Thermogravimetric characteristics of water-soluble organic matter from atmospheric aerosols collected in a rural–coastal area

Abstract

The influence of chemical composition on the thermal degradation behaviour of water-soluble organic compounds (WSOC) from fine atmospheric aerosols collected over a period of 12 months was investigated by using thermogravimetric (TG) analysis. The obtained results were further compared to those of Suwannee River Humic Acid (SRHA) and Fulvic Acid (SRFA) standards, which were obtained from the International Humic Substances Society (IHSS). Prior to the TG analyses, the WSOC samples were separated into hydrophobic acids and hydrophilic acids fractions by using a XAD-8/XAD-4 isolation procedure. The data reported in this study were illustrative of the very complex nature of aerosol WSOC hydrophobic acids samples (three to six peaks of thermal degradation) in comparison to that of SRHA and SRFA standards (two degradation peaks). A pseudo-first order mathematical approach was applied to estimate the apparent activation energy of the different multi-step degradation regions. It was found that the degradation processes of the SRHA and SRFA standards occurred with lower apparent activation energies than those obtained for the aerosol WSOC hydrophobic acids samples, although these latter samples appear to exhibit the most thermally labile structural component. The WSOC hydrophobic acids showed a trend towards high apparent activation energies at high temperatures, thus suggesting the decomposition of very stable structural units with relatively strong bond energies above ca. 460 °C. The thermal profiles presented in this study are unique in showing the annual evolution of the thermal-oxidative properties of the fine aerosol WSOC hydrophobic acids samples, and in providing important supplementary information on the structural stability of the bulk aerosol WSOC.