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Abstract The interaction between nitrogen monoxide (NO) and organic peroxy radicals (RO 2 ) greatly impacts the formation of highly oxygenated organic molecules (HOM), the key precursors of secondary organic aerosols. It has been thought that HOM production can be significantly suppressed by NO even at low concentrations. Here, we perform dedicated experiments focusing on HOM formation from monoterpenes at low NO concentrations (0 – 82 pptv). We demonstrate that such low NO can enhance HOM production by modulating the RO 2 loss and favoring the formation of alkoxy radicals that can continue to autoxidize through isomerization. These insights suggest that HOM yields from typical boreal forest emissions can vary between 2.5%-6.5%, and HOM formation will not be completely inhibited even at high NO concentrations. Our findings challenge the notion that NO monotonically reduces HOM yields by extending the knowledge of RO 2 -NO interactions to the low-NO regime. This represents a major advance towards an accurate assessment of HOM budgets, especially in low-NO environments, which prevails in the pre-industrial atmosphere, pristine areas, and the upper boundary layer. 

The main nucleating vapor in the atmosphere is thought to be sulfuric acid (H₂SO₄), stabilized by ammonia (NH₃). However, in marine and polar regions, NH₃is generally low, and H₂SO₄is frequently found together with iodine oxoacids [HIO_x, i.e., iodic acid (HIO₃) and iodous acid (HIO₂)]. In experiments performed with the CERN CLOUD (Cosmics Leaving OUtdoor Droplets) chamber, we investigated the interplay of H₂SO₄and HIO_xduring atmospheric particle nucleation. We found that HIO_xgreatly enhances H₂SO₄(-NH₃) nucleation through two different interactions. First, HIO₃strongly binds with H₂SO₄in charged clusters so they drive particle nucleation synergistically. Second, HIO₂substitutes for NH₃, forming strongly bound H₂SO₄-HIO₂acid-base pairs in molecular clusters. Global observations imply that HIO_xis enhancing H₂SO₄(-NH₃) nucleation rates 10- to 10,000-fold in marine and polar regions.

 

Biogenic vapors form new particles in the atmosphere, affecting global climate. The contributions of monoterpenes and isoprene to new particle formation (NPF) have been extensively studied. However, sesquiterpenes have received little attention despite a potentially important role due to their high molecular weight. Via chamber experiments performed under atmospheric conditions, we report biogenic NPF resulting from the oxidation of pure mixtures of β-caryophyllene, α-pinene, and isoprene, which produces oxygenated compounds over a wide range of volatilities. We find that a class of vapors termed ultralow-volatility organic compounds (ULVOCs) are highly efficient nucleators and quantitatively determine NPF efficiency. When compared with a mixture of isoprene and monoterpene alone, adding only 2% sesquiterpene increases the ULVOC yield and doubles the formation rate. Thus, sesquiterpene emissions need to be included in assessments of global aerosol concentrations in pristine climates where biogenic NPF is expected to be a major source of cloud condensation nuclei.

 

Abstract. Aerosol particles have an important role in Earth'sradiation balance and climate, both directly and indirectly throughaerosol–cloud interactions. Most aerosol particles in the atmosphere areweakly charged, affecting both their collision rates with ions and neutralmolecules, as well as the rates by which they are scavenged by other aerosolparticles and cloud droplets. The rate coefficients between ions and aerosolparticles are important since they determine the growth rates and lifetimesof ions and charged aerosol particles, and so they may influence cloudmicrophysics, dynamics, and aerosol processing. However, despite theirimportance, very few experimental measurements exist of charged aerosolcollision rates under atmospheric conditions, where galactic cosmic rays inthe lower troposphere give rise to ion pair concentrations of around 1000 cm−3. Here we present measurements in the CERN CLOUD chamber of therate coefficients between ions and small (<10 nm) aerosol particlescontaining up to 9 elementary charges, e. We find the rate coefficient of asingly charged ion with an oppositely charged particle increases from 2.0(0.4–4.4) × 10−6 cm3 s−1 to 30.6 (24.9–45.1) × 10−6 cm3 s−1 for particles with charges of 1 to9 e, respectively, where the parentheses indicate the ±1σuncertainty interval. Our measurements are compatible with theoreticalpredictions and show excellent agreement with the model ofGatti and Kortshagen (2008). 

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 of 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. 

Abstract. Currently, the complete chemical characterization of nanoparticles(< 100 nm) represents an analytical challenge, since these particlesare abundant in number but have negligible mass. Several methods forparticle-phase characterization have been recently developed to betterdetect and infer more accurately the sources and fates of sub-100 nmparticles, but a detailed comparison of different approaches is missing.Here we report on the chemical composition of secondary organic aerosol(SOA) nanoparticles from experimental studies of α-pinene ozonolysisat −50, −30, and −10 ∘C and intercompare the results measured by differenttechniques. The experiments were performed at the Cosmics Leaving OUtdoorDroplets (CLOUD) chamber at the European Organization for Nuclear Research(CERN). The chemical composition was measured simultaneously by fourdifferent techniques: (1) thermal desorption–differential mobility analyzer(TD–DMA) coupled to a NO3- chemical ionization–atmospheric-pressure-interface–time-of-flight (CI–APi–TOF) massspectrometer, (2) filter inlet for gases and aerosols (FIGAERO) coupled to anI− high-resolution time-of-flight chemical ionization mass spectrometer(HRToF-CIMS), (3) extractive electrospray Na+ ionizationtime-of-flight mass spectrometer (EESI-TOF), and (4) offline analysis offilters (FILTER) using ultra-high-performance liquid chromatography (UHPLC)and heated electrospray ionization (HESI) coupled to an Orbitraphigh-resolution mass spectrometer (HRMS). Intercomparison was performed bycontrasting the observed chemical composition as a function of oxidationstate and carbon number, by estimating the volatility and comparing thefraction of volatility classes, and by comparing the thermal desorptionbehavior (for the thermal desorption techniques: TD–DMA and FIGAERO) andperforming positive matrix factorization (PMF) analysis for the thermograms.We found that the methods generally agree on the most important compoundsthat are found in the nanoparticles. However, they do see different parts ofthe organic spectrum. We suggest potential explanations for thesedifferences: thermal decomposition, aging, sampling artifacts, etc. Weapplied PMF analysis and found insights of thermal decomposition in theTD–DMA and the FIGAERO. 

Abstract. Iodine species are important in the marine atmosphere foroxidation and new-particle formation. Understanding iodine chemistry andiodine new-particle formation requires high time resolution, highsensitivity, and simultaneous measurements of many iodine species. Here, wedescribe the application of a bromide chemical ionization mass spectrometer(Br-CIMS) to this task. During the iodine oxidation experiments in theCosmics Leaving OUtdoor Droplets (CLOUD) chamber, we have measured gas-phaseiodine species and sulfuric acid using two Br-CIMS, one coupled to aMulti-scheme chemical IONization inlet (Br-MION-CIMS) and the other to aFilter Inlet for Gasses and AEROsols inlet (Br-FIGAERO-CIMS). From offlinecalibrations and intercomparisons with other instruments, we havequantified the sensitivities of the Br-MION-CIMS to HOI, I2, andH2SO4 and obtained detection limits of 5.8 × 106,3.8 × 105, and 2.0 × 105 molec. cm−3,respectively, for a 2 min integration time. From binding energycalculations, we estimate the detection limit for HIO3 to be1.2 × 105 molec. cm−3, based on an assumption of maximumsensitivity. Detection limits in the Br-FIGAERO-CIMS are around 1 order ofmagnitude higher than those in the Br-MION-CIMS; for example, the detectionlimits for HOI and HIO3 are 3.3 × 107 and 5.1 × 106 molec. cm−3, respectively. Our comparisons of the performanceof the MION inlet and the FIGAERO inlet show that bromide chemicalionization mass spectrometers using either atmospheric pressure or reducedpressure interfaces are well-matched to measuring iodine species andsulfuric acid in marine environments. 

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.