Scanning Electrical Mobility Spectrometer Research Articles

Modular Multiplatform Compatible Air Measurement System (MoMuCAMS): a new modular platform for boundary layer aerosol and trace gas vertical measurements in extreme environments

Abstract. The Modular Multiplatform Compatible Air Measurement System (MoMuCAMS) is a newly developed in situ aerosol and trace gas measurement platform for lower-atmospheric vertical profiling. MoMuCAMS has been primarily designed to be attached to a Helikite, a rugged tethered balloon type that is suitable for operations in cold and windy conditions. The system addresses the need for detailed vertical observations of atmospheric composition in the boundary layer and lower free troposphere, especially in polar and alpine regions. The MoMuCAMS encompasses a box that houses instrumentation, a heated inlet, a single-board computer to transmit data to the ground for in-flight decisions and a power distribution system. The enclosure can accommodate various combinations of instruments within its weight limit (e.g., 20 kg for a 45 m3 balloon). This flexibility represents a unique feature, allowing for the study of multiple aerosol properties (number concentration, size distribution, optical properties, chemical composition and morphology), as well as trace gases (e.g., CO, CO2, O3, N2O) and meteorological variables (e.g., wind speed and direction, temperature, relative humidity, pressure). Different instrumental combinations are therefore possible to address the specific scientific focus of the observations. It is the first tethered-balloon-based system equipped with instrumentation providing a size distribution for aerosol particles within a large range, i.e., from 8 to 3370 nm, which is vital to understanding atmospheric processes of aerosols and their climate impacts through interaction with radiation and clouds. Here we present a characterization of the specifically developed inlet system and previously unreported instruments, most notably the miniaturized scanning electrical mobility spectrometer and a near-infrared carbon monoxide monitor. As of December 2022, MoMuCAMS has been tested during two field campaigns in the Swiss Alps in winter and fall 2021. It was further deployed in Fairbanks, Alaska, USA, in January–February 2022, as part of the ALPACA (Alaskan Layered Pollution and Chemical Analysis) campaign and in Pallas, Finland, in September–October 2022, as part of the PaCE2022 (Pallas Cloud Experiment) study. Three cases from one of the Swiss Alpine studies are presented to illustrate the various observational capabilities of MoMuCAMS. Results from the first two case studies illustrate the breakup of a surface-based inversion layer after sunrise and the dilution of a 50–70 m thick surface layer. The third case study illustrates the capability of the system to collect samples at a given altitude for offline chemical and microscopic analysis. Overall, MoMuCAMS is an easily deployable tethered-balloon payload with high flexibility, able to cope with the rough conditions of extreme environments. Compared to uncrewed aerial vehicles (drones) it allows for observation of aerosol processes in detail over multiple hours, providing insights into their vertical distribution and processes, e.g., in low-level clouds, that were difficult to obtain beforehand.

Fast SEMS size distribution measurements under down-scan operation

Particle size distributions obtained with a Scanning Electrical-Mobility Spectrometer (SEMS) are based on the response of charged particles in an applied electric field. A complete operation of SEMS instrument for particle sizing involves the exposure of charged particles to an exponentially increasing voltage (up-scan) followed by an exponentially decreasing voltage (down-scan). Accurate calculation of particle size distributions from SEMS measurements requires knowledge of the instrument transfer function or Kernel as a function of its operating conditions and subsequently, the inversion of an integral equation. A semi-theoretical transfer function calculation approach based on particle arrival-times was shown to accurately characterize fast up-scan SEMS operation. In this paper, we extend the arrival-time calculation approach for SEMS down-scan operation. The combination of a decreasing voltage over time and a parabolic velocity field results in particle trajectories that are fundamentally different from the up-scan operation. However, when smearing due to downstream transport of particles to the detector is considered, the down-scan transfer functions are seen to be a mirror image of the up-scan transfer functions in the mobility space. This suggests that the inversion algorithms optimized for fast up-scan operation could also be applicable for fast down-scan operation. The combination of near real-time up-scan and down-scan transfer functions should enable fast, and accurate size distribution measurements under a range of operating conditions.

Diffusional transfer function for the scanning electrical mobility spectrometer (SEMS)

The scanning electrical mobility spectrometer (SEMS), or scanning mobility particle sizer (SMPS), uses the differential mobility analyzer (DMA) operated in scanning mode to measure particle size di...

Scanning DMA Data Analysis I. Classification Transfer Function

The scanning electrical mobility spectrometer (SEMS; also known as the scanning mobility particle sizer, SMPS) enables rapid particle size distribution measurements with a differential mobility ana...

Scanning DMA data analysis II. Integrated DMA-CPC instrument response and data inversion

Analysis of scanning electrical mobility spectrometer (SEMS) or SMPS data requires coupling the scanning differential mobility analyzer (DMA) transfer function with the response functions for the instrument plumbing and the detector. In the limit of plug flow (uniform velocity) within the DMA, the scanning DMA transfer function has the same form as that for constant voltage. Most SEMS/SMPS data analysis uses this model, though previous studies have shown that boundary layers distort the transfer function during scanning DMA measurements. Part I determined the instantaneous transfer function during scanning of the TSI Model 3081 A long column DMA by modeling the flows, fields, and particle trajectories within the actual DMA geometry. This study (Part II) combines that transfer function with empirical data on the efficiencies and delay time distributions of the plumbing and detector of the SEMS/SMPS to determine the instantaneous rate at which particles are counted, and integrates the count rate over the finite counting time interval to obtain the integrated SEMS/SMPS response function. Simulations using this geometrical model are compared with those obtained using traditional, idealized DMA models for scan rates ranging from slow (240 s) to very fast (10 s), and with measurements of monodisperse calibration aerosols. Data inversion studies show that both increasing and decreasing voltage scans can be used to determine the particle size distribution, even with fast scans.

Towards near real-time SEMS size distribution measurements under up-scan operation

Near real-time particle size distribution measurements with a scanning electrical mobility spectrometer (SEMS) requires a fast scan DMA operation. The classification characteristics of the DMA when operated with an up-scan voltage can be established from calculations of particle arrival-times at the sample port of the DMA. This approach, referred to as the arrival-time transfer function (ATF) calculation approach, has been shown to accurately calculate DMA mobility transfer functions under up-scan operation for a range of scan times. Fast SEMS operation, however, are further complicated by downstream effects of particle smearing, plumbing delays (offset time) and counting time interval of the particle counter. These effects have thus far been quantified empirically. In this paper, a theoretical framework is proposed to minimize empirical corrections. The effects of particle smearing, plumbing delays (offset time) and counting time interval are incorporated in determining SEMS transfer functions, which are then used to predict the signal response to a known upstream size distribution under different operating conditions. The predicted signals are then validated with experimental data with scan times as fast 5 s. Using the calculated SEMS transfer functions and a regularization-based inversion algorithm, we are able to accurately invert SEMS signals from a scan as fast as 5 s. Our proposed SEMS transfer function calculation approach, thus, suggests that near real-time, accurate size distributions may be possible with SEMS measurements.

A multiple charging correction algorithm for scanning electrical mobility spectrometer data

Accuracy of particle size distribution measurements from scanning electrical mobility spectrometers (SEMS) is critically dependent on the quality of SEMS data inversion. A critical element of SEMS data inversion is the consideration of the charged particle fraction in the sample flow. In particular, when larger, multiply-charged particles are present in the measured aerosol, significant errors in the calculation of size distributions are possible if their contribution is not correctly accounted for. While ignoring the contribution of multiply charged particles may be acceptable when the particles are mostly in the ultrafine size range, a substantial error is possible when a significant fraction of particles are larger than 100nm. Accurate calculation of size distributions from SEMS data is possible when an inertial impactor is used to eliminate the contribution of multiply-charged particles larger than a cut-size and an iterative multiple-charge correction (MCC) algorithm is used during the data inversion process. The effectiveness of this approach is, however, strongly dependent on the relationship between the aerodynamic and the electrical mobility diameters of the particles. Here, we demonstrate the limitations of the existing inversion algorithms and propose an alternative MCC algorithm for size distribution calculation from non-ideal SEMS data. In the proposed approach, the zeroth order singly charged particle size distribution is fit using a Gumbel distribution function, and the resultant fit is used to correct for the multiply-charged contribution to the SEMS data. The effectiveness of the proposed approach is tested for a range of particle size distribution scenarios and validated with experimental data.

Improved Inversion of Scanning Electrical Mobility Spectrometer Data Using a New Multiscale Expectation Maximization Algorithm

The accuracy of particle size distributions obtained from scanning electrical mobility spectrometer (SEMS) measurements is strongly dependent on the accurate consideration of the instrument characteristics in the formulation of the SEMS problem and the effective inversion of the resulting SEMS equation. The estimation of size distributions from SEMS measurements requires a solution of the discretized form of the Fredholm integral equation of the first kind. The often ill-conditioned nature of the linear inverse problem coupled with the possible presence of measurement noise complicates these calculations. The use of standard inversion approaches, such as nonnegative least squares (NNLS) or regularization-based algorithms, requires SEMS measurements with significant signal-to-noise ratio or some a priori knowledge of the shape of the sampled size distribution. These severe constraints for SEMS measurements can be relaxed with the new multiscale expectation-maximization SEMS inversion method introduced here...

Aircraft observations of aerosol composition and ageing in New England and Mid‐Atlantic States during the summer 2002 New England Air Quality Study field campaign

Aerosol chemical composition, size distribution, and optical properties were measured during 17 aircraft flights in New England and Middle Atlantic States as part of the summer 2002 New England Air Quality Study field campaign. An Aerodyne aerosol mass spectrometer (AMS) was operated with a measurement cycle of 30 s, about an order of magnitude faster than used for ground‐based measurements. Noise levels within a single measurement period were sub μg m−3. Volume data derived from the AMS were compared with volume measurements from a Passive Cavity Aerosol Spectrometer (PCASP) optical particle detector and a Twin Scanning Electrical Mobility Spectrometer (TSEMS); calculated light scattering was compared with measured values from an integrating nephelometer. The median ratio for AMS/TSEMS volume was 1.25 (1.33 with an estimated refractory component); the median ratio for AMS/nephelometer scattering was 1.18. A dependence of the AMS collection efficiency on aerosol acidity was quantified by a comparison between AMS and PCASP volumes in two high sulfate plumes. For the entire field campaign, the average aerosol concentration was 11 μg m−3. Compared with monitoring data from the IMPROVE network, the organic component made up a large fraction of total mass, varying from 70% in clean air to 40% in high concentration sulfate plumes. In combination with other optical and chemical measurements, the AMS gave information on secondary organic aerosol (SOA) production and the time evolution of aerosol light absorption. CO is taken as a conservative tracer of urban emissions and the ratios of organic aerosol and aerosol light absorption to CO examined as a function of photochemical age. Comparisons were made to ratios determined from surface measurements under conditions of minimal atmospheric processing. In air masses in which the NOx to NOy ratio has decreased to 10%, the ratio of organic aerosol to CO has quadrupled indicating that 75% of the organic aerosol is secondary. Also, the ratio of light absorption to CO has more than doubled, which is interpreted as an equivalent increase in the light absorption efficiency of black carbon due to aerosol ageing.

A simple and versatile mini-arc plasma source for nanocrystal synthesis

Nanocrystals in the lower-nanometer-size range are attracting growing interest due to their unique properties. A simple and versatile atmospheric direct current mini-arc plasma source has been developed to produce nanoparticles as small as a few nanometers. The nanoparticles are formed by direct vaporization of solid precursors followed by a rapid quenching. Both semiconductor tin oxide and metallic silver nanoparticles have been produced at rates of 1–10 mg/h using the mini-arc source. Transmission electron microscopy and X-ray diffraction analyses indicate that most nanoparticles as produced are nonagglomerated and crystalline. Size distributions of nanoparticles measured with an online scanning electrical mobility spectrometer are broader than the self-preserving distribution, suggesting that the nanoparticle growth is coagulation-dominated, and that the particles experience a range of residence times. The electrical charges carried by as-produced aerosol nanoparticles facilitate the manipulation of nanoparticles. The new mini-arc plasma source hence shows promise to accelerate the exploration of nanostructured materials.

Analysis of nanostructured coatings synthesized by ballistic impaction of nanoparticles

SiC nanostructured coatings were synthesized by ballistic impaction of nanoparticles using a process called hypersonic plasma particle deposition (HPPD). X-ray diffraction spectra of typical samples showed the presence of crystalline SiC and Si. Grain sizes obtained through transmission electron microscopy showed particles in the sub 10 nm range with primarily crystalline β-SiC and some crystalline Si particles present. These results correlate well with particle size distributions measured using an aerosol sampling probe coupled to a scanning electrical mobility spectrometer. Interestingly, particle size distributions indicated only small changes in the particle size distributions when Si deposition was compared to SiC. Examination of adhesion characteristics highlighted the importance of a chemically bound interlayer during SiC deposition on Mo and steel substrates.

Gas Sensors Based on Tin Oxide Nanoparticles Synthesized from a Mini-Arc Plasma Source

Miniaturized gas sensors or electronic noses to rapidly detect and differentiate trace amount of chemical agents are extremely attractive. In this paper, we report on the fabrication and characterization of a functional tin oxide nanoparticle gas sensor. Tin oxide nanoparticles are first synthesized using a convenient and low-cost mini-arc plasma source. The nanoparticle size distribution is measured online using a scanning electrical mobility spectrometer (SEMS). The product nanoparticles are analyzed ex-situ by high resolution transmission electron microscopy (HRTEM) for morphology and defects, energy dispersive X-ray (EDX) spectroscopy for elemental composition, electron diffraction for crystal structure, and X-ray photoelectron spectroscopy (XPS) for surface composition. Nonagglomerated rutile tin oxide (SnO2) nanoparticles as small as a few nm have been produced. Larger particles bear a core-shell structure with a metallic core and an oxide shell. The nanoparticles are then assembled onto an e-beam lithographically patterned interdigitated electrode using electrostatic force to fabricate the gas sensor. The nanoparticle sensor exhibits a fast response and a good sensitivity when exposed to 100 ppm ethanol vapor in air.

Synthesis of Tin Oxide Nanoparticles Using a Mini-arc Plasma Source

AbstractMiniaturized electronic noses to rapidly detect and differentiate trace amount of chemical agents are extremely attractive. Use of tin oxide nanoparticles as sensing elements has been proved to significantly improve both the response time and the sensitivity of gas sensors or electronic noses. In this paper, we report the synthesis of pure tin oxide nanoparticles using a simple, convenient, and low-cost mini-arc plasma source. The nanoparticle size distribution is measured online using a scanning electrical mobility spectrometer (SEMS). The product nanoparticles are analyzed ex-situ by high resolution transmission electron microscopy (HRTEM) for morphology, crystal structure, and defects. Non-agglomerated rutile tin oxide (SnO2) nanoparticles as small as a few nm have been produced, with rounded shapes and some faceting on the lowest energy surfaces.

Fast Mixing Condensation Nucleus Counter: Application to Rapid Scanning Differential Mobility Analyzer Measurements

Condensation nucleus counters (CNCs) exhibit slower time response than expected due to mixing effects within the detector.This mixing produces an exponential distribution of delay times with a characteristic mixing time m that ranges from 0.1 s to 0.9 s for commonly used instruments and limits their usefulness for measuring rapidly changing aerosols. Moreover, when used as detectors in the scanning electrical mobility spectrometer (SEMS; also known as scanning mobility particle sizer, SMPS), CNCs limit the speed with which size distribution measurements can be made. In order to overcome this limitation, a new, fast-response mixing CNC (MCNC) has been developed and characterized. The time response of this new detector and TSI Models 3025 and 3010 CNCs has been measured using a spark source to generate an aerosol pulse. The mixing induced response smearing of this new detector, m , of this instrument is 0.058 s, which is significantly shorter than either of the other instruments tested. Its lower detection limit is about 5 nm diameter. The high aerosol flow rate of the MCNC (0.65 l min -1 ), fast time response, and low detection limit make it an ideal detector for SEMS/SMPS measurements. Using this MCNC as a detector for the SEMS, size distribution measurements over the 5 nm to 140 nm range have been made in 3 s with minimal distortion. The size distribution of a coagulation aerosol was effectively recovered by deconvolution with scans as short as 1 s. Uncertainties in the 1 s scans result, in part, from electronics problems in the scanning DMA.

Characterization of submicron aerosol size distributions from time‐resolved measurements in the Atlantic Stratocumulus Transition Experiment/Marine Aerosol and Gas Exchange

As part of the Atlantic Stratocumulus Transition Experiment/Marine Aerosol and Gas Exchange (ASTEX/MAGE), ship‐based aerosol size distribution measurements have been carried out with a scanning electrical mobility spectrometer (SEMS) aboard the R/V Oceanus. The fast time resolution of these measurements illustrates some of the short‐timescale variability of both the marine background aerosol and the anthropogenically influenced continental air masses. The resulting total number concentrations are largely in agreement with those from a collocated condensation nucleus counter (CNC). The aerosol size distributions provide characteristic signatures for different atmospheric conditions, showing low‐concentration bimodal distributions in cleaner air masses and higher‐concentration single‐mode distributions in air masses with apparent recent continental influence.

Nanoparticle formation using a plasma expansion process

We describe a process in which nanosize particles with u narrow size distribution are generated by expanding a thermal plasma carrying vapor-phase precursors through a nozzle. The plasma temperature and velocity profiles are characterized by enthalpy probe measurements. by calorimetric energy balances. and by a model of the nozzle flow. Aerosol samples are extracted from the flow downstream of the nozzle by means of a capillary probe interfaced to a two-stage ejection diluter. The diluted aerosol is directed to a scanning electrical mobility spectrometer (SEMS) which provides on-line size distributions down to particle diameters of 4 nmt. We have generated silicon, carbon, and silicon carbide particles with number mean diameters of about 10 not or less, and we have obtained some correlations between the product and the operating conditions. Inspection of the size distributions obtained in the experiments, together with the modeling results, suggests that under our conditions silicon carbide formation is initiated by nucleation of extremely small silicon particles from supersaturated silicon vapor, followed by chemical reactions at the particle surfaces involving carbon-containing species from the gas phase.

Asymmetric Instrument Response Resulting from Mixing Effects in Accelerated DMA-CPC Measurements

Several method have been proposed to accelerate the measurements made with differential mobility analyzers (DMA), including the Scanning Electrical Mobility Spectrometer (SEMS) and the Scanning Mobility Particle Sizer (SMPS). Wang and Flagan (1990) developed a data analysis procedure that accounts for the migration of the particles through a time-varying electric field and the delay associated with transport from the analyzer column outlet to the detection point. Experience using a variety of detectors and scan rates has indicated that the instrument response depends on the plumbing configuration if a condensation particle counter (CPC) is used as a detector. When a sharply peaked distribution is analyzed, the apparent breadth of the measured distribution depends on the detector used and on the scan rate. Size distributions measured with an increasing voltage scan exhibit a tail toward large particle sizes, while decreasing voltage scans produce a more pronounced tail on the small particle end of the distribution. The reduction in peak height with increasing scan rate can be attributed to particle retention in the plumbing between the outlet of the DMA analyzer column and the point in the CPC where the particles are detected optically. This paper examines the smearing of the transfer function of the SEMS as a result of flow non-idealities in the system. A model has been developed to predict the distortion of the transfer function in terms of the particle residence time distribution within the instrument. Results of this model are compared with calibration data as a function of the detector employed and the scan rate. We include laboratory observations of the phenomenon and examine the use of data inversion techniques to retrieve the true size distribution.

Electrical mobility measurements of fine-particle formation during chamber studies of atmospheric photochemical reactions

New approaches have been applied to electrical mobility measurement of ultrafine aerosol particles in smog chamber studies of secondary aerosol formation. With several mobility classifiers operating in parallel, rapid new particle formation was followed in the photochemical oxidation of dimethyl disulfide. When foreign particles were present before reaction was initiated, multiple bursts of nucleation and oscillations in the concentrations of 3.4-nm particles were observed. Later experiments used the scanning electrical mobility spectrometer to make high-resolution particle size distribution measurements. With this measurement method, the rapid growth of nuclei from their initial appearance at 10-nm size was followed in hydrocarbon/NO_x and hydrocarbon/NO_x/SO_2 reactions. Again, multiple bursts of nucleation were observed in some experiments, and insights were gained into particle growth mechanisms.

Inversion of aerosol data from the epiphaniometer

The epiphaniometer is a new instrument for continuous monitoring of the Fuchs surface of aerosol particles using the attachment of neutral radioactive 211Pb atoms to the particles. Since the 211Pb atoms have a half-life of 36.1 min, the ability of the epiphaniometer to detect short-term fluctuations in the Fuchs surface of the aerosol is limited. To overcome this drawback a general algorithm developed for the inversion of size distribution data from conventional aerosol instruments is adapted for the inversion of data from the epiphaniometer. This algorithm allows resolution of dramatic changes in the aerosol Fuchs surface occurring over a time scale of minutes. The method is applied to data collected during outdoor smog chamber experiments, with changes in aerosol concentration of several orders of magnitude occurring within a time period of 5 min. The inverted epiphaniometer data are in agreement with the corresponding data from a Scanning Electrical Mobility Spectrometer (SEMS) and an Optical Particle Counter.