Environ. Sci. Technol. 2004, 38, 4973-4978
Gas-Particle Partitioning of Polycyclic Aromatic Hydrocarbons in Urban, Adjacent Coastal, and Continental Background Sites of Western Greece ELENI TERZI AND CONSTANTINI SAMARA* Environmental Pollution Control Laboratory, Chemistry Department, Aristotle University of Thessaloniki, GR-541 24 Thessaloniki, Greece
Particle- and gas-phase polycyclic aromatic hydrocarbons (PAHs) were collected from an urban, an adjacent coastal, and a continental background site located in Eordea basin, western Greece, to investigate their gas/ particle distributions. Thirteen two- to six-ring PAHs, included in the U.S. EPA priority pollutant list, were determined in 24-h integrated glass fiber filters and polyurethane foam samples. At the prevailing ambient temperature levels, the three-ringed species (phenanthrene, anthracene) and the four-ringed fluoranthene and pyrene were primarily found in the gas phase. Conversely, the fiveand six-ring PAHs were mainly associated with the particle phase. Gas/particle partitioning coefficients, KP, were calculated, and their relationship with the subcooled liquid vapor pressure poL of individual PAHs was investigated. Despite the large variability among samples, a good linear relationship between log KP and log poL was obtained for all sampling sites following the equation log KP ) mr log poL + br. In the majority of sampling events, particularly in the adjacent coastal and the continental background sites, slopes (mr) were found to be shallower than the value of -1, which has been suggested as reflecting equilibrium partitioning. The deviations from predicted aerosol behavior observed in the present study may be attributed to several reasons, such as the presence of nonexchangeable PAH fraction, nonequilibrium as well as different particle characteristics.
Introduction Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental contaminants that are formed during the incomplete combustion of fossil fuels and wood. Major PAH sources to the atmosphere include automotive emissions, home heating, energy production and certain industrial processes, biomass burning, and municipal and industrial waste incineration. The carcinogenic nature of PAHs along with their continual and widespread atmospheric emission has led to intensive study of these compounds. Atmospheric PAH concentrations may vary greatly, from very low usually found in remote and rural areas to significantly higher most often observed in populated and industrialized urban locations (1-4). The atmospheric fate, transport, and transformation of PAHs in the atmosphere are primarily governed by their gas/ * Corresponding author e-mail:
[email protected]. 10.1021/es040042d CCC: $27.50 Published on Web 09/03/2004
2004 American Chemical Society
particle (g/p) partitioning. Wet and dry deposition, photolysis, and reactions with other pollutants act differently on gasand particle-bound PAHs. The g/p partitioning of atmospheric PAHs depends on ambient temperature, the nature and concentration of ambient aerosols, and the interactions between the compound and the aerosol (5-8). This distribution between the two phases is described by the partitioning coefficient KP
KP ) (F/TSP)/A
(1)
where F and A are the analyte concentrations in the particle and the gas phase, respectively (ng m-3), and TSP is the total suspended particulate matter concentration (µg m-3) (9, 10). Two different mechanisms have been used to explain the gas-particle partitioning of PAHs: physical adsorption onto the aerosol surface and absorption into the aerosol organic matter. Both mechanisms lead to a linear relationship between log KP and log poL
log KP ) mr log poL + br
(2)
where poL is the compound’s subcooled liquid vapor pressure (6, 7, 10-12). Under certain conditions, the value of mr could be an indication of whether adsorption or absorption is the dominant mechanism that determines the g/p partitioning of SOCs (8). The intercept br depends mainly on properties associated with the aerosol (7, 11, 12). At equilibrium, the slope for either adsorption or absorption should be near -1 given the assumptions that, for adsorption, the difference between the enthalpies of desorption and volatilization and the number of available adsorption sites must remain constant over a compound class, while, for absorption, the activity coefficients must remain constant over a compound class (7, 10, 12). However, the slopes mr yielded from several field measurements worldwide are largely diverse, and while some are close to the theoretical value, significant deviations have also been reported (2-4, 13-16). Many reasons have been used to explain these deviations, and several investigators argue that such deviations do not always indicate disequilibrium (3, 8). Moreover, there is a number of sampling limitations, which might lead to steeper or shallower slopes than -1 (12). This study is part of a larger project concerning the emissions and depositional fluxes of PAHs to western Macedonia, Greece, an area with intensive lignite burning for power generation. Gas-particle partitioning is an important factor in determining the mode of deposition (i.e. dry and wet deposition, air-water exchange). Several studies conducted in Greece during the last two decades were focused primarily on particle-phase PAHs (17-21), while there are extremely limited data available concerning the g/p distribution of these compounds (14, 15). This study represents the largest data set of gas and particulate phase PAHs, TSP, and temperatures assessed simultaneously in urban, adjacent coastal, and continental background sites in Greece. The data set offers a unique perspective into the question of equilibrium partitioning of PAHs in various atmospheres.
Experimental Section Area Description. The study was conducted at three sites located in the Eordea Basin, west Macedonia, Greece (Figure 1) chosen to encompass a range of aerosol types and potential PAH sources. Four large lignite-burning power plants (total installed capacity 4048 MW) are operated in the Basin producing approximately 2/3 of the country’s electric power VOL. 38, NO. 19, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Summary Data of Sampling in Eordea Basin, Mean (SD) sampling period no. of GFF-PUF pairs TSP (µg m-3) ambient T (°C)
FIGURE 1. Map of the studied area. 1: Petrana, 2: Vegoritis, 3: Kozani. requirements. In addition to lignite burning in the power stations, other PAH sources in the area include urban emissions (vehicle exhausts, residential heating), uncontrolled refuse burning, and bush fires during late summer and early fall. The climate in the Eordea Basin is continental Mediterranean characterized by low temperatures during winter and higher during summer (min and max daily temperatures -1.3-6.3 °C and 20.1-28.5 °C, respectively). Prevailing winds are weak (0.5-3 m s-1) with NW and SE directions resulting in inefficient dispersion of atmospheric pollutants and short-range transport (22). The continental background sampling site Petrana was located on top of a 200 m hill, at a distance of 12-15 km S of the two largest power plants (total installed capacity 2700 MW), within a rural area with only 750 inhabitants. The adjacent coastal sampling site Vegoritis was located about 50 m from the coast of the lake Vegoritis, at the outskirts of a small town with about 1100 inhabitants. Several restaurants are located along the coast of the lake. A power station (600 MW) is also established 13 km SW of this sampling site. The urban sampling site, Kozani, was situated in the center of Kozani (a city with 47000 inhabitants), about 2 km west of Petrana, and was directly impacted by urban sources, such as traffic emissions and domestic heating. Sampling Description. In Petrana and Vegoritis, sampling of particle- and gas-phase PAHs took place every sixth day over the period October 2000 to October 2001. In Kozani, the sampling campaign lasted from January 2002 to November 2002. Particle- and gas-phase PAH samples were collected using identical low-volume air samplers (Andersen PS-1). The samplers were located about 3.5 m above ground level. Air volumes of 312-409 m3 were drawn at 0.28 m3 min-1 for 24 h through round glass fiber filters (GFFs) of 10 cm diameter followed by 6 cm diameter × 7.5 cm thick polyurethane foam (PUF) plugs. Prior to sampling, GFFs were heated at 400 °C for 6 h to remove any organic contaminant, conditioned in a temperature and humidity-controlled atmosphere for 48 h, and weighed in an analytical balance for gravimetric determination of TSP. The PUF plugs were Soxhlet extracted for 12 h using acetonitrile and for another 12 h using nhexane. After drying, they were placed in glass jars with aluminum foil-lined lids thoroughly precleaned with acetone and n-hexane. After TSP sampling, loaded GFFs and PUFs were separately wrapped in aluminum foils, placed into solvent-rinsed glass jars, and transported to the laboratory. All samples were stored in -20 °C until extraction (less than two weeks). 4974
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Petrana
Vegoritis
Kozani
10/2000-10/ 2001 59
10/2000-10/ 2001 51
1/2002-11/ 2002 47
41.6 (22.2) 15.2 (7.0)
44.5 (21.8) 14.9 (8.1)
92.4 (37.1) 16.4 (7.2)
It has been reported that sampling artifacts including particle blow-off from the GFFs, adsorption to the GFFs, breakthrough in the absorbent and degradation reactions of PAHs with ozone and other reactive gases in the sample stream may alter the true gas and particle concentrations of PAHs (5, 23, 24). Several techniques have been used to minimize these artifacts, such as back-up filters and more recently diffusion denuders (25, 26). In the present study, possible formation of sampling artifacts was examined by using back-up filters and split PUFs, according to suggested procedures (23, 27, 28). Back-up filters were used at regular intervals during the sampling campaign. The PAH concentrations found on the second filter rarely exceeded the detection limits, therefore, indicating no significant adsorption of gaseous PAHs on the filter. Breakthrough to the second half of the PUF plug was low accounting on average for only 11 ( 5% (n)6) for phenanthrene and much less for the heavier PAHs. This is consistent to literature data indicating that the losses of SOCs through PUF plugs are not significant by using sample volumes and flow rates similar to those used in this study (3, 27-32). Naphthalene, acenaphthene, and fluorene were omitted from this work since the sampling method used is considered inappropriate for their collection (33). Analytical Procedure. PAHs collected on GFFs and PUFs were ultrasonically extracted using acetonitrile. A three-step sequential extraction scheme was employed for GFFs using 120 mL (3 × 40 mL) of acetonitrile for 60 min (3 × 20 min). A two-step sequential extraction scheme was employed for PUFs using 300 mL (2 × 150 mL) of acetonitrile for 120 min (2 × 60 min). After solvent removal in a rotary evaporator, PAHs were determined by reversed phase high-pressure liquid chromatography with programmable fluorescence detection (HPLC/FLD). Details about chromatographic separation and detection conditions are given elsewhere (34). The system was calibrated with the NIST SRM 1647c containing 16 PAHs (naphthalene, Np; acenaphthylene, Acenp; acenaphthene, Ace; fluorene, F; phenanthrene, Ph; anthracene, An; fluoranthene, Fl; pyrene, Py; benzo[a]anthracene, B[a]An; chrysene, Chry; benzo[b]fluoranthene, B[b]Fl; benzo[k]fluoranthene, B[k]Fl; benzo[a]pyrene, B[a]Py; dibenz[a,h]anthracene, dB[a,h]An; benzo[ghi]perylene, B[ghi]Pe and indeno[1,2,3-cd]pyrene, IPy) plus benzo[e]pyrene, B[e]Py, which is frequently used as a reference PAH compound. Acenaphthylene, although contained in the standard, was not determined since it is only weakly fluorescent. Quality Assurance. Laboratory and field blanks of GFFs and PUFs were used to check for potential contamination of samples. The concentrations of all targeted PAHs in filter blanks were below the analytical detection limits. Naphthalene, phenanthrene, anthracene, and pyrene were detected in PUF blanks at very low concentrations, which were subtracted from sample concentrations. Percent recoveries ( SD of the 16 PAHs contained in the NIST SRM 1647 spiked at appropriate concentrations were found to range between 64 ( 9% (Np) and 106 ( 14% (B[a]Py) in GFFs and between 53 ( 3% (Chry) and 100 ( 12% (Py) in PUFs. Data were corrected for recoveries lower than 85% (2, 30). The accuracy
TABLE 2. Particle- and Gas-Phase Concentrations of PAHs in the Eordea Basin (ng m-3) particle-phase, mean (SD)
Ph An Fl Py B[a]An Chry B[e]Py B[b]Fl B[k]Fl B[a]Py dB[a,h]An B[ghi]Pe IPy ΣPAH
gas-phase, mean (SD)
Petrana
Vegoritis
Kozani
Petrana
Vegoritis
Kozani
0.169 (0.156) 0.030 (0.034) 0.095 (0.163) 0.082 (0.100) 0.033 (0.034) 0.049 (0.060) nd 0.084 (0.104) 0.038 (0.045) 0.053 (0.098) 0.020 (0.021) 0.074 (0.075) 0.094 (0.152) 0.814 (0.843)
0.195 (0.122) 0.165 (0.223) 0.169 (0.213) 0.156 (0.174) 0.251 (0.300) 0.212 (0.309) 0.504 (0.829) 0.397 (0.497) 0.256 (0.435) 0.231 (0.361) 0.085 (0.110) 0.374 (0.440) 0.390 (0.489) 3.332 (3.753)
0.220 (0.060) 0.130 (0.162) 0.140 (0.082) 0.119 (0.102) 0.055 (0.068) 0.138 (0.165) 0.367 (0.409) 0.270 (0.290) 0.123 (0.137) 0.105 (0.114) 0.053 (0.047) 0.473 (0.485) 0.297 (0.268) 2.491 (2.255)
2.034 (1.342) 0.097 (0.155) 0.623 (0.273) 0.216 (0.221) 0.040 (0.037) 0.031 (0.030) nd 0.008 (0.007) 0.002 (0.002) 0.003 (0.002) 0.003 (0.003) 0.006 (0.005) 0.008 (0.004) 3.060 (1.801)
9.625 (3.530) 0.894 (0.743) 2.395 (1.617) 1.810 (1.127) 0.129 (0.149) 0.129 (0.107) 0.162 (0.135) 0.026 (0.016) 0.008 (0.006) 0.011 (0.014) 0.003 (0.002) 0.011 (0.009) 0.010 (0.007) 14.39 (5.952)
13.40 (3.422) 1.333 (0.862) 3.508 (1.367) 4.834 (2.296) 0.431 (0.259) 0.309 (0.142) 0.250 (0.105) 0.033 (0.019) 0.010 (0.006) 0.009 (0.009) 0.006 (0.007) 0.010 (0.009) 0.008 (0.004) 24.14 (6.449)
TABLE 3. Analysis of Variancea of ΣPAH Concentrationsb factor
particle-phase ΣPAH
gas-phase ΣPAH
TSP
sampling site season
++ ++
++ -
++ ++
a One-way ANOVA. b -, not significant at the 0.05 level. +, significant at the 0.05 level. ++, significant at the 0.01 level.
and precision of the extraction and analytical procedures employed were additionally assessed by analyzing the NIST SRM 1649 “urban dust”. The recoveries of the 13 targeted PAHs from this Certified Reference Material were between 51 ( 10% for IPy and 120 ( 15% for B[e]Py.
FIGURE 2. ΣΡΑΗ concentrations in particle and gas phase for the cold and the warm time period (cold period: October 16-April 15, warm period: April 16-October 15).
Results and Discussion TSP and PAH Concentration Levels. Mean TSP concentrations found at the three sampling sites during the sampling campaigns are summarized in Table 1 along with 24 h-average ambient temperature values. Mean PAH concentrations in the particle and the gas phase are reported in Table 2. For calculating descriptive statistics, PAH concentrations below detection limits were considered as equal to the half of the detection limit. It is apparent that all 13 PAHs were detectable in the particle- and the gas-phase at all three sampling sites, except B[e]Py that was not detected in any of the two phases at Petrana. Spatial and seasonal variations were assessed by employing the one-way ANOVA statistical test (Table 3). TSP as well as particle- and gas-phase ΣPAH concentrations exhibited significant spatial differences suggesting that the three sites are affected by sources of different strength. Surprisingly, the particle-phase PAH concentrations in the coastal adjacent site Vegoritis were similar to those found in the urban site Kozani, although TSP levels in Kozani were 2-fold as high as in Vegoritis. This might suggest the presence of PAH enriched particle sources in Vegoritis, likely wood burning and meat grilling in the restaurants situated along the coast of the lake (35, 36). The seasonal effect was significant for TSP and particle-phase ΣPAH. The increase in particulate PAH loadings during colder weather is likely due to wintertime sources (residential heating, cold engine start of vehicles, etc.) as well as to the prevailing meteorology (i.e. lower inversions) during winter. Moreover, atmospheric photochemical reactions of PAHs with ozone and nitrogen oxides are more intense during summer and can decrease their ambient concentrations. The lack of significant seasonality of the gas-phase ΣPAH might be attributable to the temperature-dependent g/p partitioning and/or potential volatilization of PAHs from contaminated surfaces (4). In general, the observed particle-phase concentrations of PAHs
determined in the present study are comparable to the concentrations found at other urban and rural locations in Greece (14, 17-21). Both the particle- and the gas-phase concentrations varied widely during the sampling periods as a result of variations in emissions from different sources and the prevailing atmospheric conditions. The total PAH concentrations (ΣPAH) in the particle- and the gas-phase found at the three sampling sites during the cold and the warm time period of each sampling campaign are presented in Figure 2. Unrelated to season, at all three sampling sites, gaseous ΣPAH concentrations were significantly higher (p68%). Of the four-ring PAHs, Fl and Py were mostly found in the gas phase (>89% and >73%, respectively), whereas B[a]An and Chry were distributed almost equally between the two phases. A shift of the distribution of B[a]An and Chry to the gas phase was observed in Kozani probably due to the higher ambient temperatures in this urban site (Table 1). The log KP-log poL Relationship. When log KP is regressed against the log poL of a group of SOCs, useful information about the partitioning of the compounds can be extracted from the slope mr and the intercept br of the trend line. Usually, literature log poL values are used after correction for the average ambient temperature of each sampling event. Because different log poL sources are used and the individual SOCs included may also be different among studies, direct comparison of the obtained mr and br values is difficult. In VOL. 38, NO. 19, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Relative distribution of PAHs in the gas and particle phase during the cold and the warm time period (cold period: October 16-April 15, warm period: April 16-October 15).
FIGURE 4. Log KP vs log poL plots for each sampling site. Plots in the first row were obtained by using poL values from ref 40. Plots in the second row were obtained by using poL values from ref 37 . the present study, temperature dependent poL values were calculated from two different literature sources: ref 40 as reported in ref 12, available for 10 targeted PAHs, and ref 37, available for all 13 targeted PAHs. Only those PAHs having concentrations above the detection limits for a given sample were used in the regression of log KP vs log poL. The plots derived for the whole number of sampling events at each sampling site are presented in Figure 4. Good linear relationships between log KP and log poL were found, with R2 values in the range 0.706-0.761, when poL values were calculated from ref 12, and between 0.648 and 0.754, when poL values were obtained from ref 37. However, the regression slopes mr and intercepts br were different in the two approaches, with ref 12 poL values yielding mr values closer to -1 and more negative intercepts. The mean and range 4976
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values of mr and br yielded from the two approaches are summarized in Table 4. Similar mr values with those found in the urban site of the present study have also been reported by other investigators (2, 3, 13). Based on Table 4 and Figure 4, it is apparent that the mr values at the nonurban sites of the present study were relatively shallower than those found at the urban site. Gustafson and Dickhut (4) also observed shallower slopes at a rural site in comparison to urban/ industrial sites. On the contrary, Cotham and Bidleman (2) found shallower slopes in the urban area of Chicago and steeper slopes in the rural site of their study. Finally, similar mr values were yielded for urban Chicago and over Lake Michigan (3). In all sampling events of the present study, mr deviated significantly from -1. Factors that can account for slopes
FIGURE 5. mr values obtained from Yamasaki’s poL data as a function of ambient temperature.
TABLE 4. Summary Statistics of mr and br Values slope, mr
intercept, br
location
site character
min
max
mean
min
max
mean
reference
source of poL data
Petrana Vegoritis Kozani Petrana Vegoritis Kozani Chicago UWGB Elizabeth River York River Hampton Haven Beach Chicago Lake Michigan Manchester Clapham Austwick Athens Athens Chicago Chicago Los Angeles Houston Elizabeth
continental background adjacent coastal urban continental background adjacent coastal urban urban rural industrialized semi-urban urban adjacent coastal urban adjacent coastal urban rural rural urban urban urban urban urban urban urban
-0.76 -1.11 -0.90 -0.48 -0.64 -0.74 -1.04 -1.13
-0.29 -0.33 -0.44 -0.23 -0.31 -0.40 -0.38 -0.75
-0.50 -0.58 -0.61 -0.32 -0.46 -0.53 -0.69 -1.00 -1.04 -0.97 -1.03 -0.65 -0.64 -0.67 -0.71 -0.71 -0.77
-4.23 -7.28 -5.49 -3.38 -4.36 -5.09 -6.09 -6.19
-2.57 -3.17 -3.93 -1.98 -2.82 -3.49 -2.98 -4.72
-3.37 -4.08 -4.77 -2.75 -3.58 -4.44 -4.61 -5.74 -10.3 -10.5 -10.9 -7.67 -3.47 -3.71 -5.2 -4.9 -5.1
present study
Yamasaki et al. (1984)
present study
Paasivirta et al. (1999)
2
Yamasaki et al. (1984)
4
Sonnefeld et al. (1983)
3
Liu (1994)
13
Yamasaki et al. (1984)
14 15 16
Sonnefeld et al. (1983) Paasivirta et al. (1999)
39
Offenberg et al. (1999)
-0.77 -0.81 -0.87 -0.69 -1.49 -1.06 -1.17 -1.19
-0.62 -0.58 -0.71 -0.47 -0.16 -0.753 -0.445 -0.664
-0.78 -0.69 -0.91 -0.85 -0.98
shallower than -1 include the following: (a) increasing aerosol concentrations or decreasing temperature during sampling, (b) the presence of nonexchangeable PAHs on atmospheric particles, (c) slow gas-to-particle sorption of PAHs, (d) varying differences in the “excess” heat of desorption (heat of desorption minus heat of vaporization) within a class of compounds, (e) varying activity coefficients in organic matter within a class of compounds, and (f) varying number of available adsorption sites. Moreover, Goss and Schwarzenbach (8) and Simcik et al. (3) suggested that even in the case of equilibrium, several other factors may cause the slopes mr to deviate from -1. Temperature fluctuations during sampling events might have contributed to the shallow slopes observed in this study, but this effect could not be eliminated since a 24 h sampling period was necessary in order to collect an appreciable amount of sample. However, there are samples for which the temperature changes did not exceed 4 °C and yet exhibit very shallow slopes, in contrast to samples for which the temperature fluctuations were much higher than 4 °C during sampling but have steeper slopes. Thus, there is little evidence that temperature changes during sampling are responsible for the shallow slopes. It has been demonstrated that aerosols equilibrate faster with SOCs under warm conditions than cold conditions and that low molecular weight PAHs attain equilibrium more quickly than the heavier compounds (38). According to Pankow and Bidleman (12) slow kinetics may lead to artificially shallow slopes when relatively clean particles enter a contaminated atmosphere since the volatile compounds
-5.4 -5.6 -5.4 -5.67 -6.50 -5.73 -5.34 -6.22
-4.8 -4.4 -4.9 -4.52 -3.20 -4.30 -3.76 -3.38
-5.93 -5.16 -4.80 -4.82 -4.89
will reach equilibrium faster than the low volatility compounds. Nonequilibrium can be concluded from the regression plots of Figure 4 (row b), in which steeper slopes would derive if the less volatile PAHs dB[a,h]An, B[ghi]Pe, and IPy were omitted. Slow sorption kinetics of the heavier PAH members to combustion aerosols might have affected the slopes at Vegoritis and Kozani, since both these sites are influenced by local sources. Locally released PAHs need to equilibrate to the ambient temperature and particles present, and this can result in slopes shallower than -1. Nonexchangeability might also be a possible reason for the shallow slopes observed, since during combustion the more volatile PAH species may become trapped inside particles during formation resulting in higher log(F/TSP)/A values than would be expected (3). The samples at Petrana are considered as representing aged aerosols; therefore, one would expect steeper slopes than those observed. However, low gas-phase concentrations of PAHs at Petrana could contribute to slow sorption kinetics of PAHs to background aerosols in the local atmosphere (4). Another possible explanation for the very shallow slopes observed at this site throughout the sampling period might be the nonexchangeable fraction of PAHs that can increase as the particulate matter ages and is expected to be larger for samples from remote areas than for samples from urban areas (6). Moreover, at low levels of TSP, such as those found at Petrana, even a few percent of nonexchangeable compound can cause significant deviations to the slopes of the plots (12). At higher temperatures, volatile compounds favor the gas phase, and therefore, nonexhangeable PAHs would have a larger influence on KP than VOL. 38, NO. 19, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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at lower temperatures (4, 12). The mr values were found to be positively correlated with mean temperature at p < 0.01 (Figure 5). This effect was more important in Petrana (R2)0.331), indicating that slow sorption kinetics and the nonexchangeable fraction of PAHs affect this site in a higher degree than the other two sites. Noticeable variability was also found for br values (Table 4). The factors that affect the slope mr will also change br. The intercept br depends mainly on properties associated with the aerosol. For adsorptive partitioning, br is related to the number of adsorption sites on the particle surface and surface-chemical interactions. For absorptive partitioning, br is associated with the fraction of organic matter on the particle surface and the activity coefficient of the absorbing compound in the organic matter (7, 11, 12). The mr-br Relationship. Measured values of mr and br are expected to exhibit some degree of interrelation which can be described by a linear regression of the form br ) msmr + bs (12). For the data sets of Petrana, Vegoritis, and Kozani the obtained slope values (i.e. ms) were 3.21, 3.45, and 2.43, the intercept bs values were -1.77, -2.11, and -3.29, and R2 values were 0.65, 0.71, and 0.48 (significant at p
