‘‘labile.’’ Complexes that undergo dissociation during the measurement period are defined as kinetically ‘‘labile.’’ In voltammetry, it is essential that the presence of
ligands should not affect the diffusion coefficient of the aquo ion in solution nor should the ligand adsorb onto the working electrode where it may complex metal ion or may
change the rates of analytical redox reactions being monitored. Natural complexing agents such as humic or fulvic acids were shown to exhibit properties that render such
Ž complexation studies difficult or often impossible to interpret quantitatively Filella et
. al., 1990 . These complexants tend to form electroactive metal–ligand complexes, cause
complex dissociation during the voltammetric measurements andror adsorb on the electrode surface. These effects are evident from the shifts in peak current potentials
andror half-width relative to those observed in their absence. The same interferences, however, can be used to prove the presence of these natural complexants in atmospheric
samples, thus supplementing evidences obtained from spectroscopic measurements. In this paper, we attempt to make use of these interference effects in anodic stripping
voltammetry to substantiate the presence of humic-like substances in fog water. To enable comparisons with results available for aquatic humic matter, the pH of fog water
is adjusted to that of most aquatic environment.
2. Methods
Fog sampling was carried out at the FISBAT field station of S. Pietro Capofiume, in the Po Valley, Italy. The area is characterised by high fog occurrence during the
fall–winter period and also by a high aerosol loaded air due to the high levels of pollution. In addition, the fog system in the Po Valley is characterised by high
atmospheric stability and long residence time of the air masses.
An automated, computer-driven, sampling system was set up to concurrently sample fog droplets and interstitial particles. The sampling system was essentially composed of:
Ž . Ž
. a Particulate Volume Monitor PVM-100 Gerber, 1991 for detecting the presence
of fog; Ž .
b temperature sensor to detect subfreezing conditions; and Ž .
c stainless steel fog droplets string collector. The sampling of fog droplets was performed using an active string collector designed
Ž .
by Fuzzi et al. 1997 . This instrument collects fog droplets suspended in an air stream created by a fan, by impacting them on cylindrical strings. Sampling flow rate is 17 m
3
min
y1
, and the collection efficiency is 43 of the actual fog liquid water content Ž
. LWC , with calculated 50 cut-off for each individual string, with an aerodynamic
diameter of ca. 6 mm. All parts that come in contact with the fog droplets, including the sampling strings, were made of stainless steel, which does not give problems of artefact
formation and adsorption on the surfaces for organic compounds. The sampling cam- paign lasted from the beginning of November 1996 to the end of March 1997,
corresponding to the fog season in the Po Valley. During this period, 17 fog samples
were obtained, 11 of which were used for the experiments. Sampling duration ranged from 2 to 19 h.
Fog samples were first weighed and pH was then determined in a small aliquot of the sample. The sample was then filtered through a 47-mm quartz filter and the water
Ž .
soluble organic carbon in the filtrate WSOC was determined with a total carbon
F
Ž .
analyser TOC Astro 2100 liquid module using potassium hydrogen phthalate solutions for calibration. TOC liquid measurements had an analytical error of 5. The concentra-
Ž
q y
2y
. tion of inorganic ions NH , NO , SO
was determined by ion chromatography. The
4 3
4
Ž . total concentration of Cu II in filtered fog samples was determined by atomic absorp-
tion spectrometry. All vials, sampling bottles and Petri dishes used for sampling, collection and storage of the aerosol and liquid samples were of Pyrex glass and were
pre-cleaned and conditioned. The pH of the samples and model solution was adjusted using Britton–Robinson buffer solution. The stock buffer solution was composed of
acetic acid, phosphoric acid and boric acid at a concentration of 0.04 M each.
Ž .
y5
The copper II solution of 1.00 = 10
M was prepared from analytical grade Ž
. copper-nitrate Merck with sequential dilution in calibrated flasks using MilliQ water
Ž .
Ž .
containing 0.10 M potassium-nitrate Reanal . The humic acid standard
Aldrich solution of 100 mg dm
y3
was prepared gravimetrically using 100 ml of a solution made of 10.0 ml Britton–Robinson buffer and 90.0 ml of 0.10 M potassium nitrate and pH
adjusted to 9.00. While we are aware that the pH of fog water is mainly acidic, our objective was not to study copper complexation in fog water but to compare the
electrochemical behaviour of dissolved organic carbon present in fog water to that of
Ž .
natural humic matter in aquatic environments e.g. in seawater . Throughout the
experiments, 3-h equilibrium time was allowed for copper complexation before analysis. One milliliter of the buffer solution was added to 9.00 ml of fog samples and model
solution and the pH was then adjusted to 9.00 with 1.00 M sodium-hydroxide solution. The ionic strength of all solutions was adjusted to 0.10 M potassium-nitrate concentra-
tion. The instrument was an AUTOLAB electrochemical instrument with a dropping mercury electrode, the deposition potential was y1.000 V, deposition time of 60.0 s.
The volume of the sample cell was 10.0 ml. The potential was scanned from y1.000 to 0.000 V, purge time was 120 s with nitrogen, equilibrium time was set to 30.0 s.
3. Results and discussion
