Impacts of Aerosol Aging on Laser Desorption/Ionization in Single-Particle Mass Spectrometers

Single-particle mass spectrometry (SPMS) has been widely used for characterizing the chemical mixing state of ambient aerosol particles. However, processes occurring during particle ablation and ionization can influence the mass spectra produced by these instruments. These effects remain poorly characterized for complex atmospheric particles. During the 2005 Study of Organic Aerosols in Riverside (SOAR), a thermodenuder was used to evaporate the more volatile aerosol species in sequential temperature steps up to 230°C; the residual aerosol particles were sampled by an aerosol mass spectrometer (AMS) and a single-particle aerosol time-of-flight mass spectrometer (ATOFMS). Removal of the secondary species (e.g., ammonium nitrate/sulfate) through heating permitted assessment of the change in ionization patterns as the composition changed for a given particle type. It was observed that a coating of secondary species can reduce the ionization efficiency by changing the degree of laser absorption or particle ablation, which significantly impacted the measured ion peak areas. Nonvolatile aerosol components were used as pseudo-internal standards (or “reference components”) to correct for this LDI effect. Such corrected ATOFMS ion peak areas correlated well with the AMS measurements of the same species up to 142°C. This work demonstrates the potential to accurately relate SPMS peak areas to the mass of specific aerosol components.

Copyright 2014 American Association for Aerosol Research     

A New Time-of-Flight Aerosol Mass Spectrometer (TOF-AMS)—Instrument Description and First Field Deployment

We report the development and first field deployment of a new version of the Aerosol Mass Spectrometer (AMS), which is capable of measuring non-refractory aerosol mass concentrations, chemically speciated mass distributions and single particle information. The instrument was constructed by interfacing the well-characterized Aerodyne AMS vacuum system, particle focusing, sizing, and evaporation/ionization components, with a compact TOFWERK orthogonal acceleration reflectron time-of-flight mass spectrometer. In this time-of-flight aerosol mass spectrometer (TOF-AMS) aerosol particles are focused by an aerodynamic lens assembly as a narrow beam into the vacuum chamber. Non-refractory particle components flash-vaporize after impaction onto the vaporizer and are ionized by electron impact. The ions are continuously guided into the source region of the time-of-flight mass spectrometer, where ions are extracted into the TOF section at a repetition rate of 83.3 kHz. Each extraction generates a complete mass spectrum, which is processed by a fast (sampling rate 1 Gs/s) data acquisition board and a PC. Particle size information is obtained by chopping the particle beam followed by time-resolved detection of the particle evaporation events. Due to the capability of the time-of-flight mass spectrometer of measuring complete mass spectra for every extraction, complete single particle mass spectra can be collected. This mode provides quantitative information on single particle composition. The TOF-AMS allows a direct measurement of internal and external mixture of non-refractory particle components as well as sensitive ensemble average particle composition and chemically resolved size distribution measurements. Here we describe for the first time the TOF-AMS and its operation as well as results from its first field deployment during the PM _2.5 Technology Assessment and Characterization Study—New York (PMTACS-NY) Winter Intensive in January 2004 in Queens, New York. These results show the capability of the TOF-AMS to measure quantitative aerosol composition and chemically resolved size distributions of the ambient aerosol. In addition it is shown that the single particle information collected with the instrument gives direct information about internal and external mixture of particle components.     

Calibration of a particle mass spectrometer using polydispersed aerosol particles

Routine calibrations of online aerosol chemical composition analyzers are important for assessing data quality during field measurements. The combination of a differential mobility analyzer (DMA) and condensation particle counter (CPC) is a reliable, conventional method for calibrations. However, some logistical issues arise, including the use of radioactive material, quality control, and deployment costs. Herein, we propose a new, simple calibration method for a particle mass spectrometer using polydispersed aerosol particles combined with an optical particle sizer. We used a laser-induced incandescence–mass spectrometric analyzer (LII-MS) to test the new method. Polydispersed aerosol particles of selected chemical compounds (ammonium sulfate and potassium nitrate) were generated by an aerosol atomizer. The LII section was used as an optical particle sizer for measuring number/volume size distributions of polydispersed aerosol particles. The calibration of the MS section was performed based on the mass concentrations of polydispersed aerosol particles estimated from the integration of the volume size distributions. The accuracy of the particle sizing for each compound is a key issue and was evaluated by measuring optical pulse height distributions for monodispersed ammonium sulfate and potassium nitrate particles as well as polystyrene latex particles. A comparison of the proposed method with the conventional DMA-CPC method and its potential uncertainties are discussed.

Copyright © 2018 American Association for Aerosol Research     

Dominant Mechanisms that Shape the Airborne Particle Size and Composition Distribution in Central California

The size and composition of ambient airborne particulate matter is reported for winter conditions at five locations in (or near) the San Joaquin Valley in central California. Two distinct types of airborne particles were identified based on diurnal patterns and size distribution similarity: hygroscopic sulfate/ammonium/nitrate particles and less hygroscopic particles composed of mostly organic carbon with smaller amounts of elemental carbon. Daytime PM₁₀ concentrations for sulfate/ammonium/nitrate particles were measured to be 10.1 μ g m^?3, 28.3 μ g m^?3, and 52.8 μ g m^?3 at Sacramento, Modesto and Bakersfield, California, respectively. Nighttime concentrations were 10–30% lower, suggesting that these particles are dominated by secondary production. Simulation of the data with a box model suggests that these particles were formed by the condensation of ammonia and nitric acid onto background or primary sulfate particles. These hygroscopic particles had a mass distribution peak in the accumulation mode (0.56–1.0 μ m) at all times. Daytime PM₁₀ carbon particle concentrations were measured to be 9.5 μ g m^?3, 15.1 μ g m^?3, and 16.2 μ g m^?3 at Sacramento, Modesto, and Bakersfield, respectively. Corresponding nighttime concentrations were 200–300% higher, suggesting that these particles are dominated by primary emissions. The peak in the carbon particle mass distribution varied between 0.2–1.0 μ m. Carbon particles emitted directly from combustion sources typically have a mass distribution peak diameter between 0.1–0.32 μ m. Box model calculations suggest that the formation of secondary organic aerosol is negligible under cool winter conditions, and that the observed shift in the carbon particle mass distribution results from coagulation in the heavily polluted concentrations experienced during the current study. The analysis suggests that carbon particles and sulfate/ammonium/nitrate particles exist separately in the atmosphere of the San Joaquin Valley until coagulation mixes them in the accumulation mode.     

Coincidence in Time-of-Flight Aerosol Spectrometers: Phantom Particle Creation

When using time-of-flight aerosol spectrometers, particle size measurement is based upon a particle's transit time between two laser beams. The particle's transit time is assumed to be the time difference between the two pulses of light that are produced as the particle passes through the two laser beams. Particle coincidence, which occurs when a second particle crosses the first laser beam before the first particle crosses the second laser beam, has a complex effect upon the measured size distribution. As a result of coincidence, time-of-flight aerosol spectrometers can replace real particles of one size with spurious, or phantom, particles of a different size in the measured distribution. When partial detection of a particle occurs, i.e., only one pulse from a particle is detected, another particle producing a pulse that occurs while the timer is open can cause the recording of a randomly sized phantom particle. The creation of these phantom particles, which we termed “open-timer” phantom particles, has been investigated theoretically and experimentally in a commercially available time-of-flight aerosol spectrometer. The number of these open-timer phantom particles was found to increase with particle size and aerosol concentration. In addition, the instrument's detection logic affects the number and size of the phantom particles. These are most apparent in the tails and minima of the measured distribution. In order to minimize phantom particle creation, the concentration of partially detected particles must be minimized. Strategies to reduce phantom particle concentration involve reducing the concentration of small particles near the optical detection threshold of the spectrometer.     

Real-Time Measurements of Nonmetallic Fine Particulate Matter Adjacent to a Major Integrated Steelworks

A campaign took place in Wales (UK) in the spring of 2006 to characterize emissions from a major steelworks through atmospheric measurements. At no time during the measurements was the 24-h air quality standard for PM₁₀ exceeded. However, real-time measurements of single particles by aerosol time-of-flight mass spectrometry (ATOFMS) allowed detection of particulate matter from the steelworks, which could be associated with specific emission areas within the works from measurements of wind direction. Three main wind sectors were identified with possible sources of emissions of fine nonmetallic particulate matter (PM < 1 μm). Characterization of the aerosol composition by a high resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) of the nonrefractory material associated with the specific plumes is also reported, along with results from other real-time techniques. The ATOFMS detected for the first time a unique elemental sulfur-rich particle type, likely to originate from the blast furnaces. AMS results, supported also by laboratory studies, confirm this finding by reporting elevated mass ratios m/z 64/48 and m/z 64/80. Two other novel ATOFMS particulate types were found to be associated with steelworks emissions. One was characterized by nitrogen-containing organic species, aromatic compounds, and high-molecular-weight (MW) polycyclic aromatic hydrocarbons (PAHs) and was associated with the sources in the area of the hot and cold mills. The second was found to be rich in organic carbon internally mixed with elemental carbon, nitrate, sulfate, and PAHs with lower MW. These particle types were likely related to the coke ovens and the basic oxygen steelmaking plant.

Copyright 2012 American Association for Aerosol Research     

Comparison of Two Aerodynamic Lenses as an Inlet for a Single Particle Laser Ablation Mass Spectrometer

By means of a newly designed portable aerosol mass spectrometer SPLAT (Single Particle Laser Ablation Time-of-flight mass spectrometer) for the analysis of single atmospheric aerosol particles we investigated the system performance in dependency on two different aerodynamic lenses (Liu and Schreiner type) capable of focusing particles with diameters ranging from 80 nm to 800 nm and 300 nm to 3000 nm, respectively. By using the pressure regulated Schreiner lens, the instrument is independent of variations in atmospheric pressure which would lead to changing dynamical properties of the aerosol particles. Active pressure control inside the inlet system facilitates airborne measurements without complicated corrections. With the Liu setup no pressure regulation was used. Here the overall efficiency of our instrument was 7% while with the Schreiner setup 2% was achieved. The Liu lens setup is optimal for measuring submicron particles at low particle concentrations. To detect supermicron particles the Schreiner lens setup is favored. Together with these experiments we present key details of the SPLAT setup and its characterization. Our instrument is able to measure simultaneously the size and the chemical composition of individual aerosol particles larger than 300 nm in diameter. It uses forward scattered light of single aerosol particles at two positions to determine their vacuum aerodynamic diameter from the flight time between the two lasers. Chemical analysis of the particles is done by laser ablation mass spectrometry utilizing a bipolar time-of-flight mass spectrometer.     

An Aerosol Chemical Speciation Monitor (ACSM) for Routine Monitoring of the Composition and Mass Concentrations of Ambient Aerosol

We present a new instrument, the Aerosol Chemical Speciation Monitor (ACSM), which routinely characterizes and monitors the mass and chemical composition of non-refractory submicron particulate matter in real time. Under ambient conditions, mass concentrations of particulate organics, sulfate, nitrate, ammonium, and chloride are obtained with a detection limit <0.2 μg/m³ for 30 min of signal averaging. The ACSM is built upon the same technology as the widely used Aerodyne Aerosol Mass Spectrometer (AMS), in which an aerodynamic particle focusing lens is combined with high vacuum thermal particle vaporization, electron impact ionization, and mass spectrometry. Modifications in the ACSM design, however, allow it to be smaller, lower cost, and simpler to operate than the AMS. The ACSM is also capable of routine stable operation for long periods of time (months). Results from a field measurement campaign in Queens, NY where the ACSM operated unattended and continuously for 8 weeks, are presented. ACSM data is analyzed with the same well-developed techniques that are used for the AMS. Trends in the ACSM mass concentrations observed during the Queens, NY study compare well with those from co-located instruments. Positive Matrix Factorization (PMF) of the ACSM organic aerosol spectra extracts two components: hydrocarbon-like organic aerosol (HOA) and oxygenated organic aerosol (OOA). The mass spectra and time trends of both components correlate well with PMF results obtained from a co-located high resolution time-of-flight AMS instrument.     

Development of an Aerosol Mass Spectrometer for Size and Composition Analysis of Submicron Particles

The importance of atmospheric aerosols in regulating the Earth's climate and their potential detrimental impact on air quality and human health has stimulated the need for instrumentation which can provide real-time analysis of size resolved aerosol, mass, and chemical composition. We describe here an aerosol mass spectrometer (AMS) which has been developed in response to these aerosol sampling needs and present results which demonstrate quantitative mea surement capability for a laboratory-generated pure component NH₄ NO₃ aerosol. The instrument combines standard vacuum and mass spectrometric technologies with recently developed aerosol sampling techniques. A unique aerodynamic aerosol inlet (developed at the University of Minnesota) focuses particles into a narrow beam and efficiently transports them into vacuum where aerodynamic particle size is determined via a particle time-of-flight (TOF) measurement. Time-resolved particle mass detection is performed mass spectrometrically following particle flash vaporization on a resistively heated surface. Calibration data are presented for aerodynamic particle velocity and particle collection efficiency measurements. The capability to measure aerosol size and mass distributions is compared to simultaneous measurements using a differential mobility analyzer (DMA) and condensation particle counter (CPC). Quantitative size classification is demonstrated for pure component NH₄ NO₃ aerosols having mass concentrations 0.25_mu g m -3. Results of fluid dynamics calculations illustrating the performance of the aerodynamic lens are also presented and compared to the measured performance. The utility of this AMS as both a laboratory and field portable instrument is discussed.     

Characterization of Short-Term Particulate Matter Events by Real-Time Single Particle Mass Spectrometry

Single particle measurements were made in Baltimore, Maryland from March to December 2002 using a real-time single particle mass spectrometer, RSMS-3. Particle composition classes were identified that indicated how the aerosol composition changed with time. The results were compared with collocated instruments giving particle number concentrations and size distributions, sulfate, nitrate, organic, and elemental carbon mass concentrations and total mass. Examination of these measurements revealed several particulate matter (PM) events in which the 24 h averaged PM _2.5 mass exceeded 30 μ g/m ³ . Three of these events were studied in further detail by comparing number and mass concentrations obtained by RSMS-3 with standard methods. For all three events, the number concentrations obtained with RSMS-3 and a scanning mobility particle sizer were highly correlated (R ² ～ 0.7). For the event characterized by a high sulfate mass concentration, the RSMS-3 provided an accurate measure of time-dependent nitrate and carbon mass concentrations, but not for sulfate and total mass. For the two events characterized by high carbon mass concentrations (one from a transcontinental wildfire and the other from stagnation during a period of high traffic), RSMS-3 provided an accurate measure of time-dependent nitrate mass, carbon mass and total mass when the aerosol was not dominated by particles outside the size limit of RSMS-3. While the time dependencies were strongly correlated, the absolute mass or number concentrations determined by RSMS-3 were sometimes off by a constant value, which permitted the relative detection efficiencies of some particle classes to be estimated. Other factors that inhibit reconciliation of mass- and number- based concentration measurements are discussed including the difficulty of detecting ammonium sulfate by laser ablation/ionization and the varying size ranges of different particle measurement methods.     

Airborne Deployment of an Instrument for the Real-Time Analysis of Single Aerosol Particles

An instrument is described that provides real-time chemical analysis of the composition of individual aerosol particles. A differentially pumped aerosol inlet transfers particles from the ambient atmosphere into the source region of a time-of-flight mass spectrometer where they impact on a heated surface and the resulting vapors are ionized by electron ionization prior to mass analysis. Labora tory calibration studies demonstrated that the instrument was capable of detecting particles with diameters greater than approximately 0.4mu m. The instrument was operated on the NASA DC-8 research aircraft as part of the 1996 SUbsonic aircraft: Contrail and Cloud Effects Special Study (SUCCESS) mission with the intent of studying the chemical composition of upper tropospheric particles. More than 25,000 aerosol particle mass spectra were recorded during 19 mission flights. Although approximately 120 of those spectra showed clear evidence of sulfate, nitrate, and other inorganic materials, the remaining spectra contained only mass peaks consistent with water. Moreover, particles were detected only while traversing clouds. These results are not consistent with expectations of the size, quantity, or composition of upper tropospheric particles. It is likely, however, that a subisokinetic aircraft sampling inlet resulted in the collection of only very large ice particles. This situation would account for both the observed preponder ance of water-only spectra and the apparent lack of particles outside of clouds. Despite the sampling problem, the instrument was able to chemically speciate aerosols directly sampled from a medium altitude aircraft. These represent the first examples of aerosol particles chemically speciated in real time from an airborne platform.     

Characterization of Single Particles in Urban Atmospheric Aerosols Over Nagoya Based on Mass Spectral Analysis

Mass spectral analysis of single aerosol particles was performed using a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) in the urban area of Nagoya, Japan, for five days in the autumn of 2010. An ion-series analysis was applied to the single-particle mass spectra; the sum of the masses at Δ of 1, 3, 4, and 5 divided by the total mass of organics (f_Δ1,3-5) was used as an indicator of the degree of oxygenation of the organics. The organic mass fraction (F_org) and f_Δ1,3-5 of single particles varied substantially, and the results for the entire period indicate that two groups of particles with high and low f_Δ1,3-5 were present. The organics in particles with high F_org were less oxygenated, whereas those with low F_org were more oxygenated; these different particles were associated with hydrocarbon-like organic aerosols (HOA) and low-volatility oxygenated organic aerosols (LV-OOA), respectively, based on the PMF analysis of the MS mode data. The size distributions of the two types of particles overlapped substantially. The degree of the overlap depends on the equivalent particle diameter used, indicating the importance of single-particle analysis in understanding the size-composition relationship in detail. Alteration of the aerosol mixing state was clearly indicated by the changes in f_Δ1,3-5 and F_org; the homogeneous conditions changed to externally mixed conditions from the evening until noon of the next day. Possible causes of this alteration include the condensation of oxygenated organics and ammonium nitrate onto pre-existing particles. This study demonstrated the usefulness of single-particle analysis based on electron-ionization aerosol mass spectrometry.

Copyright 2015 American Association for Aerosol Research     

An Intensive Study of the Size and Composition of Submicron Atmospheric Aerosols at a Rural Site in Ontario,Canada

Atmospheric sampling was conducted at a rural site near Egbert, about 70 km north of Toronto, Ontario, Canada from March 27 to May 8, 2003 to characterize the physical and chemical properties of the ambient aerosol in near real-time. The instrumentation included a tapered element oscillating microbalance (TEOM), an ultrafine condensation particle counter (UCPC), a scanning mobility particle sizer (SMPS), an aerodynamic particle sizer (APS), an aerosol mass spectrometer (AMS), and a particulate nitrate monitor (R&P 8400N) for aerosol measurements. Gas-phase non-methane hydrocarbon compounds (NMHCs) were measured by gas chromatograph-flame ionization detection (GC-FID). Filter samples were also collected for analysis of inorganic ions by ion chromatography (IC). Aerosol properties varied considerably depending upon meteorological conditions and airmass histories. For example, urban and industrial emissions advected from the south strongly influenced the site occasionally, resulting in higher particulate mass with the higher fractions of nitrate and organics. Cleaner northwesterly winds carried aerosols with relatively higher fractions of organics and sulfate. The AMS derived mass size distributions showed that the inorganic species in the particles with vacuum aerodynamic diameters between about 60 nm and 600 nm had mass modal vacuum aerodynamic diameters around 400–500 nm. The particulate organics often exhibited two modes at about 100 nm and 425 nm, more noticeable during fresh pollution events. The small organic mode was well correlated with gas-phase nonmethane hydrocarbons such as ethylbenzene, toluene, and propene, suggesting that the likely sources of small organic particles were combustion related emissions. The particulate nitrate exhibited a diurnal variation with higher concentrations during dark hours and minima in the afternoon. Particulate sulfate and organics showed evidence of photochemical processing with higher levels of sulfate and oxygenated organics in the afternoon. Reasonable agreement among all of the co-located measurements is found, provided the upper size limit of the AMS is considered.     

Aircraft-based operation of an aerosol mass spectrometer: Measurements of tropospheric aerosol composition

The Aerodyne quadrupole aerosol mass spectrometer was deployed on the Falcon twin jet research aircraft operated by Deutsches Zentrum für Luft- und Raumfahrt (DLR). This was the first deployment of an AMS in a jet aircraft. Aerosol mass concentration measurements in the troposphere up to altitudes of about 11 km were performed within two measurement flights on 12 and 14 May 2003 over southern Germany. Background aerosol data were gained up to 6 km, while aircraft exhaust aerosol was be sampled at higher altitudes on 14 May, indicating the presence of sulfuric acid and unburned hydrocarbons in the exhaust particles. The boundary layer aerosol on 12 May was found to be composed of 49% organics, 12% sulfate, 15% ammonium, and 24% nitrate by mass. The upper edge of the boundary layer was marked by a sharp decrease of nitrate and ammonium at an altitude of 3 km, while sulfate and organics decreased to a much lesser degree. On 14 May, the boundary layer aerosol was composed of 23% organics, 20% sulfate, 24% ammonium, and 33% nitrate by mass, and the boundary layer reached up to about 5000 m and had no sharp upper edge. The size distributions indicated internal mixtures of ammonium sulfate and –nitrate in the boundary layer, while the organics were externally mixed. Additionally, a smaller mode consisting only of ammonium sulfate, was detected. This bimodal structure of ammonium sulfate was also detected above the boundary layer in 6 km altitude on 14 May.     

Using ATOFMS to Determine OC/EC Mass Fractions in Particles

Historically, obtaining quantitative chemical information using laser desorption ionization mass spectrometry for analyzing individual aerosol particles has been quite challenging. This is due in large part to fluctuations in the absolute ion signals resulting from inhomogeneities in the laser beam profile, as well as chemical matrix effects. Progress has been made in quantifying atomic species using high laser powers, but very few studies have been performed quantifying molecular species. In this study, promising results are obtained using a new approach to measure the fraction of organic carbon (OC) associated with elemental carbon (EC) in aerosol particles using single particle laser desorption ionization. A tandem differential mobility analyzer (TDMA) is used to generate OC/EC particles by size selecting EC particles of a given mobility diameter and then coating them with known thicknesses of OC measured using a second DMA. The mass spectra of the OC/EC particles exiting the second DMA are measured using an ultrafine aerosol time-of-flight mass spectrometer (UF-ATOFMS). A calibration curve is produced with a linear correlation (R² = 0.98) over the range of OC/EC ion intensity ratios observed in source and ambient studies. Importantly, the OC/EC values measured in ambient field tests with the UF-ATOFMS show a linear correlation (R² = 0.69) with OC/EC mass ratios obtained using semi-continuous filter based thermo-optical measurements. The calibration procedure established herein represents a significant step toward quantification of OC and EC in sub-micron ambient particles using laser desorption ionization mass spectrometry.     

Nanoparticle Chemical Composition During New Particle Formation

The nano aerosol mass spectrometer (NAMS) was deployed at a coastal site in Lewes, Delaware, to measure the composition of 21 nm mass normalized (18 nm mobility) diameter nanoparticles during new particle formation (NPF) events. NAMS provides a quantitative measure of the atomic composition of individual nanoparticles. NAMS analysis of ambient particles showed only a small change in particle composition during NPF events in Lewes compared with off-event (before and/or after the event). The N mole fraction increased 15% on-event, whereas the C mole fraction decreased 25%, suggesting an enhanced inorganic component to the aerosol during NPF. The measured changes in atomic composition constrain the possible changes in molecular composition. To explore these constraints, an apportionment algorithm was applied to the atomic composition data. This algorithm partitions the atomic composition into sulfate, nitrate, and ammonium on the basis of the atomic abundance of S, N, and O and into organic matter on the basis of C and residual O after removing contributions to inorganic species. Particles were fully neutralized both on- and off-event. The nitrate to sulfate ratio during NPF ranged from 0.7 to 1.4, suggesting that ammonium nitrate is important to particle growth in this environment. Nonetheless, nanoparticles had a significant organic fraction, and upper limits for cationic amine content were determined. The relatively small changes in total particle composition on-event versus off-event suggest that observed changes in particle hygroscopicity and volatility during NPF at other locations may be linked to subtle changes in particle composition or to shifts in the character of the organic content.     

Evaluation of Composition-Dependent Collection Efficiencies for the Aerodyne Aerosol Mass Spectrometer using Field Data

In recent years, Aerodyne aerosol mass spectrometers (AMS) have been used in many locations around the world to study the size-resolved, nonrefractory chemical composition of ambient particles. In order to obtain quantitative data, the mass or (number) of particles detected by the AMS relative to the mass (or number) of particles sampled by the AMS, i.e., the AMS collection efficiency (CE) must be known. Previous studies have proposed and used parameterizations of the AMS CE based on the aerosol composition and sampling line relative humidity. Here, we evaluate these parameterizations by comparing AMS mass concentrations with independent measurements of fine particle volume or particle-into-liquid sampler (PILS) ion chromatography measurements for 3 field campaigns with different dominant aerosol mixtures: (1) acidic sulfate particles, (2) aerosol containing a high mass fraction of ammonium nitrate, and (3) aerosol composed of primarily biomass burning emissions. The use of the default CE of 0.5 for all campaigns resulted in 81–90% of the AMS speciated and total mass concentrations comparing well with fine particle volume or PILS measurements within experimental uncertainties, with positive biases compared with a random error curve. By using composition-dependent CE values (sometimes as a function of size) which increased the CE for the above aerosol types, the fraction of data points within the measurement uncertainties increased to more than 92% and the mass concentrations decreased by ～5–15% on an average. The CE did not appear to be significantly dependent on changes in organic mass fraction although it was substantial in the 3 campaigns (47, 30, and 55%).

Copyright 2012 American Association for Aerosol Research     

Chemically Resolved Particle Fluxes Over Tropical and Temperate Forests

Chemically resolved submicron (PM₁) particle mass fluxes were measured by eddy covariance with a high resolution time-of-flight aerosol mass spectrometer over temperate and tropical forests during the BEARPEX-07 and AMAZE-08 campaigns. Fluxes during AMAZE-08 were small and close to the detection limit (<1 ng m^?2 s^?1) due to low particle mass concentrations (<1 μg m^?3). During BEARPEX-07, concentrations were five times larger, with mean mid-day deposition fluxes of ?4.8 ng m^?2 s^?1 for total nonrefractory PM₁ (V_ex,PM1 = ?1 mm s^?1) and emission fluxes of +2.6 ng m^?2 s^?1 for organic PM₁ (V_ex,org = +1 mm s^?1). Biosphere–atmosphere fluxes of different chemical components are affected by in-canopy chemistry, vertical gradients in gas-particle partitioning due to canopy temperature gradients, emission of primary biological aerosol particles, and wet and dry deposition. As a result of these competing processes, individual chemical components had fluxes of varying magnitude and direction during both campaigns. Oxygenated organic components representing regionally aged aerosol deposited, while components of fresh secondary organic aerosol (SOA) emitted. During BEARPEX-07, rapid in-canopy oxidation caused rapid SOA growth on the timescale of biosphere-atmosphere exchange. In-canopy SOA mass yields were 0.5–4%. During AMAZE-08, the net organic aerosol flux was influenced by deposition, in-canopy SOA formation, and thermal shifts in gas-particle partitioning. Wet deposition was estimated to be an order of magnitude larger than dry deposition during AMAZE-08. Small shifts in organic aerosol concentrations from anthropogenic sources such as urban pollution or biomass burning alters the balance between flux terms. The semivolatile nature of the Amazonian organic aerosol suggests a feedback in which warmer temperatures will partition SOA to the gas-phase, reducing their light scattering and thus potential to cool the region.

Copyright 2013 American Association for Aerosol Research     

Size Measurements of Latex Particles by Laser Aerosol Spectrometer

Size measurements of PSL (polystyrene latex) particles in a size range from 0.109 to 0.330 μm were made by laser aerosol spectrometer (PMS, LAS-X). The results were compared with those by electron microscopy. For example, the geometric standard deviation, σ_g, of nominally 0.176-μm PSL particles was measured as 1.05, assuming that their sizes distribute log-normally. The value of 1.05 was very close to 1.02 measured by electron microscopy. It was found that the spectrometer had very high size resolution, although the size resolution of the light scattering type spectrometer has been said to be poor. For some samples of PSL particles, however, there were large differences between particle sizes measured by LAS-X and those by electron microscopy. It was also found that LAS-X had a problem in calibration of size response curve.     

Secondary Organic Aerosol Coating Formation and Evaporation: Chamber Studies Using Black Carbon Seed Aerosol and the Single-Particle Soot Photometer

We report a protocol for using black carbon (BC) aerosol as the seed for secondary organic aerosol (SOA) formation in an environmental chamber. We employ a single-particle soot photometer (SP2) to probe single-particle SOA coating growth dynamics and find that SOA growth on nonspherical BC aerosol is diffusion-limited. Aerosol composition measurements with an Aerodyne high resolution time-of-flight aerosol mass spectrometer (AMS) confirm that the presence of BC seed does not alter the composition of SOA as compared to self-nucleated SOA or condensed SOA on ammonium sulfate seed. We employ a 3-wavelength photoacoustic soot spectrometer (PASS-3) to measure optical properties of the systems studied, including fullerene soot as the surrogate BC seed, nucleated naphthalene SOA from high-NO_x photooxidation, and nucleated α-pinene SOA from low-NO_x photooxidation. A core-and-shell Mie scattering model of the light absorption enhancement is in good agreement with measured enhancements for both the low- and high-NO_x α-pinene photooxidation systems, reinforcing the assumption of a core-shell morphology for coated BC particles. A discrepancy between measured and modeled absorption enhancement factors in the naphthalene photooxidation system is attributed to the wavelength-dependence of refractive index of the naphthalene SOA. The coating of high-NO_x α-pinene SOA decreases after reaching a peak thickness during irradiation, reflecting a volatility change in the aerosol, as confirmed by the relative magnitudes of f₄₃ and f₄₄ in the AMS spectra. The protocol described here provides a framework by which future studies of SOA optical properties and single-particle growth dynamics may be explored in environmental chambers.

Copyright 2013 American Association for Aerosol Research