2.6 Source characterisation of build-up and wash-off
2.6.2 Qualitative approach to source characterisation
Past research studies on source characterisation have surmised that each pollutant source produces pollutants with specific characteristics (Ong, et al., 2007). In the event such characteristics can be identified, then these can be used to distinguish sources of pollutants in a pollutant mixture.
Source characterisation has been done in several studies by visually comparing the physical and chemical characteristics of the pollutant mixture with the signatures of possible contributing sources. Stout et al. (2005) used that qualitative approach to identify the sources of oil spills. They compared the chromatograms resulting from gas chromatography–mass spectrometry (GC/MS) and gas chromatography–flame ionisation detector (GC/FID) for possible sources of oils and oil spill. The submitted sample was considered as a probable source of oil spill if the chromatogram pattern of oil spill virtually overlays with that of the oil sample submitted for comparison, (Stout et al. 2005). Adachi and Tainosho (2004) evaluated possible sources of road surface build-up pollutants by analysing their physical and chemical characteristics such as morphology, particle diameter and metal composition. In their study, brake dust, road paint and tyre tread samples were considered as possible sources. However, contribution from each source to the road surface build-up was not identified using this approach.
Use of metal concentrations in a sample to characterise the sources
Several studies have investigated the trends in concentrations of individual metal elements in road build-up in order to characterise sources into broad categories such as industrial sources.
Jensen and Laxen (1985) analysed urban road build-up samples for the presence of Pb in central London. A relatively low concentration level (approximately 1000 µgg- 1
) has been observed in samples collected away from roads and other obvious immediate sources of Pb. By contrast, samples close to the newspaper printing industry had Pb levels greater than 10000µgg-1. The contribution of Pb from the newspaper printing industry located nearby was a significant source in road build-up. Therefore, reduction in concentration with the distance from the study site indicates that a particular source can be considered as a source of road deposit. Geographic
Information System (GIS) techniques are also helpful in demonstrating spatial variation in metals related to industries as used by Chang et al. (2009). Kim et al. (1998) noted that high concentrations of Zn appear in road dust due to the use of Zn and its compounds in the manufacturing of alloys, parchment papers, glass, batteries and electrical apparatus. However, in other land uses such as residential areas such a kind of trend in metal concentrations could not be noted to identify the sources of pollutants.
In addition, enrichment factors (EFs) have been introduced to discriminate between oceanic, terrestrial and potentially other elemental sources in the atmosphere (Reimann and de Caritat, 2005). The usage of EFs has been extended to discriminate between natural and anthropogenic sources of elements. To calculate the EFs, firstly the concentration ratio of interest element to the reference element in the pollutant mixture (road build-up) must be calculated. Then it can be compared with the same elemental concentration ratio present in the natural sample (local soil) to check whether the pollutant mixture is affected by anthropogenic activities. It is an input from the reference soil if EF value is close to unity for any metal. The sample is considered as enriched with that metal, if EF value is greater than 1. Thereby, it can be concluded that the sample is polluted by the anthropogenic sources, but not by natural sources. The EFs can be represented by following generalized equation (Eq. 2.3) given by Zoller et al. (1974),
EF = (Y/X)sample
(Y/X)reference Eq. 2.3
where, Y is the metal element of interest and X is the reference metal.
The reference metal X is selected based on three assumptions. These are: 1) the reference metal has a lower variability in concentration relative to the elements of interest; 2) reference metals have approximately comparable spatial distributions; and 3) elemental ratios are relatively homogeneous in different soils (Reimann and de Caritat, 2005).
Prior to computing EFs, it would be essential to test whether the assumptions are valid for the data set. Otherwise, high EFs do not exactly represent the anthropogenic influence on metals (Reimann and de Caritat, 2005). Zhou et al. (2007) evaluated the first assumption using coefficient of variance (CV*) (Eq. 2.4) (Reimann and de Caritat 2005).
median(xi) Eq. 2.4
where xi is the individual concentrations of metals.
To evaluate the second assumption they used scatter-plots to investigate the spatial variations among the different reference metals. The third assumption was not validated for their data set.
As reference metals, Al, Li, Fe, Mn and Ce have been used in different studies (Atiemo et al., 2011; Covelli and Fontolan, 1997; Fergusson and Kim, 1991; Tanner et al., 2000; Zhou, et al., 2007). Once the EFs are determined for suitability for analysis with a suitable reference metal and sample, it can be used to distinguish anthropogenic and natural sources. It does not, however, identify the type of anthropogenic activity and does not quantify the anthropogenic inputs.
Use of hydrocarbon concentrations in a sample to characterise the sources
Takada, et al. (1990) identified 64 PAH compounds using capillary GC/MS and identified their contributing sources in road build-up in the Tokyo metropolitan area. They visually compared the chromatograms of possible pollutant sources of road build-up (vehicle exhaust, asphalt/concrete and combustion products from steam generators) with the chromatogram for the road build-up. This showed similar patterns of results between road vehicle exhaust and build-up having an abundance of alkyl-substituted PAHs and sulphur heterocyclic compounds. It can be concluded that vehicle exhaust is a major source of PAHs in road build-up. In addition, specific pollutant compound ratios common to sources (diagnostic ratios) such as total methyl phenanthrenes (MP) to phenanthrene (P) (Barrick and Prahl, 1987; Youngblood and
Blumer, 1975) and methyldibenzothiophenes (MDBTP) to MP ratio have been used to confirm the conclusions from the chromatograms.
Takada, et al. (1990) further studied the road build-up in residential areas. Alkyl- substituted PAHs and sulphur heterocyclic compounds were found to be depleted in road build-up in contrast to the industrial areas (Takada, et al., 1990). They concluded that combustion products from steam generators are responsible for the PAHs. Similarly, chromatograms of asphalt particles (the other possible source of road build-up they considered) and road build-up were compared. Observations showed that asphalt is not a major contributor to road build-up in Tokyo. This was proved by comparing diagnostic ratios of MP/P and MDBTP/MP of asphalt road particles and concrete road particles. These ratios should be higher in asphalt road particles than concrete road particles if asphalt is a major source. Therefore, MP/P and MDBTP/MP ratios can be used to distinguish mobile sources such as vehicle exhaust from asphalt road particles along with GC/MS results. However, they failed to quantify their source contributions.
Tripathi et al. (2009) conducted a study on PAH pollution in the sediments of river Gomti, India. Concentration profiles of 16 priority PAHs were obtained by using HPLC and GC/MS. Particularly, the ratio of low molecular weight PAHs/high molecular weight PAHs (LMW/HMW) was used as a source indicator to differentiate petrogenic and pyrogenic sources. This is because petrogenic sources contain HMW PAHs which are formed under high temperature (emission of petroleum related material) and pyrogenic sources contain LMW PAHs which are formed from burning of fuels (wood and coal). In some sampling locations in this study, the LMW/HMW ratio is less than 1 which indicates the petrogenic origin of PAHs. Overall there was a mixed trend of pyrogenic and petrogenic sources. This conclusion was strengthened by the following diagnostic ratios and compared with past research findings (Table 2.3).
Table 2-3 Diagnostic ratios for pyrogenic and petrogenic sources (Adapted from Tripathi, et al. (2009))
Diagnostic ratio Pyrogenic Petrogenic Reference Tripathi, et al. (2009)
FLU/PYR >1 <1 Baumard et al. (1998) 0.2-5.28
PHE/ANT <10 >15 Baumard, et al. (1998) 0.06-104.44
NAP/ PHE >1 <<1 Dahle et al. (2003) 0.04-17.03
ANT/(ANT+PHE) >0.1 <0.1 Yunker et al. (2002) 0.01-0.95
FLU/(FLU+ PYR) >0.5 <0.5 Yunker, et al. (2002) 0.16-0.84
LMW/HMW, FLU/ PYR, BaA/CHR and BbF/BkF ratios were used by Zhang et al. (2008) to identify PAH sources in urban runoff. Road dust and canopy throughfall were considered as possible contributing sources of PAHs to the runoff. Each 16 priority PAHs were determined using GC/MS and the above ratios were calculated to discriminate pollutant origins. Whether the dominant source was petrogenic or pyrogenic was decided using LMW/HMW ratio as in Tripathi, et al. (2009). Further classification based on diagnostic ratios used by Zhang, et al. (2008) is given in Table 2.4.
Diagnostic ratios which have been selected to distinguish sources of pollutant mixture have comparable thermodynamic partitioning and kinetic transfer properties (Dickhut et al., 2000). Yunker, et al. (2002) noted that PAHs having molecular weight between 276 and 202 have the greatest stability and these are good indicators of petroleum and combustion sources, while those 278 and 228 provide slight assurance of identification. Further, the ratio calculations are restricted to given molecular mass to minimize the confusing factors such as volatility, water solubility and adsorption (Readman et al. 1987).
Table 2-4 Diagnostic ratios to identify PAH sources
Diagnostic ratio Condition Source Reference
Mai et al. (2003), Soclo et al. (2000) PHE/ANT <10 >10 >15 Pyrogenic Coal and wood combustion Petrogenic Yunker, et al. (2002) Takada, et al. (1990) ANT/(ANT+PHE) (Mass -178 gmol-1) <0.1 >0.1 Petroleum Combustion Budzinski et al. (1997) FLU/( FLU+PYR) (Mass -202 gmol-1) = 0.5 0.4<r<0.5 >0.5 Petroleum combustion transition point Vehicle and crude oil combustion Grass, wood or coal combustion Budzinski, et al. (1997) Yunker, et al. (2002) Yunker, et al. (2002) BaA/( BaA+CHR) (Mass 228 gmol-1) <0.2 0.2<r<0.35 >0.35 Petroleum Petroleum or combustion combustion Yunker, et al. (2002) Sicre et al. (1987) IND/(IND+BgP) (Mass- 276 gmol-1) <0.2 0.2<r<0.5 >0.5 Petroleum Vehicle and crude oil combustion Grass, wood or coal combustion
Yunker, et al. (2002) Less frequently used ratios