Virtual Impactor Research Articles

Computational Fluid Dynamics Simulation, Microelectromechanical System Fabrication, and Radio-Frequency Evaluation of the PM2.5 Fine Dust Sensor Based on the Surface Acoustic Wave Resonator

To classify 2.5-µm-scale fine dust and measure its concentration, research was conducted to produce a fine-dust sensor utilizing computational fluid dynamics simulation and microelectromechanical system processes, and the measurement system was manufactured for the sensor. The virtual impactor was designed to classify particles in the air above and below 2.5 µm, and the cut-off diameter of 2.5 µm at an acceleration nozzle length of 83.2 µm was obtained by verifying this impactor through flow-analysis simulation. The microheater was then designed to measure the mass of the classified particle by adsorbing it onto the surface acoustic wave resonator placed on the lower part of the flow channel using thermophoretic force. The conditions where the classified particles are completely adsorbed on the surface acoustic wave resonator through the heater were calculated using numerical modeling and the results were verified through the simulation. The fine dust sensor based on the designed surface acoustic wave resonator is composed of the upper, middle, and lower parts including the heater, virtual impactor, and surface acoustic wave resonator and SMA connector, respectively, and the micro-fan controlling the total flow. Each part was produced using the microelectromechanical system process, assembled, and finally finished. Moreover, the evaluation system was produced in this study to assess the developed sensor. The flow of air controlled through the flowmeter was sprayed into a dust bottle filled with hollow silica powder to scatter the powder, which flowed into the system with the air, making the environment similar to one with fine dust present. The change in radio-frequency resonance characteristics, such as those before and after exposure to the fine dust environment, and insertion loss after the surface cleaning were observed, and a sensitivity of, detection limit of, and restoration rate after surface cleaning exceeding 99.9% were obtained.

A combined virtual impactor and field-effect transistor microsystem for particulate matter separation and detection

Ambient suspended particulate matter (PM) (primarily with particle diameter 2.5 µm or less, i.e., PM2.5) can adversely affect ecosystems and human health. Currently, optical particle sensors based on light scattering dominate the portable PM sensing market. However, the light scattering method has poor adaptability to different-sized PM and adverse environmental conditions. Here, we design and develop a portable PM sensing microsystem that consists of a micromachined virtual impactor (VI) for particle separation, a thermophoretic deposition chip for particle collection, and an extended-gate field-effect transistor (FET) for particle analysis. This system can realize on-site separation, collection, and analysis of aerosol particles without being influenced by environmental factors. In this study, the design of the VI is thoroughly analyzed by numerical simulation, and mixtures of different-sized silicon dioxide (SiO2) particles are used in an experimental verification of the performance of the VI and FET. Considering the low cost and compact design of the whole system, the proposed PM analysis microsystem has potential for PM detection under a wide range of conditions, such as heavily polluted industrial environments and for point-of-need outdoor and indoor air quality monitoring.

Performance Evaluation of a Virtual Impactor with an Additional Pretreatment Structure for Particle Separation

ABSTRACT In this paper, a proposed method based on a microfluidic chip that can arrange the collected particles in radial positions according to their sizes before virtual impactor separation was introduced. The main objective of this paper was to design a novel micro-separator structure composed of two major parts: a virtual impactor, and a pretreatment that consists of a sheath flow and a backward-facing step flow configuration. In the separation process, the particles firstly were aggregated on the middle channel by the sheath flow, then ordered in radial position of backward-facing step flow, and finally separated in virtual impactor. To determine the effects of the pretreatment, both simulations and experiments were conducted. A parameter optimization was performed on the micro-separator where the cut-off size of the virtual impactor was 2.5 µm. The results demonstrated that when the flow ratio between inlet flow rate and the total flow rate was set to 50% and the expansion ratio (Er) was 2. The cut-off size of the micro-separator was reduced by 20.77% from 2.436 µm to 1.93 µm, and the wall loss (WL) was acceptable compared with the result only using PM2.5 virtual impactor. The effect of pretreatment on PM1 and PM5 virtual impactor also saw an improvement of collection efficiency even when the additional pretreatment had varying Reynold numbers and nozzle sizes. Accordingly, the stk50 value was reduced by 44.39% from 0.669 to 0.372 by adding pretreatment before the virtual impactor.

Performance Evaluation of a Low Cost Air Quality Sensor over a Wide Concentration Range

The recent research advancements in the field of chemical sensors, distributed sensing and wireless networking have led to development of several low-cost optical aerosol sensors. These sensors are based on the principle of particle light scattering, and are dependent on several parameters - temperature, relative humidity, particle size, and particle concentration. However, the linearity of the response of these sensors over a wide concentration range has not been well studied. We evaluated a low-cost particulate matter measuring sensor (BlueSky Air Monitor Model 8143 TSI Inc) in different environments. The particulate matter concentration of PM10, PM4, PM2.5 and PM1 were compared with reference measurements using a Dust Trak Model 8534 TSI Inc- a widely used light scattering instrument. Several methodologies were developed to generate wide concentration of aerosols. The concentration ranges for the lower end were created in indoor environmental conditions (0.003-0.004 mg/m3) and for the higher end were created in a chamber with e-cigarette vaping (0.006- 30 mg/m3). The intermediate concentrations (0.003- 2 mg/m3) were obtained using a Concentrated Air Particulate System (CAPS) consisting of a virtual impactor setup that magnifies ambient concentration by an adjustable enrichment factor between 5-20. Results of the present study indicate that the BlueSky low cost sensor compares well with the reference instrument (R2 = 0.92; Intercept ~0; Slope = 0.75). The novel design of the CAPS including maintaining an uniform temperature and humidity makes it an ideal design for characterizing low cost sensors over a wide range of concentrations. The findings of the present study can be used to design more robust methodologies for laboratory testing for low cost air quality sensors at different concentrations.

Dozens of virtual impactor orbits eliminated by the EURONEAR VIMP DECam data mining project

Context. Massive data mining of image archives observed with large etendue facilities represents a great opportunity for orbital amelioration of poorly known virtual impactor asteroids (VIs). There are more than 1000 VIs known today; most of them have very short observed arcs and many are considered lost as they became extremely faint soon after discovery. Aims. We aim to improve the orbits of VIs and eliminate their status by data mining the existing image archives. Methods. Within the European Near Earth Asteroids Research (EURONEAR) project, we developed the Virtual Impactor search using Mega-Precovery (VIMP) software, which is endowed with a very effective (fast and accurate) algorithm to predict apparitions of candidate pairs for subsequent guided human search. Considering a simple geometric model, the VIMP algorithm searches for any possible intersection in space and time between the positional uncertainty of any VI and the bounding sky projection of any image archive. Results. We applied VIMP to mine the data of 451,914 Blanco/DECam images observed between 12 September 2012 and 11 July 2019, identifying 212 VIs that possibly fall into 1286 candidate images leading to either precovery or recovery events. Following a careful search of candidate images, we recovered and measured 54 VIs in 183 DECam images. About 4,000 impact orbits were eliminated from both lists, 27 VIs were removed from at least one list, while 14 objects were eliminated from both lists. The faintest detections were around V ∼ 24.0, while the majority fall between 21 < V < 23. The minimal orbital intersection distances remains constant for 67% detections, increasing for eight objects and decreasing for ten objects. Most eliminated VIs (70%) had short initial arcs of less than five days. Some unexpected photometric discovery has emerged regarding the rotation period of 2018 DB, based on the close inspection of longer trailed VIs and the measurement of their fluxes along the trails. Conclusions. Large etendue imaging archives represent great assets to search for serendipitous encounters of faint asteroids and VIs.

A new optical-based technique for real-time measurements of mineral dust concentration in PM<sub>10</sub> using a virtual impactor

Abstract. Atmospheric mineral dust influences Earth's radiative budget, cloud formation, and lifetime; has adverse health effects; and affects air quality through the increase of regulatory PM10 concentrations, making its real-time quantification in the atmosphere of strategic importance. Only few near-real-time techniques can discriminate dust aerosol in PM10 samples and they are based on the dust chemical composition. The online determination of mineral dust using aerosol absorption photometers offers an interesting and competitive alternative but remains a difficult task to achieve. This is particularly challenging when dust is mixed with black carbon, which features a much higher mass absorption cross section. We build on previous work using filter photometers and present here for the first time a highly time-resolved online technique for quantification of mineral dust concentration by coupling a high-flow virtual impactor (VI) sampler that concentrates coarse particles with an aerosol absorption photometer (Aethalometer, model AE33). The absorption of concentrated dust particles is obtained by subtracting the absorption of the submicron (PM1) aerosol fraction from the absorption of the virtual impactor sample (VI-PM1 method). This real-time method for detecting desert dust was tested in the field for a period of 2 months (April and May 2016) at a regional background site of Cyprus, in the Eastern Mediterranean. Several intense desert mineral dust events were observed during the field campaign with dust concentration in PM10 up to 45 µg m−3. Mineral dust was present most of the time during the campaign with an average PM10 of about 8 µg m−3. Mineral dust absorption was most prominent at short wavelengths, yielding an average mass absorption cross section (MAC) of 0.24±0.01 m2 g−1 at 370 nm and an absorption Ångström exponent of 1.41±0.29. This MAC value can be used as a site-specific parameter for online determination of mineral dust concentration. The uncertainty of the proposed method is discussed by comparing and validating it with different methods.

Improving impact monitoring through Line Of Variations densification

We propose a densification algorithm to improve the Line Of Variations (LOV) method for impact monitoring, which can fail when the information is too little, as it may happen in difficult cases. The LOV method uses a 1-dimensional sampling to explore the uncertainty region of an asteroid. The close approaches of the sample orbits are grouped by time and LOV index, to form the so-called returns, and each return is analysed to search for local minima of the distance from the Earth along the LOV. The strong non-linearity of the problem causes the occurrence of returns with so few points that a successful analysis can be prevented. Our densification algorithm tries to convert returns with length at most 3 in returns with 5 points, properly adding new points to the original return. Due to the complex evolution of the LOV, this operation is not necessarily achieved all at once: in this case the information about the LOV geometry derived from the first attempt is exploited for a further attempt. Finally, we present some examples showing that the application of our method can have remarkable consequences on impact monitoring results, in particular about the completeness of the virtual impactors search.

Development of a multi-slit virtual impactor as a high-volume bio-aerosol sampler

Virtual impactors are capable of aspirating aerosols in the air, concentrating particles of a certain size, and discharging these particles through a minor flow. In general, commonly used virtual impactor has a cut-off size of 1 μm or more, a concentration ratio of 10–20 times, and an operating flowrate of 10 L/min; it is used to increase the detection probability of various aerosol measuring equipment. In this study, we developed a special-purpose virtual impactor aimed at the mass capturing of aerosols, including pathogens released by human respiration or coughing. For this purpose, we designed a virtual impactor exceeding the typical operating range, with a cut-off size of 0.3 μm and a concentration ratio of 13.3. Five multi-slit nozzles were adopted to indicate a cut diameter of 0.3 μm for a high aerosol flowrate of 170 L/min. Clean air was injected near the nozzle wall of the virtual impactor to minimize wall loss and further reduce the cut-off size. The performance of the designed virtual impactor was verified through numerical analyses and experimentation. As a result, aerosol aspirated at 170 L/min of flowrate was effectively concentrated in the minor flow with a cut-off size of 0.3 μm.

Inertial Impaction Technique for the Classification of Particulate Matters and Nanoparticles: A Review

Inertial impactors are applied widely to classify particulate matters (PMs) and nanoparticles (NPs) with desired aerodynamic diameters for further analyses due to their sharp cutoff characteristics, simple design, easy operation, and high collection ability. A few hundred papers have been published since the 1860s that addressed the characteristics and applications of the inertial impactors. In the last 30 years, our group has also carried out lots of studies to contribute to the design and the improvement of inertial impactors. With our understanding of inertial impactors, this article reviews previous studies of some typical types of the inertial impactors including conventional impactors, cascade impactors, and virtual impactors and the parameters for design consideration of these devices. The article also reviews some applications of the inertial impactors, which are mass concentration measurement, mass and number distribution measurement, personal exposure measurement, particulate matter control, and powder classification. The synthesized knowledge of the inertial impactor in this study can help researchers to design an inertial impactor with an accurate cutoff diameter, a sharp collection efficiency curve, and no particle bounce and particle overloading effects for long-term use for PM classification and control purposes.

Physical and chemical properties of aerosol particles and cloud residuals on Mt. Åreskutan in Central Sweden during summer 2014

The size distribution, volatility and hygroscopicity of ambient aerosols and cloud residuals were measured with a differential mobility particle sizer (DMPS) and a volatility–hygroscopicity tandem differential mobility analyser (VHTDMA) coupled to a counterflow virtual impactor (CVI) inlet during the Cloud and Aerosol Experiment at Åre (CAEsAR) campaign at Mt. Åreskutan during summer 2014. The chemical composition of particulate matter (PM) and cloud water were analysed offline using thermo-optical OC/EC analysis and ion chromatography. The importance of aerosol particle size for cloud droplet activation and subsequent particle scavenging was clearly visible in the measured size distributions. Cloud residuals were shifted towards larger sizes compared to ambient aerosol, and the cloud events were followed by a size distribution dominated by smaller particles. Organics dominated both PM (62% organic mass fraction) and cloud water (63% organic mass fraction) composition. The volatility and hygroscopicity of the ambient aerosols were representative of homogeneous aged aerosol with contributions from biogenic secondary organics, with median volume fraction remaining (VFR) of 0.04–0.05, and median hygroscopicity parameter κ of 0.16–0.24 for 100–300 nm particles. The corresponding VFR and κ for the cloud residuals were 0.03–0.04 and 0.18–0.20. The chemical composition, hygroscopicity and volatility measurements thus showed no major differences between the ambient aerosol particles and cloud residuals. The VFR and κ values predicted based on the chemical composition measurements agreed well with the VHTDMA measurements, indicating the bulk chemical composition to be a reasonable approximation throughout the size distribution. There were indications, however, of some more subtle changes in time scales not achievable by the offline chemical analysis applied here. Further, online observations of aerosol and cloud residual chemical composition are therefore warranted.

Variation characteristics of final size-segregated PM emissions from ultralow emission coal-fired power plants in China

The ultralow emission (ULE) retrofits for Chinese coal-fired power plants (CFPPs) are nearing completion. Large-scale and rapid retrofits have resulted in significant changes in the emission level and characteristics of particulate matter (PM). To investigate the variation characteristics of final three size fractions PM (PM2.5, PM10−2.5, PM>10) emissions, we conducted field tests at the outlets of wet flue gas desulfurization (WFGD) and wet electrostatic precipitator (WESP) by a pair of two-stage virtual impactors in eight representative ULE CFPPs. Our results indicate that, after WESP installations, the mass concentrations of final PM are significantly reduced and those of the final total ions and elements decrease as most individual chemical compositions are reduced. WESP presents an excellent removal performance for large particle sizes and high PM concentrations. SO42− is the major ionic component at both the outlets of WFGD and WESP, and its proportion in total ions is reduced to some extent through WESP. Furthermore, the average mass contents of SO42− and most elements in PM2.5 are significantly lower than those in PM10−2.5 and PM>10 whether at the WFGD-outlets or WESP-outlets. By comparison, chemical profiles of PM have substantially changed after ULE retrofits, and those after WFGD (e.g., sulfate, Zn, Pb, and Cu) have also changed relative to existing data. The end-tail emission factors (EFs) of PM2.5, PM10, and PMtotal under the typical ULE technical routes of WESP are calculated in time, and the corresponding EFs are in the range of 2.82–8.97, 15.7–27.6, and 38.6–61.7 g t−1, respectively. We believe the latest detailed PM EFs and the associated chemical profiles provided in this study are more representative of the current emission situations of Chinese CFPPs.

Direct Online Mass Spectrometry Measurements of Ice Nucleating Particles at a California Coastal Site

AbstractThe formation of ice in clouds can strongly impact cloud properties and precipitation processes during storms, including atmospheric rivers. Sea spray aerosol (SSA) particles are relatively inefficient as ice nucleating particles (INPs) compared to mineral dust. However, due to the vast coverage of the Earth's surface by the oceans, a number of recent studies have focused on identifying sources of marine INPs, particularly in regions lacking a strong influence from dust. This study describes the integration, validation, and application of a system coupling a continuous flow diffusion chamber with a single particle mass spectrometer using a pumped counterflow virtual impactor to remove nonnucleated particles and selectively measure the composition of INPs with a detection efficiency of 3.10 × 10−4. In situ measurements of immersion freezing INP composition were made at a coastal site in California using the integrated system. Mineral dust particles were the most abundant ice crystal residual type during the sampling period and found to be ice active despite having undergone atmospheric processing. SSA were more abundant in ambient measurements but represented only a minor fraction of the ice crystal residual population at −31 °C. Notably, the SSA particles that activated were enriched with organic nitrogen species that were likely transferred from the ocean. Calculations of ice nucleation active site densities were within good agreement with previous studies of mineral dust and SSA.

Influence of clean air and inlet configuration on the performance of slit nozzle virtual impactor

A two-partitioned horizontal inlet was developed for improving the collection efficiency and minimizing the wall loss problem in slit virtual impactor. The two-partitions were provided to simultaneously supply both aerosol and clean air to the virtual impactor. Both numerical and experimental investigations were carried out on the developed inlet configuration by considering different flowrate ratios of aerosol to clean air. The horizontal inlet was helpful in reducing the cutoff diameter, whereas the clean air prevented the particle deposition on the virtual impactor walls. The performance of two-partitioned horizontal inlet was compared with the conventional vertical inlet configuration for PM2.5, PM5 and PM10 virtual impactors. All the operating conditions and geometric parameters, such as the inlet flowrate; the width of collection nozzle; the width, length and span of acceleration nozzle; and the distance between collection and acceleration nozzles, were kept the same and only the inlet configuration was changed. The major-to-total flowrate ratio was kept at 0.9 and minor-to-total flowrate ratio at 0.1. It was observed that by using the two-partitioned horizontal inlet configuration, the cutoff diameters for PM2.5, PM5 and PM10 virtual impactors, were reduced by 16%, 10% and 11%, respectively, while the wall loss of particles near the cutoff size in all three cases were reduced from 16% to about 1%.

Vertically-stacked MEMS PM2.5 sensor for wearable applications

Exposure to fine airborne particulate matter (PM), i.e., PM with an aerodynamic diameter (AD) less than 2.5 μm, is associated with many adverse health effects, including impaired pulmonary function, asthma, cardiovascular diseases, and Alzheimer. Consequently, there is a strong need for the development of small, low-cost wearable PM2.5 sensors that can be used by regular citizens to monitor their PM exposure. This work presents the design, fabrication, and experimental evaluation of a stacked-channel wearable direct-read micro-electro-mechanical system (MEMS) PM2.5 sensor that directly measures the mass concentration of the ambient PM by deposition on a mass-sensing resonator. The sensor employs a vertically stacked air-microfluidic channel geometry and an out-of-plane vertical virtual impactor (VVI), fabricated using a novel delayed deep reactive ion etch (DDRIE) process. As a result, this new device not only allows for a greater level of miniaturization than the previously reported air-microfluidic PM sensors, but also significantly reduces internal fouling due to PM accumulation in its channels. The small footprint of the sensor (27 mm × 14 mm × 2 mm) allows for the integration of the sensor into a wearable or cellular platform. An opto-gravimetric method was used to evaluate the collection efficiency of the VVI, which confirmed its 50% cutpoint at 2.5 μm. The experimental evaluation of the functionality of the sensor confirmed the sensitivity of the new design at 7 Hz/min per μg/m3, which is the highest sensitivity of direct-read mass-based PM2.5 sensor presented to this date. Such sensitivity allows for the limit of detection (LOD) of one μg/m3 within 7 min of integration time at a noise level of 50 Hz.

Versatile aerosol concentration enrichment system (VACES) operating as a cloud condensation nuclei (CCN) concentrator: development and laboratory characterization

Abstract. The ability of atmospheric aerosol particles to act as cloud condensation nuclei (CCN) depends on many factors, including particle size, chemical composition and meteorological conditions. To expand our knowledge of CCN, it is essential to understand the factors leading to CCN activation. For this purpose, a versatile aerosol concentrator enrichment system (VACES) has been modified to select CCN at different supersaturations. The VACES enables sampling non-volatile CCN particles without altering their chemical and physical properties. The redesigned VACES enriches CCN particles by first passing the aerosol flow to a new saturator and then to a condenser. The activated particles are concentrated by an inertial virtual impactor and then can be returned to their original size by diffusion drying. For the calibration, the saturator temperature was fixed at 52 ∘C and the condenser temperature range was altered from 5 to 25 ∘C to obtain activation curves for NaCl particles of different sizes. Critical water vapour supersaturations can be calculated using the 50 % cut point of these curves. Calibration results have also shown that CCN concentrations can be enriched by a factor of approx. 17, which is in agreement with the experimentally determined enrichment factor of the original VACES. The advantage of the redesigned VACES over conventional CCN counters (both static and continuous flow instruments) lies in the substantial enrichment of activated CCN, which facilitates further chemical analysis.

Enrichment of submicron sea-salt-containing particles in small cloud droplets based on single-particle mass spectrometry

Abstract. The effects of the chemical composition and size of sea-salt-containing particles on their cloud condensation nuclei (CCN) activity are incompletely understood. We used a ground-based counterflow virtual impactor (GCVI) coupled with a single-particle aerosol mass spectrometer (SPAMS) to characterize chemical composition of submicron (dry diameter of 0.2–1.0 µm) and supermicron (1.0–2.0 µm) sea-salt-containing cloud residues (dried cloud droplets) at Mount Nanling, southern China. Seven cut sizes (7.5–14 µm) of cloud droplets were set in the GCVI system. The highest number fraction of sea-salt-containing particles was observed at the cut size of 7.5 µm (26 %, by number), followed by 14 µm (17 %) and the other cut sizes (3 %–5 %). The submicron sea-salt-containing cloud residues contributed approximately 20 % (by number) at the cut size of 7.5 µm, which was significantly higher than the percentages at the cut sizes of 8–14 µm (below 2 %). This difference was likely involved in the change in the chemical composition. At the cut size of 7.5 µm, nitrate was internally mixed with over 90 % of the submicron sea-salt-containing cloud residues, which was higher than sulfate (20 %), ammonium (below 1 %), amines (6 %), hydrocarbon organic species (2 %), and organic acids (4 %). However, at the cut sizes of 8–14 µm, nitrate, sulfate, ammonium, amines, hydrocarbon organic species, and organic acids were internally mixed with > 90 %, > 80 %, 39 %–84 %, 71 %–86 %, 52 %–90 %, and 32 %–77 % of the submicron sea-salt-containing cloud residues. The proportion of sea-salt-containing particles in the supermicron cloud residues generally increased as a function of cut size, and their CCN activity was less influenced by chemical composition. This study provided a significant contribution towards a comprehensive understanding of sea-salt CCN activity.

Numerical Investigation of Collection Efficiency of Virtual Impactor with Electro-Aerodynamic Lens

Numerical Investigation of Collection Efficiency of Virtual Impactor with Electro-Aerodynamic Lens

Concept for an electrostatic focusing device for continuous ambient pressure aerosol concentration

Abstract. We present a concept for enhancing the concentration of charged submicron aerosol particles in a continuous-flow stream using in situ electrostatic focusing. It is proposed that electrostatic focusing can enable the continuous, isothermal concentration of aerosol particles at ambient pressure, without altering their chemical composition. We model this approach theoretically and demonstrate proof of concept via laboratory measurements using a prototype. The prototype design consists of a nozzle-probe flow system analogous to a virtual impactor. The device was tested in the laboratory using submicron, monodisperse stearic acid particles. Particles were charged using a unipolar charger then concentrated using a cylindrical electrostatic immersion lens to direct the charged submicron particles into the sample probe. Under applied lens voltages ranging from 0 V to 30 kV, aerosol concentration increased up to 15 %. Observed particle enrichment varied as a function of voltage and particle diameter. These results suggest that an imposed electric field can be used to increase aerosol concentration in a continuous flow. This approach shows promise in increasing the effective enriched size range of virtual impactors or other continuous-flow methods of collection.

Computational fluid dynamics study of the effects of flow and geometry parameters on a linear-slit virtual impactor for sampling and concentrating aerosols

Virtual impactors can be used as aerosol concentrators as well as particle size classifiers. The flow field and particle trajectory inside a linear slit-type virtual impactor were investigated using a commercial computational fluid dynamics software package. Effects of flow and slit geometry, including the inlet flow (Qin), the ratio of minor to total flow (r), and the configuration of the taper-lip slit nozzle and the collection slit nozzle, were studied. A new modified Stokes number (Stkc) for the virtual impactor was proposed to predict the d50 at different r values. Stkc,500.5 was found to be retained at approximately 0.9 under different Qin or r values, and it can be considered the characteristic performance parameter for the slit virtual impactor. The numerical simulation results show that the particle loss starts increasing when Stkc0.5 is larger than 2.0 due to a particle-crossing phenomenon. The range of the maximum concentration factor (CFmax) is influenced by the particle-crossing phenomenon. An expression was also developed to predict the size range of particles with the maximum CF under different inlet flow rates. Regarding the geometrical configuration, the arc-lip nozzle could postpone particle-crossing and lessen the internal losses of coarse particles, whereas it increased particle losses near d50. However, the divergent collection slit nozzle could effectively decrease particle losses on the wall of the nozzle without this side effect. The designing guidelines of a linear-slit virtual impactor are briefly summarized based on these findings.

In-cloud formation of secondary species in iron-containing particles

Abstract. The increase in secondary species through cloud processing potentially increases aerosol iron (Fe) bioavailability. In this study, a ground-based counterflow virtual impactor coupled with a real-time single-particle aerosol mass spectrometer was used to characterize the formation of secondary species in Fe-containing cloud residues (dried cloud droplets) at a mountain site in southern China for nearly 1 month during the autumn of 2016. Fe-rich, Fe-dust, Fe-elemental carbon (Fe-EC), and Fe-vanadium (Fe-V) cloud residual types were obtained in this study. The Fe-rich particles, related to combustion sources, contributed 84 % (by number) to the Fe-containing cloud residues, and the Fe-dust particles represented 12 %. The remaining 4 % consisted of the Fe-EC and Fe-V particles. It was found that above 90 % (by number) of Fe-containing particles had already contained sulfate before cloud events, leading to no distinct change in number fraction (NF) of sulfate during cloud events. Cloud processing contributed to the enhanced NFs of nitrate, chloride, and oxalate in the Fe-containing cloud residues. However, the in-cloud formation of nitrate and chloride in the Fe-rich type was less obvious relative to the Fe-dust type. The increased NF of oxalate in the Fe-rich cloud residues was produced via aqueous oxidation of oxalate precursors (e.g., glyoxylate). Moreover, Fe-driven Fenton reactions likely increase the formation rate of aqueous-phase OH, improving the conversion of the precursors to oxalate in the Fe-rich cloud residues. During daytime, the decreased NF of oxalate in the Fe-rich cloud residues was supposed to be due to the photolysis of Fe-oxalate complexes. This work emphasizes the role of combustion Fe sources in participating in cloud processing and has important implications for evaluating Fe bioavailability from combustion sources during cloud processing.