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Atmospheric Environment 42 (2008) 7737–7745

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Atmospheric Environment

journal homepage: www.elsevier .com/locate/atmosenv

Organochlorine pesticides in soils and air of southern Mexico:Chemical profiles and potential for soil emissions

Fiona Wong a,b, Henry A. Alegria c, Liisa M. Jantunen d, Terry F. Bidleman d,*,Miguel Salvador-Figueroa e, Gerardo Gold-Bouchot f, Victor Ceja-Moreno f,Stefan M. Waliszewski g, Raul Infanzon g

a Science and Technology Branch, Environment Canada, Toronto, 4905 Dufferin Street, Toronto, ON M3H 5T4, Canadab Department of Chemistry, University of Toronto at Scarborough, 1265 Military Trail, Toronto, ON M1C 1A4, Canadac Department of Environmental Science, Policy and Geography, University of South Florida St. Petersburg, 140 7th Ave. S., St. Petersburg, FL 33701, USAd Centre for Atmospheric Research Experiments, Science and Technology Branch, Environment, Canada, 6248 Eighth Line, Egbert, ON L0L 1N0, Canadae Departamento de Biotecnologıa, Facultad de Ciencias Quımicas, Universidad Autonoma de Chiapas, Carretera a Puerto Madero, Km. 2,Tapachula, Chiapas, Mexicof Centro de Investigacion y de Estudios Avanzados del IPN, Merida, Yucatan, 97310, Mexicog Instituto de Medicina Forense, Universidad de Veracruz, SS. Juan Pablo II s/n, Boca del Rıo, Veracruz, Mexico

a r t i c l e i n f o

Article history:Received 29 October 2007Received in revised form 1 May 2008Accepted 6 May 2008

Keywords:Soil–air exchangeOrganochlorine pesticidesFugacityChiralEnantioselective degradation

* Corresponding author. Tel.: þ1 705 458 3322; faE-mail address: terry.bidleman@ec.gc.ca (T.F. Bid

1352-2310/$ – see front matter Crown Copyright �doi:10.1016/j.atmosenv.2008.05.028

a b s t r a c t

The extent of organochlorine pesticides (OCs) contamination in southern Mexico wasinvestigated in this study. Biweekly air samplings were carried out in two sites in thestate of Chiapas (during 2002–2003), and one in each state of Veracruz and Tabasco(during 2003–2004). Corresponding to the air sampling locations, soil samples werealso collected to gauge the soil–air exchange of OCs in the region.

PDDTs in soils

ranged from 0.057 to 360 ng g�1 whereas those in air ranged from 240 to 2400 pg m�3.DDT and metabolite DDE were expressed as fractional values, FDDTe¼ p,p0-DDT/(p,p0-DDTþ p,p0-DDE) and FDDTo¼ p,p0-DDT/(p,p0-DDTþ o,p0-DDT). FDDTe in soils ranged from0.30 to 0.69 while those in air ranged from 0.45 to 0.84. FDDTe in air at a farm in Chiapas(0.84) was closer to that of technical DDT (0.95) which is suggestive of fresh DDT input.Enantiomer fractions (EF) of o,p0-DDT in air were racemic at all locations (0.500–0.504).However, nonracemic o,p0-DDT was seen in the soils (EFs¼ 0.456–0.647). Fugacities ofOCs in soil (fs) and air (fa) were calculated, and the fugacity fraction, ff¼ fs/(fsþ fa) ofDDTs ranged from 0.013 to 0.97 which indicated a mix of net deposition (ff< 0.5) andvolatilization (ff> 0.5) from soil among the sites. It is suggested that DDTs in Mexicoair are due to a combination of ongoing regional usage and re-emission of old DDT resi-dues from soils. Total toxaphene in soils ranged from 0.066 to 69 ng g�1 while levels inair ranged from 6.2 to 230 pg m�3. Chromatographic profiles of toxaphenes in both airand soil showed depletion of Parlar congeners 39 and 42. Fugacity fractions of toxaphenewere within the equilibrium range or above the upper equilibrium threshold boundary.These findings suggested that soil emission of old residues is the main source of toxa-phenes to the atmosphere. Results from this study provide baseline data for establishinga long-term OC monitoring program in Mexico.

Crown Copyright � 2008 Published by Elsevier Ltd. All rights reserved.

x: þ1 705 458 3301.leman).

2008 Published by Elsevier Ltd. All rights reserved.

F. Wong et al. / Atmospheric Environment 42 (2008) 7737–77457738

1. Introduction

Mexico is known for its long history of organochlorinepesticides (OCs) usage in malaria control and agriculture.During 1974–1991, 60,000 tons of OCs were sprayed, andDDT accounted for w10% of these products (Lopez-Carrilloet al., 1996). Among Latin American countries, Mexico hadthe highest OC consumption (Lopez-Carrillo et al., 1996)and has stopped using DDT for malaria control since 2000(Anonymous, 2001) and chlordane since 2003 (Moody,2003) under North American Regional Action Plans initi-ated by the North American Commission for EnvironmentalCooperation (NACEC 1997a,b). However, OC levels in theblood of Mexican populations continue to be high. It isreported that the sum of DDT compounds (

PDDT) in Mexi-

cans was 3–18 times higher than in Swedes or Green-landers (Waliszewski et al., 2004a). Work carried out insouthern Mexico (Alegria et al., 2006, in press; Shen et al.,2005) showed that

PDDT and toxaphene concentrations

in ambient air were 1–2 orders of magnitude above levelsin the Laurentian Great Lakes and arctic regions.

The high levels of OCs observed may be due to ongoingusage as well as emission of old residues from soil. Soils arean important sink and source for persistent organic pollut-ants to the atmosphere. Dalla Villa et al. (2006) monitoredDDT concentrations in surface and deep soils of Brazil, andfound that 50% of the p,p0-DDT dissipated over time butthere was no increase of the degradation product, p,p0-DDEin the surface soils. The authors suggested that p,p0-DDTdissipated faster than its degradation rate and that volatil-ization was the dominant pathway. Several soil–airexchange studies have been carried out in the U.S. and Can-ada, where most OCs have been banned for decades. Inves-tigations at farms in Ontario (Kurt-Karakus et al., 2006;Meijer et al., 2003), British Columbia (Bidleman et al.,2006), California (Spencer et al., 1996) and the southernU.S. (Bidleman and Leone, 2004a,b), have demonstratedthe emissions of ‘‘legacy’’ OCs from soils. Such studieshave not been done in Mexico where DDT and other OCswere only recently deregistered. Dissipation rates of OCsfrom tropical soils are often faster than in temperateclimates, due to increased volatility and degradation rates(Khan, 1994). Lalah et al. (2001) reported that DDT half-lives in tropical soils of Kenya ranged from 65 to 250 d,

Table 1Summary of soil sampling locations and soil organic carbon content

Location Description Latitude (north),longitude (west)

Tapachula (TP) farm Low altitude farm 14.89, �92.29TP cemetery Cemetery 14.89, �92.29TP park City park 14.89, �92.29Chiapas mountain farm High altitude

organic farm15.11, �92.24

Chiapas high altitudebackground

Non-urban,non-farm site

15.11, �92.24

Chiapas low altitudebackground

Non-urban,non-farm site

14.63, �92.24

Veracruz Residential andurban sites

19.18, �96.17

Tabasco Near a fish farm,rural site

17.86, �91.79

compared to years to decades in California (Spencer et al.,1996), the Netherlands (Martijn et al., 1993) and New Zea-land (Boul et al., 1994). Half-lives for p,p0-DDE due to vola-tilization alone from tilled and untilled soil ranged from 11to 80 yr in southern U.S. soils (Scholtz and Bidleman, 2007)and 200 yr was estimated for the volatilization half-lifeofP

DDT from a high organic matter soil in Ontario(Kurt-Karakus et al., 2006).

This is the second of two papers that gauges the extentof OCs contamination in southern Mexico. In the first paper,OCs in the ambient air of southern Mexico were presentedfrom an air sampling campaign conducted between 2002and 2004 (Alegria et al., in press). Here, we report theOCs in soils of southern Mexico, chemical profiles in airand soils, and the potential for the soils to act as a sourceof OCs to the atmosphere.

2. Methods

2.1. Soil and air sampling

Soil samples (n¼ 46) were collected at five locations insouthern Mexico during 2003–2006. The locations were:(i) the city of Tapachula (TP), Chiapas at a cemetery(n¼ 1), a park (n¼ 1), and a banana farm outside of Tapa-chula (n¼ 5); (ii) an organic coffee farm (n¼ 9) in themountain ranges of Chiapas; (iii) Chiapas background,a combination of soils from the high (n¼ 5) and low(n¼ 6) altitude rural areas of Chiapas; (iv) the city of Vera-cruz, which includes two residential (n¼ 9), a roadside(n¼ 3) and a beach (n¼ 4) site and (v) a fish farm in therural area of Tabasco (n¼ 3). Each sample represented3–4 grab samples that were collected at the soil surface(0–10 cm). These grab samples were pooled and sievedthrough a 2-mm stainless steel mesh to provide a homoge-neous mixture. Portions were dried at 105 �C for 48 h todetermine the dry weight. Organic carbon was determinedby combustion and measurement of evolved CO2 after acid-ification to remove carbonates (Chemisar Laboratories,Guelph, ON, Canada). A summary of the sampling locationsand organic carbon content of the samples is presented inTable 1.

An air sampling survey was carried out during 2002–2004. Biweekly air samples were collected over 24-h

No. of samples Sample ID # Organic carbon content (%)

5 1–5 0.8–1.81 11 1.31 12 2.39 6–8, 13–18 2.8–7.2

5 9–10, 19–21 2.4–4.8

6 22–27 1.0–2.1

16 28–43 0.5–1.9

3 44–46 3.9–6.4

F. Wong et al. / Atmospheric Environment 42 (2008) 7737–7745 7739

periods at: (a) a private residence in suburban Tapachula(n¼ 14); (b) at the Chiapas mountain farm (n¼ 19), soilsampling location (ii) (Soil#6–8, 13–18); (c) Veracruz City(n¼ 20), at one of the residential soil sampling sites(Soil#28–32); (d) Tabasco (n¼ 14), at the soil sampling loca-tion (Soil#44–46). Air samples were collected by drawing airthrough a glass fiber filter followed by polyurethane foam(PUF) plugs. Details of the air sampling campaign aredescribed in Alegria et al. (in press).

2.2. Extraction and cleanup

Soil samples of 30 g were spiked with 20 ng [2H6]-a-hexachlorocyclohexane (HCH), 20 ng [13C10]-heptachlorexo-epoxide (HEPX), 20 ng [13C10]-trans-nonachlor (TN),20 ng [13C12]-dieldrin (DIEL), and 100 ng [2H8]-p,p0-DDT.Granular anhydrous sodium sulfate was mixed thoroughlyinto the sample to remove water. Extraction was achievedby Soxhlet apparatus using w400 mL dichloromethane(DCM) for w18 h (Harner et al., 1999). Blanks (n¼ 6) wererun by extracting sodium sulfate and treating it as a sample.The extracts were concentrated to 1 mL by using a rotaryevaporator and blowing down with a gentle stream ofnitrogen and solvent exchanging to isooctane. Extractionof air samples, and cleanup and quantitative analysis ofair and soil extracts, followed procedures in Alegria et al.(in press).

2.3. Chiral analysis

Chiral analysis of trans- and cis-chlordanes (TC, CC) ando,p0-DDT in soil and air samples was done by gas chroma-tography – electron capture negative ion mass spectro-metry on a MS-Engine using different chiral columns. TCand CC were run on a primary column, with confirmationon a secondary column which had different elution charac-teristics (Bidleman et al., 2002). Enantiomers of o,p0-DDTcould only be resolved on the secondary column. Theprimary column was Betadex-120 (BDX, 30 m, 0.25 mmi.d., 0.25 mm film thickness, Supelco Corporation, Belle-fonte, PA, U.S.). The secondary column was BGB-172 (BGB,30 m, 0.25 mm i.d., 0.25 mm film thickness, BGB AnalytikAG, Switzerland). Details of the analysis are described else-where (Kurt-Karakus et al., 2005). Chiral results wereexpressed as the enantiomer fraction, EF¼ peak areas ofthe (þ)/[(þ)þ (�)] enantiomers. EF is considered a superiordescriptor over the enantiomer ratio (De Geus et al., 2000;Harner et al., 2000; Ulrich et al., 2003). A racemic mixturehas an EF¼ 0.500 whereas preferential degradation of the(þ) or (�) enantiomer yields EFs <0.500 and >0.500,respectively.

2.4. Quality control

Instrumental detection limits (IDLs) were estimated byinjecting low concentrations of target analytes until a smallpeak at w3:1 signal:noise ratio was obtained. For soils, IDLsare expressed in ng g�1 considering 30 g of soil extractedand a final sample volume of 1 mL. For toxaphenes, thesample volume was 0.15 mL. The IDL for the HCHs was0.0027 ng g�1; chlordanes and TN 0.001 ng g�1; DIEL

0.027 ng g�1; ENDOs and ESUL 0.001–0.015 ng g�1; DDTs0.008.0–0.11 ng g�1; and

PTOX 0.060 ng g�1. Peaks above

IDLs were not observed in the blanks for most of the OCs,although low levels of ENDO I, II and ESUL were found.The limits of detection (LOD) for the three endosulfans,calculated as the mean blankþ 3� SD, were 0.013, 0.038and 0.013 ng g�1. Recovery percentages for the spikedsurrogates were: [2H6]-a-HCH: 79�10%; [13C10]-HEPX:92�12%; [13C10]-TN: 80� 6%; [13C12]-DIEL: 86�14%;[2H8]-p,p0-DDT: 86�12%.

For air, IDLs and recovery percentages are reported inAlegria et al. (in press).

Ion ratios for each enantiomer peak were required to fallwithinthe 95% confidence interval of standards fora satisfac-tory analysis; otherwise, the result was rejected. Decisionsas to whether a particular sample contained racemic or non-racemic residues were made by determining whether its EFwas significantly different from the mean EF of standards atp< 0.05. Standard EFs� SD were: TC¼ 0.500� 0.002,CC¼ 0.501�0.003, o,p0-DDT¼ 0.501�0.003. The primarycolumn for TC and CC chiral analysis was BDX and thesecondary column was BGB. Confirmation on the BGBcolumn was done for TC (n¼ 29) and CC (n¼ 23). Theaverage difference in EFs determined by the two columnswas 0.12% for TC and 0.32% for CC.

3. Results and discussion

3.1. OC residues in soils and air

Soils: concentration of OC (ng g�1, dry weight) are givenin Supporting Information 1 (SI-1) as the arithmeticmean (AM) and the number of positive (þ)/total samples(N). Results are grouped according to the geographicregion, except for Tapachula (TP), where results are pre-sented for individual sampling sites (TP farm, TP cemeteryand TP park). The dominant OCs detected were

PDDTs

(sum of o,p0-DDE, p,p0-DDE, o,p0-DDD, p,p0-DDD, o,p0-DDT,p,p0-DDT) and

PTOX, which together accounted for 83–

100% of the total OCs measured. DDTs were the most abun-dant OCs with the highest frequency of detection (44 out of46 samples). There was great variability among the sites,with

PDDT ranging from 0.057 to 360 ng g�1. Highest

PDDTs were found at TP cemetery (360 ng g�1), which

were 1–3 orders of magnitude greater than the other sites.The

PDDTs at the two farms – Chiapas mountain farm

(19 ng g�1) and TP farm (14 ng g�1) were next in abun-dances. The urban sites – TP park (5.1 ng g�1) and Veracruz(6.5 ng g�1) had intermediate levels of

PDDTs. The

PDDTs

in the Chiapas high altitude background (0.36 ng g�1) andTabasco (0.17 ng g�1) soils were one magnitude lowerthan the other sites, and the least were at the Chiapaslow altitude background site (0.057 ng g�1). Interestingly,the high

PDDTs in TP cemetery corresponded to the find-

ings of Harner et al. (1999) who reported OCs from soils oftwo Alabama cemeteries that were about the same orhigher than the farm soils in the region.

TheP

DDTs in the Chiapas mountain farm soils variedgreatly, with a high relative standard deviation (184%).This farm occupies a large area of over 800 hectares andthe nine samples taken were spread out across this

F. Wong et al. / Atmospheric Environment 42 (2008) 7737–77457740

area. DDTs in Chiapas high altitude background soils weresix times greater than Chiapas low altitude backgroundsoils. During this study, higher DDT concentrations in airwere found at the Chiapas mountain site than in TP (Ale-gria et al., in press). Therefore the high altitude area ofChiapas may have received a greater amount of DDTfrom atmospheric deposition than the low altitude region.The

PDDTs in Veracruz soils also had high relative stan-

dard deviation (166%) due to the low levels ofP

DDTsdetected in the Veracruz beach (Soil#36–39, average0.014 ng g�1) and roadside (Soil#33–35, average0.105 ng g�1) samples. The levels in the Veracruz beachsoil were lower than in the Chiapas low altitude back-ground soils (0.057 ng g�1) and they may reflect back-ground levels in Veracruz.

Herrera-Portugal et al. (2005) reported that soils froma DDT-contaminated area in Chiapas contained 350–117,00 ng g�1 (averaged 4760� 3500 ng g�1) of

PDDTs.

These levels are much higher than those in this study,which has an overall range from 0.057 to 360 ng g�1 ofP

DDTs. The residential soils from the Herrera-Portugalet al. study may have been affected by use of DDT formalaria control. The same authors also found the meanblood DDT concentration of the children living in thisarea were w7 times greater than those living in an areawith low DDT exposure where DDTs in soils were w20times lower (300–840 ng g�1, average 190� 270 ng g�1).This mean value is similar to the

PDDT in the TP cemetery

soil (360 ng g�1, SI-1). It was suggested that the elevatedblood levels were partly due to the greater DDT residues insoils of the highly exposed area.

PDDTs in agricultural soils

of Puebla, Central Mexico averaged 54 ng g�1 and frequencyfor detecting p,p0-DDT and o,p0-DDT was 100% (Waliszewskiet al., 2004b). For comparison, the two farms sampled in thisstudy showed mean

PDDT levels of 14–19 ng g�1 (SI-1).

DDTs in soils of Costa Rica were much lower than southernMexico with maximum concentration of 1 ng g�1 p,p0-DDEand undetectable p,p0-DDT (Daly et al., 2007a).

TheP

TOX (sum of 7-Cl, 8-Cl and 9-Cl homologs) was thehighest in Tapachula, with the highest residues found at TPpark (69 ng g�1), followed by TP farm (17 ng g�1) and TPcemetery (15 ng g�1). Next in

PTOX abundance were

samples from the Chiapas mountain farm (1.3 ng g�1), Chia-pas high altitude background (0.96 ng g�1), Veracruz(0.16 ng g�1) and Chiapas low altitude background(0.13 ng g�1). Tabasco contained the lowest residues(0.066 ng g�1). Overall, the

PTOX found in southern Mexico

soils is lower than concentrations reported in southern U.S.soils, which averaged 690 ng g�1 in Alabama, Louisiana andTexas (Bidleman and Leone, 2004b) and 180 ng g�1 in SouthCarolina and Georgia (Kannan et al., 2003).

TheP

ENDOs (sum of ENDO I, ENDO II and ESUL) werefound at low levels in the blanks and only samples withvalues above the LOD were considered. The

PENDOs

were the highest at the Chiapas high altitude backgroundsite (1.1 ng g�1) followed by TP park (0.20 ng g�1). At theother sites,

PENDOs were barely above the LOD, and levels

after subtracting the average blanks were: TP cemetery(0.094 ng g�1), Chiapas mountain farm (0.075 ng g�1),Veracruz (0.041 ng g�1), TP farm (0.032 ng g�1), Tabasco(0.018 ng g�1). The

PENDOs in Chiapas low altitude

background soils were below the LOD. In general, the rangeofP

ENDOs is similar to the range reported in the lowelevation soils collected from Costa Rica (Daly et al., 2007b).

TheP

CHLs (sum of TC, CC and TN) were highest atVeracruz (0.13 ng g�1) and TP park (0.10 ng g�1). Levels atthe other sites were quite uniform, ranging from 0.014 to0.044 ng g�1. DIEL had the greatest concentration at TPpark (0.056 ng g�1), followed by Chiapas mountain farm(0.043 ng g�1) and TP cemetery (0.028 ng g�1). DIEL wasnot detectable in other sites. The

PCHLs in the Mexican

soils were orders of magnitude below levels in the mid-western U.S. (Aigner et al., 1998) and comparable to thelower end of residues in southern U.S. soils (Bidlemanand Leone, 2004a) and Costa Rica soils (Daly et al.,2007a). The

PHCHs (sum of a-HCH and g-HCH) were

more uniformly distributed with the highest level foundin the TP park (0.046 ng g�1) and cemetery (0.045 ng g�1)sites. Levels at the other sites ranged from 0.012 to0.033 ng g�1. The Chiapas low altitude background sitehas no detectable HCHs and no b-HCH was detected inany of the samples. Overall, OCs in soils of the Chiapasmountain farm, Tapachula (including the TP farm, park,and cemetery) and Veracruz were higher than those inChiapas high and low altitude background, and Tabascosites.

Air: atmospheric levels of the OC pesticides are reportedin Alegria et al. (in press) and are summarized here. TheP

DDTs andP

TOX were the most abundant. Highestconcentrations (pg m�3) of

PDDTs were found at the Chia-

pas mountain farm (2400 pg m�3) followed by Veracruz(1200 pg m�3), Tapachula (550 pg m�3) and Tabasco(240 pg m�3). The high concentration of

PDDT at the Chia-

pas mountain farm is consistent with the relatively highconcentrations measured in soils. According to the prop-erty owner of the coffee plantation, there had been nopesticide applications for the past 20 years. Hence, theorigin of the DDTs is unclear. Perhaps, they are due tocurrent usage at surrounding farms or atmospheric trans-port from neighbouring countries. Next in abundance wasP

ENDOs, in the order: Tapachula (340 pg m�3), Chiapasmountain farm (230 pg m�3), Veracruz (98 pg m�3) andTabasco (92 pg m�3). The

PTOX in Tapachula (230 pg m�3)

was much higher than at other stations: Chiapas mountainfarm (41 pg m�3), Veracruz (19 pg m�3) and Tabasco(6.2 pg m�3). The

PCHLs were slightly higher in the two

urban locations, Tapachula (12 pg m�3) and Veracruz(12 pg m�3) than the non-urban ones, Chiapas mountainfarm (5.8 pg m�3) and Tabasco (7.2 pg m�3).

3.2. Profiles of chemical constituents in soil and air

3.2.1. DDTs (p,p,-DDT, p,p,-DDE, and o,p,-DDT)Proportion of DDT congeners are used to gauge the

age and source of the DDT residues. A high proportion ofthe parent compound, p,p0-DDT indicates a fresh signaturewhile a low proportion implies an old residue. There areno definitive guidelines as to what constitutes a highand low ratio, but comparisons can be made among theregions studied here. Relative amounts of DDT congenerswere expressed as fractional values, which are preferredto ratios (Ulrich et al., 2003; De Geus et al., 2000).

F. Wong et al. / Atmospheric Environment 42 (2008) 7737–7745 7741

These are: (i) FDDTe¼ p,p0-DDT/(p,p0-DDTþ p,p0-DDE) and(ii) FDDTo¼ p,p0-DDT/(p,p0-DDTþo,p0-DDT) (Kurt-Karakuset al., 2006). Calculation was performed only when bothspecies in a fraction were detected in the sample.

Fig. 1A shows the FDDTe in soils and air. FDDTe of technicalDDT (0.95) is included for reference, assuming the WorldHealth Organization (W.H.O.) reported composition fortechnical DDT: 77% p,p0-DDT, 15% o,p0-DDT, 4% p,p0-DDE(WHO, 1989). In soils, the highest FDDTe was observed atChiapas high altitude background (0.69� 0.14), followedby Chiapas mountain farm (0.63� 0.17), TP park (0.58)and Veracruz (0.54� 0.05). This indicated that the level ofthe parent compound, p,p0-DDT, was greater than itsmetabolite, p,p0-DDE at the first three sites and approxi-mately equal to p,p0-DDE in Veracruz. Although Chiapashigh altitude background soils had relatively low concen-trations of

PDDT, the proportion of p,p0-DDT was about

the same as at the Chiapas mountain farm. The high valuesof FDDTe in the Chiapas mountain farm and high backgroundsoils are consistent with the dominance of p,p0-DDT in theChiapas mountain farm air (see below). In addition to the

TP farm TPcemetery

TP park Tapachula(TP)

Chiapasmountain

farm

FD

DT

e

0.0

0.2

0.4

0.6

0.8

1.0

Tapachula(TP)

FD

DT

o

0.0

0.2

0.4

0.6

0.8

1.0

TP farm TPcemetery

TP park Chiapasmountain

farm

Soils Air

Soils Air

A

B

Fig. 1. FDDTe and FDDTo in soils and air. (A) FDDTe¼ p,p0-DDT/(p,p0-DDTþ p,p0-DDE). (Bpas low background and Tabasco soils.

higherP

DDT concentrations, this further suggests thatthis area is receiving fresh DDT. FDDTe for TP farm and TPcemetery were lower, with values of 0.30� 0.04 and 0.35,respectively, which indicate a more aged DDT signature atthese sites. No p,p0-DDT was detected in the Tabasco andChiapas low altitude background soils and therefore FDDTe

was not determined. Confounding these interpretations isthe fact that degradation of p,p0-DDT in soils may varygreatly depending on the specific soil type and regionalcondition. For example, FDDTe in 21 farm soils in the mid-western U.S. ranged from 0.32 to 0.87, with a median valueof 0.59 (Aigner et al., 1998). Data for 82 farm soils in thesouthern U.S. showed FDDTe ranging from 0.04 to 0.73with a median of 0.33 (Bidleman and Leone, 2004a; Kannanet al., 2003).

Air samples collected at the four sites differed greatly inconcentrations of

PDDTs and also in the distribution of

DDT congeners. FDDTe at the Chiapas mountain farm sitewas high, averaging 0.83� 0.04 (Fig. 1A). This lies betweenthe FDDTe of technical DDT (0.95) and the predicted FDDTe forvaporized technical DDT (0.74), estimated from the above

Chiapashigh

altitudebkgd

Veracruz Tabasco

Technical DDT

Technical DDT vapour

Chiapaslow

altitudebkgd

ND ND

Technical DDT

TechnicalDDT vapour

ND NDChiapas

highaltitudebkgd

Veracruz TabascoChiapaslow

altitudebkgd

) FDDTo¼ p,p0-DDT/(p,p0-DDTþo,p0-DDT). ND¼ not determined for the Chia-

F. Wong et al. / Atmospheric Environment 42 (2008) 7737–77457742

composition and the ratio of liquid-phase vapour pressuresat 20 �C (Kurt-Karakus et al., 2006). Such a high proportionof p,p0-DDT in air, and high

PDDTs concentrations is

strongly suggestive of local usage or regional transport offresh DDT. The second highest FDDTe was found at theTabasco rural site (0.74� 0.13). Here the FDDTe was alsoclose to the predicted composition of vaporized technicalDDT although concentrations of

PDDTs were the lowest

of all four sites. Tapachula (0.55� 0.08) and Veracruz(0.45� 0.07) had lower FDDTe, which suggest some contri-bution of older residues at these sites. Air samples takenfrom the same site in Tapachula during 2000–2001 hada mean FDDTe of 0.51�0.11 (Alegria et al., 2006), whichagreed with this study.

In locations where p,p0-DDT and p,p0-DDT were measur-able in both soils and air (Tapachula, the high mountainfarm and background sites, and Veracruz), FDDTe in airwas similar to or slightly higher than FDDTe in soils(Fig. 1A). If concentrations of DDTs in air were due solelyto soil emissions, the expected FDDTe in air would besubstantially lower than in soil due to the lower liquid-phase vapour pressure of p,p0-DDT (about 15% of thep,p0-DDE vapour pressure, Kurt-Karakus et al., 2006).Thus, levels and proportions of DDTs in air are probablyinfluenced by both atmospheric transport of fresh DDT aswell as soil emission of old residues, with more or lesscontribution from these source types at the different sites.

FDDTo ranged from 0.77 to 0.91 in soils and 0.76 to 0.88 inair (Fig. 1B). The ranges in both are similar to FDDTo in tech-nical DDT (0.84). FDDTo in technical DDT vapour is muchlower (0.54), due to the lower liquid-phase vapour pressureof p,p0-DDT (27% of the o,p0-DDT vapour pressure,Kurt-Karakus et al., 2006). If soil emissions were drivingair concentrations of o,p0-DDT in air, the expected FDDTo inair would be closer to 0.54, since FDDTo in soils and technicalDDT were similar. Again, this suggests that both fresh DDTusage and soil emissions are responsible for observed airconcentrations and profiles. The above conclusions are

Toxaphene

0

0.5

1

1.5

2

2.5

3

Parlar 26 Parlar 39 Parlar 42

% n

orm

alized

to

P

arlar 40+

41

Technical standard

Soils

Air a

Fig. 2. Proportions of toxaphene congeners (26, 39, 42, 44, 50, 63) normalized to Paronly (Table 1).

based on the W.H.O. composition of technical DDT (seeabove) and ratios of liquid-phase vapour pressures (Kurt-Karakus et al., 2006); however, technical DDT with quitedifferent percentages of o,p0-DDT have been reported(Metcalf, 1955). Mexico manufactured DDT, but the compo-sition of the technical product is not known.

3.2.2. ToxaphenesProfiles of four octachlorobornanes, 8-Cl (Parlar conge-

ners 26, 39, 42 and 44) and two nonachlorobornanes(Parlar 50 and 63) were normalized to the total amountof Parlar 40þ 41 (which were only partly resolved). Fig. 2compares the congener proportions in air (Tapachula andChiapas only) and all soils to the technical standard. Theproportions of Parlar 26, 42, 44 and 50 in technical toxa-phene found in this study agreed well with those reportedfrom an air intercalibration study involving 10 laboratories(Bidleman et al., 2004). Air samples showed depletion ofParlar 39, 42 and 63 and enrichment of Parlar 44, whilethe proportions of Parlar 26 and 50 in air were similar tothose in technical toxaphene. Compared to technical toxa-phene, soils showed depletion of Parlar 26, 39 and 42,about the same proportion of Parlar 44, and enrichmentof Parlar 50 and 63. Altered profiles in soil relative to tech-nical toxaphene are due to differences in volatilities andstabilities of the congeners. Comparing the two most stablecongeners, Parlar 26 has a liquid-phase vapour pressureabout three times higher than Parlar 50 (Bidleman andLeone, 2004b), and for this reason Parlar 50 may preferen-tially accumulate in soils. This argument may also apply toParlar 63, which has a vapour pressure about six timeslower than Parlar 26. Parlar 39, 42 and 44 have similarvapour pressures, but 39 and 42 are more labile and subjectto degradation. This probably explains their depletion insoil, and through volatilization, also in air. Similar profileshave been noted previously in air and soils of southernU.S., and the log soil/air concentration ratio (log Q) wasinversely correlated with the log liquid-phase vapour

Congeners

Parlar 44 Parlar 50 Parlar 63 b

lar (40þ 41). a, Chiapas and Tapachula air samples only; b, soil samples#1–12

F. Wong et al. / Atmospheric Environment 42 (2008) 7737–7745 7743

pressure (log PL, Pa) of the congener (Bidleman and Leone,2004b). For the six congeners in the Mexico soil and airsamples, log Q was also inversely correlated with log PL

with r2¼ 0.87. Hence, it is concluded that toxaphenes inair of southern Mexico are largely due to soil emissionand not current usage.

3.2.3. ChlordanesRatios of TC/CC for air samples at the four sites were as

follows: Tapachula: 1.2� 0.2, Veracruz: 1.2� 0.2, Chiapasmountain farm: 0.9� 0.3 and Tabasco: 0.8� 0.1. The TC/CCratio in technical chlordane is w1.2, and when weightedfor the slightly higher volatility of TC, the predictedratio from vaporization of technical chlordane is 1.6(Jantunen et al., 2000). The TC/CC ratios in the two urbanareas (Tapachula and Veracruz) were closer to the ratioexpected for technical chlordane, whereas those at thetwo non-urban sites (Chiapas mountain farm and Tabasco)showed more depletion of TC. The

PCHL was low in all

the soils and ratios of TC/CC were not obtainable for mostof them due to either TC or CC being below detection. Thosesoils with detectable residues showed a narrow range of TC/CC (0.7–0.8).

3.3. Enantiomer signatures of chiral organochlorine pesticides

EFs of TC, CC and o,p0-DDT were determined for airsamples, but only the EFs of o,p0-DDT are reported for soilsdue to the low concentrations of chlordanes. EFs of TC andCC in all air samples were racemic, with mean values of0.497–0.504 and relative standard deviations of 0.4–1.1%.This indicated recent usage or emissions from former ter-miticide applications. Agricultural soils generally containednonracemic chlordanes (Aigner et al., 1998; Eitzer et al.,2003; Wiberg et al., 2001), whereas chlordanes in theindoor air of termiticide-treated houses were racemic(Jantunen et al., 2000; Leone et al., 2001). Chlordanes inambient air of the Great Lakes region tend to be nonracemicin rural areas (Gouin et al., 2007; Shen et al., 2005; Ulrichand Hites, 1998; Bidleman and Falconer, 1999) and closerto racemic in the city of Toronto (Shen et al., 2005).Similarly, racemic (Jantunen et al., 2000) and nonracemic(Shen et al., 2005; Venier and Hites, 2007) chlordaneshave been reported in air of the southern U.S. The differen-tiation seems to depend on urban (termiticide) vs. rural(soil emission) sources. Within the Great Lakes region,more nearly racemic chlordanes were found in the city ofToronto than in rural areas, probably due to former termi-ticide usage (Gouin et al., 2007; Shen et al., 2005). Forcomparison, EFs of TC in air and soils from Costa Ricaaveraged 0.488� 0.009 (n¼ 9) and 0.498� 0.012 (n¼ 4);EFs of CC in air and soil averaged 0.513� 0.014 (n¼ 6)and 0.501�0.001 (n¼ 2) (Daly et al., 2007a).

For o,p0-DDT, EFs in all air samples were racemic, withmean values 0.500–0.504, which agrees with a previousreport of racemic o,p0-DDT in air samples from Tapachulaand Chetumal, Mexico, and Belmopan, Belize (Shenet al., 2005). The same workers noted a tendency for o,p0-DDT to be nonracemic, with depletion of the (þ) enantiomer(EFs< 0.500) at stations running from east to west along theCanada–U.S. border. However, the EFs of o,p0-DDT were only

determined at five of those stations, so it is difficult to gener-alize about EFs of o,p0-DDT in U.S. and Canadian air.

EFs of o,p0-DDT in most soil samples were nonracemic(with mean values 0.456–0.647), even though mean valuesat a particular site were racemic in some cases (Chiapasmountain farm and Veracruz, SI-2). It is known that degrada-tion of o,p0-DDT in soils is ambivalent, with residues that canbe racemic or depleted in the (þ) or (�) enantiomers. Thishas been reported in studies from the midwest (Aigneret al., 1998) and southern (Wiberg et al., 2001) U.S. and inbackground soils from 32 countries (Kurt-Karakus et al.,2005). Such ambivalence was seen in the Mexican soils,where EFs ranged on either side of 0.500 (SI-2). As themean value of EF does not necessarily reflect the extent ofdegradation, deviation from racemic value (DEVrac) is alsoreported. DEVrac is defined as the absolute value of thedifference between 0.500 and sample0s EF (Kurt-Karakuset al., 2005). SI-2 shows that Chiapas mountain farm, Chiapashigh altitude background and TP cemetery have the smallestDEVrac values (0.011–0.019), which suggest they have theleast enantioselective degradation of o,p0-DDT. DEVrac wasgreater for TP park, TP farm and Veracruz, (0.044–0.147)are greater, which indicates greater enantioselective degra-dation. A lower DEVrac for a soil could be interpreted asindicative of fresher DDT, although it may simply be due tolack of enantioselective microbial degradation. Bidlemanand Leone (2004a) found a correlation between the EFs ofo,p0-DDT in soils of the southern U.S. and in air samples taken40 cm above those soils. Such a correlation was not foundhere, and several interpretations are possible. (i) The airsamples were taken at 3–15 m above ground (Alegria et al.,2006) and were probably influenced more by the regionalconcentrations and EFs. (ii) The ambivalence in o,p0-DDTenantiomer degradation preference in soils may lead toemissions with different EFs, which when combined resultin near-racemic values. (iii) Regional soil emissions do notconstitute a sufficiently strong source to influence the atmo-spheric concentrations and EFs of o,p0-DDT.

3.4. Soil–air exchange

Fugacities (f, Pa) in soil and air were estimated for DDTsand toxaphenes using the method described in Daly et al.(2007a). These calculations were made only when thecompound was detectable in both soils and air. Octanol–air partition coefficients (KOA) as functions of temperaturewere taken from Shoeib and Harner (2002), and the drysolid density was assumed to be 2650 kg m�3 (Daly et al.,2007a). Results were expressed as fugacity fraction (ff)which equals fs/(fsþ fa). ff¼ 0.500 indicates that thecompound is at soil–air equilibrium. ff> 0.500 indicatesnet volatilization from soil to air, and <0.500 indicatesnet deposition from air to soil.

It is noted that only the gas phase chemicals are consid-ered for the air–soil exchange, but here the air concentra-tion includes the particle phase as the filter and the PUFwere extracted together. The overall effect should beminimal as it is predicted that only 0.01–7.3% of the OCpesticides are associated with the particle phase at 25 �C(Shoeib and Harner, 2002), and all of our sampling locationshad mean annual temperatures ranging from 23 to 30 �C.

0.0

0.2

0.4

0.6

0.8

1.0

TP farm TPcemetery

TP park Chiapasmountain

farm

Chiapashigh altitude

bkgd

Chiapaslow altitude

bkgd

Veracruz Tabasco

Fu

gacit

y F

ra

ctio

n (

ff) Equilibrium 0.15<ff<0.85

Net Depositionff<0.15

Net Volatilization ff>0.85Mean DDT Toxaphene

Fig. 3. Fugacity fractions (ff) ofP

DDTs andP

TOX; ff¼ fsoil/(fsoilþ fair). The ff of DDTs are means of p,p0-DDT, p,p0-DDE, p,p0-DDD and o,p0-DDT. The ff of DDTs atTabasco and Chiapas low altitude background site are represented by p,p0-DDE only.

F. Wong et al. / Atmospheric Environment 42 (2008) 7737–77457744

DDTs: Fig. 3 presents the ff for DDTs andP

TOX. The ff offour frequently detected DDT compounds (p,p0-DDT, p,p0-DDE, p,p0-DDD and o,p0-DDT) were calculated individually,and the ff in Fig. 3 represent the mean of these four. Vera-cruz soils are represented by the two residential sites asDDTs in the other Veracruz sites were non-detectable.The ff of DDTs at Chiapas mountain farm, Chiapas highand low altitude background, and Tabasco ranged from0.006 to 0.092, which indicates there was net depositionfrom air to the soils. Although ff< 0.500 suggests net depo-sition, an error analysis by Daly et al. (2007a) indicated thatff in the window of 0.500� 0.350 may not representa significant departure from equilibrium. The ff which fallwithin this range are at TP farm (0.514), TP park (0.277)and Veracruz (0.306). TP cemetery had the highestff (0.968), well above the equilibrium threshold of 0.85.This implies that net volatilization was taking place. Sinceair concentrations at the cemetery are assumed to be thesame as in the nearby residential area where the airsamples were collected (Alegria et al., in press), ff at thecemetery does not provide unequivocal evidence of soilvolatilization as air concentrations of DDTs may not beuniform throughout the area. The great variability in ff ofDDTs suggests that the soil and air were generally indisequilibrium. While some regions were undergoing netdeposition, others showed net volatilization.

Toxaphenes: the ff of toxaphenes at the Chiapas high andlow altitude background sites are below 0.15 which indi-cate there is net deposition. ff at all other sites were eitherwithin the equilibrium range or above the upper equilib-rium threshold boundary of 0.85. This suggests theirtendencies to volatilize from soils and consistent with thetoxaphene profiles discussed above which point to soilemission as the source of toxaphenes to the ambient air.

4. Conclusions

Organochlorine pesticides (OCs) have shown wide-spread occurrence in soils and air of southern Mexico.The

PDDT residues in soils were low to moderate at

the sites investigated here, but much higher levelshave been reported in malarious areas of Chiapas(Herrera-Portugal et al., 2005). Soil–air exchange ofP

DDT at our sampling sites indicated net depositionin some cases and equilibrium or net volatilization inothers. Proportions of p,p0-DDT, o,p0-DDT and p,p0-DDEvaried among the sites and were consistent with a mixof fresh emissions and revolatilization from soils. Profilesof toxaphene components in air and soil showed deple-tion of two labile congeners, suggesting old residuesrather than current usage. Low concentrations of chlor-danes in soils, racemic composition of TC and CC in air,and TC/CC ratios in urban air close to those in technicalchlordane all point to ongoing chlordane usage or evapo-ration of residues from termiticide applications ratherthan soil emissions. This study provides some baselinedata of OCs in southern Mexico. Further investigationswhich span the entire country would better characterizethe spatial variability, as well as assess the role of tropicalsoils in the recycling of OCs in the environment.

Acknowledgement

Funding for this study was provided through a grant(GL975592-01-0) from the U.S. EPA Great Lakes NationalProgram Office (Melissa Hulting, Program Officer, GrantGL975592). Travel support to Miguel Salvador-Figueroaand Victor Ceja-Moreno to visit Environment Canada labo-ratories was provided by the U.S. EPA and the NACEC. Wethank the helpful comments from Frank Wania, JianminMa and Tom Harner. We thank the students of Prof. Salva-dor-Figueroa for assistance and we are grateful to the farmowners for access to their properties.

Appendix. Supporting information

Supplementary data associated with this article canbe found, in the online version, at doi:10.1016/j.atmosenv.2008.05.028.

F. Wong et al. / Atmospheric Environment 42 (2008) 7737–7745 7745

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