Formation of highly oxygenated organic aerosol in the atmosphere: Insights from the Finokalia Aerosol Measurement Experiments

[1] Aged organic aerosol (OA) was measured at a remote coastal site on the island of Crete, Greece during the Finokalia Aerosol Measurement Experiments (FAME-08 and FAME-09), which were part of the EUCAARI intensive campaigns. Quadrupole aerosol mass spectrometers (Q-AMSs) were employed to measure the size-resolved chemical composition of non-refractory submicron aerosol (NR-PM1), and to estimate the extent of oxidation of the OA. The experiments provide unique insights into ambient oxidation of aerosol by measuring at the same site but under different photochemical conditions. NR-PM1 concentrations were about a factor of three lower during FAME-09 (winter) than during FAME-08 (summer). The OA sampled was significantly less oxidized and more variable in composition during the winter than during the early summer. Lower OH concentrations in the winter were the main difference between the two campaigns, suggesting that atmospheric formation of highly oxygenated OA is associated with homogeneous photochemical aging.</p

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

An Iodide-Adduct High-Resolution Time-of-Flight Chemical-Ionization Mass Spectrometer: Application to Atmospheric Inorganic and Organic Compounds

A high-resolution time-of-flight chemical-ionization mass spectrometer (HR-ToF-CIMS) using Iodide-adducts has been characterized and deployed in several laboratory and field studies to measure a suite of organic and inorganic atmospheric species. The large negative mass defect of Iodide, combined with soft ionization and the high mass-accuracy (<20 ppm) and mass-resolving power (<i>R</i> > 5500) of the time-of-flight mass spectrometer, provides an additional degree of separation and allows for the determination of elemental compositions for the vast majority of detected ions. Laboratory characterization reveals Iodide-adduct ionization generally exhibits increasing sensitivity toward more polar or acidic volatile organic compounds. Simultaneous retrieval of a wide range of mass-to-charge ratios (m/Q from 25 to 625 Th) at a high frequency (>1 Hz) provides a comprehensive view of atmospheric oxidative chemistry, particularly when sampling rapidly evolving plumes from fast moving platforms like an aircraft. We present the sampling protocol, detection limits and observations from the first aircraft deployment for an instrument of this type, which took place aboard the NOAA WP-3D aircraft during the Southeast Nexus (SENEX) 2013 field campaign

Formation of Secondary Organic Aerosol from the Direct Photolytic Generation of Organic Radicals

The immense complexity inherent in the formation of secondary organic aerosol (SOA)due primarily to the large number of oxidation steps and reaction pathways involvedhas limited the detailed understanding of its underlying chemistry. As a means of simplifying such complexity, here we demonstrate the formation of SOA through the photolysis of gas-phase alkyl iodides, which generates organic peroxy radicals of known structure. In contrast to standard OH-initiated oxidation experiments, photolytically initiated oxidation forms a limited number of products via a single reactive step. As is typical for SOA, the yields of aerosol generated from the photolysis of alkyl iodides depend on aerosol loading, indicating the semivolatile nature of the particulate species. However, the aerosol was observed to be higher in volatility and less oxidized than in previous multigenerational studies of alkane oxidation, suggesting that additional oxidative steps are necessary to produce oxidized semivolatile material in the atmosphere. Despite the relative simplicity of this chemical system, the SOA mass spectra are still quite complex, underscoring the wide range of products present in SOA

Gas-Phase Ozonolysis of Selected Olefins: The Yield of Stabilized Criegee Intermediate and the Reactivity toward SO₂

The gas-phase reaction of ozone with olefins represents an important path for the conversion of unsaturated hydrocarbons in the atmosphere. The current interest is focused on the formation of stabilized Criegee intermediates (sCI) and possible further reactions of sCI. We report results from the ozonolysis of 2,3-dimethyl-2-butene (TME), trans-2-butene and 1-methyl-cyclohexene (MCH) carried out in an atmospheric pressure flow tube at 293 ± 0.5 K and RH = 50% using chemical ionization atmospheric pressure interface time-of-flight (CI-APi-TOF) mass spectrometry to detect H₂SO₄ produced from SO₂ oxidation by sCI. The yields of sCI were found to be in good agreement with recently observed data: 0.62 ± 0.28 (TME), 0.53 ± 0.24 (trans-2-butene) and 0.16 ± 0.07 (MCH). The rate coefficients for sCI + SO₂ from our experiment, (0.9–7.7) × 10^–13 cm³ molecule^–1 s^–1, are within the range of recommendations from indirect determinations as given so far in the literature. Our study helps to assess the importance of sCI in atmospheric chemistry, especially for the oxidation of SO₂ to H₂SO₄

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

<p>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 (<i>f</i>_m/z =, i.e., the ratio of the organic signal at a given <i>m/z</i> 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.</p> <p>Copyright © 2018 American Association for Aerosol Research</p

Real-Time Characterization of Aerosol Particle Composition above the Urban Canopy in Beijing: Insights into the Interactions between the Atmospheric Boundary Layer and Aerosol Chemistry

Despite extensive efforts into the characterization of air pollution during the past decade, real-time characterization of aerosol particle composition above the urban canopy in the megacity Beijing has never been performed to date. Here we conducted the first simultaneous real-time measurements of aerosol composition at two different heights at the same location in urban Beijing from December 19, 2013 to January 2, 2014. The nonrefractory submicron aerosol (NR-PM₁) species were measured in situ by a high-resolution aerosol mass spectrometer at near-ground level and an aerosol chemical speciation monitor at 260 m on a 325 m meteorological tower in Beijing. Secondary aerosol showed similar temporal variations between ground level and 260 m, whereas much weaker correlations were found for the primary aerosol. The diurnal evolution of the ratios and correlations of aerosol species between 260 m and the ground level further illustrated a complex interaction between vertical mixing processes and local source emissions on aerosol chemistry in the atmospheric boundary layer. As a result, the aerosol compositions at the two heights were substantially different. Organic aerosol (OA), mainly composed of primary OA (62%), at the ground level showed a higher contribution to NR-PM₁ (65%) than at 260 m (54%), whereas a higher concentration and contribution (15%) of nitrate was observed at 260 m, probably due to the favorable gas–particle partitioning under lower temperature conditions. In addition, two different boundary layer structures were observed, each interacting differently with the evolution processes of aerosol chemistry

Observation of Fullerene Soot in Eastern China

This work reports the observation of a series of fullerene ions, indicating the occurrence of fullerene soot (FS) in ambient air for the first time using an Aerodyne soot particle-aerosol mass spectrometer (SP-AMS) deployed in eastern China. We found the distribution of these ions showed a pattern almost identical with that of an Alfa Aesar FS standard. Although the SP-AMS may provide only a semiquantitative measurement of the FS, the measured concentrations can still reflect the temporal variations of airborne fullerenes. Combining results from factor analyses and meteorological data, we identified the petrochemical plants situated northeast of the site as the major source responsible for the FS-like ions. Our findings indicate the general presence of FS in ambient air, especially in oil and gas production regions. The SP-AMS technique may offer new insights into characterizing fullerene-related species in other environmental samples, as well

Effect of Pellet Boiler Exhaust on Secondary Organic Aerosol Formation from α‑Pinene

Interactions between anthropogenic and biogenic emissions, and implications for aerosol production, have raised particular scientific interest. Despite active research in this area, real anthropogenic emission sources have not been exploited for anthropogenic-biogenic interaction studies until now. This work examines these interactions using α-pinene and pellet boiler emissions as a model test system. The impact of pellet boiler emissions on secondary organic aerosol (SOA) formation from α-pinene photo-oxidation was studied under atmospherically relevant conditions in an environmental chamber. The aim of this study was to identify which of the major pellet exhaust components (including high nitrogen oxide (NO_<i>x</i>), primary particles, or a combination of the two) affected SOA formation from α-pinene. Results demonstrated that high NO_<i>x</i> concentrations emitted by the pellet boiler reduced SOA yields from α-pinene, whereas the chemical properties of the primary particles emitted by the pellet boiler had no effect on observed SOA yields. The maximum SOA yield of α-pinene in the presence of pellet boiler exhaust (under high-NO_<i>x</i> conditions) was 18.7% and in the absence of pellet boiler exhaust (under low-NO_<i>x</i> conditions) was 34.1%. The reduced SOA yield under high-NO<i>_x</i> conditions was caused by changes in gas-phase chemistry that led to the formation of organonitrate compounds

Effects of Chemical Complexity on the Autoxidation Mechanisms of Endocyclic Alkene Ozonolysis Products: From Methylcyclohexenes toward Understanding α‑Pinene

Formation of highly oxidized, multifunctional products in the ozonolysis of three endocyclic alkenes, 1- methylcyclohexene, 4-methylcyclohexene, and α-pinene, was investigated using a chemical ionization atmospheric pressure interface time-of-flight (CI-APi-TOF) mass spectrometer with a nitrate ion (NO₃^–) based ionization scheme. The experiments were performed in borosilicate glass flow tube reactors at room temperature (<i>T</i> = 293 ± 3 K) and at ambient pressure. An ensemble of oxidized monomer and dimer products was detected, with elemental compositions obtained from the high-resolution mass spectra. The monomer product distributions have O/C ratios from 0.8 to 1.6 and can be explained with an autocatalytic oxidation mechanism (=autoxidation) where the oxygen-centered peroxy radical (RO₂) intermediates internally rearrange by intramolecular hydrogen shift reactions, enabling more oxygen molecules to attach to the carbon backbone. Dimer distributions are proposed to form by homogeneous peroxy radical recombination and cross combination reactions. These conclusions were supported by experiments where H atoms were exchanged to D atoms by addition of D₂O to the carrier gas flow. Methylcyclohexenes were observed to autoxidize in accordance with our previous work on cyclohexene, whereas in α-pinene ozonolysis different mechanistic steps are needed to explain the products observed