RESUMO
Online mass spectrometry techniques, such as extractive electrospray ionization mass spectrometry (EESI-MS), present an attractive alternative for analyzing aerosol molecular composition due to reduced aerosol sample collection and handling times and improved time resolution. Recent studies show a dependence of EESI-MS sensitivity on particle size and mixing state. This study measured authentic sea spray aerosol (SSA) components generated during a phytoplankton bloom, specifically glycerol, palmitic acid, and potassium ions. We demonstrate temporal variability and trends dependent on specific biological processes occurring in seawater. We found that the EESI-MS sensitivity, after adjusting for pressure variations at the inlet and normalizing to the reagent ion, critically depends on the sample's relative humidity. Relevant SSA species exhibited heightened sensitivity at an elevated relative humidity near the deliquescence relative humidity of sea salt and poorer sensitivity with sparse detection below the efflorescence relative humidity. Modeling the reagent ion's diffusive depth demonstrates that the sample aerosol particle viscosity governs the relative humidity dependence because it modulates the particle's coagulation efficiency and distance the reagent ion diffuses and reacts with components in the particle bulk. The effects of particle size and mixing state are discussed, revealing improved sensitivity of phase-separated components present along the particle surface. This work highlights the importance of the particle phase state in detecting and quantifying molecular components within authentic and complex aerosol particles and the utility of EESI-MS for measuring SSA composition.
RESUMO
Plastics have become ubiquitous in the world's oceans, and recent work indicates that they can transfer from the ocean to the atmosphere in sea spray aerosol (SSA). Hazardous chemical residues in plastics, including bisphenol-A (BPA), represent a sizable fraction of consumer plastics and have been measured consistently in air over both terrestrial and marine environments. However, the chemical lifetimes of BPA and mechanisms by which plastic residues degrade with respect to photochemical and heterogeneous oxidation processes in aerosols are unknown. Here, we present the photosensitized and OH-initiated heterogeneous oxidation kinetics of BPA in the aerosol phase consisting of pure-component BPA and internal mixtures of BPA, NaCl, and dissolved photosensitizing organic matter. We found that photosensitizers enhanced BPA degradation in binary-component BPA + photosensitizer aerosol mixtures when irradiated in the absence of OH. OH-initiated degradation of BPA was enhanced in the presence of NaCl with and without photosensitizing species. We attribute this enhanced degradation to greater mobility and thus reaction probability between BPA, OH, and reactive chlorine species (RCS) formed through reaction between OH and dissolved Cl- in the more liquid-like aerosol matrix in the presence of NaCl. Addition of the photosensitizers in the ternary-component BPA + NaCl + photosensitizer aerosol led to no enhancement in the degradation of BPA following light exposure compared to the binary-component BPA + NaCl aerosol. This was attributed to quenching of triplet state formation by dissolved Cl- in the less viscous aqueous aerosol mixtures containing NaCl. Based upon measured second-order heterogeneous reaction rates, the estimated lifetime of BPA with respect to heterogeneous oxidation by OH is one week in the presence of NaCl compared to 20 days in the absence of NaCl. This work highlights the important heterogeneous and photosensitized reactions and the role of phase state, which affect the lifetimes of hazardous plastic pollutants in SSA with implications for understanding pollutant transport and exposure risks in coastal marine environments.
