture the critical aerosol dynamics of newly formed atmospheric particles. Using a soft x-ray bipolar ion source in a compact housing designed to optimize both nanoparticle charging and transmission efficiency as a charge conditioner, the nSEMS has enabled measurement of the contributions of both neutral and ion-mediated nucleation to new particle formation.

Different characteristics of individual particles from light-duty diesel vehicle at the launching and idling state by AAC-SPAMS

The rapid development of cities and economic prosperity greatly motivates the growth of vehicular exhaust particles, especially the diesel-exhausted particles from the large fleet of passenger and freight, which present profound implications on climate, air quality, and biological health (e.g., pulmonary, autoimmune and cardiovascular diseases). As important physiochemical properties of atmospheric aerosols, however, the mixing state and effective density of individual particles emitted from diesel-powered vehicles under different driving conditions and their environmental implications remain uncertain. Here, a single-particle aerosol mass spectrometer (SPAMS) was used to investigate the chemical composition and vacuum aerodynamic diameter (Dva), along with the aerodynamic diameter (Da) from an aerodynamic aerosol classifier (AAC), to determine the effective density of primary particles emitted from a light- duty diesel vehicle (LDDV) under the launching and idling engine states. Interestingly, the particle types and effective density appear to vary significantly with the engine status. A single particle type of Ca-rich particles, named Na-Ca-PAH, was predominant in the idling state, whose chemical components may be affected by the lubricants and incomplete combustion, contributing to a higher effective density (0.66 ± 0.21 g cm−3). In contrast, launching particles exhibited a lower effective density (0.34 ± 0.17 g cm−3) because of the substantial elemental carbon (EC). In addition, the effective density depends not only on the particle size but also on the chemical components with various abundances. EC and Ca play opposite roles in the effective density of LDDV emissions. Notably, a higher proportion of polycyclic aromatic hydrocarbons (PAHs) was observed in the idling particles, contributing to 78 ± 1.2%. Given the high contribution to these PAH-containing particles in the idling state, indispensable precautions should be taken at bus stops or waiting for pedestrians. This study provides more comprehensive insights into the initial characteristics of LDDV particles due to the launching and idling states, which is beneficial for improving the model results of source apportionment and understanding its environmental behavior regarding human health.

Measurement of Density and Shape for Single Black Carbon Aerosols in a Heavily Polluted Urban Area

Black carbon (BC) aerosol imposes adverse effects on atmospheric visibility, climate, and health. The particle density and morphology are often needed to investigate the mixing state and aging process of BC particles. A method, combining an aerodynamic aerosol classifier (AAC), a differential mobility analyzer (DMA), a single-particle soot photometer (SP2) and a single particle aerosol mass spectrometer (SPAMS), was developed to determine the density and dynamic shape factor (χ) of ambient BC particles with three different aerodynamic diameters (Da, 200 nm, 350 nm, and 500 nm) in Shanghai, China, a typical urban area. The BC particles were either classified as “BC-dominated particle” which is mainly made of black carbon or “BC-mixed particle” which is a mixture of both BC and non-BC substances. The results show that BC-dominated particles whose BC core mass (~2.2 fg) was almost equal to particle mass (~2.3 fg) were observed in particles with 200 nm Da. The morphology of these BC-dominated particles was near-spherical (χ ≈ 1.02), indicating that they had undergone rapid morphology modification from the initial highly irregular morphology to near-spherical shape. Most BC particles with 350 nm or 500 nm Da were BC-mixed particles. Combining the effective densities (1.62~1.77 g cm-3) and average single particle mass spectra of particle, the ammonium sulfate and ammonium nitrate were found to be the main secondary substances of these BC-mixed particles, indicating that condensation of inorganic species such as nitrates and sulfates could play a significant role in the aging process of fresh BC in Shanghai. Generally, the morphology and density information of single BC particle is crucial to identify the mixing state and aging process of BC aerosols.

Generating an aerosol of homogeneous, non-spherical particles and measuring their bipolar charge distribution

The Aerodynamic Aerosol Classifier (AAC) and Differential Mobility Analyzer (DMA) are aerosol classifiers commonly used to generate a monodispersed aerosol by selecting particles within a narrow range of relaxation times or electrical mobilities, respectively. However, generating an aerosol of homogeneous particles, which has narrow ranges of particle mass, mobility and relaxation time simultaneously, with either of these methods is challenging. Particles classified by the DMA are often not homogeneous (or monodispersed) due to multiply-charged particles. While the AAC overcomes this challenge for spherical particles, homogeneity is not achieved with non-spherical particles due to their effective density varying with particle size. This study demonstrates using an AAC and DMA in tandem to generate an aerosol of homogeneous, non-spherical particles. This approach is validated using scanning electron microscope (SEM) images and electrical mobility measurements of the tandem-classified particles to highlight their homogeneity. To limit the effects of multiple charging during DMA classification, only a subset of DMA and AAC setpoints are permitted. While this subset is not representative of “average” non-spherical particles from the same aerosol source, this subset of low-density particles deviates the most from spherical morphology, and thus, provides insights into the upper bound of other particle properties, such as charging. Using this approach to select homogeneous particles, the bipolar charge distribution of low-density soot aggregates is then measured using another DMA. This AAC-DMA-DMA approach is demonstrated to measure up to 17 individual charge states (i.e. −8 to +8) after neutralization (with 85Kr) of size-resolved, soot aggregates with mobility diameters between 80 and 433 nm. The low-density soot aggregates obtain higher charges than predicted by theory, which overestimates the uncharged fraction (by 0.042–0.069) and, to a lesser extent, the single charge fractions (by up to 0.037) of the low-density soot aggregates, while underestimating their proportion of multiple charging (by up to 0.135 cumulatively at one particle size or up to 0.039 at one multiple charge state and size). These charging discrepancies represent an upper bound of the bipolar charging of average aggregates from the same source of flame soot.