Atmospheric fate of oil matter adsorbed on sea salt particles under UV light

Abstract:

The presence of liquid petroleum hydrocarbons at the sea water surface is an important source of marine pollution. An oil spill in sea-water will most likely occur due to an involuntary accident from tankers, offshore platforms, etc. However, a large amount of oil is also deliberately spilled in sea-water during the clean-out process of tank vessels (e.g. for the Mediterranean Sea, 490,000 tons/yr). Moreover, the pollution caused by an oil spill does not only affect the aquatic environment but also is of concern for the atmospheric environment. A portion of the oil matter present at the sea-water surface is transported into the atmosphere viaevaporation and adsorption at the surface of sea spray particles. Few studies are related to the presence of oil matter in airborne particles resulting from their adsorption on sea salt aerosols.

We observed that the non-volatile oil matter was adsorbed at the surface of sea-salt crystals (av. size of 1.1 µm). Due to their small size, these particles can have a significant residence time in the atmosphere. The hydrocarbon matter adsorbed at the surface of these particles can also be transformed by catalyzers present in the atmosphere (i.e. UV, OH, O₃, ...). In this work, we focused on the photo-oxidation rates of the C₁₆ to C₃₀alkanes present in these particles.

We utilized a bubble column reactor, which produced an abundance of small sized bubbles. These bubbles generated droplets upon bursting at the air-salt water interface. These droplets were then further dried up and lifted to the top of the column where they were collected as particles. These particles were incubated in a controlled reactor in either dark conditions or under UV-visible light. The difference of alkane content analyzed by GC-MS between the particles exposed to UV or the particles not exposed to UV indicated that up to 20% in mass was lost after 20 min of light exposure. The degradation kinetics varied for each range of alkanes (C_16-20, C_21-25, C_26-30) from 20 to 60 µg L^-1 min^-1. To observe the effect on air composition when samples are exposed to solar light, experiments were conducted under controlled atmospheric conditions: oxygen free or with O₃ gas. The results showed the importance of the photo-transformation processes of oil in airborne particles and its relation to the gaseous nature of the ambient atmosphere.