Properties of atmospheric humic-like substances – water system

摘要

Urban-type PM_2.5-fraction aerosol samples were collected and samples of pureatmospheric humic-like substances (HULIS) were isolated from them.Atmospheric concentrations of organic carbon (OC), water soluble organiccarbon (WSOC) and HULIS were determined, and UV/Vis spectroscopicproperties, solubility and conductivity of HULIS in aqueous samples wereinvestigated. Atmospheric concentrations of OC and WSOC were 8.5 and 4.6 μg m−3, respectively. Hydrophilic WSOC accounted for 39% ofWSOC, carbon in HULIS made up 47% of WSOC, and 14% of WSOC wasretained on the separation column by irreversible adsorption. Overallaverage molecular mass and aromatic carbon abundance of HULIS were estimatedfrom molar absorptivity to be 556 Da and 12%, respectively. Both resultsare substantially smaller than for standard reference fulvic acids, whichimply different mechanisms for the formation processes of atmospheric HULISand aquatic or terrestrial humic matter. HULIS were found to be watersoluble as ionic unimers with a saturation concentration of 2–3 g l−1.Their solubility increased again with total HULIS concentration being aboveapproximately 4 g l−1, which was most likely explained by the formationof HULIS aggregates. Solubility increased linearly from approximately 5 upto 20 g l−1 of dissolved HULIS concentration. The ionic dissolution wasconfirmed by electrochemical conductivity in the investigated concentrationinterval. Limiting molar conductivity was extrapolated and this was utilizedto determine the apparent dissociation degree of HULIS for differentconcentrations. The dissociation degree was further applied to derive theconcentration dependence of the van't Hoff factor of HULIS. The van't Hofffactor decreased monotonically with HULIS concentration; the decrease wassubstantial for dilute solutions and the relationship became weak for ratherconcentrated solutions.