Hyytiälä, Finland, aerosol chemical composition, 2012-2014

aerosol chemical composition, Hyytiälä, Finland, 2012-201

Isomer-Resolved Mobility-Mass Analysis of α‑Pinene Ozonolysis Products

Highly oxygenated organic molecules (HOMs) are important sources of atmospheric aerosols. Resolving the molecular-level formation mechanisms of these HOMs from freshly emitted hydrocarbons improves the understanding of aerosol properties and their influence on the climate. In this study, we measure the electrical mobility and mass-to-charge ratio of α-pinene oxidation products using a secondary electrospray-differential mobility analyzer-mass spectrometer (SESI-DMA-MS). The mass-mobility spectrum of the oxidation products is measured with seven different reagent ions generated by the electrospray. We analyzed the mobility-mass spectra of the oxidation products C9–10H14–18O2–6. Our results show that acetate and chloride yield the highest charging efficiencies. Analysis of the mobility spectra suggests that the clusters have 1–5 isomeric structures (i.e., ion-molecule cluster structures with distinct mobilities), and the number is affected by the reagent ion. Most of the isomers are likely cluster isomers originating from binding of the reagent ion to different sites of the molecule. By comparing the number of observed isomers and measured mobilities and collision cross sections between standard pinanediol and pinonic acid to the values observed for C10H18O2 and C10H16O3 produced from oxidation of α-pinene, we confirm that pinanediol and pinonic acid are the only isomers for these elemental compositions in our experimental conditions. Our study shows that the SESI-DMA-MS produces new information from the first steps of oxidation of α-pinene

Enhanced Aerosol Source Identification by Utilizing High Molecular Weight Signals in Aerosol Mass Spectra

The aerosol mass spectrometer (AMS) has significantly expanded our understanding of aerosol chemical composition over the past few decades. However, most studies have made limited use of the high molecular weight (HMW) signals in the mass spectra due to their low intensities and multiple overlapping peaks. Using long time-of-flight (LTOF) AMS measurements at a boreal forest site in Finland, we utilize the high resolution of the LTOF and the newly developed binPMF approach to explore the potential of the HMW range to improve source identification. During our measurements, inorganics (primarily sulfate) contributed ∼30% and oxygenated organic aerosol (OOA) contributed ∼60% to aerosol mass. The remaining ∼10% were attributed to specific organic aerosol types: hydrocarbon-like OA (HOA) ∼3%, biomass burning OA (BBOA) ∼0.06%, and a pollution plume from the Kola Peninsula ∼5%. None of these factors were separated using traditional unit-mass resolution PMF (m/z 12–150 Th). The small BBOA factor was only extracted using binPMF on the 100–225 amu m/z range, and using this BBOA temporal profile, we could constrain the BBOA factor for the lower masses, revealing a prominent signal at m/z 60 (a tracer for levoglucosan). We encourage AMS users to experiment with the presented approach with their own datasets

Extending the Range of Detectable Trace Species with the Fast Polarity Switching of Chemical Ionization Orbitrap Mass Spectrometry

Chemical ionization (CI) atmospheric pressure interface mass spectrometry is a unique analytical technique for its low detection limits, softness to preserve molecular information, and selectivity for particular classes of species. Here, we present a fast polarity switching approach for highly sensitive online analysis of a wide range of trace species in complex samples using selective CI chemistries and high-resolution mass spectrometry. It is achieved by successfully coupling a multischeme chemical ionization inlet (MION) and an Orbitrap Fourier transform mass spectrometer. The capability to flexibly combine ionization chemistries from both polarities effectively extends the detectability compared to using only one ionization chemistry, as commonly used positive and negative reagent ions tend to be sensitive to different classes of species. We tested the performance of the MION-Orbitrap using reactive gaseous organic species generated by α-pinene ozonolysis in an environmental chamber and a standard mixture of 71 pesticides. Diethylammonium and nitrate are used as reagent ions in positive and negative polarities. We show that with a mass resolving power of 280,000, the MION-Orbitrap can switch and measure both polarities within 1 min, which is sufficiently fast and stable to follow the temporal evolution of reactive organic species and the thermal desorption profile of pesticides. We detected 23 of the 71 pesticides in the mixture using only nitrate as the reagent ion. Facilitated by polarity switching, we also detected 47 pesticides using diethylammonium, improving the total number of detected species to 59. For reactive organic species generated by α-pinene ozonolysis, we show that combining diethylammonium and nitrate addresses the need to measure oxygenated molecules in atmospheric environments with a wide range of oxidation states. These results indicate that the polarity switching MION-Orbitrap can promisingly serve as a versatile tool for the nontargeted chemical analysis of trace species in various applications

Real-Time Detection of Arsenic Cations from Ambient Air in Boreal Forest and Lake Environments

We present the first observation of airborne organic and inorganic arsenic cations, detected in real time within the boreal forest in Hyytiälä, Finland, and over nearby Lake Kuivajärvi. The technique of atmospheric-pressure interface time-of-flight mass spectrometry provides online, <i>in situ</i> monitoring as well as chemical information about the arsenic species, identified as protonated trimethylarsine oxide (AsC₃H₁₀O⁺) and AsO­(H₂O)_<i>n</i>⁺ clusters (<i>n</i> = 0–4). Quantum chemical calculations confirm that the proposed cations are stable under atmospheric conditions. Our most remarkable discovery is that minimal arsenic appeared during spring 2011 until after the ground began to thaw, triggering a sharp increase in airborne arsenic levels as snowmelt flooded the soil with water and stimulated microbial activity. These findings reveal that volatile arsenic species, detected here as atmospheric ions, link the biogeochemical cycling of arsenic through air, soil, water, and living organisms

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

Gas-to-Particle Partitioning of Products from Ozonolysis of Δ³‑Carene and the Effect of Temperature and Relative Humidity

Formation of oxidized products from Δ3-carene (C10H16) ozonolysis and their gas-to-particle partitioning at three temperatures (0, 10, and 20 °C) under dry conditions (<2% RH) and also at 10 °C under humid (78% RH) conditions were studied using a time-of-flight chemical ionization mass spectrometer (ToF-CIMS) combined with a filter inlet for gases and aerosols (FIGAERO). The Δ3-carene ozonolysis products detected by the FIGAERO-ToF-CIMS were dominated by semivolatile organic compounds (SVOCs). The main effect of increasing temperature or RH on the product distribution was an increase in fragmentation of monomer compounds (from C10 to C7 compounds), potentially via alkoxy scission losing a C3 group. The equilibrium partitioning coefficient estimated according to equilibrium partitioning theory shows that the measured SVOC products distribute more into the SOA phase as the temperature decreases from 20 to 10 and 0 °C and for most products as the RH increases from <2 to 78%. The temperature dependency of the saturation vapor pressure (above an assumed liquid state), derived from the partitioning method, also allows for a direct way to obtain enthalpy of vaporization for the detected species without accessibility of authentic standards of the pure substances. This method can provide physical properties, beneficial for, e.g., atmospheric modeling, of complex multifunctional oxidation products

The Formation of Highly Oxidized Multifunctional Products in the Ozonolysis of Cyclohexene

The prompt formation of highly oxidized organic compounds in the ozonolysis of cyclohexene (C₆H₁₀) was investigated by means of laboratory experiments together with quantum chemical calculations. The experiments were performed in borosilicate glass flow tube reactors coupled to a chemical ionization atmospheric pressure interface time-of-flight mass spectrometer with a nitrate ion (NO₃^–)-based ionization scheme. Quantum chemical calculations were performed at the CCSD­(T)-F12a/VDZ-F12//ωB97XD/aug-cc-pVTZ level, with kinetic modeling using multiconformer transition state theory, including Eckart tunneling corrections. The complementary investigation methods gave a consistent picture of a formation mechanism advancing by peroxy radical (RO₂) isomerization through intramolecular hydrogen shift reactions, followed by sequential O₂ addition steps, that is, RO₂ autoxidation, on a time scale of seconds. Dimerization of the peroxy radicals by recombination and cross-combination reactions is in competition with the formation of highly oxidized monomer species and is observed to lead to peroxides, potentially diacyl peroxides. The molar yield of these highly oxidized products (having O/C > 1 in monomers and O/C > 0.55 in dimers) from cyclohexene ozonolysis was determined as (4.5 ± 3.8)%. Fully deuterated cyclohexene and <i>cis</i>-6-nonenal ozonolysis, as well as the influence of water addition to the system (either H₂O or D₂O), were also investigated in order to strengthen the arguments on the proposed mechanism. Deuterated cyclohexene ozonolysis resulted in a less oxidized product distribution with a lower yield of highly oxygenated products and <i>cis</i>-6-nonenal ozonolysis generated the same monomer product distribution, consistent with the proposed mechanism and in agreement with quantum chemical modeling

Molecular Composition of Oxygenated Organic Molecules and Their Contributions to Organic Aerosol in Beijing

The understanding at a molecular level of ambient secondary organic aerosol (SOA) formation is hampered by poorly constrained formation mechanisms and insufficient analytical methods. Especially in developing countries, SOA related haze is a great concern due to its significant effects on climate and human health. We present simultaneous measurements of gas-phase volatile organic compounds (VOCs), oxygenated organic molecules (OOMs), and particle-phase SOA in Beijing. We show that condensation of the measured OOMs explains 26–39% of the organic aerosol mass growth, with the contribution of OOMs to SOA enhanced during severe haze episodes. Our novel results provide a quantitative molecular connection from anthropogenic emissions to condensable organic oxidation product vapors, their concentration in particle-phase SOA, and ultimately to haze formation