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The number concentration and properties of aerosol particles serving as cloud condensation nuclei (CCN) are important for understanding cloud properties, including in the tropical Atlantic marine boundary layer (MBL), where marine cumulus clouds reflect incoming solar radiation and obscure the low-albedo ocean surface. Studies linking aerosol source, composition, and water uptake properties in this region have been conducted primarily during the summertime dust transport season, despite the region receiving a variety of aerosol particle types throughout the year. In this study, we compare size-resolved aerosol chemical composition data to the hygrocopicity parameter κ derived from size-resolved CCN measurements made during the Elucidating the Role of Clouds-Circulation Coupling in Climate (EUREC4A) and Atlantic Tradewind Ocean-Atmosphere Mesoscale Interaction Campaign (ATOMIC) campaigns from January to February 2020. We observed unexpected periods of wintertime long-range transport of African smoke and dust to Barbados. During these periods, the accumulation-mode aerosol particle and CCN Number concentrations as well as the proportions of dust and smoke particles increased, whereas average κ slightly decreased (κ = 0.46 +/- 0.10) from marine background conditions (κ = 0.52 +/- 0.09) when the particles were mostly composed of marine organics and sulfate. Size-resolved chemical analysis shows that smoke particles were the major contributor to the accumulation mode during long-range transport events, indicating that smoke is mainly responsible for the observed increase in CCN number concentrations. Earlier studies conducted at Barbados have mostly focused on the role of dust in CCN, but our results show that aerosol hygroscopicity and CCN number concentrations during wintertime long-range transport events over the tropical North Atlantic are also affected by African smoke. Our findings highlight the importance of African smoke for atmospheric processes and cloud formation over the Caribbean. In the file “Dust_Mass_Conc_Royer2022” dust mass concentrations in grams per meter^3 are provided for each day of sampling. These data were used to generate Figure 2a in the manuscript. The file “Particle_Type_#fract_Royer2022” contains data obtained through CCSEM/EDX analysis and used to generate the temporal chemistry plot (Figure 4) provided in the manuscript. The data contains particle numbers for each particle type identified on stage 3 of the sampler, total particle numbers analyzed for the entire stage 3 sample, as well as particle number fractions in % values. In the file “Size-resolved_chem_Royer2022” we provide particle # and number fraction (%) values used to generate size-resolved chemistry plots in the manuscript (Figures 5a and 5b). The file includes all particle numbers and number fractions for sea salt, aged sea salt, dust+sea salt, dust, dust+smoke, smoke, sulfate, and organic particles in each size bin from 0.1 through 8.058 um during cumulative clean marine periods and CAT Event 1 as described in the manuscript. The file “K_at_0.16S_Royer2022” contains κ values calculated at 0.16% supersaturation (S) throughout the entire sampling period. These data were specifically used to generate the plot in Figure 7a. The file “CCN#_at_0.16S_Royer2022” contains cloud condensation nuclei (CCN) values calculated at 0.16% supersaturation (S) throughout the entire sampling period. These data were used to create the CCN portion of the plot in Figure 7b. 

Abstract. The number concentration and properties of aerosol particles serving ascloud condensation nuclei (CCN) are important for understanding cloudproperties, including in the tropical Atlantic marine boundary layer (MBL), where marine cumulus clouds reflect incoming solar radiation and obscure thelow-albedo ocean surface. Studies linking aerosol source, composition, andwater uptake properties in this region have been conducted primarily duringthe summertime dust transport season, despite the region receiving a varietyof aerosol particle types throughout the year. In this study, we comparesize-resolved aerosol chemical composition data to the hygroscopicityparameter κ derived from size-resolved CCN measurements made duringthe Elucidating the Role of Clouds–Circulation Coupling in Climate (EUREC4A) and Atlantic Tradewind Ocean-Atmosphere Mesoscale Interaction Campaign (ATOMIC) campaigns from January to February 2020. Weobserved unexpected periods of wintertime long-range transport of Africansmoke and dust to Barbados. During these periods, the accumulation-mode aerosol particle and CCN number concentrations as well as the proportions ofdust and smoke particles increased, whereas the average κ slightlydecreased (κ=0.46±0.10) from marine backgroundconditions (κ=0.52±0.09) when the submicron particles were mostly composed of marine organics and sulfate. Size-resolved chemicalanalysis shows that smoke particles were the major contributor to theaccumulation mode during long-range transport events, indicating that smokeis mainly responsible for the observed increase in CCN numberconcentrations. Earlier studies conducted at Barbados have mostly focused onthe role of dust on CCN, but our results show that aerosol hygroscopicity and CCN number concentrations during wintertime long-range transport events over the tropical North Atlantic are also affected by African smoke. Ourfindings highlight the importance of African smoke for atmospheric processesand cloud formation over the Caribbean. 

Thirdhand smoke (THS) persists in locations where smoking previously occurred and can be transported into non-smoking environments, leading to non-smoker exposure. Laboratory experiments using high-resolution mass spectrometry demonstrate that deposited particulate matter (PM) and smoke-exposed surrogate lung lining fluid (LLF) are substantial, chemically-complex reservoirs of gas-phase THS emissions, including hazardous air pollutants, polycyclic aromatic compounds, and nitrogen/oxygen-containing species. Both PM and LLF are persistent real-world THS reservoirs that chemically evolve over time, and can act as vehicles for the transport and emission of reactive pollutants and their reaction byproducts (e.g., acrolein). Deposited PM on clothes, furnishings, bodies, and/or airways will emit volatile to semi-volatile gases over long lifetimes, which can re-partition to other indoor materials and increase their overall persistence. On the other hand, LLF off-gassing consists predominantly of volatile organic compounds in amounts influenced by their aqueous solubilities, and their persistence in breath will be prolonged by re-distribution across internal aqueous reservoirs, as corroborated by multicompartment modeling in this study. 

Abstract. Mass accommodation is an essential process for gas–particle partitioning oforganic compounds in secondary organic aerosols (SOA). The massaccommodation coefficient is commonly described as the probability of a gasmolecule colliding with the surface to enter the particle phase. It is oftenapplied, however, without specifying if and how deep a molecule has topenetrate beneath the surface to be regarded as being incorporated into thecondensed phase (adsorption vs. absorption). While this aspect is usuallynot critical for liquid particles with rapid surface–bulk exchange, it canbe important for viscous semi-solid or glassy solid particles to distinguishand resolve the kinetics of accommodation at the surface, transfer acrossthe gas–particle interface, and further transport into the particle bulk. For this purpose, we introduce a novel parameter: an effective massaccommodation coefficient αeff that depends on penetrationdepth and is a function of surface accommodation coefficient, volatility,bulk diffusivity, and particle-phase reaction rate coefficient. Applicationof αeff in the traditional Fuchs–Sutugin approximation ofmass-transport kinetics at the gas–particle interface yields SOApartitioning results that are consistent with a detailed kinetic multilayermodel (kinetic multilayer model of gas–particle interactions in aerosols and clouds, KM-GAP; Shiraiwa et al., 2012) and two-film model solutions (Modelfor Simulating Aerosol Interactions and Chemistry, MOSAIC;Zaveri et al., 2014) but deviate substantially from earlier modelingapproaches not considering the influence of penetration depth and relatedparameters. For highly viscous or semi-solid particles, we show that the effective massaccommodation coefficient remains similar to the surface accommodationcoefficient in the case of low-volatility compounds, whereas it can decrease byseveral orders of magnitude in the case of semi-volatile compounds. Such effectscan explain apparent inconsistencies between earlier studies deriving massaccommodation coefficients from experimental data or from molecular dynamicssimulations. Our findings challenge the approach of traditional SOA models using theFuchs–Sutugin approximation of mass transfer kinetics with a fixed massaccommodation coefficient, regardless of particle phase state and penetrationdepth. The effective mass accommodation coefficient introduced in this studyprovides an efficient new way of accounting for the influence of volatility,diffusivity, and particle-phase reactions on SOA partitioning in processmodels as well as in regional and global air quality models. While kineticlimitations may not be critical for partitioning into liquid SOA particlesin the planetary boundary layer (PBL), the effects are likely important foramorphous semi-solid or glassy SOA in the free and upper troposphere (FT–UT)as well as in the PBL at low relative humidity and low temperature. 

Abstract. Polycyclic aromatic hydrocarbons (PAHs) are carcinogenic air pollutants. The dispersion of PAHs in the atmosphere is influenced by gas–particle partitioning and chemical loss. These processes are closely interlinked and may occur at vastly differing timescales, which complicates their mathematical description in chemical transport models. Here, we use a kinetic model that explicitly resolves mass transport and chemical reactions in the gas and particle phases to describe and explore the dynamic and non-equilibrium interplay of gas–particle partitioning and chemical losses of PAHs on soot particles. We define the equilibration timescale τeq of gas–particle partitioning as the e-folding time for relaxation of the system to the partitioning equilibrium. We find this metric to span from seconds to hours depending on temperature, particle surface area, and the type of PAH. The equilibration time can be approximated using a time-independent equation, τeq≈1kdes+kads, which depends on the desorption rate coefficient kdes and adsorption rate coefficient kads, both of which can be calculated from experimentally accessible parameters. The model reveals two regimes in which different physical processes control the equilibration timescale: a desorption-controlled and an adsorption-controlled regime. In a case study with the PAH pyrene, we illustrate how chemical loss can perturb the equilibrium particulate fraction at typical atmospheric concentrations of O3 and OH. For the surface reaction with O3, the perturbation is significant and increases with the gas-phase concentration of O3. Conversely, perturbations are smaller for reaction with the OH radical, which reacts with pyrene on both the surface of particles and in the gas phase. Global and regional chemical transport models typically approximate gas–particle partitioning with instantaneous-equilibration approaches. We highlight scenarios in which these approximations deviate from the explicitly coupled treatment of gas–particle partitioning and chemistry presented in this study. We find that the discrepancy between solutions depends on the operator-splitting time step and the choice of time step can help to minimize the discrepancy. The findings and techniques presented in this work not only are relevant for PAHs but can also be applied to other semi-volatile substances that undergo chemical reactions and mass transport between the gas and particle phase. 

Abstract. In the aqueous phase, fine particulate matter can form reactive species (RS)that influence the aging, properties, and health effects of atmosphericaerosols. In this study, we explore the RS yields of aerosol samples froma remote forest (Hyytiälä, Finland) and polluted urban locations(Mainz, Germany; Beijing, China), and we relate the RS yields to differentchemical constituents and reaction mechanisms. Ultra-high-resolution massspectrometry was used to characterize organic aerosol composition, electronparamagnetic resonance (EPR) spectroscopy with a spin-trapping technique wasapplied to determine the concentrations of ⚫OH,O2⚫-, and carbon- or oxygen-centered organic radicals, anda fluorometric assay was used to quantify H2O2. The aqueousH2O2-forming potential per mass unit of ambient PM2.5(particle diameter < 2.5 µm) was roughly the same for allinvestigated samples, whereas the mass-specific yields of radicals werelower for sampling sites with higher concentrations of PM2.5. Theabundances of water-soluble transition metals and aromatics in ambientPM2.5 were positively correlated with the relative fraction of⚫OH and negatively correlated with the relative fraction ofcarbon-centered radicals. In contrast, highly oxygenated organic molecules(HOM) were positively correlated with the relative fraction ofcarbon-centered radicals and negatively correlated with the relativefraction of ⚫OH. Moreover, we found that the relative fractionsof different types of radicals formed by ambient PM2.5 were comparableto surrogate mixtures comprising transition metal ions, organichydroperoxide, H2O2, and humic or fulvic acids. The interplay oftransition metal ions (e.g., iron and copper ions), highly oxidized organicmolecules (e.g., hydroperoxides), and complexing or scavenging agents (e.g.,humic or fulvic acids) leads to nonlinear concentration dependencies inaqueous-phase RS production. A strong dependence on chemical compositionwas also observed for the aqueous-phase radical yields oflaboratory-generated secondary organic aerosols (SOA) from precursormixtures of naphthalene and β-pinene. Our findings show how thecomposition of PM2.5 can influence the amount and nature ofaqueous-phase RS, which may explain differences in the chemical reactivityand health effects of particulate matter in clean and polluted air. 

The Atlantic Tradewind Ocean-Atmosphere Mesoscale Interaction Campaign (ATOMIC) took place from 7 January to 11 July 2020 in the tropical North Atlantic between the eastern edge of Barbados and 51∘ W, the longitude of the Northwest Tropical Atlantic Station (NTAS) mooring. Measurements were made to gather information on shallow atmospheric convection, the effects of aerosols and clouds on the ocean surface energy budget, and mesoscale oceanic processes. Multiple platforms were deployed during ATOMIC including the NOAA RV Ronald H. Brown (RHB) (7 January to 13 February) and WP-3D Orion (P-3) aircraft (17 January to 10 February), the University of Colorado's Robust Autonomous Aerial Vehicle-Endurant Nimble (RAAVEN) uncrewed aerial system (UAS) (24 January to 15 February), NOAA- and NASA-sponsored Saildrones (12 January to 11 July), and Surface Velocity Program Salinity (SVPS) surface ocean drifters (23 January to 29 April). The RV Ronald H. Brown conducted in situ and remote sensing measurements of oceanic and atmospheric properties with an emphasis on mesoscale oceanic–atmospheric coupling and aerosol–cloud interactions. In addition, the ship served as a launching pad for Wave Gliders, Surface Wave Instrument Floats with Tracking (SWIFTs), and radiosondes. Details of measurements made from the RV Ronald H. Brown, ship-deployed assets, and other platforms closely coordinated with the ship during ATOMIC are provided here. These platforms include Saildrone 1064 and the RAAVEN UAS as well as the Barbados Cloud Observatory (BCO) and Barbados Atmospheric Chemistry Observatory (BACO). Inter-platform comparisons are presented to assess consistency in the data sets. Data sets from the RV Ronald H. Brown and deployed assets have been quality controlled and are publicly available at NOAA's National Centers for Environmental Information (NCEI) data archive (https://www.ncei.noaa.gov/archive/accession/ATOMIC-2020, last access: 2 April 2021). Point-of-contact information and links to individual data sets with digital object identifiers (DOIs) are provided herein.