Centrifugal Particle Mass Analyzer Research Articles

Dilution versus fractionation: Separation technologies hyphenated with spICP-MS for characterizing metallic nanoparticles in aerosols

The presence of metal salts has become one of the major limitations for measuring metallic nanoparticles (NPs) in single particle inductively coupled plasma mass spectrometry (spICP-MS). Their presence generates a background signal in spICP-MS that can be overlapped with the signal intensity of small particles, leading to inaccurate NP size distributions. To overcome this analytical problem, sample pretreatment methods (e.g. dilution or fractionation) have been applied to liquid samples before spICP-MS measurements to minimize the ionic interference. However, the number of studies focused on reducing the presence of metal salts in aerosol characterization is limited. In this contribution, we investigated three different technologies coupled to an ICP-MS for online separating metallic NPs signals from ionic interference signals of metal salts in the aerosol. A rotating disk diluter (RDD) was used for the online dilution of the aerosol, while a differential mobility analyzer (DMA) and a centrifugal particle mass analyzer (CPMA) were used for the online fractionation of specific-sized NPs in the aerosol. The results from the analysis of 100 nm gold NPs (AuNPs) mixed with gold salts (Au3+, mass ratio 1:25) showed the particle size limit of detection decreased from 78 nm to 61, 50, and 33 nm by using RDD, CPMA, and DMA respectively. In addition, it was found that the separation performance of AuNPs was in the order of DMA > RDD > CPMA. The methods used in this study based on spICP-MS have the potential to characterize directly NPs in complex aerosols containing metal salts.

Characterization of size-resolved effective density of atmospheric particles in an urban atmosphere in Southern China

Effective density (ρeff) is one of the most important physical properties of atmospheric particles, providing important references in exploring the emissions and aging processes of fresh particles. In this study, a combined system of differential mobility analyzer, centrifugal particle mass analyzer, and condensation particle counter was used to periodically measure the ρeff of atmospheric particles in Shenzhen from Oct. 2021 to Jan. 2022. Results showed that the ρeff of particles with various size presented a bimodal distribution, which could be divided into main density (ρm, main peak, corresponding to relatively dense particles after aging) and sub density (ρs, sub peak, corresponding to fresh particles). The occurrence frequencies of ρs of particles with diameters of 50 and 80 nm were less than 20%, but were as high as about 40% of that with diameters from 120 to 350 nm. The ρm showed increasing trend with the size of particles, while ρs decreased as the increasing of the size of particles. The ρeff on pollution day varied significantly with chemical compositions. The increasing of the proportion of sulfate could promote the increasing of ρeff, while black carbon and organic matter caused opposite effects, which may be related to various factors, including the difference of the material density and morphology of various chemical components. The ρeff of 50, 80 and 120 nm particles decreased considerably during the new particle formation event, indicating that organic condensation was an important contributor to new particle growth.

High-accuracy effective density measurements of sodium methanesulfonate and aminium chloride nanoparticles using a particulate calibration standard

Methanesulfonate and aminium salts are commonly found in ambient nanoparticles and are often used as calibration standards. However, the effective densities of the particles generated from these standards are required if they are to be used to calibrate aerosol mass spectrometers or to estimate hygroscopic growth factors from electrodynamic balance experiments. A centrifugal particle mass analyzer and scanning mobility particle sizer were used in tandem (CPMA-SMPS) to measure the effective density of five salts. The effective densities were determined to be: sodium methanesulfonate, 1474 ± 13 kg m−3; methylammonium chloride, 1236 ± 29 kg m−3, monoethanolamine hydrochloride, 1136 ± 26 kg m−3; 1,4-diaminobutane dihydrochloride, 1135 ± 33 kg m−3; and NaCl was found to have a size-dependent effective density due to its non-spherical shape. It is also shown how Santovac® vacuum pump oil can be used to accurately calibrate the CPMA-SMPS system for particles less than 100 nm in diameter, which is a size range that has been problematic for calibration techniques relying on polystyrene latex spheres.Copyright © 2023 American Association for Aerosol Research

Laboratory studies of ice nucleation onto bare and internally mixed soot–sulfuric acid particles

Abstract. Soot particles are potential candidates for ice-nucleating particles in cirrus cloud formation, which is known to exert a net-warming effect on climate. Bare soot particles, generally hydrophobic and fractal ones, mainly exist near emission sources. Coated or internally mixed soot particles are more abundant in the atmosphere and have a higher probability of impacting cloud formation and climate. However, the ice nucleation ability of coated soot particles is not as well understood as that of freshly produced soot particles. In this laboratory study, two samples, a propane flame soot and a commercial carbon black, were used as atmospheric soot surrogates and coated with varying wt % of sulfuric acid (H2SO4). The ratio of coating material mass to the mass of bare soot particles was controlled and progressively increased from less than 5 wt % to over 100 wt %. Both bare and coated soot particle ice nucleation activities were investigated with a continuous-flow diffusion chamber operated at mixed-phase and cirrus cloud conditions. The mobility diameter and mass distribution of size-selected soot particles with/without H2SO4 coating were measured by a scanning mobility particle sizer and a centrifugal particle mass analyser running in parallel. The mixing state and morphology of soot particles were characterized by scanning electron microscopy and transmission electron microscopy. In addition, the evidence of the presence of H2SO4 on a coated soot particle surface is shown by energy-dispersive X-ray spectroscopy. Our study demonstrates that H2SO4 coatings suppress the ice nucleation activity of soot particles to varying degrees depending on the coating thickness, but in a non-linear fashion. Thin coatings causing pore filling in the soot aggregate inhibits pore condensation and freezing. Thick coatings promote particle ice activation via droplet homogeneous freezing. Overall, our findings reveal that H2SO4 coatings will suppress soot particle ice nucleation abilities in the cirrus cloud regime, having implications for the fate of soot particles with respect to cloud formation in the upper troposphere.

Bimodal distribution of size-resolved particle effective density: results from a short campaign in a rural environment over the North China Plain

Abstract. Effective density is one of the most important physical properties of atmospheric particles. It is closely linked to particle chemical composition and morphology and could provide special information on particle emissions and aging processes. In this study, size-resolved particle effective density was measured with a combined differential mobility analyzer–centrifugal particle mass analyzer–condensation particle counter (DMA–CPMA–CPC) system in autumn 2019 as part of the Multiphase chemistry experiment in Fogs and Aerosols in the North China Plain (McFAN). With a newly developed flexible Gaussian fit algorithm, frequent (77 %–87 %) bimodal distribution of particle effective density is identified, with a low-density mode (named sub-density mode) accounting for 22 %–27 % of the total number of observed particles. The prevalence of the sub-density mode is closely related to fresh black carbon (BC) emissions. The geometric mean for the main density mode (ρ‾eff,main) increases from 1.18 ± 0.10 g cm−3 (50 nm) to 1.37 ± 0.12 g cm−3 (300 nm) due to a larger fraction of high-density components and a more significant restructuring effect at large particle sizes but decreases from 0.89 ± 0.08 g cm−3 (50 nm) to 0.62 ± 0.12 g cm−3 (300 nm) for the sub-density mode (ρ‾eff,sub), which could be mainly ascribed to the agglomerate effect of BC. ρ‾eff,main and ρ‾eff,sub show similar diurnal cycles with peaks in the early afternoon, mainly attributed to the increasing mass fraction of high material density components associated with secondary aerosol production, especially of secondary inorganic aerosol (SIA). To investigate the impact of chemical composition, bulk particle effective density was calculated based on measured chemical composition (ρeff,ACSM) and compared to the average effective density at 300 nm (ρ‾eff,tot,300nm). The best agreement between the two densities is achieved when assuming a BC effective density of 0.60 g cm−3. The particle effective density is highly dependent on SIA and BC mass fractions. The influence of BC on the effective density is even stronger than SIA, implying the importance and necessity of including BC in the estimate of effective density for ambient particles.

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.

Optimized instrument configurations for tandem particle mass analyzer and single particle-soot photometer experiments

Tandem centrifugal particle mass analyzer (CPMA)-single-particle soot photometer (SP2) arrangements provide a measure of the non-refractory components of soot. In such a system, the CPMA is used to classify particles by mass-to-charge ratio (i.e., total particle mass mp), and the SP2 measures the mass of rBC (i.e., mrBC) within each particle. Data inversion can then be used to compute the two-dimensional total particle-non-refractory black carbon (rBC) (mp-mrBC) distribution, mapping out the distribution of non-refractory material on rBC particles. He present study derives the optimum sampling settings (i.e., the number of CPMA setpoints per decade, the number of SP2 bins per decade, CPMA resolution, and the number of SP2 counts per CPMA setpoint) to accurately reconstruct mp-mrBC distributions. Monte Carlo sampling is used to realize the sensitivity of reconstruction error to experimental inputs across a range of distribution widths. General recommendations for a typical aged atmospheric aerosol include: (i) large numbers of SP2 bins per decade (ns' = 64), (ii) a preference for higher SP2 sampling counts per CPMA setpoint (Ns > 104), (iii) a moderate number of CPMA setpoints per decade (nc’ = 3 to 8), (iv) CPMA resolution around 1.0, and (v) a high CPMA flow rate (1.5 L/min). These settings generally result in reconstruction accuracies below 3%. These recommendations vary based on the concentration of the aerosol, the time available for the measurement, and the width of the distribution. Optimized CPMA-SP2 settings are presented for example distributions as a function of aerosol concentration and desired measurement time.

Mixing characteristics of black carbon aerosols in a coastal city using the CPMA-SP2 system

The mixing state of black carbon aerosols (BC) has the largest uncertainty for climate forcing evaluation. In this study, a tandem observation, combining a centrifugal particle mass analyzer (CPMA) and a single particle soot photometer (SP2), was conducted to investigate the mixing state of BC in a coastal city in eastern China. First, particles with specific masses (1.0 fg, 2.1 fg, 4.5 fg, 9.7 fg, and 20.1 fg) were selected using CPMA and consecutively measured by SP2 to determine the corresponding BC mass (MBC) of the particle. The mass ratio (MR) of the coating matter to the BC core was used to indicate the coating thickness and aging degree of BC. The results showed that the MR value obtained from the CPMA-SP2 system was well correlated with the mixing state condition derived from the traditional lag-time method. However, the lag-time method tends to misclassify thickly to thinly coated BC as particles with larger masses. The MR generally increased as the mass of BC-containing particles increased. MR showed a clear diurnal pattern with a minimum at 8:00 and a maximum at 13:00 due to photochemical process during the daytime. The larger BC-containing particles had a much weaker diurnal pattern because other secondary processes may contribute more to the coatings for large BC-containing particles. The MR maintained a high value when the air mass was stagnant in the local area with high pollutant concentrations and high relative humidity (RH), while the MR was at its lowest value when the air mass was derived from the marine area with high RH and low pollutant concentrations. These observations suggest that the increase in MR is the result of the combined effect of high RH and high pollutant concentrations.

Using two-dimensional distributions to inform the mixing state of soot and salt particles produced in gas flares

Gas flaring is a common practice in the oil and gas industry, where droplets of flowback water with varying levels of dissolved salts (mainly composed of sodium and chloride) often become entrained in the flared gas. In this study, we examine the mixing state of the aerosol produced by a laboratory flare with and without entrained droplets of sodium chloride solutions. The resultant aerosol is cross-examined using several different methods, including: transmission electron microscopy (TEM), tandem measurements using a CPMA and a differential mobility analyzer (DMA), tandem measurements using a centrifugal particle mass analyzer (CPMA) and a single particle soot photometer (SP2), and Raman spectroscopy. A focus is placed on two-dimensional distributions of properties and the kind of morphological information contained therein. The TEM and CPMA-SP2 measurements both show that the majority of soot particles were internally mixed with salt, while TEM and CPMA-DMA measurements indicate that there are also a large number of isolated salt particles.

Effects of biomass burning and photochemical oxidation on the black carbon mixing state and light absorption in summer season

To investigate the effects of biomass burning and photochemical oxidation on the black carbon (BC) mixing state and light absorption, a tandem centrifugal particle mass analyzer (CPMA) and single-particle soot photometer (SP2) system was used to measure ambient fine particulate BC during EXPLORE-YRD 2018 campaign (EXPeriment on the eLucidation of the atmospheric Oxidation capacity and aerosol foRmation, and their Effects in Yangtze River Delta). The mass ratio (MR) of the non-BC coating to BC core, a morphology-independent parameter to quantify the mixing state of BC-containing aerosols, is derived. Two types of periods were identified, namely non-significant biomass-burning period (NB), and enhanced biomass burning period (EB). The MR was higher during EB (2.3 ± 0.5), compared with during the NB (2.0 ± 0.5). MR showed bimodal diurnal variation with peaks in the early morning and early afternoon, mainly due to the biomass burning and photochemical processes, respectively. The light-absorbing enhancement ranged from 1.0 to 1.19 when MR was from 1.4 to 3.4. The relationship of rBC mixing state and its optical properties was parameterized, which can further be used in the regional model.

Mass and Number Size Distributions of rBC in Snow and Firn Samples From Pine Island Glacier, West Antarctica

AbstractAn extended‐range Single Particle Soot Photometer (SP2) coupled to a Marin‐5 nebulizer was used to measure the refractory black carbon (rBC) mass and number size distributions in 1,004 samples from a West Antarctica snow/firn core. The SP2 was calibrated using Aquadag and a Centrifugal Particle Mass Analyzer for BC particles ranging from 0.5 to 800 fg. Our results indicate a significant contribution of rare, large particles of mass‐equivalent diameter (DBC) > 500 nm to the total rBC mass (36%), while small particles (DBC < 100 nm) are abundant but contribute <8% to total rBC mass. We observed a primary mass median diameter of 162 ± 40 nm, smaller than reported for snow in other regions of the globe but similar to East Antarctica rBC size distributions. In addition, we observed other modes at 673, 1,040, and >1,810 nm (uncontained mode). We compared two sets of samples from different seasons (wet vs. dry) and observed that dry season concentrations are 3.4 and 2 times that of the wet season in the ranges of 80 nm < DBC < 500 nm (small particles) and 500 nm < DBC < 2,000 nm (large particles), respectively, while number of particles in the dry season is 3.5 and 2 times that of the wet season for the same size ranges. Millimeter thick melt layers have been observed in some samples, although they did not change the observed median diameter. This study provides the first detailed rBC mass and number size distribution from West Antarctica.

Effect of potassium chloride addition on soot formation during ethylene pyrolysis in a flow reactor

In this work, we investigated the effect of potassium chloride (KCl) addition on soot formation during ethylene pyrolysis in a flow reactor by using micro-orifice probe sampling in tandem with a series of systematic characterizations. The particle size distributions measured by a scanning mobility particle sizer showed that, with the addition of KCl, the nucleation of soot particles was not affected while the size growth of soot particles was inhibited. Then the morphology and mass of soot particles were examined by a transmission electron microscopy (TEM) and a centrifugal particle mass analyzer, respectively. The TEM images and mass-mobility relationship further showed that, soot particles were generally more compact and the primary particle size distributions were narrower when KCl was added. To understand what happened to the particle growth process, the charge characteristics were examined via tandem differential mobility analyzers. The number of particles with both double and triple positive charges was found to decrease with the increase of KCl concentration, indicating that the multi-charged particles were neutralized by KCl, which could lead to slower coagulation rates and thus smaller particles.

Effective density and metals content of particle emissions generated by a diesel engine operating under different marine fuels

A differential mobility analyzer along with a centrifugal particle mass analyzer was employed to study the effect of fuel type on effective density for nascent and non-volatile particles generated by a marine diesel engine. The engine was operated at 1500 rpm at loads between 10% and 100%. Three different fuels were evaluated: diesel fuel, distillate marine oil Grade A, and intermediate fuel oil. A combination of a thermodenuder and a catalytic stripper was used to remove semi-volatile materials in order to measure the denuded particulate effective densities. Particle mass concentrations were quantified using thermal-optical analysis as well as integrated particle size distribution. Relatively good agreement between IPSD and TOA was observed for undenuded particle mass concentrations, however IPSD underestimated the mass concentrations measured for denuded particles by approximately 76%. The results revealed that more than 93% of undenuded particles from diesel and DM-A fuels were composed of semi-volatile materials. The denuded effective density values at some test conditions were significantly higher than the effective density of soot particle from other sources specifically for particles smaller than 100 nm generated from IFO. We hypothesized that the high effective density values are in part due to large proportion of metal-containing tarballs. By comparing the measured effective densities with a literature effective density function for soot, we were able to predict the metal mass concentrations measured by ICP-MS with reasonable accuracy in most cases.

Morphology and size of soot from gas flares as a function of fuel and water addition

A large-scale, laboratory turbulent diffusion flame was used to study the effects of fuel composition on soot size and morphology. The burner and fuels are typical of those used in the upstream oil and gas industry for gas flaring, a practice commonly used to dispose of excess gaseous hydrocarbons. Fuels were characterized by their carbon-to-hydrogen ratio (from 0.264 to 0.369) and their volumetric higher heating value (HHVv) (from 35.8 to 75.2 MJ/m3). Transmission electron microscopy (TEM) was used to assess primary particle and aggregate size, showing that the scaling of primary particle size to aggregate size was roughly the same for all of the considered fuels (dp = 16.3(da,100 [nm]/100)0.35). However, fuels with higher HHVv produced substantially larger soot aggregates. A scanning mobility particle sizer (SMPS) was also used (i) to measure mobility diameter distributions and (ii) in tandem with a centrifugal particle mass analyzer (CPMA) to determine the two-dimensional mass-mobility and effective density-mobility distributions using a new inversion approach. The new approach was shown to improve internal consistency of inferred morphological parameters, though with a shift relative to median-based analysis of the tandem data. Raman spectroscopy was used to quantify the degree of graphitization in the soot nanostructure. The addition of water to the fuel consistently reduced the soot yields but did not affect other morphological parameters. Larger aggregates also tended to have larger primary particles and higher Raman D/G ratios suggesting larger graphitic domains.

Mixing characteristics of refractory black carbon aerosols at an urban site in Beijing

Abstract. Black carbon aerosols play an important role in climate change because they directly absorb solar radiation. In this study, the mixing state of refractory black carbon (rBC) at an urban site in Beijing in the early summer of 2018 was studied with a single-particle soot photometer (SP2) as well as a tandem observation system with a centrifugal particle mass analyzer (CPMA) and a differential mobility analyzer (DMA). The results demonstrated that the mass-equivalent size distribution of rBC exhibited an approximately lognormal distribution with a mass median diameter (MMD) of 171 nm. When the site experienced prevailing southerly winds, the MMD of rBC increased notably, by 19 %. During the observational period, the ratio of the diameter of rBC-containing particles (Dp) to the rBC core (Dc) was 1.20 on average for Dc=180 nm, indicating that the majority of rBC particles were thinly coated. The Dp∕Dc value exhibited a clear diurnal pattern, with a maximum at 14:00 LST and a Dp growth rate of 2.3 nm h−1; higher Ox conditions increased the coating growth rate. The microphysical properties of rBC were also studied. Bare rBC particles were mostly found in fractal structures with a mass fractal dimensions (Dfm) of 2.35, with limited variation during both clean and polluted periods. The morphology of rBC changed with coating thickness increasing. When the mass ratio of nonrefractory matter to rBC (MR) was <1.5, rBC-containing particles were primarily found in external fractal structures, and they changed to a core–shell structure when MR>6, at which point the measured scattering cross section of rBC-containing particles was consistent with that based on the Mie-scattering simulation. We found that only 28 % of the rBC-containing particles were in core–shell structures with a particle mass of 10 fg in the clean period but that proportion increased considerably, to 45 %, in the polluted period. Due to the morphology change, the absorption enhancement (Eabs) was 12 % lower than that predicted for core–shell structures.

Characterising mass-resolved mixing state of black carbon in Beijing using a morphology-independent measurement method

Abstract. Refractory black carbon (rBC) in the atmosphere is known for its significant impacts on climate. The relationship between the microphysical and optical properties of rBC remains poorly understood and is influenced by its size and mixing state. Mixing state also influences its cloud scavenging potential and thus atmospheric lifetime. This study presents a coupling of a centrifugal particle mass analyser (CPMA) and a single-particle soot photometer (SP2) for the morphology-independent quantification of the mixing state of rBC-containing particles, used in the urban site of Beijing as part of the Air Pollution and Human Health–Beijing (APHH-Beijing) project during winter (10 November–10 December 2016) and summer (18 May–25 June 2017). This represents a highly dynamic polluted environment with a wide variety of conditions that could be considered representative of megacity area sources in Asia. An inversion method (used for the first time on atmospheric aerosols) is applied to the measurements to present two-variable distributions of both rBC mass and total mass of rBC-containing particles and calculate the mass-resolved mixing state of rBC-containing particles, using previously published metrics. The mass ratio between non-rBC material and rBC material (MR) is calculated to determine the thickness of a hypothetical coating if the rBC and other material followed a concentric sphere model (the equivalent coating thickness). The bulk MR (MRbulk) was found to vary between 2 and 12 in winter and between 2 and 3 in summer. This mass-resolved mixing state is used to derive the mass-weighted mixing state index for the rBC-containing particles (χrBC). χrBC quantifies how uniformly the non-rBC material is distributed across the rBC-containing-particle population, with 100 % representing uniform mixing. The χrBC in Beijing varied between 55 % and 70 % in winter depending on the dominant air masses, and χrBC was highly correlated with increased MRbulk and PM1 mass concentration in winter, whereas χrBC in summer varied significantly (ranging 60 %–75 %) within the narrowly distributed MRbulk and was found to be independent of air mass sources. In some model treatments, it is assumed that more atmospheric ageing causes the BC to tend towards a more homogeneous mixture, but this leads to the conclusion that the MRbulk may only act as a predictor of χrBC in winter. The particle morphology-independent and mass-based information on BC mixing used in this and future studies can be applied to mixing-state-aware models investigating atmospheric rBC ageing.

Ejector-based nanoparticle sampling from pressures down to 20 mbar

The application of standard online instrumentation, such as scanning mobility particle sizer (SMPS), centrifugal particle mass analyzer or aerosol particle mass analyzer (CPMA/APM), and electrical low-pressure impactor (ELPI+) to low-pressure processes is only possible with extensive modification of the devices and extensive calibrations. A low-pressure ejector is a suitable device to transfer aerosol nanoparticles from low-pressure regions to atmospheric pressure and allows the direct use of standard online instrumentation. In this work, a commercial low-pressure ejector is investigated in the pressure range from 20–180 mbar with fully-sintered and size-selected nanoparticles (15–80 nm) in order to extend the application range of online instruments to low-pressure processes and open up a new variety of analysis methods. Results are compared to our previous work which was limited to pressures above 120 mbar. A change in particle size during the measurements for fully-sintered silver particles was not observed. A particle dilution factor between 60–6500 was found. High particle losses in the ejector for large particle sizes are compensated by a lower gas dilution factor.

Effective density and hygroscopicity of protein particles generated with spray-drying process

Laboratory investigations of aerosol physico-chemical properties are usually initiated by spray-drying processes. Morphology of the generated particles may vary largely under different spray-drying conditions, which may further affect the characterization of particle properties. However, only limited data is available on the morphology of nebulized submicron particles, particularly for protein particles. To fill this gap, a centrifugal particle mass analyzer coupled with a scanning mobility particle sizer was used to analyze the effective density, an indicator of particle morphology, of nebulized protein particles (bovine serum albumin and ovalbumin) in a size range from 40 to 200 nm. Results indicate that the effective density of the protein particles generated via spray-drying processes is mainly determined by the competition between the removal of liquid water and the redistribution of the solute monomers in the sprayed droplets. Factors increasing the time needed for solute monomers redistribution (e.g. lower solution concentration and larger particle dry size) or decreasing the time needed for liquid water removal (e.g. higher drying rate) may result in a lower particle effective density, meaning a semi-solid structure or partially hollow morphology. This is also confirmed by the particle hygroscopicity measured with a nano-particle hygroscopic tandem differential mobility analyzer and a high humidity tandem differential mobility analyzer. Our results suggest that the factors affecting the morphology of particles may be complex and coupled with each other, highlighting the importance of monitoring particle effective densities in studies using aerosol particles generated via spray-drying processes.

Effect of fuel composition on properties of particles emitted from a diesel–natural gas dual fuel engine

The effective density and mixing state of particles emitted from a natural gas–diesel dual fuel engine are investigated. Measurements were conducted at three different fuel compositions including 100% diesel fuel (0% NG), 75% diesel–25% natural gas (25% NG) and 50% diesel–50% NG (50% NG). The particle effective density was measured using a differential mobility analyzer in series with a centrifugal particle mass analyzer. A catalytic stripper at 350 °C was employed upstream of the centrifugal particle mass analyzer in order to remove the semi-volatile material from the solid particles to measure the effective density of non-volatile particles as well as the particle mixing state. A scanning mobility particle sizer was used to measure the particle size distribution. The particle mass concentration was also measured using several techniques including cavity-attenuated phase-shift particulate matter single-scattering albedo, laser-induced incandescence, thermal-optical analysis, photoacoustic spectroscopy, and integrated particle size distribution. The semi-volatile number and mass fractions are found to be lower than 15%. The effective density functions of particles at 0% and 25% NG are within 6% of each other; however, the effective density values of particles at 50% NG are lower than those of the 0% NG by up to 35%. The mass-mobility exponent varies in the range of 2.42–2.51 and 2.38–2.54 for undenuded and denuded particles, respectively. For the mass concentration measurements, photoacoustic spectroscopy agrees with thermal-optical analysis within 5%, while all the other techniques measure up to 50% deviations relative to thermal-optical analysis.