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Abstract Exposure to anthropogenic atmospheric aerosol is a major health issue, causing several million deaths per year worldwide. The oxidation of aromatic hydrocarbons from traffic and wood combustion is an important anthropogenic source of low-volatility species in secondary organic aerosol, especially in heavily polluted environments. It is not yet established whether the formation of anthropogenic secondary organic aerosol involves mainly rapid autoxidation, slower sequential oxidation steps or a combination of the two. Here we reproduced a typical urban haze in the ‘Cosmics Leaving Outdoor Droplets’ chamber at the European Organization for Nuclear Research and observed the dynamics of aromatic oxidation products during secondary organic aerosol growth on a molecular level to determine mechanisms underlying their production and removal. We demonstrate that sequential oxidation is required for substantial secondary organic aerosol formation. Second-generation oxidation decreases the products’ saturation vapour pressure by several orders of magnitude and increases the aromatic secondary organic aerosol yields from a few percent to a few tens of percent at typical atmospheric concentrations. Through regional modelling, we show that more than 70% of the exposure to anthropogenic organic aerosol in Europe arises from second-generation oxidation. 

Abstract During summer, ammonia emissions in Southeast Asia influence air pollution and cloud formation. Convective transport by the South Asian monsoon carries these pollutant air masses into the upper troposphere and lower stratosphere (UTLS), where they accumulate under anticyclonic flow conditions. This air mass accumulation is thought to contribute to particle formation and the development of the Asian Tropopause Aerosol Layer (ATAL). Despite the known influence of ammonia and particulate ammonium on air pollution, a comprehensive understanding of the ATAL is lacking. In this modelling study, the influence of ammonia on particle formation is assessed with emphasis on the ATAL. We use the EMAC chemistry-climate model, incorporating new particle formation parameterisations derived from experiments at the CERN CLOUD chamber. Our diurnal cycle analysis confirms that new particle formation mainly occurs during daylight, with a 10-fold enhancement in rate. This increase is prominent in the South Asian monsoon UTLS, where deep convection introduces high ammonia levels from the boundary layer, compared to a baseline scenario without ammonia. Our model simulations reveal that this ammonia-driven particle formation and growth contributes to an increase of up to 80% in cloud condensation nuclei (CCN) concentrations at cloud-forming heights in the South Asian monsoon region. We find that ammonia profoundly influences the aerosol mass and composition in the ATAL through particle growth, as indicated by an order of magnitude increase in nitrate levels linked to ammonia emissions. However, the effect of ammonia-driven new particle formation on aerosol mass in the ATAL is relatively small. Ammonia emissions enhance the regional aerosol optical depth (AOD) for shortwave solar radiation by up to 70%. We conclude that ammonia has a pronounced effect on the ATAL development, composition, the regional AOD, and CCN concentrations. 

Abstract Aircraft observations have revealed ubiquitous new particle formation in the tropical upper troposphere over the Amazon^1,2and the Atlantic and Pacific oceans^3,4. Although the vapours involved remain unknown, recent satellite observations have revealed surprisingly high night-time isoprene mixing ratios of up to 1 part per billion by volume (ppbv) in the tropical upper troposphere⁵. Here, in experiments performed with the CERN CLOUD (Cosmics Leaving Outdoor Droplets) chamber, we report new particle formation initiated by the reaction of hydroxyl radicals with isoprene at upper-tropospheric temperatures of −30 °C and −50 °C. We find that isoprene-oxygenated organic molecules (IP-OOM) nucleate at concentrations found in the upper troposphere, without requiring any more vapours. Moreover, the nucleation rates are enhanced 100-fold by extremely low concentrations of sulfuric acid or iodine oxoacids above 10⁵ cm⁻³, reaching rates around 30 cm⁻³ s⁻¹at acid concentrations of 10⁶ cm⁻³. Our measurements show that nucleation involves sequential addition of IP-OOM, together with zero or one acid molecule in the embryonic molecular clusters. IP-OOM also drive rapid particle growth at 3–60 nm h⁻¹. We find that rapid nucleation and growth rates persist in the presence of NO_xat upper-tropospheric concentrations from lightning. Our laboratory measurements show that isoprene emitted by rainforests may drive rapid new particle formation in extensive regions of the tropical upper troposphere^1,2, resulting in tens of thousands of particles per cubic centimetre. 

Iodine oxoacids are recognised for their significant contribution to the formation of new particles in marine and polar atmospheres. Nevertheless, to incorporate the iodine oxoacid nucleation mechanism into global simulations, it is essential to comprehend how this mechanism varies under various atmospheric conditions. In this study, we combined measurements from the CLOUD (Cosmic Leaving OUtdoor Droplets) chamber at CERN and simulations with a kinetic model to investigate the impact of temperature, ionisation, and humidity on iodine oxoacid nucleation. Our findings reveal that ion-induced particle formation rates remain largely unaffected by changes in temperature. However, neutral particle formation rates experience a significant increase when the temperature drops from +10 ^oC to −10 ^oC. Running the kinetic model with varying ionisation rates demonstrates that the particle formation rate only increases with a higher ionisation rate when the iodic acid concentration exceeds 1.5 × 10⁷ cm^sup>−3, a concentration rarely reached in pristine marine atmospheres. Consequently, our simulations suggest that, despite higher ionisation rates, the charged cluster nucleation pathway of iodic acid is unlikely to be enhanced in the upper troposphere by higher ionisation rates. Instead, the neutral nucleation channel is likely to be the dominant channel in that region. Notably, the iodine oxoacid nucleation mechanism remains unaffected by changes in relative humidity from 2% to 80%. However, under unrealistically dry conditions (below 0.008% RH at +10 ^oC), iodine oxides (I₂O₄ and I₂O₅) significantly enhance formation rates. Therefore, we conclude that iodine oxoacid nucleation is the dominant nucleation mechanism for iodine nucleation in the marine and polar boundary layer atmosphere. 

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. 

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 now 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. 

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.