ARTICLE pubs.acs.org/est
Dechlorane Plus in the Atmosphere and Precipitation near the Great Lakes Amina Salamova and Ronald A. Hites* School of Public and Environmental Affairs, Indiana University, Bloomington, Indiana 47405, United States
bS Supporting Information ABSTRACT: Air (vapor and particle) and precipitation samples were collected at five sites (two urban, one rural, and two remote) on the shores of the Great Lakes from January 1, 2005 to December 31, 2009 as a part of the Integrated Atmospheric Deposition Network (IADN). The concentrations of the syn and anti isomers of Dechlorane Plus (DP), a highly chlorinated flame retardant, were measured in these samples. The highest concentrations of these compounds were generally observed at the rural site at Sturgeon Point, New York, which is located near DP’s manufacturing facility in Niagara Falls, New York, and at the urban site at Cleveland, Ohio. A multiple linear regression model was applied to the concentrations of these compounds in the vapor phase, particle phase, precipitation, and for the three phases combined. This regression resulted in an overall (three phases combined) doubling time for the anti-DP isomer of 9.5 ( 3.6 years, but for the syn- and total DP (syn + anti) concentrations, the overall regression was not statistically significant. These results suggest that there has been no significant change in the atmospheric concentrations of these compounds over the 20052009 time period. The effect of distance from the source in Niagara Falls was highly significant; for example, doubling the distance from Niagara Falls decreased the DP concentrations by about 30%. The effect of the number of people living and working within a 25-km radius of the sampling site (population density) was also highly significant but small; for example if this population doubled or halved, then the atmospheric DP concentrations would increase or decrease by only a few percent.
’ INTRODUCTION Dechlorane Plus (DP, C18H12Cl12) is a highly chlorinated flame retardant that was introduced by Hooker Chemical (now known as OxyChem) in the 1960s as a replacement for Dechlorane (also known as Mirex).1 The technical DP mixture consists of syn- and anti-DP isomers in a ratio of 1:3 (i.e., the fraction of the anti isomer is 75%).1 DP is used as an additive flame retardant in wire and cable coatings, in hard plastic connectors used in televisions and computers, and in furniture.2 Although DP is a high production volume chemical and has been used for decades, it was first detected in the atmosphere, sediments, and fish of the North America Great Lakes only in 2006.1 After this finding, DP has been detected in air,35 sediments,68 precipitation,4 biota,912 house dust,13 tree bark,4,14 and humans15 in different parts of the world, suggesting that DP is a worldwide contaminant. An identification of a DP source in China16 resulted in several studies on DP in the Chinese environment.1719 Remarkably, M€oller et al. have recently detected DP near Antarctica, suggesting that DP is a global pollutant susceptible to long-range atmospheric transport.20 In addition, several DP-related substances, some formed during DP’s synthesis and some formed from DP dechlorination, have also been identified in the Great Lakes.21 According to environmental tests performed by OxyChem, the only DP manufacturer in the United States, DP is resistant to biological and photolytic r 2011 American Chemical Society
degradation and will persist in the environment.22 DP has been identified by the European Commission as a possible replacement for the now restricted decabromodiphenyl ether (BDE-209) flame retardant.2 Clearly, the measurement of DP levels in the environment is important. In this study, we report the concentrations of the syn- and anti-DP isomers at the five United States Integrated Atmospheric Deposition Network (IADN) sites located on the shores of Lakes Superior, Michigan, and Erie. These vapor, particle, and precipitation samples were collected from 2005 to 2009 (inclusive) and analyzed by gas chromatographic mass spectrometry. The goal of this study was to provide insights on the long-term spatial and temporal distribution trends of DP in the Great Lakes atmosphere. The spatial distribution of the individual syn- and anti-DP isomer concentrations at U.S. IADN sites are reported separately for the vapor, particle, and precipitation phases for the first time. In addition, this is the first investigation of the temporal trends of atmospheric DP concentrations in this region using a harmonic multiple
Received: August 8, 2011 Accepted: October 18, 2011 Revised: October 12, 2011 Published: November 07, 2011 9924
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regression technique that includes terms for seasonality, population, distance from the assumed source, and time.
’ EXPERIMENTAL SECTION Sampling. The locations of the United States IADN sampling sites are shown in the Supporting Information (Figure S1). The two urban sites are in Chicago, IL (41.8344°N, 87.6247°W) and Cleveland, OH (41.4921°N, 81.6785°W). A rural site is located at Sturgeon Point, NY (42.6931°N, 79.0550°W). The two remote sites are at Sleeping Bear Dunes, MI (44.7611°N, 86.0586°W) and Eagle Harbor, MI (47.4631°N, 88.1497°W). The IADN Web site provides detailed information on air sampling procedures and site operations (www.msc.ec.gc.ca/iadn). The atmospheric samples discussed here were collected from January 1, 2005 to December 31, 2009, covering five continuous years. A modified Anderson high-volume air sampler (General Metal Works, model GS2310) was used to collect air samples for 24 h every 12 days at a flow rate giving a total sample volume of about 820 m3. The vapor phase was collected on Amberlite XAD-2 resin (Supelco, Bellefonte, PA; 2060 mesh, precleaned with a series of solvents for several days) held in a stainless steel cartridge, and particles were collected on Whatman quartz fiber filters (QM-A, 20.3 25.4 cm, heated at 450 °C before use). Details of the sampling procedures and site operations can be found elsewhere.23 Precipitation samples were collected using a MIC, wet-only sampler (MIC Co., Thornhill, ON). This sampler consists of a 46 46 cm stainless steel funnel connected to a 30 cm long by 1.5 cm i.d. glass column (ACE Glass, Vineland, NJ) wet packed with XAD-2 resin. The sampler is normally covered; it opens when a precipitation event is sensed by a conductivity grid located outside of the sampler. Precipitation flows through the funnel and the XAD-2 column into a large carboy used to measure the total precipitation volume. Both particulate and dissolved organic phase compounds are collected by the XAD-2 column. The funnel and the interior of the sampler are kept at 15 ( 5 °C to melt snow collected in the sampler and to prevent the XAD column from freezing. Details on the performance of these samplers are provided elsewhere.24 Precipitation events are integrated over each calendar month. Analysis. A detailed description of the sample treatment and chemical analysis procedures for the air and precipitation samples has been given elsewhere.25 In summary, the samples were Soxhlet extracted for 24 h with a 1:1 acetone/n-hexane mixture. DP was analyzed as a part of a polybrominated diphenyl ether (PBDE) analysis scheme, and a recovery standard including known amounts of BDE-77, BDE-166, and 13C12BDE-209 was spiked into the sample. The extract was reduced in volume by rotary evaporation, the solvent was exchanged to n-hexane, and fractionated on a column containing 3.5% w/w water deactivated silica gel (3.0% w/w for precipitation samples). This column was eluted with 25 mL of n-hexane (fraction 1) and 25 mL of a 1:1 n-hexane/ dichloromethane mixture (fraction 2). After N2 (4.8 Z grade, Praxair) blow down, the samples were spiked with the quantitation internal standard, which included a known amount of BDE-118. For chemical analysis, the samples were further concentrated by N2 blow down to about 100 μL. The samples were analyzed for the syn- and anti-DP isomers on an Agilent 6890 series gas chromatograph coupled to an Agilent 5973 mass spectrometer using helium as the carrier gas. The mass
Figure 1. Concentrations of syn-DP, anti-DP, and ΣDP in the atmospheric vapor phase, particle phase, and precipitation. The thin black lines represent the median, and the thick red lines represent the mean; the boxes represent the 25th and 75th percentiles; the whiskers represent the 5th and 95th percentiles. Site abbreviations: EH Eagle Harbor, CH Chicago, SB Sleeping Bear Dunes, CL Cleveland, SP Sturgeon Point.
spectrometer was operated in the electron capture negative ionization mode, the ions at m/z 651.8 and 653.8 (M + 4 and M + 6) were used to detect the syn- and anti-DP isomers, and the ions at m/z 78.9 and 80.9 (Br) were used to detect BDE118. The response factors for both DP isomers were determined using a calibration standard solution containing syn- and anti-DP and BDE-118. The details on the instrumental analysis, quality control and quality assurance procedures, and the materials used in this study are provided in the Supporting Information.
’ RESULTS AND DISCUSSION Atmospheric Concentrations. Both the syn- and anti-DP isomers were found in the gas, particle, and precipitation phases; the concentrations and percent detection of anti-DP were typically greater than those of syn-DP. Figure 1 shows the spatial distributions of these two isomers and of their sum (here abbreviated as ΣDP) in the vapor, particle, and precipitation phases at the five U.S. IADN sites. The highest mean syn-, anti-, and ΣDP concentrations were measured at Sturgeon Point (SP) and Cleveland (CL) in all three phases. As expected,1 the concentrations of DP were generally higher in the particle phase than in the vapor phase. Table 1 summarizes the arithmetic mean concentrations at each site and for each phase and the ANOVA results for the logtransformed concentrations. The Tukey posthoc test was used to distinguish statistically different spatial categories. These ANOVA results indicate that the concentrations measured at Sturgeon Point and Cleveland were significantly higher than those at the other sites. The concentrations measured at Chicago (CH) were lower, but most of the time not significantly, than at Sturgeon Point and Cleveland; the concentrations at Chicago were significantly higher than those at Sleeping Bear Dunes (SB). DP concentrations at the very remote site at Eagle Harbor (EH) were usually lower than those at Sleeping Bear 9925
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Table 1. Arithmetic Mean ( Standard Error of the syn-DP, anti-DP, and ΣDP Concentrations in the Vapor Phase (pg/m3), Particle Phase (pg/m3), and Precipitation (ng/L) at the Five U.S. IADN Sitesa Eagle Harbor concn.
*
Chicago
% det
concn.
*
Sleeping Bear Dunes % det
Cleveland
concn.
*
% det
concn.
*
Sturgeon Point % det
concn.
*
% det 42
syn-DP
vapor
0.08 ( 0.02
c
10
0.66 ( 0.16
ab
100
0.14 ( 0.03
bc
18
0.88 ( 0. 19
a
16
0.98 ( 0. 25
a
anti-DP
vapor
0.16 ( 0.10
c
19
0.41 ( 0.05
b
100
0.15 ( 0.03
c
27
0.67 ( 0.15
ab
33
1.9 ( 0.5
a
63
ΣDP
vapor
0.17 ( 0.08
b
23
1.03 ( 0.14
a
70
0.22 ( 0.04
b
30
1.4 ( 0.3
a
35
2.5 ( 0.6
a
67 89
syn-DP
particle
0.08 ( 0.01
d
37
1.1 ( 0.2
b
100
0.26 ( 0.05
c
64
2.1 ( 0.2
a
93
5.9 ( 1.3
b
anti-DP
particle
0.12 ( 0.02
d
55
1.6 ( 0.2
b
99
0.38 ( 0.10
c
76
3.9 ( 0.5
a
99
17 ( 3
a
91
ΣDP
particle
0.22 ( 0.03
d
64
2.6 ( 0.3
b
100
0.57 ( 0.14
c
81
5.8 ( 0.6
a
100
21 ( 4
ab
97
syn-DP
precip
0.05 ( 0.02
c
28
0.22 ( 0.09
a
86
0.03 ( 0.01
b
45
0.13 ( 0.03
a
87
0.23 ( 0.08
a
90
anti-DP
precip
0.11 ( 0.04
d
70
0.59 ( 0.14
b
100
0.06 ( 0.02
c
89
0.37 ( 0.07
a
98
0.70 ( 0.24
a
94
ΣDP
precip
0.05 ( 0.01
c
67
0.76 ( 0.18
b
100
0.05 ( 0.01
c
90
0.49 ( 0.10
a
100
0.89 ( 0.30
ab
99
a
The column headed with an asterisk gives the ANOVA results for the logarithmically transformed concentrations using Tukey’s post hoc analysis; the concentrations are not significantly different for those locations sharing the same letter. The percent detects for each compound in each phase is also given.
Dunes, but in many cases these two concentrations were statistically indistinguishable from one another. This spatial distribution pattern is different from the urban effect pattern observed for polybrominated diphenyl ether (PBDE) concentrations measured at the same Great Lakes sites.3,4 PBDE concentrations were highest for samples collected in Chicago and Cleveland, but for DP, the highest concentrations were observed at Sturgeon Point. In fact, the single highest concentrations of the syn- and anti-DP isomers were measured in an atmospheric particle phase sample collected at Sturgeon Point on September 28, 2008 (110 and 230 pg/m3, respectively). The generally elevated levels of DP at Sturgeon Point are clearly related to the source of DP: The Sturgeon Point site is located only 50 km south of the OxyChem DP manufacturing plant in the city of Niagara Falls, NY.1 There are no studies that have reported concentrations for the syn- and anti-DP isomers in the atmosphere separately and only a few that have reported separate concentrations for vapor and particle phases; this limits the direct comparison of our data with the literature. Our results are consistent with those of Hoh et al.,1 where the highest mean ΣDP concentrations (34 ( 24 pg/m3, syn- + anti-DP and vapor + particle phases) were observed at Sturgeon Point, followed by the concentrations at Cleveland. Hoh et al.1 did not see any statistical difference between average ΣDP concentrations at Sturgeon Point and Cleveland, which is also consistent with our results. The ΣDP concentrations detected in this study are also similar to the concentrations reported pre3 viously at the same sites by Venier and Hites (0.820 pg/m in 3 the vapor + particle phases over the period of 20032006) and by Salamova and Hites (0.151.6 pg/m3 in the vapor phase, 0.2020 pg/m3 in the particle phase, and 53522 pg/L in precipitation in samples collected in 20032007).4 These previous studies have also reported a spatial distribution of DP concentrations similar to the one observed here with the highest DP concentrations measured at Sturgeon Point. The concentrations of ΣDP measured at Sturgeon Point in this study are much lower than those reported near the recently identified DP source in China (390430 pg/m3 in the gas phase and 730026 000 pg/m3 in the particle phase).16 The
concentrations measured at the rural sites at Eagle Harbor and at Sleeping Bear Dunes are similar to the concentrations measured at the Chinese rural sites (average of 3.5 pg/m3).5,26 Our results are also similar to the DP levels reported in air from Europe (0.811 pg/m3).27 The ΣDP levels observed at the remote sites in this study are within the range reported in air over the East Greenland Sea and over the northern and southern Atlantic Ocean.20 Seasonality, Time, Population, and Distance from Source Effects. To investigate the effects of seasonality, time, human population density, and distance from DP’s source on the concentrations of DP isomers and ΣDP in the three phases, we used a harmonic multiple regression method that had been applied previously to investigate the changes in the concentrations of several persistent organic pollutants at these sites.28,29 In the first step of this approach, a harmonic multiple regression equation with seasonality, population, distance from a source, and time terms was used to fit the natural logarithms of concentrations in each phase: lnðCÞ ¼ a0 þ a1 sinðztÞ þ a2 cosðztÞ þ a3 log2 ðpopÞ þ a4 lnðdistÞ þ a5 t
ð1Þ
where C is the DP concentration in the gas phase, particle phase, or precipitation, t is time expressed in Julian days starting from January 1, 2005, z = 2π/365.25 (which fixes the periodicity at one year), pop is the number of people living and working within a 25-km radius of the sampling site, dist is the distance (in km) of the sampling site from the OxyChem DP manufacturing facility in Niagara Falls, NY, a0 is an intercept which rationalizes the units, a1 and a2 describe the seasonal variations in the concentration with time, a3 describes the change in concentration as a function of population, a4 is a coefficient describing the effect of distance from the OxyChem source, a5 is a first-order rate constant (in days1) used for the calculation of a halving or doubling time, and ε is the regression residual. The details on the derivation of this equation and of the calculations of halving or doubling times (time periods required for concentrations to decrease by a factor of 2 or to double) have been given elsewhere.29 The calculations for 9926
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0.283 ( 0.118 5.81 ( 0.48 precip ΣDP
297
precip
N is the number of samples included in the regression. The halving and doubling times (t1/2, in years; doubling time is noted with the negative sign) were calculated from the coefficient of the time term (a5). A blank cell indicates that the coefficients were not statistically significant at p < 0.05.
a
28.0 0.1 9.5 16.2 0.452 ( 0.073 0.054 ( 0.007
0.303 ( 0.080
0.439 ( 0.072
0.032 ( 0.008
0.052 ( 0.006
218 precip syn-DP
anti-DP
293
655 particle ΣDP
5.57 ( 0.48
0.269 ( 0.116
0.280 ( 0.129 4.53 ( 0.48
0.645 ( 0.050 0.083 ( 0.005 1.44 ( 0.33
1.70 ( 0.34 563
anti-DP
629
particle
particle
syn-DP
vapor ΣDP
336
1.15 ( 0.54 316 vapor anti-DP
196 vapor syn-DP
2.2
12.8
27.0
0.1
0.2
3.1
1.7
5.9
9.2
3.7
43.3 0.1 14.8 28.3
15.9
44.6 0.1 19.4
0.1
35.0 0.5 13.3
25.0
2.47 ( 1.14 0.754 ( 0.051
0.536 ( 0.050 0.072 ( 0.005
0.080 ( 0.005
0.414 ( 0.062 0.055 ( 0.007
2.98 ( 1.43
7.70 ( 3.55
21.1
0.1
24.7 1.6 9.6
0.325 ( 0.156
0.332 ( 0.112
0.523 ( 0.064 0.041 ( 0.007
4.89 ( 1.47 0.381 ( 0.087 0.047 ( 0.010
6.37 ( 3.06
10.8
0.1
24.1 3.2
2.7
15.7 0.7
0.7 7.9
2.0 7.5
12.3
5.5
3.88 ( 1.17
seas dist pop t 1/2 ( err a3 ( err3 a2 ( err2
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a1 ( err1 a0 ( err0 N phase compound
regression parameters
Table 2. Coefficients for All Regressions Using eq 1 with Their Standard Errorsa
a4 ( err4
a5 ( err5 104
halving time (years)
partial r2 (%)
time
total r2 (%)
Environmental Science & Technology
this multiple regression were done using Minitab 16, which returned the coefficients, their standard errors, and the r2 values associated with each term (see Table 2). Once the coefficients a0 to a4 had been determined, the data were normalized to the same scale by subtracting the variations caused by local human population, seasonal factors, distance from the presumed source, and the intercept: lnðCÞ a0 a1 sinðztÞ a2 cosðztÞ a3 log2 ðpopÞ a4 lnðdistÞ ¼ ε0
ð2Þ This process resulted in so-called “partial residuals”, ε0 , which were then combined for all the phases and regressed as a function of time (in Julian days) using ε0 ¼ a00 þ a05 t
ð3Þ 0
This gives the best estimate of the rate (a 5) at which the concentrations of syn-, anti-, and ΣDP were decreasing or increasing. The details on this data analysis approach have been given elsewhere.28,29 This additional step allows us to combine the data for all three phases at all sites and to calculate an average of the halving or doubling times using the number of data points in each phase as weights. This approach integrates all the available DP data for all three phases (vapor, particle, and precipitation) and all five sampling sites and provides an overall temporal trend for DP concentrations in the Great Lakes region. All of the regressions using eq 1 for both of the DP isomeric and for the ΣDP concentrations were statistically significant at p < 0.0001; note that the relatively modest total r2 values are offset by the high number of samples (N, see Table 2) in each regression. Among all the regressions, there were only a few where the seasonal terms (a1 and a2) of the harmonic regression were statistically significant, and even then their contributions to the overall regression were small. The same lack of a clear effect was observed for the time term. Among the nine regressions, only three gave statistically significant time coefficients (a5) that could be used to calculate halving or doubling times, and again their contribution to the overall regression was small, suggesting that DP concentrations are not changing significantly with time during the sampling period. In the vapor phase, the concentrations of anti-DP and ΣDP are increasing with doubling times of 3.9 ( 1.2 and 6.4 ( 3.1 years, respectively; see Table 2. The latter value is similar to the doubling time value observed for the vapor phase for pentabromoethylbenzene (PBEB) concentrations (about 8 years) measured at these sites.28 In the particle phase, the concentrations of syn-DP are decreasing with the halving times of 7.7 ( 3.6 years; see Table 2. This value is similar to the halving times reported for particle phase total PBDE (about 7 years)28 and total DDT (about 6 years)29 concentrations at the same sites. However, these times are lower than the halving times observed for phenanthrene and chrysene (about 10 years)29 and total endosulfans (about 11 years)29 in the particle phase, and higher than the halving times of HCHs in the particle phase (about 45 years).29 In precipitation, no temporal concentration changes were observed for either of the DP isomers or for ΣDP. Overall, the absences of clear temporal effects suggest that there are continued inputs of DP into the environment and that these inputs have not changed much over the last 5 years. The analysis of the partial residuals for the combined phases and sites is similarly equivocal; see Figure 2. In this case, the 9927
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Figure 2. Regression of partial residuals versus sampling date for the three phases combined (vapor, red squares; particles, green triangles; and precipitation, yellow circles) for (A) the syn-DP isomer, (B) the antiDP isomer, and (C) ∑DP. NS: not significant at p < 0.05.
regression vs time (or date) was only statistically significant for the anti-DP concentrations and resulted in a doubling time of 9.5 ( 3.6 years. The regressions for both the syn-DP and ΣDP concentrations for the combined phases were not statistically significant at p < 0.05. Again, this lack of clear temporal effects suggests that the recent inputs of DP into the environment have not changed significantly. Unlike the temporal effects, all of the concentrations measured in this study show strong relationships to the local human population density and to the distance from DP’s manufacturing site in Niagara Falls, NY. These observations indicate that DP has sources associated with both its manufacturing plant in Niagara Falls and with human population. The dependence of DP concentrations on human population density at the sites may be related to the use of DP in electrical equipment, the abundance of which is proportional to population. The average population term (a3) for the regressions of syn-, antiDP, and ΣDP concentrations is 0.057 ( 0.007, and it contributes
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Figure 3. Fraction of the anti-DP (fanti) as a function of distance from Niagara Falls, New York in (A) vapor, (B) particles, and (C) precipitation. The error bars represent standard errors. Site abbreviations: EH Eagle Harbor, CH Chicago, SB Sleeping Bear Dunes, CL Cleveland, SP Sturgeon Point.
about half to the total error of the regression. This is highly significant but lower than the contribution of the population term to the total r2 for total PBDE concentrations (up to 80%).28 The positive sign of the population term indicates that DP concentrations increase with increasing population density, presumably because an increase in population comes with an increase in the use of electronic equipment. The scale of this term can be described by following two calculations: If the population at Eagle Harbor went up from 500 to 1000 people, then the concentration of DP would increase by about 10%, which is not much. At the other end of the population spectrum, if the population at Chicago went down from 3.6 million to 1.8 million people, then the concentration of DP would decrease by 20%, which is not much either. The average distance term (a4) is 0.49 ( 0.07 for the syn-, anti-, and ΣDP concentrations in the three phases. This term also 9928
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contributes about half to the total r2 of the regression, which is higher than the effect observed for total PBDE concentrations.28 The negative sign of the distance term indicates that as the distance from DP’s source in Niagara Falls, NY, increases, the DP concentrations will decrease. If the distance relationship were simply radial dilution, one would expect a model of the form30 C dist 2 ¼ ð4Þ C0 dist0
’ ASSOCIATED CONTENT
bS
Supporting Information. Additional experimental details on instrumental analysis, quality control and quality assurance, materials, and a map of the sampling sites. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
where the zero subscripts represent the initial concentration or distance. Taking the logarithms of the both sides, we get lnðCÞ lnðC0 Þ ¼ 2lnðdistÞ þ 2lnðdist0 Þ
ð5Þ
Thus, a regression of ln(C) vs ln(dist) should be linear with a slope of 2. In previous studies, of various compounds in tree bark, the value of this slope has been significantly higher than 2. For DP concentrations measured as a function of distance from Niagara Falls, NY, this slope was 1.33 ( 0.11;14 for PBDE concentrations measured as a function of distance from southern Arkansas, this slope was 1.81 ( 0.24;31 and for toxaphene concentrations measured as a function of distance from Memphis, TN, this slope was 1.01 ( 0.15.30 The regressions for the atmospheric DP concentrations reported here (see a4 in Table 2) gave an average slope of about 0.5, which was significantly higher than any of these values. This difference might be explained by the different media (air vs tree bark) and by the use of only five different distances in our model as opposed to 2040 distances in the tree bark studies. Nevertheless, distance is still an important factor in explaining the decrease in DP concentrations; for example, doubling the distance from Niagara Falls would decrease these concentrations by about 30%. DP Isomer Profiles. The fraction of the anti-DP isomer (fanti) was calculated as the ratio of the concentration of the anti-DP isomer divided by the sum of the concentrations of the syn- plus anti-DP isomers in the gas, particle, and precipitation phases. It is known that fanti is about 0.75 in the commercial product from OxyChem.1 Figure 3 shows the average fanti values as a function of distance from Niagara Falls for the gas, particle, and precipitation phases. Only the particle phase regression shows a significant relationship between distance and fanti. The intercept of this regression is close to the fraction of the anti isomer in the commercial product, but 1000 km away from the source, this fraction has decreased significantly to about 0.50. This observation suggests either the preferential atmospheric degradation of the anti isomer during long-range transport or isomerization of the anti isomer to the syn isomer. M€oller et al. also observed a decrease in fanti with increasing distance from Western European DP sources, which suggests that the syn isomer is more stable toward photodegradation, leading to strereoselective degradation during atmospheric transport.20 In fact, Sverko et al. observed a higher stability of the syn isomer toward photodegradation in a laboratory study.32 In the vapor and precipitation phases, the relationship of fanti with the distance from Niagara Falls is not statistically significant, indicating that in these phases, these two isomers have similar atmospheric persistences. This is consistent with the findings of Venier and Hites3 and Qui and Hites,14 who reported a similar atmospheric fate for the two DP isomers in the atmosphere and in tree bark.
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