Silicon-containing secondary organic aerosols in ambient air (Si-SOA)

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This Pump-Priming project will initiate a new collaboration with a leading Chinese research group led by Prof Pingqing Fu at Institute of Atmospheric Physics (IAP-CAS). Our aim is to provide definitive evidence, for the first time, on the formation of silicon-containing secondary organic aerosols (Si-SOAs) via photochemical atmospheric processing by using the novel techniques at the IAP-CAS and the UoB. The wider project context is the oxidation of Si-containing volatile organic compounds (Si-VOCs), which are widely used in personal care products and industrial applications. Si-VOCs are the most abundant VOCs in indoor air and its concentration in Chinese megacities can be over 10 microgram per cubic meter (which is extremely high). Si-VOCs can be oxidised by hydroxyl radicals and form secondary organic aerosols (SOAs), which contributes to the regional haze pollution. Our current NERC project Sources and Emissions of Air Pollutants in a Chinese Megacity - AIRPOLL-Beijing (NE/N007190/1) (2016 - 2020) integrates recent modelling, flux measurements, satellite retrievals, tower vertical profile measurements, and chemical transport modelling to provide thorough understanding of the sources and emissions of air pollutants in Beijing, at unprecedented detail and accuracy. As part of this project awarded to the University of Birmingham, we analyzed the chemical composition of individual particles collected during the winter campaign (Nov-Dec 2016) and found that >50% the fine particles contain silicon with mass fraction > 0.01. This suggests that silicon may play an important role in the formation of fine particles, which cause widespread smog / haze and lead to adverse human health effects. We also discovered that silicon is present as a coating, rather than as a solid grain, in individual sulphur-rich particles collected downwind of Chinese megacities, suggesting that the silicon is not directly emitted from primary sources (i.e., formed in the atmosphere). These new findings promoted us to hypothesize that silicon in the secondary particles in ambient air is formed from chemical processing of gas phase Si-containing VOCs. Si-SOA project will rigorously test this hypothesis. To do this, we will apply a series of novel techniques to study silicon in fine particles for the first time. We will use STEM (Scanning Transmission Electron Microscopy) and NanoSIMS (Nanoscale Secondary Ion Mass Spectrometer) to speciate silicon in atmospheric fine particles and confirm whether silicon in individual secondary particles is present as organosilicon. If confirmed, thiswill provide "smoking gun" on the formation of silicon organic aerosol by atmospheric processing. We will also apply the ESI-HRMS (Electrospray ionisation - high resolution mass spectrometer) to identify Si-containing organic molecules in fine particles and then gas chromatography - mass spectrometer to quantify the concentration of Si-SOAs identified by ESI-HRMS in selected fine particle samples collected during the APHH-China campaigns. Si-SOA project will add value to the AIRPOLL-Beijing by confirming the presence and quantify the concentration of siloxane oxidation products in fine particles. It will provide a better understanding on the sources of secondary organic aerosols, a key objective of AIRPOLL-Beijing. The project will consist of PI / research staff exchanges to plan and carry out the experiments in detail, followed by discussions on publication(s). This proposal has been developed following discussions between SHI and FU at meetings in Beijing in May 2017, which was initiated by discovery of silicon as coating in individual secondary sulphate-rich particles and the detection of silicon in over 50% of individual fine particles (see above). In addition to the specific science goals, Si-SOA will nurture a developing collaboration between UK groups and leading Chinese researchers at IAP, with potential for future links.