Aerosol Mass Spectrometry (AMS) Global Database

<p>This database is a repository of submicron non-refractory (NR-PM1) ambient aerosol composition data collected with the Aerodyne Aerosol Mass Spectrometers (AMS) and Aerodyne Aerosol Chemical Speciation Monitors (ACSM) at locations around the world. These data are useful to evaluate aerosol models, compare concentrations and other properties across locations, and other applications.</p

Model Evaluation of Secondary Chemistry due to Disinfection of Indoor Air with Germicidal Ultraviolet Lamps

Air disinfection using germicidal ultraviolet light (GUV) has received increasing attention during the COVID-19 pandemic. GUV uses UVC lamps to inactivate microorganisms, but it also initiates photochemistry in air. However, GUV’s indoor-air-quality impact has not been investigated in detail. Here, we model the chemistry initiated by GUV at 254 (“GUV254”) or 222 nm (“GUV222”) in a typical indoor setting for different ventilation levels. Our analysis shows that GUV254, usually installed in the upper room, can significantly photolyze O3, generating OH radicals that oxidize indoor volatile organic compounds (VOCs) into more oxidized VOCs. Secondary organic aerosol (SOA) is also formed as a VOC-oxidation product. GUV254-induced SOA formation is of the order of 0.1–1 μg/m3 for the cases studied here. GUV222 (described by some as harmless to humans and thus applicable for the whole room) with the same effective virus-removal rate makes a smaller indoor-air-quality impact at mid-to-high ventilation rates. This is mainly because of the lower UV irradiance needed and also less efficient OH-generating O3 photolysis than GUV254. GUV222 has a higher impact than GUV254 under poor ventilation due to a small but significant photochemical production of O3 at 222 nm, which does not occur with GUV254

Effect of Vaporizer Temperature on Ambient Non-Refractory Submicron Aerosol Composition and Mass Spectra Measured by the Aerosol Mass Spectrometer

<div><p>Aerodyne Aerosol Mass Spectrometers (AMS) are routinely operated with a constant vaporizer temperature (<i>T</i>_vap) of 600°C in order to facilitate quantitative detection of non-refractory submicron (NR-PM₁) species. By analogy with other thermal desorption instruments, systematically varying <i>T</i>_vap may provide additional information regarding NR-PM₁ chemical composition and relative volatility, and was explored during two ambient studies. The performance of the AMS generally and the functional integrity of the vaporizer were not negatively impacted during vaporizer temperature cycling (VTC) periods. NR-PM₁ species signals change substantially as <i>T</i>_vap decreases with that change being consistent with previous relative volatility measurements: large decreases in lower volatility components (e.g., sulfate, organic aerosol [OA]) with little, if any, decrease in higher volatility components (e.g., nitrate, ammonium) as <i>T</i>_vap decreases. At <i>T</i>_vap < 600°C, slower evaporation was observed as a shift in particle time-of-flight distributions and an increase in “particle beam blocked” (background) concentrations. Some chemically reduced (i.e., C<i>_x</i>H<i>_y</i>⁺) OA ions at higher <i>m/z</i> are enhanced at lower <i>T</i>_vap, indicating that this method may improve the analysis of some chemically reduced OA systems. The OA spectra changes dramatically with <i>T</i>_vap; however, the observed trends cannot easily be interpreted to derive volatility information. Reducing <i>T</i>_vap increases the relative O:C and CO₂⁺, contrary to what is expected from measured volatility. This is interpreted as continuing decomposition of low volatility species that decreases more slowly (as <i>T</i>_vap decreases) than does the evaporation of reduced species. The reactive vaporizer surface and the inability to reach <i>T</i>_vap much below 200°C of the standard AMS limit the ability of this method to study the volatility of oxidized OA species.</p><p>Copyright 2015 American Association for Aerosol Research</p></div

Elemental Analysis of Complex Organic Aerosol Using Isotopic Labeling and Unit-Resolution Mass Spectrometry

Elemental analysis of unit-mass resolution (UMR) mass spectra is limited by the amount of information available to definitively elucidate the molecular formula of a molecule ionized by electron impact. The problem is compounded when a mixture of organic molecules (such as those found in organic aerosols) is analyzed without the benefit of prior separation. For this reason, quadrupole mass spectrometry is not usually suited to the elemental analysis of organic mixtures. Here, we present a mathematical method for the elemental analysis of UMR mass spectra of a complex organic aerosol through the use of isotopic labeling. Quadrupole aerosol mass spectrometry was used to obtain UMR data of ¹³C-labeled and unlabeled aerosol generated by far ultraviolet (FUV) photochemistry of gas mixtures containing 0.1% of either CH₄ or ¹³CH₄ in N₂. In this method, the differences in the positions of ion groups in the resulting spectra are used to estimate the mass fraction of carbon in the aerosol, and estimation of the remaining elements follows. Analysis of the UMR data yields an elemental composition of 63 ± 7% C, 8 ± 1% H, and 29 ± 7% N by mass. Unlabeled aerosols formed under the same conditions are found by high-resolution time-of-flight aerosol mass spectrometry to have an elemental composition of 63 ± 3% C, 8 ± 1% H, 20 ± 4% N, and 9 ± 3% O by mass, in good agreement with the UMR method. This favorable comparison verifies the method, which expands the UMR mass spectrometry toolkit

Gas-Phase Carboxylic Acids in a University Classroom: Abundance, Variability, and Sources

Gas-phase carboxylic acids are ubiquitous in ambient air, yet their indoor occurrence and abundance are poorly characterized. To fill this gap, we measured gas-phase carboxylic acids in real-time inside and outside of a university classroom using a high-resolution time-of-flight chemical ionization mass spectrometer (HRToF-CIMS) equipped with an acetate ion source. A wide variety of carboxylic acids were identified indoors and outdoors, including monoacids, diacids, hydroxy acids, carbonyl acids, and aromatic acids. An empirical parametrization was derived to estimate the sensitivity (ion counts per ppt of the analytes) of the HRToF-CIMS to the acids. The campaign-average concentration of carboxylic acids measured outdoors was 1.0 ppb, with the peak concentration occurring in daytime. The average indoor concentration of carboxylic acids was 6.8 ppb, of which 87% was contributed by formic and lactic acid. While carboxylic acids measured outdoors displayed a single daytime peak, those measured indoors displayed a daytime and a nighttime peak. Besides indoor sources such as off-gassing of building materials, evidence for acid production from indoor chemical reactions with ozone was found. In addition, some carboxylic acids measured indoors correlated to CO₂ in daytime, suggesting that human occupants may contribute to their abundance either through direct emissions or surface reactions

Direct Measurements of Gas/Particle Partitioning and Mass Accommodation Coefficients in Environmental Chambers

Secondary organic aerosols (SOA) are a major contributor to fine particulate mass and wield substantial influences on the Earth’s climate and human health. Despite extensive research in recent years, many of the fundamental processes of SOA formation and evolution remain poorly understood. Most atmospheric aerosol models use gas/particle equilibrium partitioning theory as a default treatment of gas-aerosol transfer, despite questions about potentially large kinetic effects. We have conducted fundamental SOA formation experiments in a Teflon environmental chamber using a novel method. A simple chemical system produces a very fast burst of low-volatility gas-phase products, which are competitively taken up by liquid organic seed particles and Teflon chamber walls. Clear changes in the species time evolution with differing amounts of seed allow us to quantify the particle uptake processes. We reproduce gas- and aerosol-phase observations using a kinetic box model, from which we quantify the aerosol mass accommodation coefficient (α) as 0.7 on average, with values near unity especially for low volatility species. α appears to decrease as volatility increases. α has historically been a very difficult parameter to measure with reported values varying over 3 orders of magnitude. We use the experimentally constrained model to evaluate the correction factor (Φ) needed for chamber SOA mass yields due to losses of vapors to walls as a function of species volatility and particle condensational sink. Φ ranges from 1–4

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

<div><p>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.</p><p>Copyright 2014 American Association for Aerosol Research</p></div

A Technique for Rapid Gas Chromatography Analysis Applied to Ambient Organic Aerosol Measurements from the Thermal Desorption Aerosol Gas Chromatograph (TAG)

<div><p>While automated techniques exist for the integration of individual gas chromatograph peaks, manual inspection of integration quality and peak choice is still required due to drifting retention times and changing peak shapes near detection limits. The feasibility of a simplified method to obtain multiple bulk species classes from complex gas chromatography data is investigated here with data from the thermal desorption aerosol gas chromatograph (TAG). Chromatograms were divided into many “chromatography bins” containing total eluting mass spectra (both from resolved species and unresolved complex mixture [UCM]), instead of only integrating resolved peaks as is performed in the traditional chromatography analysis method. Positive matrix factorization (PMF) was applied to the mass spectra of the chromatography bins to determine major factors contributing to the observed chemical composition. PMF factors are not highly sensitive to the specific PMF error estimation method applied. Increasing the number of chromatography bins that each chromatogram was divided into improved PMF results until reaching 400 bins. Increasing the number of bins above 400 does not significantly improve the PMF results. This is likely due to 400 bin separation providing bin widths (4.6 s) that match the narrowest peak widths (4.8 s) of compounds found in the TAG chromatograms. The bin-based method took only a small fraction of the time to complete compared to peak-integrated method, significantly saving operator time and effort. Finally, high-factor solutions (e.g., 20 factors) of bin-based PMF can separate many individual compounds, homologues compound series, and UCM from chromatography data.</p><p>Copyright 2014 American Association for Aerosol Research</p></div

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

<p>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 <i>m/z</i>, 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.</p> <p>Copyright © 2017 American Association for Aerosol Research</p

Probing Atmospheric Aerosols by Multimodal Mass Spectrometry Techniques: Revealing Aging Characteristics of Its Individual Molecular Components

Detailed chemical characterization of biomass burning organic aerosol (OA) was performed using a synergistic combination of multimodal mass spectrometry techniques. OA was analyzed in situ using a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) and an extractive electrospray ionization time-of-flight mass spectrometer (EESI-MS) deployed onboard the NASA DC-8 research aircraft. Additionally, complementary filter samples of OA were collected for offline laboratory analysis using high-performance liquid chromatography interfaced with a photodiode array and an electrospray ionization high-resolution mass spectrometer (HPLC-PDA-HRMS). During a research flight on August 3, 2019, which was focused on the Williams Flats Fire, WA, the onboard HR-ToF-AMS data revealed the abundant presence of organic sulfur (OS) species as prominent components of the OA. These OS species were identified based on their unique fragmentation. Further investigation using HPLC-PDA-HRMS and MSn fragmentation allowed us to identify the molecular characteristics of these unusual OS species. The dominant OS compounds detected during the research flight were found to be alkylbenzene sulfonates. Organosulfate, nitroaromatic, and oxygenated aromatic components of OA were also identified. Guided by the HRMS results, time-resolved aging profiles of selected individual OA species were retrieved from the real-time EESI-MS data sets to evaluate their aging evolution in the emission plume. Notably, the alkylbenzene sulfonate species showed remarkable stability over 8 h of atmospheric transport. In contrast, common organosulfates displayed short apparent half-life times that were as low as 1.2 h, indicating their susceptibility to aging. The nitroaromatic and oxygenated aromatic species exhibited relatively slower aging, with average apparent half-life times of 1.8 and 2.2 h, respectively