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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.

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

Abstract. New particle formation (NPF) is a significant source of atmosphericparticles, affecting climate and air quality. Understanding the mechanismsinvolved in urban aerosols is important to develop effective mitigationstrategies. However, NPF rates reported in the polluted boundary layer spanmore than 4 orders of magnitude, and the reasons behind this variability are the subject of intense scientific debate. Multiple atmospheric vapours have beenpostulated to participate in NPF, including sulfuric acid, ammonia, aminesand organics, but their relative roles remain unclear. We investigated NPFin the CLOUD chamber using mixtures of anthropogenic vapours that simulatepolluted boundary layer conditions. We demonstrate that NPF in pollutedenvironments is largely driven by the formation of sulfuric acid–baseclusters, stabilized by the presence of amines, high ammonia concentrationsand lower temperatures. Aromatic oxidation products, despite their extremelylow volatility, play a minor role in NPF in the chosen urban environment butcan be important for particle growth and hence for the survival of newlyformed particles. Our measurements quantitatively account for NPF in highlydiverse urban environments and explain its large observed variability. Suchquantitative information obtained under controlled laboratory conditionswill help the interpretation of future ambient observations of NPF rates inpolluted atmospheres.

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. In the present-day atmosphere, sulfuric acid is the mostimportant vapour for aerosol particle formation and initial growth. However,the growth rates of nanoparticles (<10 nm) from sulfuric acidremain poorly measured. Therefore, the effect of stabilizing bases, thecontribution of ions and the impact of attractive forces on molecularcollisions are under debate. Here, we present precise growth ratemeasurements of uncharged sulfuric acid particles from 1.8 to 10 nm, performedunder atmospheric conditions in the CERN (EuropeanOrganization for Nuclear Research) CLOUD chamber. Our results showthat the evaporation of sulfuric acid particles above 2 nm is negligible,and growth proceeds kinetically even at low ammonia concentrations. Theexperimental growth rates exceed the hard-sphere kinetic limit for thecondensation of sulfuric acid. We demonstrate that this results fromvan der Waals forces between the vapour molecules and particles anddisentangle it from charge–dipole interactions. The magnitude of theenhancement depends on the assumed particle hydration and collisionkinetics but is increasingly important at smaller sizes, resulting in asteep rise in the observed growth rates with decreasing size. Including theexperimental results in a global model, we find that the enhanced growth rate ofsulfuric acid particles increases the predicted particle number concentrationsin the upper free troposphere by more than 50 %.

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,more »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.« less

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