Morphology and hygroscopicity of nanoplastics in sea spray.

The role of airborne nanoparticles in atmospheric chemistry and public health is largely controlled by particle size, morphology, surface composition, and coating. Aerosol mass spectrometry provides real-time chemical characterization of submicron atmospheric particles, but analysis of nanoplastics in complex aerosol mixtures such as sea spray is severely limited by challenges associated with separation and ionization of the aerosol matrix. Here we characterize the internal and external mixing state of synthetic sea spray aerosols spiked with 150 nm nanoplastics. Aerosols generated from pneumatic atomization and from a sea spray tank are compared. A humidified tandem differential mobility analyzer is used as a size and hygroscopicity filter, resulting in separation of nanoplastics from sea spray, and an inline high-resolution time-of-flight aerosol mass spectrometer is used to characterize particle composition and ionization efficiency. The separation technique amplified the detection limit of the airborne nanoplastics. A salt coating was found on the nanoplastics with coating thickness increasing exponentially with increasing bulk solution salinity, which was varied from 0 to 40 g kg-1. Relative ionization efficiencies of polystyrene and sea salt chloride were 0.19 and 0.36, respectively. The growth-factor derived hygroscopicity of sea salt was 1.4 at 75% relative humidity. These results underscore the importance of separating airborne nanoplastics from sea salt aerosol for detailed online characterization by aerosol mass spectrometry and characterization of salt coatings as a function of water composition. The surface coating of nanoplastic aerosols by salts can profoundly impact their surface chemistry, water uptake, and humidified particle size distributions in the atmosphere.

Amorphous phase state diagrams and viscosity of ternary aqueous organic/organic and inorganic/organic mixtures.

A Dimer Coagulation, Isolation and Coalescence (DCIC) technique is used to probe the phase behaviour and glass transition temperatures of ternary aerosol mixtures. The DCIC technique is used to perform temperature and relative humidity dependent viscosity measurements at viscosities near 5 × 106 Pa s. Measurements include organic-organic and organic-inorganic mixtures composed of sucrose-citric acid and sucrose-sodium nitrate. The data reported here add additional insight into the wide discrepancies in glass transition temperatures reported for pure sodium nitrate. The phase diagram model used in the work of Rothfuss and Petters (Phys. Chem. Chem. Phys., 2017, 19, 6532-6545) is expanded to include multiple solute components. Data and model predictions of the mixtures are in good agreement with the modified model. These measurements are compared with values from Holographic Optical Tweezer (HOT) measurements taken at room temperature. Overall, the viscosities determined from the DCIC and HOT techniques are in good agreement.

Hygroscopicity of Organic Compounds as a Function of Carbon Chain Length and Carboxyl, Hydroperoxy, and Carbonyl Functional Groups.

The albedo and microphysical properties of clouds are controlled in part by the hygroscopicity of particles serving as cloud condensation nuclei (CCN). Hygroscopicity of complex organic mixtures in the atmosphere varies widely and remains challenging to predict. Here we present new measurements characterizing the CCN activity of pure compounds in which carbon chain length and the numbers of hydroperoxy, carboxyl, and carbonyl functional groups were systematically varied to establish the contributions of these groups to organic aerosol apparent hygroscopicity. Apparent hygroscopicity decreased with carbon chain length and increased with polar functional groups in the order carboxyl > hydroperoxy > carbonyl. Activation diameters at different supersaturations deviated from the -3/2 slope in log-log space predicted by Köhler theory, suggesting that water solubility limits CCN activity of particles composed of weakly functionalized organic compounds. Results are compared to a functional group contribution model that predicts CCN activity of organic compounds. The model performed well for most compounds but underpredicted the CCN activity of hydroperoxy groups. New best-fit hydroperoxy group/water interaction parameters were derived from the available CCN data. These results may help improve estimates of the CCN activity of ambient organic aerosols from composition data.

Bubble-mediated generation of airborne nanoplastic particles.

Micro- and nanoplastic particles have been detected in most environmental compartments. The presence of microplastics in the remote marine atmosphere and close to large lakes suggests bubble mediated water-air transfer as a source of airborne microplastics, however, quantitative estimates of plastic emission from surface waters remain uncertain. In this work, we elucidate the emission of submicron polystyrene nanospheres by bubble bursting in a laboratory setting from low salinity waters (salinity 0-1.0 g kg-1), polystyrene particle diameter (103, 147 and 269 nm), aqueous particle number concentrations in the range 4 × 107-2 × 109 cm-3, and bubble formation rate (0.88-3.35 L min-1 of air). Production of polystyrene aerosols was demonstrated using a scanning mobility particle sizer and confirmed by analysis of filter samples using pyrolysis gas chromatography coupled to mass spectrometry. We show that production of polystyrene aerosol particles scales linearly with the number concentration of plastic particles in the water. Our results suggest that small amounts (0.01 g kg-1) of salt increase polystyrene particle production. To the best of our knowledge this is the first study of bubble mediated water-air transfer of plastic particles as small as 100 nm.