Abstract
Organic interfaces that exist at the sea surface microlayer or as surfactant coatings on cloud droplets are highly concentrated and chemically distinct from the underlying bulk or overlying gas phase. Therefore, they may be potentially unique locations for chemical or photochemical reactions. Recently, photochemical production of volatile organic compounds (VOCs) was reported at a nonanoic acid interface however, subsequent secondary organic aerosol (SOA) particle production was incapable of being observed. We investigated SOA particle formation due to photochemical reactions occurring at an air-water interface in presence of model saturated long chain fatty acid and alcohol surfactants, nonanoic acid and nonanol, respectively. Ozonolysis of the gas phase photochemical products in the dark or under continued UV irradiation both resulted in nucleation and growth of SOA particles. Irradiation of nonanol did not yield detectable VOC or SOA production. Organic carbon functionalities of the SOA were probed using X-ray microspectroscopy and compared with other laboratory generated and field collected particles. Carbon-carbon double bonds were identified in the condensed phase which survived ozonolysis during new particle formation and growth. The implications of photochemical processes occurring at organic coated surfaces are discussed in the context of marine SOA particle atmospheric fluxes.
Highlights
Our study suggests a new source of secondary organic aerosol (SOA) particles due to the emission of volatile organic compounds (VOCs) photochemically produced by fatty acids at the air-water interface[12]
We demonstrated SOA nucleation and growth due to dark ozonolysis of unsaturated organic gas phase products arising from photochemistry at a nonanoic acid-coated aqueous interface
The potential for SOA production was suggested in previous studies, limited chamber volumes and short residence times were not optimal for detecting new particle formation
Summary
To estimate Ana, we first assume that nonanoic acid partitioned to Teflon walls following Henry’s law as a rough estimate This assumption neglects that other organic molecules (in addition to water) may be present on the walls due to the previous experimental chamber history, at concentrations which we have shown to be negligible for measurements of SOA or VOC production. We do underline that the chamber fluxes are not derived from real marine conditions and do not consider i) losses of oxidized VOCs to chamber walls which tends toward under prediction of SOA formation, ii) that high ozone concentrations not typical for the atmosphere were employed in experiments, iii) experimental variability in relative humidity and temperature, iv) that pH and surface tension values are not within a range of ambient oceanic conditions, v) that seed aerosol or pre-existing particles in ambient air will scavenge VOCs and vi) that only pure water was employed. Future studies should aim to better constrain estimates and uncertainties of this abiotic photochemical SOA formation pathway for atmospheric application
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