Particle size and chemical composition effects on elemental analysis with the nano aerosol mass spectrometer

In the Nano Aerosol Mass Spectrometer (NAMS), particles are irradiated with a high energy laser pulse to produce a plasma that quantitatively disintegrates each particle into positively charged atomic ions. Previous work with this method used electrodynamic focusing and trapping of particles 30 nm dia. and below. In the current work, an aerodynamic focusing inlet was used to study particles between 40 and 150 nm dia. The distribution of atomic ion charge states was found to be particle size dependent, shifting toward lower charges with increasing size. This shift also affected the calibration by which elemental composition was determined from atomic ion signal intensities. Size independent calibration could be achieved by restricting the analysis to particles that gave more than 90% of the total signal intensity as multiply charged ions. This approach worked best for particles smaller than about 100 nm dia. since most spectra met this criterion. For the nanoparticles studied, the elemental mole fractions of Group I and II metals, halogens, and low atomic mass nonmetals could be determined within 10% or less of the expected value when the mole fraction was at the 1% level or greater. Some transition and heavy metals could not be quantified, while others could. Quantification appeared to be dependent on the ability of the element to be vaporized. Elements with high melting and boiling points gave particle mass spectra similar to those obtained by laser desorption ionization—mostly singly charged ions with relative intensities strongly biased toward atoms with low ionization energies.

Copyright © 2017 American Association for Aerosol Research     

Evaluation of the new capture vaporizer for aerosol mass spectrometers: Characterization of organic aerosol mass spectra

The Aerosol Mass Spectrometer (AMS) and Aerosol Chemical Speciation Monitor (ACSM) are widely used for quantifying submicron aerosol mass concentration and composition, in particular for organic aerosols (OA). Using the standard vaporizer (SV) installed in almost all commercial instruments, a collection efficiency (CE) correction, varying with aerosol phase and chemical composition, is needed to account for particle bounce losses. Recently, a new “capture vaporizer” (CV) has been shown to achieve CE～1 for ambient aerosols, but its chemical detection properties show some differences from the SV due to the increased residence time of particles and vaporized molecules inside the CV. This study reports on the properties and changes of mass spectra of OA in CV-AMS using both AMS and ACSM for the first time. Compared with SV spectra, larger molecular-weight fragments tend to shift toward smaller ions in the CV due to additional thermal decomposition arising from increased residence time and hot surface collisions. Artifact CO⁺ ions (and to a lesser extent, H₂O⁺), when sampling long chain alkane/alkene-like OA (e.g., squalene) in the CV during the laboratory studies, are observed, probably caused by chemical reactions between sampled OA and molybdenum oxides on the vaporizer surfaces (with the carbon derived from the incident OA). No evidence for such CO⁺ enhancement is observed for ambient OA. Tracer ion marker fractions (f_m/z =, i.e., the ratio of the organic signal at a given m/z to the total OA signal), which are used to characterize the impact of different sources are still present and usable in the CV. A public, web-based spectral database for mass spectra from CV-AMS has been established.

Copyright © 2018 American Association for Aerosol Research     

Ionization efficiency of evolved gas molecules from aerosol particles in a thermal desorption aerosol mass spectrometer: Laboratory experiments

Thermal desorption aerosol mass spectrometers (TDAMSs) with electron ionization are widely used for quantitative analysis of aerosol chemical composition, and the ionization efficiency of evolved gas molecules from aerosol particles is an important parameter for such analysis. We performed laboratory experiments using a custom-made TDAMS to investigate the key factors affecting ionization efficiency. Ammonium chloride (NH₄Cl) and ammonium iodide (NH₄I) were used as test compounds because their thermal decomposition products are expected to be simple (dominated by ammonia (NH₃) and hydrogen halide (HX)). The ion signals originating from NH₃ and HX were measured by altering the position of the ionizer relative to the vaporization point. The ratio of ion signal from NH₃ to that from HX increased with increasing divergence angle of evolved gas plumes, which suggests that the angular distribution of gas molecules could be an important factor affecting the ionization efficiency.

Copyright © 2018 American Association for Aerosol Research     

Ionization efficiency of evolved gas molecules from aerosol particles in a thermal desorption aerosol mass spectrometer: Numerical simulations

Thermal desorption aerosol mass spectrometers (TDAMSs) with electron ionization are widely used to quantitatively measure aerosol chemical compositions. The physical and chemical mechanisms affecting the ionization efficiency of evolved gas molecules are not fully understood. We have developed a numerical model for simulating the dynamics of gas molecules evolved from aerosol particles. The simulation model is composed of two main sections. The first section simulates the elastic collisions of the evolved gas molecules in a small region near the vaporization source (collision domain), where the mean free paths of the molecules are much shorter than those in the surrounding high vacuum environment. The second section simulates the free-molecular dynamics from the boundary of the first section to the ionizer. The ionization efficiencies of ammonia and hydrogen iodide molecules that evolved from ammonium iodide particles were evaluated. Our results suggest that the molecular collisions during the early stage of plume expansion and possible changes in the molecular velocities induced by these collisions could be an important mechanism affecting the observed variability in the ionization efficiency. However, the physical and chemical processes of the vaporization and ionization of aerosol particles in TDAMSs may be too complex to be quantitatively reproduced using simplified numerical models.

Copyright © 2019 American Association for Aerosol Research     

Evaporation rate of particles in the vaporizer of the Aerodyne aerosol mass spectrometer

We present calculations for evaporation rates of particles collected on the vaporizer of the Aerodyne aerosol mass spectrometer (AMS). These calculations provide insight on certain observed phenomena associated with the size-resolved mass spectrum (MS), because the time width of the MS signal from a particle can be limited by its evaporation rate upon contact with the vaporizer. We show that the counterintuitive weak dependence of observed MS signal widths (evaporation rates) on particle volatility is due to suppression of evaporation rates induced by latent heat release, which is more prominent at high volatilities. The same physics is responsible for the observed diminishing returns associated with increasing the vaporizer temperature to achieve narrower single particle pulses. We also show that the vaporizer typical operating temperature of 600°C is sufficient to evaporate extremely low volatility organic compounds (ELVOCs) rapidly enough to obtain reliable measurements for particles smaller than approximately 600 nm. However, the sizing resolution is compromised for large (near-micron) sizes regardless of particle volatility. Finally, our calculations indicate that the observed delayed particle signals, which lead to an artificial tail in AMS mass distributions, are not due to slow evaporation of particles deposited on a surface with lower temperature than the vaporizer, but particles bouncing in the ionizer cage and finally depositing on the vaporizer.

Copyright © 2017 American Association for Aerosol Research     

Development of an aerosol mass spectrometer lens system for PM2.5

The aerodynamic lens system of the Aerodyne Aerosol Mass Spectrometer (AMS) was analyzed using the Aerodynamic Lens Calculator. Using this tool, key loss mechanisms were identified, and a new lens design that can extend the transmission of particulate matter up to 2.5 μm in diameter (PM2.5) was proposed. The new lens was fabricated and experimentally characterized. Test results indicate that this modification to the AMS lens can significantly improve the transmission of large sized particles, successfully achieving a high transmission efficiency up to PM2.5 range.

© 2016 American Association for Aerosol Research     

Computer-controlled Raman microspectroscopy (CC-Raman): A method for the rapid characterization of individual atmospheric aerosol particles

The ability of an atmospheric aerosol particle to impact climate by acting as a cloud condensation nucleus (CCN) or an ice nucleus (IN), as well as scatter and absorb solar radiation is determined by its physicochemical properties at the single particle level, specifically size, morphology, and chemical composition. The identification of the secondary species present in individual aerosol particles is important as aging, which leads to the formation of these species, can modify the climate relevant behavior of particles. Raman microspectroscopy has a great deal of promise for identifying secondary species and their mixing with primary components, as it can provide detailed information on functional groups present, morphology, and internal structure. However, as with many other detailed spectroscopic techniques, manual analysis by Raman microspectroscopy can be slow, limiting single particle statistics and the number of samples that can be analyzed. Herein, the application of computer-controlled Raman (CC-Raman) for detailed physicochemical analysis that increases throughput and minimizes user bias is described. CC-Raman applies automated mapping to increase analysis speed allowing for up to 100 particles to be analyzed in an hour. CC-Raman is applied to both laboratory and ambient samples to demonstrate its utility for the analysis of both primary and, most importantly, secondary components (sulfate, nitrate, ammonium, and organic material). Reproducibility and precision are compared to computer controlled-scanning electron microscopy (CCSEM). The greater sample throughput shows the potential for CC-Raman to improve particle statistics and advance our understanding of aerosol particle composition and mixing state, and, thus, climate-relevant properties.

© 2017 American Association for Aerosol Research     

Laboratory characterization of an aerosol chemical speciation monitor with PM2.5 measurement capability

The Aerodyne Aerosol Chemical Speciation Monitor (ACSM) is well suited for measuring non-refractory particulate matter up to approximately 1.0 µm in aerodynamic diameter (NR-sub-PM₁). However, for larger particles the detection efficiency is limited by losses in the sampling inlet system and through the standard aerodynamic focusing lens. In addition, larger particles have reduced collection efficiency due to particle bounce at the vaporizer. These factors have limited the NR-sub-PM₁ ACSM from meeting PM_2.5 (particulate matter with aerodynamic diameter smaller than 2.5 µm) monitoring standards. To overcome these limitations, we have redesigned the sampling inlet, the aerodynamic lens, and particle vaporizer. Both the new lens and vaporizer are tested in the lab using a quadruple aerosol mass spectrometer (QAMS) system equipped with light scattering module. Our results show that the capture vaporizer introduces additional thermal decomposition of both inorganic and organic compounds, requiring modifications to the standard AMS fragmentation table, which is used to partition ion fragments to chemical classes. Experiments with mixed NH₄NO₃ and (NH₄)₂SO₄ particles demonstrated linearity in the NH₄⁺ ion balance, suggesting that there is no apparent matrix effect in the thermal vaporization-electron impact ionization detection scheme for mixed inorganic particles. Considering a typical ambient PM_2.5 size distribution, we found that 89% of the non-refractory mass is detected with the new system, while only 65% with the old system. The NR-PM_2.5 system described here can be adapted to existing Aerodyne Aerosol Mass Spectrometer (AMS) and ACSM systems.

Copyright © 2017 American Association for Aerosol Research     

Evaluation of the new capture vaporizer for aerosol mass spectrometers (AMS) through field studies of inorganic species

The aerosol mass spectrometer (AMS) and aerosol chemical speciation monitor (ACSM) are widely used for quantifying aerosol composition. The quantification uncertainty of these instruments is dominated by the collection efficiency (CE) due to particle bounce. A new “capture vaporizer” (CV) has been recently developed to achieve unit CE. In this study, we examine the performance of the CV while sampling ambient aerosols. AMS/ACSMs using the original standard vaporizer (SV) and CV were operated in parallel during three field studies. Concentrations measured with the CV (assuming CE = 1) and SV (using the composition-dependent CE of Middlebrook et al.), as well as SMPS and PILS-IC are compared. Agreement is good in all cases, verifying that CE ～ 1 in the CV when sampling ambient particles. Specific findings include: (a) The fragmentation pattern of ambient nitrate and sulfate species observed with the CV was shifted to smaller m/z, suggesting additional thermal decomposition. (b) The differences in fragmentation patterns of organic vs. inorganic nitrate and sulfur species are still distinguishable in the CV, however, with much lower signal-to-noise compared to the SV. (c) Size distribution broadening is significant, but its impact is limited in field studies since ambient distributions are typically quite broad. Consistent size distributions were measured with the SV and CV. (d) In biogenic areas, UMR nitrate is overestimated based on the default fragmentation table (～factor of 2–3 in SOAS) for both vaporizers, due to underestimation of the organic interferences. We also report a new type of small interference: artifact chloride signal can be observed in the AMS when high nitrate mass concentration is sampled with both the SV (～0.5% chloride/nitrate) or CV (～0.2% chloride/nitrate). Our results support the improved quantification with the CV AMS and characterize its chemical detection properties.

Copyright © 2017 American Association for Aerosol Research     

Cloud condensation nuclei activity and droplet formation of primary and secondary organic aerosol mixtures

Understanding the mixing behavior of anthropogenic primary and biogenic secondary organic aerosol (POA and SOA) is important for characterizing their interactions with water vapor. The following work expands upon previous studies and investigates cloud condensation nuclei (CCN) activity and droplet kinetics of α-pinene SOA formed in an environmental chamber and mixed with diesel or motor oil-diesel fuel POA. The changes in the aerosol mixing are similar to previously published work but this study provides new CCN activity and droplet information. The CCN activity of the unmixed aerosol systems are measured separately; κ = 0.15, 0.11, 0.022 for α-pinene SOA, diesel POA and motor oil-diesel fuel POA, respectively. In the α-pinene SOA + diesel POA mixture, the CCN activity, characterized by κ-hygroscopicity, decreases from κ = 0.15 to 0.06 after an initial injection of the POA but increases to κ = 0.12. The increase in CCN activity occurs after particle collision (coagulation and wall-loss) rates dominate aerosol processes in the chamber. The α-pinene SOA + motor oil-fuel POA does not readily mix and the CCN activity of the complex system increases with time (from κ = 0.022 to 0.10). An empirical equation using unit mass resolution (UMR) AMS data of two different ion fragments reasonably predicts CCN activity of the POA and SOA mixtures. CCN measurement may be a promising tool to gain additional insight into the complex mixtures of organic aerosol and subsequent interactions with water vapor.

Copyright © 2018 American Association for Aerosol Research     

Simulated sensitivity of secondary organic aerosol in the South Coast Air Basin of California to nitrogen oxides and other chemical parameters

Sensitivity of secondary organic aerosol (SOA) concentrations in the South Coast Air Basin (SoCAB) of California to nitrogen oxide (NO_x) emission is simulated using gas-phase chemistry and gas-particle partitioning modules. These modules are implemented into a three-dimensional air quality model applied for high-pollution summer meteorology and 2008 emissions. To test sensitivity, NO_x emissions in all locations and at all times are scaled by factors ranging from 0.1 to 10.0 in separate model runs. The basin-wide average SOA concentration exhibits a ‘turnover’ NO_x emission multiplicative factor, above and below which the average SOA concentration decreases. For the entire SoCAB, this critical NO_x emission factor is ～0.3; while the magnitude of SOA concentrations changes with time, this peak value (～0.2–0.3) appears to be relatively independent of the hour of the simulated day. When considering individual locations within the SoCAB, this peak factor shows a slightly broader range. Projected emissions for 2023 indicate a decrease in basin-average SOA concentration; the response at individual locations, however, can be either positive or negative, indicating the need for location-specific considerations. Ensembles of module simulations based on parameter values selected using efficient sampling techniques (Latin Hypercube method) are used to identify parameters to which SOA predictions are significantly sensitive. Total SOA predictions are most sensitive (in no particular order) to concentrations of O₃, unsaturated species formed from the gas-phase oxidation of monoaromatic compounds, and substituted products from long-chain alkane oxidation. Secondary inorganic aerosol species, likely through influencing aerosol liquid water, control at least partially the formation of SOA upwind. In addition, the rate at which unsaturated bicyclic oxidation products of monoaromatic compounds are oxidized by hydroxyl radical impacts significantly SOA prediction. These findings emphasize the need for consideration of long-chain alkanes and monoaromatic species when designing emission control strategies.

© 2018 American Association for Aerosol Research     

Near real-time measurement of carbonaceous aerosol using microplasma spectroscopy: Application to measurement of carbon nanomaterials

A sensitive, field-portable microplasma spectroscopy method has been developed for real-time measurement of carbon nanomaterials. The method involves microconcentration of aerosol on a microelectrode tip for subsequent analysis for atomic carbon using spark emission spectroscopy (SES). The spark-induced microplasma was characterized by measuring the excitation temperature (15,000–35,000 K), electron density (1.0 × 10¹⁷–2.2 × 10¹⁷ cm^?3), and spectral responses as functions of time and interelectrode distance. The system was calibrated and detection limits were determined for total atomic carbon (TAC) using a carbon emission line at 247.856 nm (C I) for various carbonaceous materials including sucrose, EDTA, caffeine, sodium carbonate, carbon black, and carbon nanotubes. The limit of detection for total atomic carbon was 1.61 ng, equivalent to 238 ng m^?3 when sampling at 1.5 L min^?1 for 5 min. To improve the selectivity for carbon nanomaterials, which mainly consist of elemental carbon (EC), the cathode was heated to 300°C to reduce the contribution of organic carbon to the total atomic carbon. Measurements of carbon nanotube aerosol at elevated electrode temperature showed improved selectivity to elemental carbon and compared well with the measurements from the thermal optical method (NIOSH Method 5040). The study shows the SES method to be an excellent candidate for development of low-cost, hand-portable, real-time instrument for measurement of carbonaceous aerosols and nanomaterials.     

Ambient aerosol composition by infrared spectroscopy and partial least squares in the chemical speciation network: Multilevel modeling for elemental carbon

Fourier transform infrared spectroscopy (FT-IR) has been used to predict elemental carbon (EC) on polytetrafluoroethylene (PTFE) filter samples from the United States Environmental Protection Agency's Chemical Speciation Network (CSN). This study provides a proof-of-principle demonstration of using multilevel modeling to determine thermal/optical reflectance (TOR) equivalent EC (a.k.a., FT-IR EC) on PTFE samples collected in the CSN. Initially, spectra from nine geographically disperse sites were pooled and calibrated directly to collocated TOR EC measurements. The FT-IR EC quantified in test samples was deemed substandard when judged against an earlier study, e.g., R² = 0.760 and median absolute deviation (MAD) = 26.7%. Upon scrutinizing each sample's absolute prediction error and squared Mahalanobis distance, Elizabeth, NJ predictions were found to exhibit atypical systematic errors, motivating the development of a multilevel classification and calibration procedure. Atypical Elizabeth spectra were distinguished from the (typical) CSN spectra by training a partial least-square discriminant analysis. Predicting EC using calibrations dedicated to either atypical or typical samples produced a satisfactory improvement in overall performance (R² = 0.886, MAD = 19.8%). Analysis of the atypical FT-IR spectra and select TOR thermal fractions suggested that Elizabeth samples contained elevated levels of diesel particulate matter as evidenced by the use of organic nitrogen functional groups for prediction, very low average OC/EC, and minimal charring during TOR speciation. FT-IR EC from the other eight sites was predominately determined by aliphatic C-H, C = C aromatic, and functional groups associated with oxidation. This study provides preliminary confirmation that FT-IR EC may be accurately determined from source-oriented calibrations under a combined classification and calibration methodology.

Copyright © 2018 American Association for Aerosol Research     

More unsaturated,cooking-type hydrocarbon-like organic aerosol particle emissions from renewable diesel compared to ultra low sulfur diesel in at-sea operations of a research vessel

The aerosol particle emissions from R/V Robert Gordon Sproul were measured during two 5-day research cruises (29 September–3 October 2014; 4–7 and 26–28 September 2015) at four engine speeds (1600 rpm, 1300 rpm, 1000 rpm, and 700 rpm) to characterize the emissions under different engine conditions for ultra low sulfur diesel (ULSD) and hydrogenation derived renewable diesel (HDRD) fuels. Organic aerosol composition and mass distribution were measured on the aft deck of the vessel directly behind the exhaust stack to intercept the ship plume. The ship emissions for both fuels were composed of alkane-like compounds (H/C = 1.94 ± 0.003, O/C = 0.04 ± 0.001, C_nH_2n) with mass spectral fragmentation patterns consistent with hydrocarbon-like organic aerosol (HOA). Single-particle mass spectra from emissions for both fuels showed two distinct HOA compositions, with one HOA type containing more saturated alkane fragments (C_nH_2n+1) and the other HOA type containing more monounsaturated fragments (C_nH_2n?1). The particles dominated by the C_nH_2n?1 fragment series are similar to mass spectra previously associated with cooking emissions. More cooking-type organic particles were observed in the ship emissions for HDRD than for ULSD (45% and 38%, respectively). Changes in the plume aerosol composition due to photochemical aging in the atmosphere were also characterized. The higher fraction of alkene or aromatic (C_nH_2n?m, m ≥ 3) fragments in aged compared to fresh plume emissions suggest that some of the semivolatile alkane-like components partition back to the vapor phase as dilution increases, while alkene or aromatic hydrocarbons contribute more mass to the particle phase due to continuing photochemical oxidation and subsequent condensation from the vapor phase.

Copyright © 2017 American Association for Aerosol Research     

The effects of molecular weight and thermal decomposition on the sensitivity of a thermal desorption aerosol mass spectrometer

In free molecular flow the slower speed of heavier molecules means that they spend more time in the ion source of a mass spectrometer. Hence the sensitivity of the thermal desorption mass spectrometers such as the Aerodyne Aerosol Mass Spectrometer (AMS) should include a term that scales as the square root of the molecular weight. Thermal decomposition on the vaporizer reduces the molecular weight prior to ionization and changes electron impact cross-sections. Thermal decomposition therefore has the potential to change the sensitivity, in some cases by more than a factor of three. Current AMS calibrations that rely upon an ammonium nitrate calibration and scaling for other components with a relative ionization efficiency may overestimate the concentration of large, thermally stable molecules and underestimate small or thermally unstable molecules. The overall sensitivity of the AMS to organics includes a partial cancellation of these effects. There is an incomplete understanding of the vaporization process, including that of ammonium nitrate.     

Characterization of a bipolar near-infrared laser desorption/ionization aerosol mass spectrometer

A novel method of soft ionization aerosol mass spectrometry (AMS), bipolar near-infrared laser desorption/ionization AMS (BP-NIR-LDI-AMS), has been developed for the on-line, real-time analysis of organic aerosols. Use of a single NIR laser pulse to desorb/ionize aerosols deposited onto an aluminum probe results in minimal analyte fragmentation to produce exclusively intact pseudomolecular ions at [M–H]^? for acidic organic analytes and [M+H]⁺ for basic organic analytes. Incorporation of a bipolar mass spectrometer with the NIR-LDI source enables simultaneous detection of acidic and basic species in organic particles. Limits of detection (total particulate mass sampled) for amino acids common to the organic fraction of atmospheric aerosols ranged from 69.1 pg for ornithine to 197 pg for serine on the positive channel, and from 17.0 pg for glycine to 100 pg for ornithine on the negative channel. From studies of the laser energy dependence of the NIR-LDI mechanism, it was found that [M–H]^? formation for oleic acid proceeds through simultaneous action of two 1064 nm photons, suggesting a surface-assisted process rather than direct photoionization, for which photon energy is insufficient. For acidic aerosol species, sensitivity was found to increase as a function of analyte acidity, while for basic species, [M+H]⁺ ion signals were detected only in the presence of a labile proton source, with the intensity of the ion signals scaling with the acidity of the proton source. The sensitivity of BP-NIR-LDI-AMS to the amino acids was rationalized in terms of their acidic/basic character, as measured by isoelectric point (pI), with the cationic sensitivity scaling proportionally with pI and the anionic sensitivity scaling inversely with pI.

© 2016 American Association for Aerosol Research     

Development of a volatility and polarity separator (VAPS) for volatility- and polarity-resolved organic aerosol measurement

Discrepancies between modeled and measured atmospheric organic aerosol (OA) have highlighted the need for in situ instrumentation to better characterize the sources, formation mechanisms, and atmospheric evolution of ambient OA. We have developed the Volatility and Polarity Separator (VAPS) for hourly measurements of volatility- and polarity-resolved OA detected using high-resolution time-of-flight mass spectrometry (HR-ToF-MS). Here, atmospheric OA is inertially impacted onto a collection cell, material is transferred onto a short transfer line located inside a gas chromatography (GC) oven, the oven is heated to provide a first-dimension separation of volatility, then thermally pulsed through a short polar GC column for a second-dimension polarity separation, and finally detected by HR-ToF-MS. This novel instrument increases the mass throughput of ambient OA in comparison to traditional GC due to shorter transfer paths and passivated coatings. Molecular separation resolution is partially sacrificed for this increased mass recovery, but the high-resolution mass spectral data recovers information such as chemical classes and even some individual compounds along with elemental composition to determine aerosol oxidation states. Different techniques for interpreting and representing VAPS data are considered and its applicability to positive matrix factorization (PMF) analysis is demonstrated.

Copyright © 2016 American Association for Aerosol Research     

Field intercomparison of the gas/particle partitioning of oxygenated organics during the Southern Oxidant and Aerosol Study (SOAS) in 2013

We present results of the first intercomparison of real-time instruments for gas/particle partitioning of organic species. Four recently-developed instruments that directly measure gas/particle partitioning in near-real time were deployed in Centreville, Alabama during the Southern Oxidant Aerosol Study (SOAS) in 2013. Two instruments were filter inlet for gases and aerosols high-resolution chemical ionization mass spectrometers (FIGAERO-HRToF-CIMS) with acetate (A-CIMS) and iodide (I-CIMS) ionization sources, respectively; the third was a semi-volatile thermal desorption aerosol GC-MS (SV-TAG); and the fourth was a high-resolution thermal desorption proton-transfer reaction mass spectrometer (HR-TD-PTRMS). Signals from these instruments corresponding to several organic acids were chosen for comparison. The campaign average partitioning fractions show good correlation. A similar level of agreement with partitioning theory is observed. Thus the intercomparison exercise shows promise for these new measurements, as well as some confidence on the measurement of low versus high particle-phase fractions. However, detailed comparison show several systematic differences that lie beyond estimated measurement errors. These differences may be due to at least eight different effects: (1) underestimation of uncertainties under low signal-to-noise; (2) inlet and/or instrument adsorption/desorption of gases; (3) differences in particle size ranges sampled; (4) differences in the methods used to quantify instrument backgrounds; (5) errors in high-resolution fitting of overlapping ion groups; (6) differences in the species included in each measurement due to different instrument sensitivities; and differences in (7) negative or (8) positive thermal decomposition (or ion fragmentation) artifacts. The available data are insufficient to conclusively identify the reasons, but evidence from these instruments and available data from an ion mobility spectrometer shows the particular importance of effects 6–8 in several cases. This comparison highlights the difficulty of this measurement and its interpretation in a complex ambient environment, and the need for further improvements in measurement methodologies, including isomer separation, and detailed study of the possible factors leading to the observed differences. Further intercomparisons under controlled laboratory and field conditions are strongly recommended.

Copyright © 2017 American Association for Aerosol Research