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Abstract. The air–sea exchange of ozone (O3) is controlled by chemistry involving halogens, dissolved organic carbon, and sulfur in the sea surface microlayer. Calculations also indicate faster ozone photolysis at aqueous surfaces, but the role of clouds as an ozone sink is currently not well established. Fast-response ozone sensors offer opportunities to measure eddy covariance (EC) ozone fluxes in the marine boundary layer. However, intercomparisons of fast airborne O3 sensors and EC O3 fluxes measured on aircraft have not been conducted before. In April 2022, the Technological Innovation Into Iodine and GV Environmental Research (TI3GER) field campaign deployed three fast ozone sensors (gas chemiluminescence and a combination of UV absorption with coumarin chemiluminescence detection, CID) together with a fast water vapor sensor and anemometer to study iodine chemistry in the troposphere and stratosphere over Colorado and over the Pacific Ocean near Hawaii and Alaska. Here, we present an instrument comparison between the NCAR Fast O3 instrument (FO3, gas-phase CID) and two KIT Fast AIRborne Ozone instruments (FAIRO, UV absorption and coumarin CID). The sensors have comparable precision < 0.4 % Hz−0.5 (0.15 ppbv Hz−0.5), and ozone volume mixing ratios (VMRs) generally agreed within 2 % over a wide range of environmental conditions: 10 < O3 < 1000 ppbv, below detection < NOx < 7 ppbv, and 2 ppmv < H2O < 4 % VMR. Both instrument designs are demonstrated to be suitable for EC flux measurements and were able to detect O3 fluxes with exchange velocities (defined as positive for upward) as slow as −0.010 ± 0.004 cm s−1, which is in the lower range of previously reported measurements. Additionally, we present two case studies. In one, the direction of ozone and water vapor fluxes was reversed (vO3 = +0.134 ± 0.005 cm s−1), suggesting that overhead evaporating clouds could be a strong ozone sink. Further work is needed to better understand the role of clouds as a possibly widespread sink of ozone in the remote marine boundary layer. In the second case study, vO3 values are negative (varying by a factor of 6–10 from −0.036 ± 0.006 to −0.003 ± 0.004 cm s−1), while the water vapor fluxes are consistently positive due to evaporation from the ocean surface and spatially homogeneous. This case study demonstrates that the processes governing ozone and water vapor fluxes can become decoupled and illustrates the need to elucidate possible drivers (physical, chemical, or biological) of the variability in ozone exchange velocities on fine spatial scales (∼ 20 km) over remote oceans. 

Abstract. Laboratory experiments addressing complex phenomena such as atmospheric new-particle formation and growth typically involve numerous instruments measuring a range of key coupled variables. In addition to independent calibration, the combined dataset provides not just constraints on the parameters of interest but also on the critical instrument calibrations. Here we find good agreement between production and loss rates of sulfuric acid (H2SO4) in an experiment performed at the CERN CLOUD chamber involving oxidation of sulfur dioxide (SO2) in the presence of ammonia (NH3) at 58 % relative humidity, driving new-particle formation and growth of particles by H2SO4+NH3 nucleation initiated by O3 photolysis via several light sources. This closure requires consistency across numerous parameters, including: the particle number and size distribution; their condensation sink for H2SO4; the particle growth rates; the concentration of H2SO4; and the nucleation coefficients for both neutral and ion-induced pathways. Our study shows that accurate agreement can be achieved between production and loss of condensable vapors in laboratory chambers under atmospheric conditions, with accuracy ultimately tied to particle number measurement (i.e. a condensation particle counter). This, in turn implies parameters such as the H2SO4 concentration and particle size distributions can be determined to a comparable precision. 

Abstract Dimethyl sulfide (DMS; CH₃SCH₃) from marine phytoplankton is a notable source of atmospheric sulfur¹. Its oxidation products include sulfuric acid (SA; H₂SO₄) and methanesulfonic acid (MSA; CH₃SO₃H), which has a higher yield than SA below 10 °C (ref. ²). Although SA is known to drive the formation of new particles³, which may subsequently grow and act as cloud condensation nuclei (CCN), the role of MSA remains unclear⁴. Here, in experiments performed under atmospheric conditions at the CERN CLOUD (Cosmics Leaving OUtdoor Droplets) chamber, we show that MSA nucleates together with ammonia (NH₃) below −10 °C, at rates comparable with SA-NH₃. Moreover, MSA and SA nucleate synergistically below −10 °C, forming multi-acid molecular clusters with NH₃. Even at ultralow NH₃levels, MSA drives particle growth at or near the kinetic limit below 9 °C and above 40% relative humidity (RH). Because MSA and SA generally coexist at similar concentrations in cool marine regions, our findings indicate that nucleation rates may be accelerated up to tenfold and growth rates up to twofold compared with SA-NH3 alone. Our global model simulations indicate that MSA can enhance CCN concentrations, especially in polar regions. We propose that MSA might be an important driver of biogenic particles in cool, pristine marine regions of both the present-day and pre-industrial atmospheres and yet is unaccounted for in global climate models⁵. 

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 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. 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. 

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.