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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.

 

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. 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. 

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. 

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 that spread across the mid-latitude Northern Hemisphere. 

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 of 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. 

Iodic acid (HIO 3 ) is known to form aerosol particles in coastal marine regions, but predicted nucleation and growth rates are lacking. Using the CERN CLOUD (Cosmics Leaving Outdoor Droplets) chamber, we find that the nucleation rates of HIO 3 particles are rapid, even exceeding sulfuric acid–ammonia rates under similar conditions. We also find that ion-induced nucleation involves IO 3 − and the sequential addition of HIO 3 and that it proceeds at the kinetic limit below +10°C. In contrast, neutral nucleation involves the repeated sequential addition of iodous acid (HIO 2 ) followed by HIO 3 , showing that HIO 2 plays a key stabilizing role. Freshly formed particles are composed almost entirely of HIO 3 , which drives rapid particle growth at the kinetic limit. Our measurements indicate that iodine oxoacid particle formation can compete with sulfuric acid in pristine regions of the atmosphere. 

Abstract. Nucleation of atmospheric vapours produces more than half of global cloudcondensation nuclei and so has an important influence on climate. Recentstudies show that monoterpene (C10H16) oxidation yieldshighly oxygenated products that can nucleate with or without sulfuric acid.Monoterpenes are emitted mainly by trees, frequently together with isoprene(C5H8), which has the highest global emission of all organicvapours. Previous studies have shown that isoprene suppresses new-particleformation from monoterpenes, but the cause of this suppression is underdebate. Here, in experiments performed under atmospheric conditions in theCERN CLOUD chamber, we show that isoprene reduces the yield ofhighly oxygenated dimers with 19 or 20 carbon atoms – which drive particlenucleation and early growth – while increasing the production of dimers with14 or 15 carbon atoms. The dimers (termed C20 and C15,respectively) are produced by termination reactions between pairs of peroxyradicals (RO2⚫) arising from monoterpenes or isoprene.Compared with pure monoterpene conditions, isoprene reduces nucleation ratesat 1.7 nm (depending on the isoprene ∕ monoterpene ratio) and approximatelyhalves particle growth rates between 1.3 and 3.2 nm. However, above 3.2 nm,C15 dimers contribute to secondary organic aerosol, and the growth ratesare unaffected by isoprene. We further show that increased hydroxyl radical(OH⚫) reduces particle formation in our chemical system ratherthan enhances it as previously proposed, since it increases isoprene-derivedRO2⚫ radicals that reduce C20 formation.RO2⚫ termination emerges as the critical step that determinesthe highly oxygenated organic molecule (HOM) distribution and the corresponding nucleation capability. Speciesthat reduce the C20 yield, such as NO, HO2 and as we showisoprene, can thus effectively reduce biogenic nucleation and early growth.Therefore the formation rate of organic aerosol in a particular region ofthe atmosphere under study will vary according to the precise ambientconditions. 

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 strongreduction 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.