Atmospheric deposition of trace metals to urban and coastal environments

Abstract

The atmospheric flux (wet and dry) of trace metals (Ni, Co. Cu, Pb, Cd, Al and Na) to the Irish Sea and the North-West of England is presented. The fluxes were calculated from determined trace metal concentrations of aerosols (Al, Fe, Mn, Cd,
Co, Cr, Cu, Na, Ni, Pb, V and Zn) and rainwaters (Ni, Co, Cu, Pb, Cd, Al and Na), which were collected using specially designed sampling equipment. The particulate material was analysed using inductively coupled plasma - mass spectroscopy (ICPMS) and also atomic absorption spectroscopy techniques after a complete aciddigestion (HFIHNO3). The dissolved rainwater metal concentrations were determined using the electroanalytical technique adsorptive cathodic stripping
voltammetry (ACSV). This technique also enabled dissolved metal Organic complexation to be determined in the rainwater.
The results indicated that the trace metal aerosol and rainwater concentrations detected for the Irish Sea and urban areas vary considerably during an annual period. This can be attributed to (i) air mass source and (ii) rainwater washout. Geometric mean aerosol concentrations (ng m 3) were calculated for the Irish Sea aerosol (Al, 210; Fe, 159; Mn, 6.2; Cd, 0.26; Co, 0.17; Cr, 2.0; Cu, 5.3; Na 1140; Ni, 4.2; Pb, 19; V, 4.1; Zn, 32) and North-West England aerosol (Al, 365;
Fe, 589; Mn, 21; Cd, 0.80; Co, 0.68; Cr, 13; Cu, 17; Na 2208; Ni, 16; Pb, 45; V, 7.7; Zn, 153) along with total (dissolved + particulate) volume weighted mean concentrations (j.ig l') for the Irish Sea rainwater (Al, 32; Cd, 0.05; Co, 0.05; Cu,
1.8; Na 1998; Ni, 1.1; Pb, 0.73) and North-West England rainwater (Al, 72; Cd, 0.14; Co, 0.15; Cu, 2.9; Na 1274; Ni, 1.7; Pb, 1.82). Air mass analysis studies illustrated the affect on the aerosol trace metal concentrations. For Fe, Mn, Cd, Cu. Pb, V and Zn the Irish Sea aerosol concentrations decreased in the order E> S > N > W. Calculated total inputs to the Irish Sea for Cu (131 t yr:'), Pb (82 t yr') and Cd (3.2 t yr.') were of similar magnitude to total riverine inputs and considerably greater than those of direct discharges (sewage and drainage) for Cu and Pb.
The fate of aerosol associated trace metals in the Irish Sea was investigated by laboratory simulation aerosol seawater/rainwater solubility studies. These studies indicated fast dissolution kinetics for crust-rich and urban-rich aerosol trace metals, with equilibrium times of - 5 minutes for Ni, Co, Cu, Pb and Cd. Similar solubility studies performed on collected Irish Sea aerosols in seawater and artificial rainwater indicated solubilities increased in the order Co (32 %) c Pb (37 %) < Ni (38 %) < Cd (46 %) < Cu (58 %) for seawater. These were correspondingly higher in artificial rainwater, Co (37 %) <Ni (41 %) <Pb (44 %) <Cd (52 %) <Cu (63 %). These solubilities were found to be both lower than field determined rainwater
solubilities, Pb (63 %) <Ni (69 %) c Co (70 %) <Cd (79 %) <Cu (82 %). This difference being attributed to condensation and evaporation cloud cycles which can reduce the pH of cloudwater to - 2.
Within the dissolved fraction, organic complexation was detected for Cu (32 %), Cd (32 %), Pb (34 %), Ni (35 %) and Co (38 %) indicating that the atmosphere is a source of potential metal complexing material. Calculation of net inputs to the Irish Sea highlights the importance of wet deposition. The wet to dry ratio for the selected metals in the Irish Sea increased in
the order Co (1.7) !~ Ni (1.7) <Cu (2) !~ Cd (2) <Pb (2.1). Net atmospheric inputs to the North - East Irish Sea are of the same magnitude for Ni (7 t yr.'), Cu (17 yr.j, Pb (10 t yr.') and Cd (0.36 t yr:') when compared to riverine inputs.