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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, in 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. 

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. 

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. 

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 (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 %.