University of Cambridge > > Centre for Atmospheric Science seminars, Chemistry Dept. > Hygroscopicity and Phase Transition of Several Kinds of Aerosols

Hygroscopicity and Phase Transition of Several Kinds of Aerosols

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If you have a question about this talk, please contact Dr Amanda Maycock.

Aerosols are small droplets or particles that suspended in the air. They have diverse sizes, phases and components, influencing the climate and people’s health through different processes. Two important properties of aerosols that correlate with these influences are hygroscopicity and phase transition.

The amount ratio of water vapour in the air at a specific temperature to the maximum value of that could hold at that temperature is defined as relative humidity (RH), and is normally given as a percentage value. 

During a RH increasing (humidifying) process, liquid aerosols absorb water and grow. Some solid particles transform into liquid droplet upper special RH (deliquescence point).

During a RH decreasing (drying) process, aerosol droplets lose water and shrink. Most inorganic aerosols transform into crystals at some special RH (efflorescence point). However, some inorganic and many low evaporating organic aerosols will lose water at a really low speed and finally transform into amorphous, gel, glassy or semi solid particles with high viscosity.

Some lab work through Fourier transform infrared spectroscopy (FTIR), Confocal Raman, Optical Tweezers and High Speed Camera will be shown at this seminar.

The efflorescence and deliquescence process of some sulphate and nitrate aerosols are detected through attenuated total reflectance (ATR) FTIR methods. 5 µm diameter aerosols are dispensed and deposited on ATR substrate, which is surrounded by a RH controlled small chamber. Room temperature efflorescence and deliquescence can be observed at 4 cm-1 resolution. The observation of sulphate and nitrates and their mixed components aerosols will be displayed.

Confocal Raman is used to detect the hygroscopicity and phase transition of 1-100 µm single aerosols deposited on substrates (quarts or PTFE ). A spectrum resolution of 1 cm-1 and spatial resolution of 1 µm can be obtained.

Aerosol Optical Tweezers (AOT) is used to levitate single 1~15 µm diameter particle by 532 nm laser through gradient scattering force. Water absorption and desorption process and some phase transition processes can be observed by changing the RH surrounds the aerosols. Cavity enhanced Raman and whispering gallery mode (WGMs) will be used to characterize the size of the droplets at a spatial resolution of nanometres. The morphology of the droplets or particles can be observed by CCD cameras at the same time. Glassy sucrose aerosol and evaporation of organic acid droplets will be talked about.

High speed camera combined with microscope system is used to observe the efflorescence and deliquescence processes of single substrate deposited sea water droplets and some inorganic droplets at microsecond time resolution and several micrometer spatial resolution.

By using the FTIR -ATR, Confocal Raman, Optical Tweezers and High Speed Camera strategies, the hygroscopicity and phase transition of several kinds of aerosols with size from several microns to hundreds microns are observed. Some interested phenomenon and corresponding parameters will be talked about.

This talk is part of the Centre for Atmospheric Science seminars, Chemistry Dept. series.

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