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

Kamila Widziewicz-Rzońca,Malwina Tytła,Grzegorz Majewski,Krzysztof Loska,Patrycja Rogula-Kopiec,Wioletta Rogula-Kozłowska

doi:10.3390/su12156196

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.

Highlights

The presence of particulate matter (PM)-bound water is one of the factors responsible for the discrepancy between its gravimetric mass and the chemically reconstructed mass calculated by summing up its chemical compounds [1,2,3,4,5,6,7]
We found that the fine PM1 fraction constitutes about 56% of the total suspended particles (TSP), which is in agreement with the findings of Rogula-Kozłowska et al (2019) [7] and gave us some tips to conclude that a significant portion of PM-bound water can be present in this size fraction
The study showed that the TSP-bound water contents collected in the Warsaw urban background location were probably impacted by the emissions from transport together with soot release from both residential heating and vehicle engine combustion

Summary

Introduction

The presence of PM-bound water (water bound to PM particles, liquid water in hygroscopic particles) is one of the factors responsible for the discrepancy between its gravimetric mass and the chemically reconstructed mass calculated by summing up its chemical compounds [1,2,3,4,5,6,7]. Water in PM may occur as loosely bound water ( adsorption water) and constitutional water—strongly bound to ambient PM and irreversibly associated with the chemical structure of PM-forming compounds under normal conditions [8,9,10] The strength of this binding will vary depending on the chemical composition of PM, and on its origin (source of PM emission). Constituents like sulfates and nitrates drive aerosol water uptake, and hydrophilic organic compounds can promote the uptake of water [13,14], especially under high humidity conditions [15] Both the amount of adsorbed water as well as the presence of secondary ammonium salts can be responsible for generating positive mass artifacts [16]; for example, gaseous NH3 and HNO3 favors the formation of NH4NO3. The results obtained from cited works suggest that the presence of water in PM is an indicator of its physical and chemical properties, but in well-planned experiments, including the characterization of the water contribution due to structurally-bound water may be used as a tracer (marker) of its probable origin [11]

Methods

Results

Conclusion