Strongly and Loosely Bound Water in Ambient Particulate Matter—Qualitative and Quantitative Determination by Karl Fischer Coulometric Method

Abstract

Simple physical characterization of water evaporation can provide detailed information regarding its component distribution in particulate matter (PM) samples. The water presence in PM can greatly influence its polarity and subsequent reaction activity, for example, in secondary inorganic and organic matter formation. In this study, the presence of PM-bound water is detected using the Karl Fischer titration method in a temperature gradient with an aim to quantitatively assess different types of water occurrence. The analyses were initiated by testing two reference materials, namely urban particulate matter 1648a and urban dust 1649b (NIST). Four different types of water were found in both NIST materials, which helped to optimize the temperature ramp program and its adjustment for real PM samples. It was found that water contents in total suspended particles (TSP) are similar to those typically occurring in urban background stations—approximately 7.12–45.13% of the TSP mass, differentiated into the following water mass contributions: 48.5% of the total water found was loosely bound water; 23.3% was attributed to the absorption water; while the missing 20% could be probably attributed to crystal water removed only above 180 °C and artifacts connected with the drift correction problem. By comparing water release curves for single PM-compounds like pure SiO2; Al2O3; NH4NO3; (NH4)2SO4 and NH4Cl with water spectra obtained for real PM samples, it was found that water in particulate matter mainly comes from the dehydration of TSP-bound crystalline like Al2O3, SiO2 and to a lesser extent from salts like NH4NO3; (NH4)2SO4 and NH4Cl. A newly used thermal ramp method was able to assess water contents from Teflon–polypropylene baked filters characterized by low melting points and therefore filter degradation even under temperatures oscillating around 200 °C. The advantage of this new work is the separation of different types of TSP-bound water contributions, facilitating and promoting further research on the origin of PM-bound water and its role in atmospheric chemistry, secondary aerosol formation and visibility.