1. Introduction
The importance of aerosol particles in climate change has been recently recognised. However, what prevents us from better assessing the direct and indirect climate forcing
of tropospheric aerosol is the limited knowledge on its chemical composition. In particular, the bulk of organic matter of continental fine aerosol and precipitation seems
to have defied identification very effectively. Even by using powerful analytical techniques such as gas chromatography-mass spectrometry, only a fraction of total
organic carbon can be attributed to individual organic compounds such as mono- and dicarboxylic acids, n-alkanes, etc. Recently, there has been increasing evidence that a
considerable fraction of organic carbon is theoretically impossible to speciate because they are made up of natural humic-like macromolecules. It was speculated that such
compounds may be predominant in Amazonian aerosol during the wet season when no
Ž .
biomass burning occurred Artaxo et al., 1990; Andreae and Crutzen, 1997 . In
temperate regions, from the residues of rainwater and melted snow, yellow, brown or Ž
. black aggregations and tar-like droplets were separated Went, 1966 . In terrestrial cloud
water some degradation products and an unidentified orange-brown material were found Ž
. Ž
. Bank and Castillo, 1987 . Mukai and Ambe 1986 extracted a brown substance having
the solubility characteristics of humic acid from airborne particulate matter collected in a rural area in Japan. Using ultrafiltration and
13
C-nuclear magnetic resonance spec- Ž
. troscopy, Havers et al. 1998 demonstrated that humic-like substances separated from
particulate matter made up a significant fraction of organic carbon and they can be characterised primarily by polysaccharide and aliphatic substructures. Using various
spectroscopic techniques such as infrared, UV–VIS or fluorescence spectrometry, we recently proved that in continental background aerosol, a significant amount of these
humic-like macromolecules were present in the fine mode, and they also accounted for a
Ž .
considerable part of water soluble organic carbon Zappoli et al., 1999 . In all the above cases, the occurrence of humic-like substances was established using separation methods
and spectroscopic or elemental analytical techniques. However, it is well known that humic substances, especially humic and fulvic acids, show high complexing ability
towards transition metal ions such as lead, copper, nickel, etc. It is therefore possible to prove the presence of humic-like substances in atmospheric samples on the basis of a
completely different principle, making use of their complexing properties in electro- chemical analysis. To the best of our knowledge, there is only one study that focuses on
Ž organic metal complexation in rainwater using complexing ligand titration Spokes et
. al., 1996 .
Transition metal complexation can be readily studied by anodic stripping voltamme- try with two different approaches. In the first method, metal is added to the organic
ligands under conditions held constant, and stability constants and coordination numbers Ž
are determined from the shift in the redox potential of the metal ion Wilson et al., .
1980 . Alternatively, reduction of the limiting current of the metal ion with increasing amount of ligands may be used to estimate stability constants. This usually requires that
the complexed metal ion should not be electroactive and should not contribute to the faradaic current measured at a potential characteristic of the aquo ion. Complexes that
do contribute to the faradaic current are operationally defined as electrochemically
‘‘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
Kategori