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1. Chemical composition of nanoparticles from <i>α</i>-pinene nucleation and the influence of isoprene and relative humidity at low temperature

   https://doi.org/10.5194/acp-21-17099-2021  

   Caudillo, Lucía ; Rörup, Birte ; Heinritzi, Martin ; Marie, Guillaume ; Simon, Mario ; Wagner, Andrea C. ; Müller, Tatjana ; Granzin, Manuel ; Amorim, Antonio ; Ataei, Farnoush ; et al ( January 2021 , Atmospheric Chemistry and Physics)

   Abstract. Biogenic organic precursors play an important role inatmospheric new particle formation (NPF). One of the major precursor speciesis α-pinene, which upon oxidation can form a suite of productscovering a wide range of volatilities. Highly oxygenated organic molecules(HOMs) comprise a fraction of the oxidation products formed. While it isknown that HOMs contribute to secondary organic aerosol (SOA) formation,including NPF, they have not been well studied in newly formed particles dueto their very low mass concentrations. Here we present gas- and particle-phase chemical composition data from experimental studies of α-pinene oxidation, including in the presence of isoprene, at temperatures(−50 and −30 ∘C) and relativehumidities (20 % and 60 %) relevant in the upper free troposphere. Themeasurements took place at the CERN Cosmics Leaving Outdoor Droplets (CLOUD)chamber. The particle chemical composition was analyzed by a thermaldesorption differential mobility analyzer (TD-DMA) coupled to a nitratechemical ionization–atmospheric pressure interface–time-of-flight(CI-APi-TOF) mass spectrometer. CI-APi-TOF was used for particle- and gas-phase measurements, applying the same ionization and detection scheme. Ourmeasurements revealed the presence of C8−10 monomers and C18−20dimers as the major compounds in the particles (diameter up to∼ 100 nm). Particularly, for the system with isoprene added,C5 (C5H10O5−7) and C15 compounds(C15H24O5−10) were detected. This observation is consistentwith the previously observed formation ofmore »such compounds in the gas phase. However, although the C5 and C15 compounds do not easily nucleate,our measurements indicate that they can still contribute to the particlegrowth at free tropospheric conditions. For the experiments reported here,most likely isoprene oxidation products enhance the growth of particleslarger than 15 nm. Additionally, we report on the nucleation rates measuredat 1.7 nm (J1.7 nm) and compared with previous studies, we found lowerJ1.7 nm values, very likely due to the higher α-pinene andozone mixing ratios used in the present study.« less
2. Molecular understanding of new-particle formation from <i>α</i>-pinene between −50 and +25 °C

   https://doi.org/10.5194/acp-20-9183-2020  

   Simon, Mario ; Dada, Lubna ; Heinritzi, Martin ; Scholz, Wiebke ; Stolzenburg, Dominik ; Fischer, Lukas ; Wagner, Andrea C. ; Kürten, Andreas ; Rörup, Birte ; He, Xu-Cheng ; et al ( January 2020 , Atmospheric Chemistry and Physics)

   Abstract. Highly oxygenated organic molecules (HOMs) contributesubstantially to the formation and growth of atmospheric aerosol particles,which affect air quality, human health and Earth's climate. HOMs are formedby rapid, gas-phase autoxidation of volatile organic compounds (VOCs) suchas α-pinene, the most abundant monoterpene in the atmosphere. Due totheir abundance and low volatility, HOMs can play an important role innew-particle formation (NPF) and the early growth of atmospheric aerosols,even without any further assistance of other low-volatility compounds suchas sulfuric acid. Both the autoxidation reaction forming HOMs and theirNPF rates are expected to be strongly dependent ontemperature. However, experimental data on both effects are limited.Dedicated experiments were performed at the CLOUD (Cosmics Leaving OUtdoorDroplets) chamber at CERN to address this question. In this study, we showthat a decrease in temperature (from +25 to −50 ∘C) results ina reduced HOM yield and reduced oxidation state of the products, whereas theNPF rates (J1.7 nm) increase substantially.Measurements with two different chemical ionization mass spectrometers(using nitrate and protonated water as reagent ion, respectively) providethe molecular composition of the gaseous oxidation products, and atwo-dimensional volatility basis set (2D VBS) model provides their volatilitydistribution. The HOM yield decreases with temperature from 6.2 % at 25 ∘C to 0.7 % at −50 ∘C. However, there is a strongreductionmore »of the saturation vapor pressure of each oxidation state as thetemperature is reduced. Overall, the reduction in volatility withtemperature leads to an increase in the nucleation rates by up to 3orders of magnitude at −50 ∘C compared with 25 ∘C. Inaddition, the enhancement of the nucleation rates by ions decreases withdecreasing temperature, since the neutral molecular clusters have increasedstability against evaporation. The resulting data quantify how the interplaybetween the temperature-dependent oxidation pathways and the associatedvapor pressures affect biogenic NPF at the molecularlevel. Our measurements, therefore, improve our understanding of purebiogenic NPF for a wide range of tropospherictemperatures and precursor concentrations.« less
3. Using GECKO-A to derive mechanistic understanding of secondary organic aerosol formation from the ubiquitous but understudied camphene

   https://doi.org/10.5194/acp-21-11467-2021  

   Afreh, Isaac Kwadjo ; Aumont, Bernard ; Camredon, Marie ; Barsanti, Kelley Claire ( January 2021 , Atmospheric Chemistry and Physics)

   Abstract. Camphene, a dominant monoterpene emitted from both biogenic and pyrogenicsources, has been significantly understudied, particularly in regard tosecondary organic aerosol (SOA) formation. When camphene represents asignificant fraction of emissions, the lack of model parameterizations forcamphene can result in inadequate representation of gas-phase chemistry andunderprediction of SOA formation. In this work, the first mechanistic study of SOA formation from camphene was performed using the Generator for Explicit Chemistry and Kinetics of Organics in the Atmosphere (GECKO-A). GECKO-A was used to generate gas-phase chemical mechanisms for camphene and two well-studied monoterpenes, α-pinene and limonene, as well as to predict SOAmass formation and composition based on gas/particle partitioning theory. Themodel simulations represented observed trends in published gas-phase reactionpathways and SOA yields well under chamber-relevant photooxidation and darkozonolysis conditions. For photooxidation conditions, 70 % of thesimulated α-pinene oxidation products remained in the gas phasecompared to 50 % for limonene, supporting model predictions andobservations of limonene having higher SOA yields than α-pinene underequivalent conditions. The top 10 simulated particle-phase products in theα-pinene and limonene simulations represented 37 %–50 % ofthe SOA mass formed and 6 %–27 % of the hydrocarbon mass reacted. Tofacilitate comparison of camphene with α-pinene and limonene, modelsimulations were run under idealized atmospheric conditions, wherein thegas-phase oxidantmore »levels were controlled, and peroxy radicals reacted equallywith HO2 and NO. Metrics for comparison included gas-phasereactivity profiles, time-evolution of SOA mass and yields, andphysicochemical property distributions of gas- and particle-phaseproducts. The controlled-reactivity simulations demonstrated that (1)in the early stages of oxidation, camphene is predicted to form very low-volatility products, lower than α-pinene and limonene, which condenseat low mass loadings; and (2) the final simulated SOA yield for camphene(46 %) was relatively high, in between α-pinene (25 %) andlimonene (74 %). A 50 % α-pinene + 50 % limonene mixture was then used as a surrogate to represent SOA formation from camphene; while simulated SOA mass and yield were well represented, the volatility distribution of the particle-phase products was not. To demonstrate the potential importance of including a parameterized representation of SOA formation by camphene in air quality models, SOA mass and yield were predicted for three wildland fire fuels based on measured monoterpene distributions and published SOA parameterizations for α-pinene and limonene. Using the 50/50 surrogate mixture to represent camphene increased predicted SOA mass by 43 %–50 % for black spruce and by 56 %–108 % for Douglas fir. This first detailed modeling study of the gas-phase oxidation of camphene and subsequent SOA formation highlights opportunities for future measurement–model comparisons and lays a foundation for developing chemical mechanisms and SOA parameterizations for camphene that are suitable for air quality modeling.« less
4. A new model mechanism for atmospheric oxidation of isoprene: global effects on oxidants, nitrogen oxides, organic products, and secondary organic aerosol

   https://doi.org/10.5194/acp-19-9613-2019  

   Bates, Kelvin H. ; Jacob, Daniel J. ( January 2019 , Atmospheric Chemistry and Physics)

   Abstract. Atmospheric oxidation of isoprene, the most abundantly emitted non-methane hydrocarbon, affects the abundances of ozone (O3), the hydroxyl radical (OH), nitrogen oxide radicals (NOx), carbon monoxide (CO), oxygenated and nitrated organic compounds, and secondary organic aerosol (SOA). We analyze these effects in box models and in the global GEOS-Chem chemical transport model using the new reduced Caltech isoprene mechanism (RCIM) condensed from a recently developed explicit isoprene oxidation mechanism. We find many similarities with previous global models of isoprene chemistry along with a number of important differences. Proper accounting of the isomer distribution of peroxy radicals following the addition of OH and O2 to isoprene influences the subsequent distribution of products, decreasing in particular the yield of methacrolein and increasing the capacity of intramolecular hydrogen shifts to promptly regenerate OH. Hydrogen shift reactions throughout the mechanism lead to increased OH recycling, resulting in less depletion of OH under low-NO conditions than in previous mechanisms. Higher organonitrate yields and faster tertiary nitrate hydrolysis lead to more efficient NOx removal by isoprene and conversion to inorganic nitrate. Only 20 % of isoprene-derived organonitrates (excluding peroxyacyl nitrates) are chemically recycled to NOx. The global yield of formaldehyde from isoprene is 22 % per carbonmore »and less sensitive to NO than in previous mechanisms. The global molar yield of glyoxal is 2 %, much lower than in previous mechanisms because of deposition and aerosol uptake of glyoxal precursors. Global production of isoprene SOA is about one-third from each of the following: isoprene epoxydiols (IEPOX), organonitrates, and tetrafunctional compounds. We find a SOA yield from isoprene of 13 % per carbon, much higher than commonly assumed in models and likely offset by SOA chemical loss. We use the results of our simulations to further condense RCIM into a mini Caltech isoprene mechanism (Mini-CIM) for less expensive implementation in atmospheric models, with a total size (108 species, 345 reactions) comparable to currently used mechanisms.« less
5. Quantitative constraints on autoxidation and dimer formation from direct probing of monoterpene-derived peroxy radical chemistry

   https://doi.org/10.1073/pnas.1812147115  

   Zhao, Yue ; Thornton, Joel A. ; Pye, Havala O. T. ( November 2018 , Proceedings of the National Academy of Sciences)

   Organic peroxy radicals (RO₂) are key intermediates in the atmospheric degradation of organic matter and fuel combustion, but to date, few direct studies of specific RO₂in complex reaction systems exist, leading to large gaps in our understanding of their fate. We show, using direct, speciated measurements of a suite of RO₂and gas-phase dimers from O₃-initiated oxidation of α-pinene, that ∼150 gaseous dimers (C_16–20H_24–34O_4–13) are primarily formed through RO₂cross-reactions, with a typical rate constant of 0.75–2 × 10⁻¹²cm³molecule⁻¹s⁻¹and a lower-limit dimer formation branching ratio of 4%. These findings imply a gaseous dimer yield that varies strongly with nitric oxide (NO) concentrations, of at least 0.2–2.5% by mole (0.5–6.6% by mass) for conditions typical of forested regions with low to moderate anthropogenic influence (i.e., ≤50-parts per trillion NO). Given their very low volatility, the gaseous C_16–20dimers provide a potentially important organic medium for initial particle formation, and alone can explain 5–60% of α-pinene secondary organic aerosol mass yields measured at atmospherically relevant particle mass loadings. The responses of RO₂, dimers, and highly oxygenated multifunctional compounds (HOM) to reacted α-pinene concentration and NO imply that an average ∼20% of primary α-pinene RO₂from OH reaction and 10% from ozonolysis autoxidize at 3–10 s⁻¹and ≥1more »s⁻¹, respectively, confirming both oxidation pathways produce HOM efficiently, even at higher NO concentrations typical of urban areas. Thus, gas-phase dimer formation and RO₂autoxidation are ubiquitous sources of low-volatility organic compounds capable of driving atmospheric particle formation and growth.

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