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Abstract Oxidation chemistry controls both combustion processes and the atmospheric transformation of volatile emissions. In combustion engines, radical species undergo isomerization reactions that allow fast addition of O 2 . This chain reaction, termed autoxidation, is enabled by high engine temperatures, but has recently been also identified as an important source for highly oxygenated species in the atmosphere, forming organic aerosol. Conventional knowledge suggests that atmospheric autoxidation requires suitable structural features, like double bonds or oxygen-containing moieties, in the precursors. With neither of these functionalities, alkanes, the primary fuel type in combustion engines and an important class of urban trace gases, are thought to have minor susceptibility to extensive autoxidation. Here, utilizing state-of-the-art mass spectrometry, measuring both radicals and oxidation products, we show that alkanes undergo autoxidation much more efficiently than previously thought, both under atmospheric and combustion conditions. Even at high concentrations of NO X , which typically rapidly terminates autoxidation in urban areas, the studied C 6 –C 10 alkanes produce considerable amounts of highly oxygenated products that can contribute to urban organic aerosol. The results of this inter-disciplinary effort provide crucial information on oxidation processes in both combustion engines and the atmosphere, with direct implications for enginemore »efficiency and urban air quality.« less

Abstract. This study presents a characterization of the hygroscopic growth behaviour and effects of different inorganic seed particles on the formation of secondary organic aerosols (SOAs) from the dark ozone-initiated oxidation of isoprene at low NOx conditions. We performed simulations of isoprene oxidation using a gas-phase chemical reaction mechanism based onthe Master Chemical Mechanism (MCM) in combination with an equilibriumgas–particle partitioning model to predict the SOA concentration. Theequilibrium model accounts for non-ideal mixing in liquid phases, includingliquid–liquid phase separation (LLPS), and is based on the AIOMFAC (Aerosol Inorganic–Organic Mixtures Functional groups Activity Coefficients) model for mixture non-ideality and the EVAPORATION (Estimation of VApour Pressure of ORganics, Accounting for Temperature,Intramolecular, and Non-additivity effects) model for pure compound vapourpressures. Measurements from the Cosmics Leaving Outdoor Droplets (CLOUD)chamber experiments, conducted at the European Organization for NuclearResearch (CERN) for isoprene ozonolysis cases, were used to aid inparameterizing the SOA yields at different atmospherically relevanttemperatures, relative humidity (RH), and reacted isoprene concentrations. To represent the isoprene-ozonolysis-derived SOA, a selection of organicsurrogate species is introduced in the coupled modelling system. The modelpredicts a single, homogeneously mixed particle phase at all relativehumidity levels for SOA formation in the absence of any inorganic seedparticles. In the presence ofmore »aqueous sulfuric acid or ammonium bisulfateseed particles, the model predicts LLPS to occur below ∼ 80 % RH, where the particles consist of an inorganic-rich liquid phase andan organic-rich liquid phase; however, this includes significant amounts of bisulfate and water partitioned to the organic-rich phase. The measurements show an enhancement in the SOA amounts at 85 % RH, compared to 35 % RH, for both the seed-free and seeded cases. The model predictions of RH-dependent SOA yield enhancements at 85 % RH vs. 35 % RH are 1.80 for a seed-free case, 1.52 for the case with ammonium bisulfate seed, and 1.06 for the case with sulfuric acid seed. Predicted SOA yields are enhanced in the presence of an aqueous inorganic seed, regardless of the seed type (ammonium sulfate, ammonium bisulfate, or sulfuric acid) in comparison with seed-free conditions at the same RH level. We discuss the comparison of model-predicted SOA yields with a selection of other laboratory studies on isoprene SOA formation conducted at different temperatures and for a variety of reacted isoprene concentrations. Those studies were conducted at RH levels at or below 40 % with reported SOA mass yields ranging from 0.3 % up to 9.0 %, indicating considerable variations. A robust feature of our associated gas–particle partitioning calculations covering the whole RH range is the predicted enhancement of SOA yield at high RH (> 80 %) compared to low RH (dry) conditions, which is explained by the effect of particle water uptake and its impact on the equilibrium partitioning of all components.« less

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.

Abstract. Iodine species are important in the marine atmosphere foroxidation and new-particle formation. Understanding iodine chemistry andiodine new-particle formation requires high time resolution, highsensitivity, and simultaneous measurements of many iodine species. Here, wedescribe the application of a bromide chemical ionization mass spectrometer(Br-CIMS) to this task. During the iodine oxidation experiments in theCosmics Leaving OUtdoor Droplets (CLOUD) chamber, we have measured gas-phaseiodine species and sulfuric acid using two Br-CIMS, one coupled to aMulti-scheme chemical IONization inlet (Br-MION-CIMS) and the other to aFilter Inlet for Gasses and AEROsols inlet (Br-FIGAERO-CIMS). From offlinecalibrations and intercomparisons with other instruments, we havequantified the sensitivities of the Br-MION-CIMS to HOI, I2, andH2SO4 and obtained detection limits of 5.8 × 106,3.8 × 105, and 2.0 × 105 molec. cm−3,respectively, for a 2 min integration time. From binding energycalculations, we estimate the detection limit for HIO3 to be1.2 × 105 molec. cm−3, based on an assumption of maximumsensitivity. Detection limits in the Br-FIGAERO-CIMS are around 1 order ofmagnitude higher than those in the Br-MION-CIMS; for example, the detectionlimits for HOI and HIO3 are 3.3 × 107 and 5.1 × 106 molec. cm−3, respectively. Our comparisons of the performanceof the MION inlet and the FIGAERO inlet show that bromide chemicalionization mass spectrometers using either atmospheric pressure or reducedpressure interfaces are well-matched to measuring iodine species andsulfuric acid in marinemore »environments.« 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.