Partitioning of Metals between the Aqueous Phase and Suspended Insoluble Material in Fog Droplets

Abstract

This paper discusses the partitioning of metals (K, Na, Ca, Mg, Al, Cu, Fe, Pb and Zn) between the aqueous phase and the suspended insoluble material in fog samples collected in the Po Valley during two extensive fields campaigns. Metals represent on average 11% of the mass of suspended insoluble matter, while the main component is carbon (both organic carbon, OC = 35%, and black carbon, BC = 8%). The unaccounted suspended matter mass is very high, on average 46%, and is attributable to non metallic species, such as O and N and of Si. The principal metals in the insoluble suspended fraction are Fe and Al (2-5%), while the contributions of other metals (Na, Mg, Cu, Pb and Zn) are lower than 1%. Ca and K exhibited high blank values and could not be detected above blank detection limit threshold. The main components in the aqueous phase are NO3- (34%), WSOC (23%), SO4(2-) (18%) and NH4+ (19%), while trace metals and remaining cations and anions accounted for less than 1% of solute mass. The main dissolved trace metals in fog droplets are Zn, Al and Fe, while the main metallic cations are Na and Ca. Fe and Al are the only metals preferentially distributed in the suspended insoluble matter of fog droplets (partitioning ratio respectively 37% and 33%). All other metals are mostly dissolved in the aqueous phase (mean partitioning ratios of Mg, Pb, Zn, Cu and Na are 69%, 70%, 77%, 81% and 87%). These findings are in agreement with literature data on metal speciation in cloud and rain samples. The dependence of partitioning ratios on pH is investigated for the different metals, with only Al showing a clear partitioning ratio decrease with increasing pH. Conversely, the other metals show no dependence or a complex and highly variable behaviour. The partitioning ratio of iron (mean 37%) observed in the Po Valley fog samples is much higher than the water extractable iron in aerosol particles (typically 1-2 %): this fact can be explained by differences in the aerosol sources and composition among sites and by chemical processes in the aqueous phase, such as complexation and redox reactions involving organic ligands (oxalate, or other organic acids as humic-like organic matter) which may promote Fe solubility.

The Chemistry of Intercepted Clouds in Northern Arizona during the North American Monsoon Season

“…Again, this appears to be largely the result of differences in air mass origin as discussed below. For comparison, trace metal concentrations for cloud and fog samples collected at a mountain site in Italy, a mountain site in Pakistan, an urban site in France, and a mountain site in Chile are also included in Table 2 (Cherif et al 1998;Cini et al 2002;Ghauri et al 2001;Mancinelli et al 2005;Schemenauer and Cereceda 1992). The sample comparison sites were chosen for their comprehensive trace metal concentration data sets.…”

“…Trace metal concentrations in the atmospheric aqueous phase have also been measured (Cini et al 2002;Collett et al 2002;Ghauri et al 2001;Mancinelli et al 2005;Parazols et al 2006;Siefert et al 1998). Trace metals usually have contributions from soil/crustal components, anthropogenic sources and marine aerosols (Husain et al 2004;Plessow et al 2001).…”

Trace metal characterization of aerosol particles and cloud water during HCCT 2010

“…In cloud water a wide range of concentrations has been observed (Deguillaume et al, 2005) which has also been found to influence the chemical reactions they take part in, as well as the composition of the aerosol after cloud processing. During the past years some studies have reported trace metal concentrations in different cloud conditions during various field measurements (Plessow et al, 2001;Cini et al, 2002;Hutchings et al, 2009;Mancinelli et al, 2005;Burkhard et al, 1992;Siefert et al, 1998;Liu et al, 2012;Bruggemann et al, 2005;Wang et al, 2014;Straub et al, 2012;Li et al, 2013;Guo et al, 2012;Rao and Collett, 1998) showing strong variability due to the location and air mass history.…”

Extractable iron and organic matter in the suspended insoluble material of fog droplets

“…HS are in fact complex mixtures of compounds spanning over a wide range of chemical structures and molecular weights, and have variable capacity to chelate or coordinate metal ions (Stumm and Morgan, 1981). As in natural surface waters, metal/organic interactions have been shown to occur also in fogwater between Cu (II) and organic complexants similar to humic or fulvic acids and between colloidal iron and oxalates (Mancinelli et al, 2005).…”

“…In a recent study Mancinelli et al (2005) investigated the partitioning of metals and carbon between water droplets and the suspended insoluble material in fog samples collected in the Po Valley. The elemental composition of suspended insoluble material of Po Valley fog showed that organic carbon (OC) is the main identified contributor to the suspended matter (35% on average), while the contribution of metals (Al, Fe, K, Na, Mg, Zn, Pb and Cu) is on average 10%, and black carbon (BC) represents on average 8%.…”