Spontaneous formation of OH oxidation products at the interfaces of pure organic droplets

There are now many evidence that OH and H₂O₂ can be spontaneously formed at the air – water interface of aqueous droplets due to the presence of a strong electric field (~10⁹ V m⁻¹). OH^– anion has been suggested that it partially exist as an ion pair (OH^...e^-), which may undergo charge separation in the presence of this electric field. This can lead to OH radical and electron production, while H₂O₂ can be formed via subsequent reactions. The presence of organic molecules on aqueous droplet surfaces and the OH radicals that are spontaneously generated can lead to oxidation products formation altering the composition of the particles.

In this work we studied the OH products formation of pure organic aerosols while they interact with water vapour. We investigated the product formation in the absence of OH radicals precursors, in the dark in the range of RH 0 – 100 %. Organic droplets, in a range of diameter 10 to 300 nm, were generated by nebulizing citric acid solutions. The particles passed through a diffusion dryer and entered inside a flow tube reactor. Particles were collected either on filters or by using a spot sampler. The particle phase analysis was performed via Ultra High-Performance Liquid Chromatography coupled with Electrospray Ionization Orbitrap Mass Spectrometry. Gas phase products were also monitored using a VOCUS mass spectrometer.

All the experiments provide evidence that organic droplets can be oxidized while interacting with water vapours. No products were observed under dry conditions. Oxidation products were formed in the particle phase and their production displayed a systematic increase with the increase of RH denoting the enhancement of OH radical generation. At complete humid conditions, products were also observed in the gas phase due to their desorption. H₂O₂ was also monitored in the gas phase confirming that interfacial OH and electron generation can lead to its formation. Results from this study are expected to significantly improve our insights on the interfacial processes that occur in atmospheric droplets and on the atmospheric multiphase oxidation chemistry.