Particle Analysis By Laser Mass Spectrometry Research Articles

Measurement report: Vanadium-containing ship exhaust particles detected in and above the marine boundary layer in the remote atmosphere

Abstract. Each year, commercial ships emit over 1.67 Tg of particulate matter (PM) pollution into the atmosphere. These ships rely on the combustion of heavy fuel oil, which contains high levels of sulfur, large aromatic organic compounds, and metals. Vanadium is one of the metals most commonly associated with heavy fuel oil and is often used as a tracer for PM from ship exhaust. Previous studies have suggested that vanadium-containing PM has impacts on human health and climate due to its toxicological and cloud-formation properties, respectively; however, its distribution in the atmosphere is not fully understood, which limits our ability to quantify the environmental implications of PM emitted by ships. Here, we present data obtained from a Particle Analysis by Laser Mass Spectrometry (PALMS) instrument on the NASA DC-8 aircraft during the 2016–2018 Atmospheric Tomography Mission (ATom) and show that ∼ 1 % of the accumulation mode particles measured in the marine boundary layer of the central Pacific and Atlantic oceans contain vanadium. These measurements, which were made without targeting ship plumes, suggest that PM emitted by ships is widespread in the atmosphere. Furthermore, we observed vanadium-containing ship exhaust particles at altitudes up to 13 km, which demonstrates that not all ship exhaust particles are immediately removed via wet deposition processes. In addition, using laboratory calibrations, we determined that most vanadium-containing ship exhaust particles can contain up to a few weight percent of vanadium. This study furthers our understanding of both the chemical composition and distribution of PM emitted by ships, which will allow us to better constrain the climate, health, and air quality implications of these particle types in the future. We note that these data were collected prior to the 2020 International Maritime Organization (IMO) sulfur regulation and stand as a reference for understanding how ship emissions have evolved in light of these regulations.

A new method to quantify mineral dust and other aerosol species from aircraft platforms using single-particle mass spectrometry

Abstract. Single-particle mass spectrometry (SPMS) instruments characterize the composition of individual aerosol particles in real time. Their fundamental ability to differentiate the externally mixed particle types that constitute the atmospheric aerosol population enables a unique perspective into sources and transformation. However, quantitative measurements by SPMS systems are inherently problematic. We introduce a new technique that combines collocated measurements of aerosol composition by SPMS and size-resolved absolute particle concentrations on aircraft platforms. Quantitative number, surface area, volume, and mass concentrations are derived for climate-relevant particle types such as mineral dust, sea salt, and biomass burning smoke. Additionally, relative ion signals are calibrated to derive mass concentrations of internally mixed sulfate and organic material that are distributed across multiple particle types. The NOAA Particle Analysis by Laser Mass Spectrometry (PALMS) instrument measures size-resolved aerosol chemical composition from aircraft. We describe the identification and quantification of nine major atmospheric particle classes, including sulfate–organic–nitrate mixtures, biomass burning, elemental carbon, sea salt, mineral dust, meteoric material, alkali salts, heavy fuel oil combustion, and a remainder class. Classes can be sub-divided as necessary based on chemical heterogeneity, accumulated secondary material during aging, or other atmospheric processing. Concentrations are derived for sizes that encompass the accumulation and coarse size modes. A statistical error analysis indicates that particle class concentrations can be determined within a few minutes for abundances above ∼10 ng m−3. Rare particle types require longer sampling times. We explore the instrumentation requirements and the limitations of the method for airborne measurements. Reducing the size resolution of the particle data increases time resolution with only a modest increase in uncertainty. The principal limiting factor to fast time response concentration measurements is statistically relevant sampling across the size range of interest, in particular, sizes D < 0.2 µm for accumulation-mode studies and D > 2 µm for coarse-mode analysis. Performance is compared to other airborne and ground-based composition measurements, and examples of atmospheric mineral dust concentrations are given. The wealth of information afforded by composition-resolved size distributions for all major aerosol types represents a new and powerful tool to characterize atmospheric aerosol properties in a quantitative fashion.

Laboratory studies of collection efficiency of sub-micrometer aerosol particles by cloud droplets on a single-droplet basis

Abstract. An experimental setup has been constructed to measure the collection efficiency (CE) of sub-micrometer aerosol particles by cloud droplets. Droplets of a dilute aqueous ammonium sulfate solution with an average radius of 21.6 μm fall freely into a chamber and collide with sub-micrometer polystyrene latex (PSL) sphere particles of known sizes and concentrations. Two relative humidity (RH) conditions, 15 ± 3 % and 88 ± 3 %, hereafter termed "low" and "high", respectively, were varied with different particles sizes and concentrations. After passing through the chamber, the droplets and aerosol particles were sent to the Particle Analysis by Laser Mass Spectrometry (PALMS) instrument to determine chemical compositions on a single-droplet basis. "Coagulated droplets" (droplets that collected aerosols) had mass spectra that contained signatures from both an aerosol particle and a droplet residual. CE values range from 2.0 × 10−1 to 1.6 for the low-RH case and from 1.5 × 10−2 to 9.0 × 10−2 for the high-RH case. CE values were, within experimental uncertainty, independent of the aerosol concentrations. CE values in this study were found to be in agreement with previous experimental and theoretical studies. To our knowledge, this is the first collection experiment performed on a single-droplet basis with atmospherically relevant conditions such as droplet sizes, droplet charges and flow.

Evaluation of DMA Size Selection of Dry Dispersed Mineral Dust Particles

Mineral dust particles play a significant role in the Earth's radiative balance via direct interaction with solar radiation and indirectly through their ability to initiate cloud formation. Many field and laboratory studies utilize a differential mobility analyzer (DMA) for particle size selection. Here we evaluate the use of a DMA to size-segregate dry dispersed mineral dust particles. We examine the post-DMA size distribution using four different techniques: a scanning mobility particle sizer (SMPS) for mobility sizing, an optical particle sizer (OPS) for optical sizing, the Particle Analysis by Laser Mass Spectrometry (PALMS) instrument for vacuum aerodynamic sizing, and electron microscopy (EM) for geometric sizing. While the SMPS measured a narrow mobility size distribution at the DMA-selected diameter, the OPS, PALMS, and EM in most cases showed broader distributions and a smaller mode size than that selected by the DMA. These techniques also observed super-micrometer particles, often extending beyond the upper size limit of a typical SMPS scan. Complicating analysis, particle shape factor (χ) was observed to be a function of mobility size, ranging from 1.3 at 500 nm to 3.1 at 1000 nm. We conclude that mobility size selection of mineral dust particles using a DMA most often does not yield particles of the desired physical size or surface area. We suggest that attempts to size-select from a broad distribution of non-spherical particles require an independent measurement downstream of the DMA to verify the actual selected size.Copyright 2015 American Association for Aerosol Research

Droplet activation, separation, and compositional analysis: laboratory studies and atmospheric measurements

Abstract. Droplets produced in a cloud condensation nuclei chamber (CCNC) as a function of supersaturation have been separated from unactivated aerosol particles using counterflow virtual impaction. Residual material after droplets were evaporated was chemically analyzed with an Aerodyne Aerosol Mass Spectrometer (AMS) and the Particle Analysis by Laser Mass Spectrometry (PALMS) instrument. Experiments were initially conducted to verify activation conditions for monodisperse ammonium sulfate particles and to determine the resulting droplet size distribution as a function of supersaturation. Based on the observed droplet size, the counterflow virtual impactor cut-size was set to differentiate droplets from unactivated interstitial particles. Validation experiments were then performed to verify that only droplets with sufficient size passed through the counterflow virtual impactor for subsequent analysis. A two-component external mixture of monodisperse particles was also exposed to a supersaturation which would activate one of the types (hygroscopic salts) but not the other (polystyrene latex spheres or adipic acid). The mass spectrum observed after separation indicated only the former, validating separation of droplets from unactivated particles. Results from ambient measurements using this technique and AMS analysis were inconclusive, showing little chemical differentiation between ambient aerosol and activated droplet residuals, largely due to low signal levels. When employing as single particle mass spectrometer for compositional analysis, however, we observed enhancement of sulfate in droplet residuals.

Persistence of organic carbon in heated aerosol residuals measured during Tropical Composition Cloud and Climate Coupling (TC4)

The Particle Analysis by Laser Mass Spectrometry (PALMS) single particle mass spectrometer was used to analyze the composition of the nonvolatile fraction of atmospheric aerosol in a number of different environments. The mass spectra of individual particles sampled through an inlet section heated to 300°C were compared to unheated particles during flights of the NASA DC‐8 aircraft during the Tropical Composition Cloud and Climate Coupling (TC4) mission. Comparisons are presented of measurements made in the marine boundary layer, the free troposphere, and the continental boundary layer over the Colombian jungle. The heated section completely removed sulfate from the aerosols except for sodium sulfate and related compounds in sea salt particles. Organic material in sea salt particles was observed to be less volatile than chlorine. Biomass burning particles were more likely to survive heating than other mixed sulfate‐organic particles. For all particle types, there was a significant contribution to the residues from carbonaceous material other than elemental carbon. These results demonstrate the remaining compositional complexity of aerosol residuals that survive heating in a thermal denuder.

Carbonaceous material in aerosol particles in the lower stratosphere and tropopause region

The Particle Analysis by Laser Mass Spectrometry (PALMS) instrument has measured the composition of single particles in the lower stratosphere. The average fraction of carbonaceous material in the stratospheric particles decreased rapidly above the tropopause. The decrease in the average carbon content was mostly associated with a sharp increase in the bottom 2 km of the stratosphere in the frequency of fairly pure sulfate particles both with or without meteoric metals. The low potassium content of the fairly pure sulfate particles is used to show that they were formed in the stratosphere and were not tropospheric particles that had lost organics because of oxidation. Of the tropospheric carbonaceous‐sulfate particles found in the stratosphere, the mass spectra had similar patterns from the upper troposphere to the maximum altitude sampled, about 19 km. A reduction in the carbon to sulfate ratio in tropospheric particles was only apparent above about 440 K potential temperature. This implies that carbon compounds can remain for months in particles larger than about 300 nm. Despite the slow rate, these data do not exclude rapid heterogeneous reactions of organics in the particles with OH or other radicals. There was no evidence of significant transfer of semivolatile organics between particles in the stratosphere. However, particles that originated in the stratosphere acquired small amounts of carbon when they were transported to the tropopause.

Single‐particle mass spectrometry of tropospheric aerosol particles

The Particle Analysis by Laser Mass Spectrometry (PALMS) instrument has measured the composition of single particles during a number of airborne and ground‐based campaigns. In the regions studied, 30% to over 80% of the aerosol mass in the free troposphere was carbonaceous material. Most of this carbonaceous material was probably organic. Although there were variations in their amounts, over 90% of accumulation mode particles away from local sources were internal mixtures of sulfates and carbonaceous material. Within this internal mixing, there was variation in the pattern of carbonaceous peaks in the spectra, especially in peaks related to organic acids. Particles with a biomass burning signature were a significant fraction of accumulation mode particles even far from fires. The accumulation mode near the ocean surface off the coasts of California and New England had significant numbers of carbonaceous‐sulfate particles whereas at Cape Grim, Tasmania, the organic‐sulfate particles were apparently all smaller than 160 nm. Three kinds of nitrate were evident: on mineral particles, on carbonaceous‐sulfate particles when the sulfate was fully neutralized, and at temperatures below about 198 K. Most mineral particles showed evidence of the uptake of nitrates and chloride. A peak that probably represents protonated pyridine appears in some spectra in the free troposphere.

Particle analysis by laser mass spectrometry (PALMS) studies of ice nuclei and other low number density particles

It has been shown that particles which effectively initiate freezing, known as ice nuclei (IN), are normally found at concentrations less than 10l−1 in the background atmosphere. The low number density of these particles has presented significant analytical challenges, and determination of the size and composition, and thus the origin, of these particles has historically relied upon electron microscopy (EM). Single particle mass spectrometers can provide better time resolution and reduced sampling artifacts. The modifications to the particle analysis by laser mass spectrometry (PALMS) instrument, a laser desorption/ionization mass spectrometer, required to efficiently size and analyze particles with very low number density, are described here. A comparison to traditional EM studies is made and future applications of this method to solve other contemporary atmospheric problems are also discussed.

Chemical composition of air masses transported from Asia to the U.S. West Coast during ITCT 2K2: Fossil fuel combustion versus biomass‐burning signatures

As part of the Intercontinental Transport and Chemical Transformation experiment in 2002 (ITCT 2K2), a National Oceanic and Atmospheric Administration (NOAA) WP‐3D research aircraft was used to study the long‐range transport of Asian air masses toward the west coast of North America. During research flights on 5 and 17 May, strong enhancements of carbon monoxide (CO) and other species were observed in air masses that had been transported from Asia. The hydrocarbon composition of the air masses indicated that the highest CO levels were related to fossil fuel use. During the flights on 5 and 17 May and other days, the levels of several biomass‐burning indicators increased with altitude. This was true for acetonitrile (CH3CN), methyl chloride (CH3Cl), the ratio of acetylene (C2H2) to propane (C3H8), and, on May 5, the percentage of particles measured by the particle analysis by laser mass spectrometry (PALMS) instrument that were attributed to biomass burning based on their carbon and potassium content. An ensemble of back‐trajectories, calculated from the U.S. west coast over a range of latitudes and altitudes for the entire ITCT 2K2 period, showed that air masses from Southeast Asia and China were generally observed at higher altitudes than air from Japan and Korea. Emission inventories estimate the contribution of biomass burning to the total emissions to be low for Japan and Korea, higher for China, and the highest for Southeast Asia. Combined with the origin of the air masses versus altitude, this qualitatively explains the increase with altitude, averaged over the whole ITCT 2K2 period, of the different biomass‐burning indicators.

Biomass‐burning particle measurements: Characteristic composition and chemical processing

The NOAA Lockheed Orion WP‐3D aircraft intercepted a forest fire plume over Utah on 19 May 2002 during the Intercontinental Transport and Chemical Transformation (ITCT) mission. Large enhancements in acetonitrile (CH3CN), carbon monoxide (CO) and particle number were measured during the fire plume interception. In the 100 s plume crossing, the Particle Analysis by Laser Mass Spectrometry (PALMS) instrument acquired 202 positive mass spectra from ionizing single particles in the 0.2–5 μm size range. These particles contained carbon, potassium, organics, and ammonium ions. No pure soot particles were sampled directly from the plume. By characterizing these particle mass spectra, a qualitative biomass‐burning particle signature was developed that was then used to identify biomass‐burning particles throughout ITCT. The analysis was extended to identify biomass‐burning particles in four other missions, without the benefit of gas‐phase biomass‐burning tracers. During ITCT, approximately 33% of the particles sampled in the North American troposphere and 37% of the particles transported from Asia, not influenced by North American sources, were identified as biomass‐burning particles. During the WB‐57 Aerosol Mission (WAM), Atmospheric Chemistry of Combustion Emissions near the Tropopause (ACCENT) and ACCENT 2000 missions, 7% of stratospheric particles were identified as biomass‐burning particles. During the Cirrus Regional Study of Tropical Anvils and Cirrus Layers – Florida Area Cirrus Experiment (CRYSTAL‐FACE) this percentage increased to 52% because the regional stratosphere was strongly affected by an active fire season.

Single particle measurements of the chemical composition of cirrus ice residue during CRYSTAL‐FACE

The first real‐time, in situ, investigation of the chemical composition of the residue of cirrus ice crystals was performed during July 2002. This study was undertaken on a NASA WB‐57F high‐altitude research aircraft as part of CRYSTAL‐FACE, a field campaign which sought to further our understanding of the relation of clouds, water vapor, and climate by characterizing, among other parameters, anvil cirrus formed about the Florida peninsula. A counter flow virtual impactor (CVI) was used to separate cirrus ice from the unactivated interstitial aerosol particles and evaporate condensed‐phase water. Residual material, on a crystal‐by‐crystal basis, was subsequently analyzed using the NOAA Aeronomy Laboratory's Particle Analysis by Laser Mass Spectrometry (PALMS) instrument. Sampling was performed from 5 to 15 km altitude and from 12° to 28° north latitude within cirrus originating over land and ocean. Chemical composition measurements provided several important results. Sea salt was often incorporated into cirrus, consistent with homogeneous ice formation by aerosol particles from the marine boundary layer. Size measurements showed that large particles preferentially froze over smaller ones. Meteoritic material was found within ice crystals, indicative of a relation between stratospheric aerosol particles and tropospheric clouds. Mineral dust was the dominant residue observed in clouds formed during a dust transport event from the Sahara, consistent with a heterogeneous freezing mechanism. These results show that chemical composition and size are important determinants of which aerosol particles form cirrus ice crystals.

Cluster Analysis of Data from the Particle Analysis by Laser Mass Spectrometry (PALMS) Instrument

We describe the use of a hierarchical clustering algorithm on mass spectra of single particles. In this method, the first cluster is found by searching a data set for the two most similar mass spectra. Then the next most similar spectra or clusters are combined sequentially until a stopping condition is met. Chemically reasonable clusters were obtained for several sets of both laboratory and ambient aerosols. Advantages of this technique include assured convergence without multiple iterations, the ability to handle clusters with widely varying numbers of elements, and good performance even when a continuum of points connects different clusters. A disadvantage of this hierarchical clustering is that it operates on a fixed data set and cannot assign spectra to clusters as the spectra are obtained.

Nitrate and oxidized organic ions in single particle mass spectra during the 1999 Atlanta Supersite Project

We measured the chemical components of 0.35–2.5 μm diameter aerosols with a Particle Analysis by Laser Mass Spectrometry (PALMS) instrument during the Atlanta Supersite Project in August 1999. This paper presents the results for nitrate and oxidized organics. More than 20% of the negative ion spectra of single particles contained nitrate ion peaks. Nitrate peaks showed a clear maximum during the morning at the time of high ambient relative humidity. Nitrate peaks also had a small local maximum during the afternoon when the gas‐phase HNO3 concentrations were high. This afternoon maximum was more significant for the particles containing soot/hydrocarbons or aluminosilicate components than for the typical organic/sulfate particles. About 45% of negative spectra contained ions indicative of the oxidized organics. Oxidized organic peaks showed a diurnal variation similar to the nitrate: a morning time maximum during the relative humidity maximum and a small maximum in the afternoon. However, in contrast to the nitrate peaks, the afternoon local maximum of oxidized organic peaks was apparent on the organic/sulfate particles. Both nitrate and oxidized organic peaks had larger ion fractions in particles with higher scattered light intensities.

Composition of individual particles in the wakes of an Athena II rocket and the space shuttle

The Particle Analysis by Laser Mass Spectrometry (PALMS) instrument was used to obtain the first in situ measurements of the composition of particles in the wakes of solid rocket motor (SRMs) launch vehicles. PALMS acquired mass spectra of over 2300 exhaust particles within the plumes of an Athena II rocket and the Space Shuttle. The majority of positive spectra indicated the presence of primary and trace components of the aluminum fuel and the combustion catalyst. Negative spectra showed chlorine from the oxidizer. Nitrate and phosphate fragments and water were common features of spectra acquired during the Space Shuttle encounters. Elemental carbon (EC) was a significant particle type observed in the Athena II plume. The data show that particles emitted by SRMs are more diverse and probably more reactive than previously considered.

Chemical components of single particles measured with Particle Analysis by Laser Mass Spectrometry (PALMS) during the Atlanta SuperSite Project: Focus on organic/sulfate, lead, soot, and mineral particles

Using a Particle Analysis by Laser Mass Spectrometery (PALMS) instrument, we have measured the chemical components of 0.35–2.5 μm in aerodynamic diameter aerosols during the Atlanta SuperSite Project in August 1999. During the whole campaign period, over a half million single‐particle mass spectra were obtained, and about 380,000 spectra were selected for our data analysis. This paper describes the results for organic/sulfate, lead, soot, and mineral dust particles. The most common feature of the Atlanta single particles was the internal mixture of organic compounds and sulfate: 70–99% of the total spectra contained ions characteristic of organic species, and 90–98% contained sulfate ions. These organic/sulfate particles often contained other chemical components such as nitrate, ammonium, halogens, metals, soot/hydrocarbons, and aluminosilicates. Various metals and metal oxides were also observed in Atlanta particles, including Li, B, Na, Mg, K, Ca, Fe, Cu, Zn, V, Sn, and Pb. In particular, small amounts of lead were observed in nearly half of the mass spectra with high signal‐to‐noise ratio. Many soot/hydrocarbon particles in Atlanta had water‐soluble species such as sulfate, nitrate, and oxidized organic components in addition to metals. Mineral particles were detected as large aluminosilicates that also often contained sulfate and nitrate. More than 50 different elements were observed in single particles in Atlanta, which might be an issue for human health effects. Many aerosol components were not strongly dependent on wind direction, indicating that these aerosols were characteristic of the metropolitan area. The mass spectra observed in Atlanta were further compared with our previous results from Idaho Hill, Colorado (relatively remote continental air). These combined results show that the internal mixture of organic components and sulfate can be a general characteristic of tropospheric aerosols in the continental region.

In situ single‐particle characterization at Cape Grim

This paper describes operation of the particle analysis by laser mass spectrometry (PALMS) instrument at Cape Grim. Several changes to the instrument improved the uniformity of ionization, simplified data analysis, and reduced the minimum analyzable particle size to 0.16 μm. Almost all of the particles analyzed by PALMS at Cape Grim during clean marine conditions contained some sea salt. Almost all of the sulfate found during clean marine conditions was mixed with sea salt rather than in separate ammonium sulfate or sulfuric acid aerosols. Metals such as Cr, Fe, and Sn were detected in a few particles even in clean marine air masses. An NO+ peak in the mass spectra was associated with Mg, Ca, and Sr. This association could be caused either by the water retention of various salts or by different reactivities. It provides a reminder that other components of sea salt besides NaCl need to be considered for heterogeneous chemistry.

On the Purity of Laboratory-Generated Sulfuric Acid Droplets and Ambient Particles Studied by Laser Mass Spectrometry

ABSTRACT Particle analysis by laser mass spectrometry (PALMS) was used to examine sulfuric acid particles representative of stratospheric sulfate aerosols (SSAs) and ambient tropospheric aerosols. Sulfuric acid particles were generated in the laboratory by condensing sulfuric acid vapors in a flow stream of particle-free dry air or nitrogen. The purest particles were produced using filtered, high-purity nitrogen in a clean glass and stainless steel system. In contrast, generation techniques using filtered compressed air, Tygon tubing, or Viton o-rings resulted in detectable organic impurities in the positive ion spectra of sulfuric acid droplets. With the PALMS instrument, the lower limit of our detection of organics is at most 0.02 wt% α-tocopherol in sulfuric acid, which in terms of surface coverage corresponds to less than one monolayer on a 0.2-μm-diameter particle. When we eventually deploy the PALMS instrument in the upper troposphere and lower stratosphere, we should be able to detect the presence of very small amounts of organic compounds in SSAs. In the atmosphere, residence times are long and tropospheric concentrations of reactive gas-phase organic compounds are high, implying that ambient sulfuric acid particles are unlikely to be pure. Other experiments using internally mixed tridecane/sulfuric acid particles confirm that ambient particles from Boulder, CO and Idaho Hill, CO contain internal mixtures of oxidized organic compounds and sulfate.