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  1. Abstract. This study presents a characterization of the hygroscopic growth behaviour and effects of different inorganic seed particles on the formation of secondary organic aerosols (SOAs) from the dark ozone-initiated oxidation of isoprene at low NOx conditions. We performed simulations of isoprene oxidation using a gas-phase chemical reaction mechanism based onthe Master Chemical Mechanism (MCM) in combination with an equilibriumgas–particle partitioning model to predict the SOA concentration. Theequilibrium model accounts for non-ideal mixing in liquid phases, includingliquid–liquid phase separation (LLPS), and is based on the AIOMFAC (Aerosol Inorganic–Organic Mixtures Functional groups Activity Coefficients) model for mixture non-ideality and the EVAPORATION (Estimation of VApour Pressure of ORganics, Accounting for Temperature,Intramolecular, and Non-additivity effects) model for pure compound vapourpressures. Measurements from the Cosmics Leaving Outdoor Droplets (CLOUD)chamber experiments, conducted at the European Organization for NuclearResearch (CERN) for isoprene ozonolysis cases, were used to aid inparameterizing the SOA yields at different atmospherically relevanttemperatures, relative humidity (RH), and reacted isoprene concentrations. To represent the isoprene-ozonolysis-derived SOA, a selection of organicsurrogate species is introduced in the coupled modelling system. The modelpredicts a single, homogeneously mixed particle phase at all relativehumidity levels for SOA formation in the absence of any inorganic seedparticles. In the presence ofmore »aqueous sulfuric acid or ammonium bisulfateseed particles, the model predicts LLPS to occur below ∼ 80 % RH, where the particles consist of an inorganic-rich liquid phase andan organic-rich liquid phase; however, this includes significant amounts of bisulfate and water partitioned to the organic-rich phase. The measurements show an enhancement in the SOA amounts at 85 % RH, compared to 35 % RH, for both the seed-free and seeded cases. The model predictions of RH-dependent SOA yield enhancements at 85 % RH vs. 35 % RH are 1.80 for a seed-free case, 1.52 for the case with ammonium bisulfate seed, and 1.06 for the case with sulfuric acid seed. Predicted SOA yields are enhanced in the presence of an aqueous inorganic seed, regardless of the seed type (ammonium sulfate, ammonium bisulfate, or sulfuric acid) in comparison with seed-free conditions at the same RH level. We discuss the comparison of model-predicted SOA yields with a selection of other laboratory studies on isoprene SOA formation conducted at different temperatures and for a variety of reacted isoprene concentrations. Those studies were conducted at RH levels at or below 40 % with reported SOA mass yields ranging from 0.3 % up to 9.0 %, indicating considerable variations. A robust feature of our associated gas–particle partitioning calculations covering the whole RH range is the predicted enhancement of SOA yield at high RH (> 80 %) compared to low RH (dry) conditions, which is explained by the effect of particle water uptake and its impact on the equilibrium partitioning of all components.« less
  2. Abstract Iodine is a reactive trace element in atmospheric chemistry that destroys ozone and nucleates particles. Iodine emissions have tripled since 1950 and are projected to keep increasing with rising O 3 surface concentrations. Although iodic acid (HIO 3 ) is widespread and forms particles more efficiently than sulfuric acid, its gas-phase formation mechanism remains unresolved. Here, in CLOUD atmospheric simulation chamber experiments that generate iodine radicals at atmospherically relevant rates, we show that iodooxy hypoiodite, IOIO, is efficiently converted into HIO 3 via reactions (R1) IOIO + O 3  → IOIO 4 and (R2) IOIO 4  + H 2 O → HIO 3  + HOI +  (1) O 2 . The laboratory-derived reaction rate coefficients are corroborated by theory and shown to explain field observations of daytime HIO 3 in the remote lower free troposphere. The mechanism provides a missing link between iodine sources and particle formation. Because particulate iodate is readily reduced, recycling iodine back into the gas phase, our results suggest a catalytic role of iodine in aerosol formation.
    Free, publicly-accessible full text available November 14, 2023
  3. Intense new particle formation events are regularly observed under highly polluted conditions, despite the high loss rates of nucleated clusters. Higher than expected cluster survival probability implies either ineffective scavenging by pre-existing particles or missing growth mechanisms. Here we present experiments performed in the CLOUD chamber at CERN showing particle formation from a mixture of anthropogenic vapours, under condensation sinks typical of haze conditions, up to 0.1 s −1 . We find that new particle formation rates substantially decrease at higher concentrations of pre-existing particles, demonstrating experimentally for the first time that molecular clusters are efficiently scavenged by larger sized particles. Additionally, we demonstrate that in the presence of supersaturated gas-phase nitric acid (HNO 3 ) and ammonia (NH 3 ), freshly nucleated particles can grow extremely rapidly, maintaining a high particle number concentration, even in the presence of a high condensation sink. Such high growth rates may explain the high survival probability of freshly formed particles under haze conditions. We identify under what typical urban conditions HNO 3 and NH 3 can be expected to contribute to particle survival during haze.
    Free, publicly-accessible full text available May 19, 2023
  4. Abstract. Extractive electrospray ionization (EESI) has been a well-knowntechnique for high-throughput online molecular characterization of chemicalreaction products and intermediates, detection of native biomolecules, invivo metabolomics, and environmental monitoring with negligible thermal andionization-induced fragmentation for over two decades. However, the EESIextraction mechanism remains uncertain. Prior studies disagree on whetherparticles between 20 and 400 nm diameter are fully extracted or if theextraction is limited to the surface layer. Here, we examined the analyteextraction mechanism by assessing the influence of particle size and coatingthickness on the detection of the molecules therein. We find that particlesare extracted fully: organics-coated NH4NO3 particles with afixed core volume (156 and 226 nm in diameter without coating) showedconstant EESI signals for NH4NO3 independent of the shell coatingthickness, while the signals of the secondary organic molecules comprisingthe shell varied proportionally to the shell volume. We also found that theEESI sensitivity exhibited a strong size dependence, with an increase insensitivity by 1–3 orders of magnitude as particle size decreasedfrom 300 to 30 nm. This dependence varied with the electrospray (ES)droplet size, the particle size and the residence time for coagulation in theEESI inlet, suggesting that the EESI sensitivity was influenced by thecoagulation coefficient between particles and ES droplets. Overall, ourresults indicate that, inmore »the EESI, particles are fully extracted by the ESdroplets regardless of the chemical composition, when they are collected bythe ES droplets. However, their coalescence is not complete and dependsstrongly on their size. This size dependence is especially relevant whenEESI is used to probe size-varying particles as is the case in aerosolformation and growth studies with size ranges below 100 nm.« less
  5. Abstract New particle formation in the upper free troposphere is a major global source of cloud condensation nuclei (CCN) 1–4 . However, the precursor vapours that drive the process are not well understood. With experiments performed under upper tropospheric conditions in the CERN CLOUD chamber, we show that nitric acid, sulfuric acid and ammonia form particles synergistically, at rates that are orders of magnitude faster than those from any two of the three components. The importance of this mechanism depends on the availability of ammonia, which was previously thought to be efficiently scavenged by cloud droplets during convection. However, surprisingly high concentrations of ammonia and ammonium nitrate have recently been observed in the upper troposphere over the Asian monsoon region 5,6 . Once particles have formed, co-condensation of ammonia and abundant nitric acid alone is sufficient to drive rapid growth to CCN sizes with only trace sulfate. Moreover, our measurements show that these CCN are also highly efficient ice nucleating particles—comparable to desert dust. Our model simulations confirm that ammonia is efficiently convected aloft during the Asian monsoon, driving rapid, multi-acid HNO 3 –H 2 SO 4 –NH 3 nucleation in the upper troposphere and producing ice nucleating particles thatmore »spread across the mid-latitude Northern Hemisphere.« less
    Free, publicly-accessible full text available May 19, 2023
  6. Aerosol particles negatively affect human health while also having climatic relevance due to, for example, their ability to act as cloud condensation nuclei. Ultrafine particles (diameter D p < 100 nm) typically comprise the largest fraction of the total number concentration, however, their chemical characterization is difficult because of their low mass. Using an extractive electrospray time-of-flight mass spectrometer (EESI-TOF), we characterize the molecular composition of freshly nucleated particles from naphthalene and β-caryophyllene oxidation products at the CLOUD chamber at CERN. We perform a detailed intercomparison of the organic aerosol chemical composition measured by the EESI-TOF and an iodide adduct chemical ionization mass spectrometer equipped with a filter inlet for gases and aerosols (FIGAERO-I-CIMS). We also use an aerosol growth model based on the condensation of organic vapors to show that the chemical composition measured by the EESI-TOF is consistent with the expected condensed oxidation products. This agreement could be further improved by constraining the EESI-TOF compound-specific sensitivity or considering condensed-phase processes. Our results show that the EESI-TOF can obtain the chemical composition of particles as small as 20 nm in diameter with mass loadings as low as hundreds of ng m −3 in real time. This was until nowmore »difficult to achieve, as other online instruments are often limited by size cutoffs, ionization/thermal fragmentation and/or semi-continuous sampling. Using real-time simultaneous gas- and particle-phase data, we discuss the condensation of naphthalene oxidation products on a molecular level.« less
  7. Free, publicly-accessible full text available January 30, 2024
  8. 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