Water adsorption on the surface of aerosols influence their atmospheric processes, such as heterogeneous chemistry and cloud condensation nuclei activity. On combustion particles, such as coal-fired fly ash, water coverage can change depending on the source of emission. In this work, we combine vibrational spectroscopy with quartz crystal microbalance (QCM) gravimetry to investigate the adsorption isotherms of water on fly ash. Because of composition variability with the sample source region, water uptake was studied at room temperature as a function of relative humidity (RH) on fly ash from several regions: United States, India, The Netherlands, and Germany. Vibrational spectroscopy shows water absorbing on the particle surface in both an ordered (ice-like) and a disordered (liquid-like) structure. The QCM, modeled using the Brunauer, Emmett, and Teller (BET) adsorption isotherm, shows that water uptake on fly ash takes place mostly on the surface of the particle. The difference in the observed water uptake can be linked to differences in composition and the density of OH binding sites on the surface of each sample. A discussion is presented on the RH required to reach monolayer coverage on each sample as well as a comparison between surface sites of fly ash samples and enthalpies of adsorption of water between the samples and mullite (3Al2O3 2SiO2), the aluminosilicate main component of fly ash.

Spectroscopic and gravimetric evaluation of water adsorption isotherms on combustion particles

2019 Spring National Meeting

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