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This article presents a validation study of the stochastic particle-resolved aerosol model PartMC with experimental data from an aerosol chamber experiment. For the experiment, a scanning mobility particle sizer and a single-particle soot photometer were used to monitor the aerosol mixing state evolution of two initially externally mixed aerosol populations of ammonium sulfate and black carbon particles undergoing agglomeration. We applied an efficient optimization algorithm (ProSRS) to determine several unconstrained simulation parameters and were able to successfully reproduce number concentrations and size distributions of mixed particles that formed by agglomeration. The PartMC modeling approach in conjunction with the optimization procedure provides a tool for detailed comparisons of chamber experiments and modeling, where aerosol mixing state is the focus of investigation. Copyright © 2019 American Association for Aerosol Research [ABSTRACT FROM AUTHOR]

Aerosol intensive optical properties, including Ångström exponents for aerosol light extinction (EAEs), scattering (SAEs) and absorption (AAEs) as well as and the single-scattering albedo (SSA), are indicators for aerosol size, chemical composition, radiative behaviour and particle sources. Derivation of these parameters requires the measurement of aerosol optical properties at multiple wavelengths, which usually involves the use of several instruments. Our study aims to quantify the uncertainties in the determination of these intensive properties using an optical closure approach. In our laboratory closure study, we measured the full set of optical properties for a range of light-absorbing particles with different properties externally mixed with ammonium sulfate to generate aerosols with controlled SSA values. The investigated absorbing particle types were fresh combustion soot emitted by an inverted flame soot generator (SOOT; fractal agglomerates), Aquadag (AQ; compact aggregates), Cabot black (BC; compact agglomerates) and an acrylic paint (magic black, shape unknown). The instruments used in this study were two cavity attenuated phase shift particle monitors for single-scattering albedo (CAPS PM SSA 's; λ=450 , 630 nm) for measuring light-extinction and light-scattering coefficients, one integrating nephelometer (λ=450 , 550, 700 nm) for light-scattering coefficients, and one tricolour absorption photometer (TAP; λ=467 , 528, 652 nm) for filter-based light-absorption coefficients. One key finding is that the coefficients of light absorption, scattering and extinction derived from combing the measurements of two independent instruments agree with measurements from single instruments; the slopes of regression lines are equal within reported uncertainties (i.e. closure is observed). Despite closure for measured absorption coefficients, we caution that the estimated uncertainties for absorption coefficients, propagated for the differential method (DM; absorption = extinction minus scattering), can exceed 100 % for atmospherically relevant SSA values (>0.9). This increasing estimated uncertainty with increasing SSA yields AAE values that may be too uncertain for measurements in the range of atmospheric aerosol loadings. We recommend using the DM for measuring AAE values when the SSA < 0.9. EAE- and SAE-derived values achieved closure during this study within stated uncertainties for extinction coefficients greater than 15 Mm -1. SSA values for 450 and 630 nm wavelengths internally agreed with each other within 10 % uncertainty for all instrument combinations and sampled aerosol types, which fulfils the defined goals for measurement uncertainty of 10 % proposed by Laj et al. (2020) for GCOS (Global Climate Observing System) applications. [ABSTRACT FROM AUTHOR]

The U. S. Environmental Protection Agency in collaboration with the U. S. Air Force Arnold Engineering Development Complex conducted the VAriable Response In Aircraft nvPM Testing (VARIAnT) 3 and 4 test campaigns to compare nonvolatile particulate matter (nvPM) emissions measurements from a variety of diffusion flame combustion aerosol sources (DFCASs), including a Cummins diesel engine, a diesel powered generator, two gas turbine start carts, a J85-GE-5 turbojet engine burning multiple fuels, and a Mini-CAST soot generator. The VARIAnT research program was devised to understand reported variability in the ARP6320A sampling system nvPM measurements. The VARIAnT research program has conducted four test campaigns to date with the VARIAnT 3 and 4 campaigns devoted to: (1) assessing the response of three different black carbon mass analyzers to particles of different size, morphology, and chemical composition; (2) characterizing the particles generated by 6 different combustion sources according to morphology, effective density, and chemical composition; and (3) assessing any significant difference between black carbon as determined by the 3 mass analyzers and the total PM determined via other techniques. Results from VARIAnT 3 and 4 campaigns revealed agreement of about 20% between the Micro-Soot Sensor, the Cavity Attenuated Phase Shift (CAPS PM SSA) monitor and the thermal-optical reference method for elemental carbon (EC) mass, independent of the calibration source used. For the LII-300, the measured mass concentrations in VARIAnT 3 fall within 18% and in VARIAnT 4 fall within 27% of the reference EC mass concentration when calibrated on a combustor rig in VARIAnT 3 and on an LGT-60 start cart in VARIAnT 4, respectively. It was also found that the three mass instrument types (MSS, CAPS PM SSA , and LII-300) can exhibit different BC to reference EC ratios depending on the emission source that appear to correlate to particle geometric mean mobility diameter, morphology, or some other parameter associated with particle geometric mean diameter (GMD) with the LII-300 showing a slightly stronger apparent trend with GMD. Systematic differences in LII-300 measured mass concentrations have been reduced by calibrating with a turbine combustion as a particle source (combustor or turbine engine). With respect to the particle size measurements, the sizing instruments (TSI SMPS, TSI EEPS, and Cambustion DMS 500) were found to be in general agreement in terms of size distributions and concentrations with some exceptions. Gravimetric measurements of the total aerosol mass produced by the various DFCAs differed from the reference EC, BC and integrated particle size distribution measured aerosol masses. The measurements of particle size distributions and single particle analysis performed using the miniSPLAT indicated the presence of larger particles (≳150 nm) having more compact morphologies, higher effective density, and a composition dominated by OC and containing ash. This increased large particle fraction is also associated with higher values of single scattering albedo measured by the CAPS PM SSA instrument and higher OC measurements. These measurements indicate gas turbine engine emissions can be a more heterogeneous mix of particle types beyond the original E−31 assumption that engine exit exhaust particles are mainly composed of black carbon. • PA and CAPS BC mass measurement methods have less variability than the LII BC method. • Gravimetric mass 30–40% greater than reference EC and 50–60% greater than BC masses. • Secondary particle mode >150 nm was observed dominated by non-volatile OC and containing ash. • Calibration of LII with a turbine combustion source more effective than other methods. [ABSTRACT FROM AUTHOR]

We present laboratory and field measurements of aerosol light extinction (σep) using an instrument that employs Cavity Attenuated Phase Shift (CAPS) spectroscopy. The CAPS extinction monitor comprises a light emitting diode (LED), an optical cavity that acts as the sample cell, and a vacuum photodiode for light detection. The particle σep is determined from changes in the phase shift of the distorted waveform of the square-wave modulated LED light that is transmitted through the optical cell. The 3-σ detection limit of the CAPS monitor under dry particle-free air is 3 Mm-1 for 1s integration time. Laboratory measurements of absolute particle extinction cross section (σext) using non-absorbing, monodisperse polystyrene latex (PSL) spheres are made with an average precision of ± 3% (2-σ) at both 445 and 632 nm. A comparison with Mie theory scattering calculations indicates that these results are accurate within the 10% uncertainty stated for the particle number density measurements. The CAPS extinction monitor was deployed twice in 2009 to test its robustness and performance outside of the laboratory environment. During these field campaigns, a co-located Multi Angle Absorption Photometer (MAAP) provided particle light absorption coefficient (σap) at 635 nm: the single scattering albedo (ω) of the ambient aerosol particles was estimated by combining the CAPS σep measured at 632 nm with the MAAP σap data. Our initial results show the high potential of the CAPS as lightweight, compact instrument to perform precise and accurate σep measurements of atmospheric aerosol particles in both laboratory and field conditions. [ABSTRACT FROM AUTHOR]

The Aerodyne Aerosol Mass Spectrometer (AMS) is a useful tool to study ambient particles. To be quantitative, the mass or (number) of particles detected by the AMS relative to the mass (or number) of particles sampled by the AMS, or the AMS collection efficiency (CE), must be known. Here we investigated the effect of particulate phase on AMS CE for ammonium nitrate, ammonium sulfate, mixed ammonium nitrate/ammonium sulfate, and ammonium sulfate particles coated with an organic liquid. Dry, solid ammonium sulfate particles were sampled with a CE of 24 ± 3%. Liquid droplets and solid particles that were thickly coated with a liquid organic were collected with a CE of 100%. Mixed phase particles, solid particles thinly coated with liquid organic, and metastable aqueous ammonium sulfate droplets had intermediate CEs. The higher CEs for liquid particles compared with solid particles were attributed to wet or coated particles tending to stick upon impact with the AMS vaporizer, while a significant fraction of solid particles bounced prior to vaporization/detection. The consistency of single particle signals indicated that the phase (and hence CE) of mixed component particles did not affect the AMS sensitivity to a particular chemical species once volatilization occurred. Particle phase might explain a significant fraction of the variable AMS CEs reported in the literature. For example, ambient particles that were liquid (e.g., composition dominated by ammonium nitrate or acidic sulfate) have been reported to be sampled with 100% CE. In contrast, most ambient particle measurements report CEs of < 100% (typically~ 50%). [ABSTRACT FROM AUTHOR]

Aerosol intensive optical properties like the Ångström exponents for aerosol light extinction, scattering and absorption, or the single-scattering albedo are indicators for aerosol size distributions, chemical composition and radiative behaviour and contain also source information. The observation of these parameters requires the measurement of aerosol optical properties at multiple wavelengths which usually implies the use of several instruments. Our study aims to quantify the uncertainties of the determination of multiple-wavelengths intensive properties by an optical closure approach, using different test aerosols. In our laboratory closure study, we measured the full set of aerosol optical properties for a range of light-absorbing aerosols with different properties, mixed externally with ammonium sulphate to generate aerosols of controlled single-scattering albedo. The investigated aerosol types were: fresh combustion soot emitted by an inverted flame soot generator (SOOT, fractal aggregates), Aquadag (AQ, spherical shape), Cabot industrial soot (BC, compact clusters), and an acrylic paint (Magic Black, MB). One focus was on the validity of the Differential Method (DM: absorption = extinction minus scattering) for the determination of Ångström exponents for different particle loads and mixtures of light-absorbing aerosol with ammonium sulphate, in comparison to data obtained from single instruments. The instruments used in this study were two CAPS PMssa (Cavity Attenuated Phase Shift Single Scattering Albedo, ? = 450, 630 nm) for light extinction and scattering coefficients, one Integrating Nephelometer (? = 450, 550, 700 nm) for light scattering coefficient and one Tricolour Absorption Photometer (TAP, ? = 467, 528, 652 nm) for filter-based light absorption coefficient measurement. Our key finding is that the coefficients of light absorption sap, scattering ssp and extinction sep from the Differential Method agree with data from single reference instruments, and the slopes of regression lines equal unity within the precision error. We found, however, that the precision error for the DM exceeds 100% for sap values lower than 10-20 Mm-1 for atmospheric relevant single scattering albedo. This increasing uncertainty with decreasing sap yields an absorption Ångström exponent (AAE) that is too uncertain for measurements in the range of atmospheric aerosol loadings. We recommend using DM only for measuring AAE values for sap > 50 Mm-1. Ångström exponents for scattering and extinction are reliable for extinction coefficients from 20 up to 1000 Mm-1 and stay within 10% deviation from reference instruments, regardless of the chosen method. Single-scattering albedo (SSA) values for 450 nm and 630 nm wavelengths agree with values from the reference method ssp (NEPH)/sep (CAPS PMSSA) with less than 10% uncertainty for all instrument combinations and sampled aerosol types which fulfil the defined goals for measurement uncertainty of 10% proposed by Laj et al., 2020 for GCOS (Global Climate Observing System) applications. [ABSTRACT FROM AUTHOR]

A fluidized bed aerosol generator has been designed and built for the purpose of generating a constant output of dry, submicrometer particles with a large number density. The output of the fluidized bed for generating aerosol particles from dry soot powder has been characterized using a differential mobility analyzer and a condensation particle counter. The particle size distribution is bimodal, with a mode in the submicrometer diameter size range and a mode in the supermicrometer diameter size range. The larger diameter mode is fully separated from the smaller mode and can thus be easily removed from the aerosol flow using impaction techniques. The distribution in the submicrometer size range is nearly log-normal, with a count median diameter falling between 0.1 and 0.3 micrometers. A number density of greater than 10[sup 5] particles cm[sup -3] of soot particles in the submicrometer range can be produced, constant to within 25% (1 standard deviation) over a 4 h time period. The number density of particles produced in the submicrometer range was found to vary with the ratio of soot to bronze beads in the bed mixture, whether or not a feed system was used, and nitrogen flow rate through the fluidized bed and feed system. [ABSTRACT FROM AUTHOR]

We report the development and validation of a new humidified aerosol single-scattering albedometer to quantify the effects of water uptake on submicrometer particle optical properties. The instrument simultaneously measures in situ aerosol light extinction (σep) and scattering (σsp) using a cavity-attenuated phase shift-single scattering albedo particulate matter (PM) monitor (CAPS-PMSSA, Aerodyne Research, Inc., Billerica, MA, USA). It retrieves by difference aerosol light absorption (σap) and directly quantifies aerosol single-scattering albedo (SSA), the aerosol "brightness." We custom built a relative humidity (RH) control system using a water vapor-permeable membrane humidifier and coupled it to the CAPS-PMSSA to enable humidified aerosol observations. Our humidified instrument (H-CAPS-PMSSA) overcomes problems with noise caused by mirror purge-flow humidification, heating, and characterizing cell RH. Careful angular truncation corrections in scattering, particularly for larger particles, were combined with empirical observations. Results show that the optimal operational size to be Dp < 400 nm. The H-CAPS-PMSSA was evaluated with several pure single-component aerosols including ammonium sulfate ((NH4)2SO4), absorbing nigrosin, and levoglucosan, an organic biomass smoke tracer. The measured σep, σsp, and the derived optical hygroscopicity parameter (κ) for size-selected ammonium sulfate are in good agreement with literature values. For dry size-selected nigrosin in the 100 < Dp < 400 nm range, SSA values increased from ∼0.3 to 0.65 with increasing Dp. The enhancement in nigrosin σap at RH = 80% was a factor of 1.05–1.20 relative to dry conditions, with the larger particles showing greater enhancement. SSA increased with RH with the largest fractional enhancement measured for the smallest particles. For polydisperse levoglucosan, we measured an optical κ of 0.26 for both light extinction and scattering and negligible absorption. Our new instrument enables reliable observations of the effects of ambient humidity on mixed aerosol optical properties, particularly for light-absorbing aerosols whose climate forcing is uncertain due to measurement gaps. Copyright © 2021 American Association for Aerosol Research [ABSTRACT FROM AUTHOR]

The Aerodyne Soot Particle Aerosol Mass Spectrometer (SP-AMS) can operate with one or both of two particle vaporizers: (1) the standard resistively heated tungsten vaporizer (TV) for detection of non-refractory particulate matter (NR-PM), and (2) an intracavity laser vaporizer (LV) for detection of absorbing, refractory materials, including refractory black carbon (rBC), metal nanoparticles, and associated coatings. In addition to using these vaporizers individually, both can be used simultaneously (dual vaporizer, DV, mode), commonly implemented by sequentially turning the laser vaporizer on (DV-Lon) and off (DV-Loff). Recent results in DV mode have shown an apparent enhancement in measured NR-PM signal during DV-Lon compared with DV-Loff, even for inorganic species unlikely to be strongly associated with rBC. The use of dual vaporizers is complicated by different collection efficiencies (CE) of the two vaporizers, potential differences in the relative ionization efficiencies (RIE) for the same species vaporized from the two vaporizers, and potential interference between the two vaporizers due to laser heating of AMS internal components. Here, we investigated the effect of the laser-heating interference on NR-PM signal. We tested a variety of laser baffles to minimize laser-heating interferences and to constrain the alignment of the laser vaporizer and we identified a baffle that meets these criteria and can be incorporated into SP-AMS systems. For well aligned standard and laser vaporizers, laser heating was found to have only a minor effect on the NR-PM ion signals. Most of the observed increases in NR-PM signals during DV-Lon are attributed to vaporizer-specific differences in the CE and RIE values of NR-PM associated with the rBC-containing particles. Copyright © 2020 American Association for Aerosol Research [ABSTRACT FROM AUTHOR]

Abstract The composition and carbon nanostructure of soot are important parameters influencing health and climate effects, and the efficacy of soot mitigation technologies. We used laser-vaporization, electron-ionization aerosol mass spectrometry (or SP-AMS) to systematically investigate relationships between aerosol mass spectra, carbon nanostructure (HRTEM), and composition (thermal-optical carbon analysis) for soot with varying physicochemical properties. SP-AMS refractory black carbon concentrations (based on C ≤ 5 + clusters) were correlated to elemental carbon (r = 0.98, p < 10−8) and equivalent black carbon (aethalometer) concentrations. The SP-AMS large carbon ( C ≥ 6 + , midcarbons and fullerene carbons) fraction was inversely correlated to fringe length (r = −0.97, p = 0.028) and linearly correlated to the fraction of refractory organic carbon that partially pyrolize during heating (r = 0.89, p < 10−4). This refractory organic carbon material was incompletely detected with conventional aerosol mass spectrometry (flash vaporization at 600 °C). This suggests that (SP-AMS) refractory carbon cluster analysis provides insight to chemical bonding and nanostructures in refractory carbon materials, lowcarbons ( C ≤ 5 + ) indicate mature soot and large carbons indicate refractory organic carbon and amorphous nanostructures related to C 5 -components. These results have implications for assessments of soot particle mixing state and brown carbon absorption in the atmosphere and enable novel, on-line analysis of engineered carbon nanomaterials and soot characteristics relevant for climate and health. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

The Handix Scientific open-path cavity ringdown spectrometer (OPCRDS) was deployed during summer 2016 in Great Smoky Mountains National Park (GRSM). Extinction coefficients from the relatively new OPCRDS and from a more well-established extinction instrument agreed to within 7%. Aerosol hygroscopic growth (f(RH)) was calculated from the ratio of ambient extinction measured by the OPCRDS to dry extinction measured by a closed-path extinction monitor (Aerodyne’s cavity-attenuated phase shift particulate matter extinction monitor [CAPS PMex]). Derived hygroscopicity (relative humidity [RH] < 95%) from this campaign agreed with data from 1995 at the same site and time of year, which is noteworthy given the decreasing trend for organics and sulfate in the eastern United States. However, maximum f(RH) values in 1995 were less than half as large as those recorded in 2016—possibly due to nephelometer truncation losses in 1995. Two hygroscopicity parameterizations were investigated using high-time-resolution OPCRDS+CAPS PMex data, and the κext model was more accurate than the gamma model. Data from the two ambient optical instruments, the OPCRDS and the open-path nephelometer, generally agreed; however, significant discrepancies between ambient scattering and extinction were observed, apparently driven by a combination of hygroscopic growth effects, which tend to increase nephelometer truncation losses and decrease sensitivity to the wavelength difference between the two instruments as a function of particle size. There was not a statistically significant difference in the mean reconstructed extinction values obtained from the original and the revised IMPROVE (Interagency Monitoring of Protected Visual Environments) equations. On average, IMPROVE reconstructed extinction was ~25% lower than extinction measured by the OPCRDS, which suggests that the IMPROVE equations and 24-hr aerosol data are moderately successful in estimating current haze levels at GRSM. However, this conclusion is limited by the coarse temporal resolution and the low dynamic range of the IMPROVE reconstructed extinction. Implications: Although light extinction, which is directly related to visibility, is not directly measured in U.S. National Parks, existing IMPROVE protocols can be used to accurately infer visibility for average humidity conditions, but during the large fraction of the year when humidity is above or below average, accuracy is reduced substantially. Furthermore, nephelometers, which are used to assess the accuracy of IMPROVE visibility estimates, may themselves be biased low when humidity is very high. Despite reductions in organic and sulfate particles since the 1990s, hygroscopicity, particles’ affinity for water, appears unchanged, although this conclusion is weakened by the previously mentioned nephelometer limitations. [ABSTRACT FROM AUTHOR]

A photo-acoustic absorption spectrometer (PAS) and a cavity ring down aerosol extinction spectrometer (CRD-AES) were used, in conjunction with Mie Theory, to measure the refractive index (RI) of absorbing polystyrene spheres (APSS). The PAS and CRD-AES were also used to measure the absorption and extinction enhancement after these APSS were coated in oleic acid. The experimental enhancements were then compared to predictions from coated-sphere Mie Theory. The measured absorption and extinction enhancements both agreed with modeled enhancements to within an average of 5%. A filter-based absorption technique (particle soot absorption photometer, PSAP) was also used to measure the absorption by the APSS and showed a significant size-dependent bias, as evidenced by the filter-based method measuring significantly lower absorption for both uncoated and coated APSS compared to the PAS. These results suggest the validity of applying photo-acoustics to measure the absorption enhancement created by semi-volatile atmospheric species coating absorbing particles. [ABSTRACT FROM AUTHOR]

We report evidence that carbon impregnated conductive silicone tubing used in aerosol sampling systems can introduce two types of experimental artifacts: (1) silicon tubing dynamically absorbs carbon dioxide gas, requiring greater than 5 minutes to reach equilibrium and (2) silicone tubing emits organic contaminants containing siloxane that are adsorbed onto particles traveling through it and onto downstream quartz fiber filters. The consequence can be substantial for engine exhaust measurements as both artifacts directly impact calculations of particulate mass-based emission indices. The emission of contaminants from the silicone tubing can result in overestimation of organic particle mass concentrations based on real-time aerosol mass spectrometry and the off-line thermal analysis of quartz filters. The adsorption of siloxane contaminants can affect the surface properties of aerosol particles; we observed a marked reduction in the water-affinity of soot particles passed through conductive silicone tubing. These combined observations suggest that the silicone tubing artifacts may have wide consequence for the aerosol community and the tubing should, therefore, be used with caution. Contamination associated with the use of silicone tubing was observed at ambient temperature and, in some cases, was enhanced by mild heating (<70°C) or pre-exposure to a solvent (methanol). Further evaluation is warranted to quantify systematically how the contamination responds to variations in system temperature, physicochemical particle properties, exposure to solvent, sample contact time, tubing age, and sample flow rates. [ABSTRACT FROM AUTHOR]

Abstract: A novel charge-based technique for classifying fractal-like aerosol agglomerates based on their morphology is demonstrated. The study discusses the application of this technique to flame soot aerosols where singly and doubly net-charged agglomerates of similar sizes were segregated using electrostatic classifiers and shown to have different morphologies. The flexibility and simplicity of this technique does not limit its application to aerosols, making it an attractive candidate for performing particle shape selection of different types of nano and micromaterials. [Copyright &y& Elsevier]

A recently developed method to rapidly quantify the elemental composition of bulk organic aerosols (OA) using a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) is improved and applied to ambient measurements. Atomic oxygen-to-carbon (O/C) ratios characterize the oxidation state of OA, and O/C from ambient urban OA ranges from 0.2 to 0.8 with a diurnal cycle that decreases with primary emissions and increases because of photochemical processing and secondary OA (SOA) production. Regional O/C approaches 0.9. The hydrogen-to-carbon (H/C, 1.4-1.9) urban diurnal profile increases with primary OA (POA) as does the nitrogen-to-carbon (N/C, ∼0.02). Ambient organic-mass-to-organic-carbon ratios (OM/OC) are directly quantified and correlate well with O/C (R² = 0.997) for ambient OA because of low N/C. Ambient O/C and OM/OC have values consistent with those recently reported from other techniques. Positive matrix factorization applied to ambient OA identifies factors with distinct O/C and OM/OC trends. The highest O/C and OM/OC (1.0 and 2.5, respectively) are observed for aged ambient oxygenated OA, significantly exceeding values for traditional chamber SOA, while laboratory-produced primary biomass burning OA (BBOA) is similar to ambient BBOA, O/C of 0.3-0.4. Hydrocarbon-like OA (HOA), a surrogate for urban combustion POA, has the lowest O/C (0.06-0.10), similar to vehicle exhaust An approximation for predicting O/C from unit mass resolution data is also presented. [ABSTRACT FROM AUTHOR]

The size-dependent particle transmission efficiency of the aerodynamic lens system used in the Aerodyne Aerosol Mass Spectrometer (AMS) was investigated with computational fluid dynamics (CFD) calculations and experimental measurements. The CFD calculations revealed that the entire lens system, including the aerodynamic lens itself, the critical orifice which defines the operating lens pressure, and a valve assembly, needs to be considered. Previous calculations considered only the aerodynamic lens. The calculations also investigated the effect of operating the lens system at two different sampling pressures, 7.8 × 104 Pa (585 torr) and 1.0 × 105 Pa (760 torr). Experimental measurements of transmission efficiency were performed with size-selected diethyl hexyl sebacate (DEHS), NH4NO3, and NaNO3 particles on three different AMS instruments at two different ambient sampling pressures (7.8 × 104 Pa, 585 torr and 1.0 × 105 Pa, 760 torr). Comparisons of the measurements and the calculations show qualitative agreement, but there are significant deviations which are as yet unexplained. On the small size end (30 nm to 150 nm vacuum aerodynamic diameter), the measured transmission efficiency is lower than predicted. On the large size end (> 350 nm vacuum aerodynamic diameter) the measured transmission efficiency is greater than predicted at 7.8 × 104 Pa (585 torr) and in good agreement with the prediction at 1.0 × 105 Pa (760 torr). [ABSTRACT FROM AUTHOR]

A light scattering module has been integrated into the current AMS instrument. This module provides the simultaneous measurement of vacuum aerodynamic diameter (dva) and scattered light intensity (RLS) for all particles sampled by the AMS above ∼180 nm geometric diameter. Particle counting statistics and correlated chemical ion signal intensities are obtained for every particle that scatters light. A single calibration curve converts RLS to an optical diameter (do). Using the relationship between dva and do the LS-AMS provides a real-time, per particle measurement of the density of the sampled aerosol particles. The current article is focused on LS-AMS measurements of spherical, non-absorbing aerosol particles. The laboratory characterization of LS-AMS shows that a single calibration curve yields the material density of spherical particles with real refractive indices (n) over a range from 1. 41 < n < 1. 60 with an accuracy of about ±10%. The density resolution of the current LS-AMS system is also shown to be 10% indicating that externally mixed inorganic/organic aerosol distributions can be resolved. In addition to the single particle measurements of dva and RLS, correlated chemical ion signal intensities are obtained with the quadrupole mass spectrometer. A comparison of the particle mass derived from the physical (RLS and dva) and chemical measurements provides a consistency check on the performance of the LS-AMS. The ability of the LS-AMS instrument to measure the density of ambient aerosol particles is demonstrated with sample results obtained during the Northeast Air Quality Study (NEAQS) in the summer of 2004. [ABSTRACT FROM AUTHOR]

Aerodynamic lens inlets have revolutionized aerosol mass spectrometry by allowing the introduction of a very narrow particle beam into a vacuum chamber for subsequent analysis. The real-time measurement of particle beam width after an aerodynamic lens is of interest for two reasons: (1) it allows a correction to be made to the measured particle concentration if the beam is so broad, due to poor focusing by non-spherical particles, that some particles miss the detection system; and (2) under constant lens pressure it can provide a surrogate particle non-sphericity measurement. For these reasons, a beam width probe (BWP) has been designed and implemented for the Aerodyne Aerosol Mass Spectrometer (AMS), although this approach is also applicable to other instruments that use aerodynamic lens inlets. The probe implemented here consists of a thin vertical wire that can be precisely positioned to partially block the particle beam at fixed horizontal locations in order to map out the width of the particle beam. A computer model was developed to optimize the BWP and interpret its experimental data. Model assumptions were found to be reasonably accurate for all laboratory-generated particle types to which the model was compared. Comparisons of particle beam width data from a number of publications are also shown here. Particle losses due to beam broadening are found to be minor for the AMS for both laboratory and ambient particles. The model was then used to optimize the choice of the BWP dimensions, and to guide its use during continuous operation. A wire diameter approximately 1.55 times larger than the beam width to be measured provides near optimal sensitivity toward both collection efficiency and surrogate non-sphericity information. Wire diameters of 0.62 mm and 0.44 mm (for the AMS “long” and “short” chambers, respectively) provide reasonable sensitivity over the expected range of particle beam widths, for both spherical and non-spherical particles. Three other alternative BWP geometries were also modeled and discussed. [ABSTRACT FROM AUTHOR]

A generalised method for the deconvolution of mass spectral data from the aerodyne aerosol mass spectrometer (AMS) is presented. In this instrument, the sampled ensemble of gas and non-refractory particle phase materials interfere with each other in the mass spectra and the data must be systematically analyzed to generate meaningful, quantitative and chemically resolved results. The method presented here is designed to arithmetically separate the raw data into partial mass spectra for distinct chemical species. This technique was developed as part of the AMS analysis tools introduced by Allan et al. (J. Geophys. Res. Atmos. 108 (2003) 4090) and is in use by most groups within the AMS users community. This technique employs a user-definable ‘fragmentation table’ for each chemical species or group of species, and examples of some tables designed for the interpretation of field data are given. The ongoing work being performed to develop and validate the tables will be presented in future publications. [Copyright &y& Elsevier]