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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. 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. 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.