Observation of metal-ligand complexes formation in deliquescent airborne particles in an urban environment

Chiara Giorioa), Valerio Di Marcoa), Markus Kalbererb,c), and Andrea Tapparoa)

a)Department of Chemical Sciences, University of Padua, via Marzolo 1, 35131 Padova, Italy

b)Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, CB2 1EW, United Kingdom

c)Department of Environmental Sciences, University of Basel, Klingelbergstrasse 27, 4056 Basel, Switzerland

Aqueous phase processing of aerosol can lead to substantial modifications of aerosol chemical and physical properties. A process potentially very important in this context is the formation of metal-organic ligand complexes in atmospheric aqueous phases, like fog/cloud droplets and deliquescent aerosol [1,2]. Many water-soluble organic compounds present in the aerosol have coordinating properties toward metal ions, which can increase their solubility [1], therefore their bioavailability and potential toxicity. Notable examples of aerosol components having coordinating properties are highly oxidized organic compounds, such as low molecular weight hydroxy-, oxo-, and dicarboxylic acids as well as dicarbonyls, and macromolecules as humic-like, fulvic-like substances and oligomers.

We investigated the formation of metal-organic ligand complexes in urban aerosol and the effect of such phenomenon on the solubility and dissolution kinetic of metals from the particles to an aqueous solution simulating fog/cloud water. Aerosol samples were collected on filters at an urban background site in the city center of Padua (Italy), in the Po Valley. Aerosol samples were then characterized for quantification of both metals, using inductively coupled plasma mass spectrometry (ICP-MS), and organic ligands, using ion chromatography (IC), direct infusion high-resolution mass spectrometry (HRMS) and headspace gas chromatography with nitrogen–phosphorous detection (HS-GC-NPD). Thermodynamic modelling, as used in a previous study in the literature [2], together with multivariate statistical analysis were used to gain a speciation picture of the equilibria in solution mimicking fog/cloud waters and the effect of the presence of organic ligands on the solubility of the metals. We investigated the influence of organic matter on the dissolution kinetics of metals in atmospheric aqueous conditions typical of a winter environment in the Po Valley atmosphere.

Preliminary results show that iron, copper and manganese are present in the aerosol in a complexed form, mainly with oxalate, malonate and succinate. Iron was the metal whose solubility was the most influenced by the presence of organic ligands. Dissolution kinetics of many metals were influenced by the environmental conditions during the campaign. In addition, we found that strongly bound cyanides were present in all aerosol samples [3], suggesting that hydrogen cyanide can be adsorbed onto aerosol liquid water and react with metal ions to form stable metal–cyanide complexes [3].

[1] Okochi H., Brimblecombe P., Sci. World J., 2, 767–786 (2002).

[2] Scheinhardt S., Müller K., Spindler G., Herrmann H., Atmos. Environ., 74, 102–109 (2013).

[3] Giorio C., Marton D., Formenton G., Tapparo A., Environ. Sci. Technol., 51, 14107–14113 (2017).