- Email: [email protected]
Detection of monomethylarsenic compounds originating from pesticide in airborne particulate matter sampled in an agricultural area in Japan
&,m,,k,;r
,%womm
Vol
21. No. I. pp. 185-W
ooo4-69al/87 t3.OO+o.M) F’qamon Journals Ltd.
1987.
Printed in Great Britain.
DETECTION OF MONOMETHYLARSENIC COMPOUNDS ORIGINATING FROM PESTICIDE IN AIRBORNE PARTICULATE MATTER SAMPLED IN AN AGRICULTURAL AREA IN JAPAN HITOSHI
MUKAI
and YOSHINARIAMBE
Chemistry and Physics Division, National Institute for Environmental Studies, Yatabe Tsukuba, Ibaraki, 305 Japan Abstract-Alkylarsenic speciesin airborne particulate matter sampled in an agricultural area in Japan were investigated.The monomethyl form of arsenic, which has not been found so far in the air, was detected in a concentration as much as 1.4 ngm-’ in a sample collected on a sunny summer day. It had a digermt size distribution from that of di- and tri-methyl forms of arsenic. The mean particle diameter containing monomethylarsenic compound was 2-4 pm, while those of the di- and/or t&methyl forms of arsenic were 0.2-0.5 pm. This monomethyl form is thought to originate from the alkylarsenic pesticide spread over rice fields, based on the relation between variation in its concentration and meteorological conditions. Alkylarsenic pesticide appears to be blown up by the wind when the land surface is dry. Further, the methylation of arsenic in nature was found to be influenced by humidity and temperature.
Kev word index: Airborne particulate matter, pesticide, methylarsenic compound, iron methane arsonate, bidmethylation.
be produced under the general conditions of the atmosphere, because the retention time of airborne particulate matter in atmosphere is not long enough for biological degradation to occur. Since iron methane arsonate used as a pesticide in Japan is a monomethylarsenic compound, if this pesticide contaminates the air, it should be detectable as the monomethyl form of As in airborne particulate matter. In this report, to identify the origin of the monomethyl form of As detected in airborne particulate matter, the relationship between its concentration and the meteorological conditions (temperature, sunlight, humidity and wind direction)and the characteristics of its size distribution are discussed. not
INTRODUCTION It is well known that air is contaminated with several volatile pesticides, e.g. DDT, BHC etc. (Stanley et 01.. 1971; Freed et al., 1972). Nonvolatile pesticides such as organic As pesticide may also contaminate the air by drift and/or wind erosion of the land where the pesticide has spread. The organic As pesticide, consisting mainly of iron methane arsonate ((CH,As0&Fe2), is used in Japan to prevent Sheath blight of rice. It is spread over rice fields as powder (0.4 % iron methane arsonate) having a mean particle diameter of 12-13 pm or 22-23 pm. In 1984, about 200 t of this pesticide (corresponding to 800 kg of iron methane arsonate) was spread over this district (Ibaraki prefecture). In this study, speciation of As compounds in airborne particulate matter was carried out and the contribution of alkylarsenic pesticide to As compounds in air was evaluated. Johnson and Braman (1975) reported that trimethyl and dimethyl forms of As were present in the air. They are considered to be produced as trimethylarsine and dimethylarsine gases by biological activity (McBride and Wolfe, 1971; Cox and Alexander, 1973). Trimcthylarsine is partially converted into trimethylarsineoxide and dimethylarsinic acid by oxygen (Parris and Brinckman, 1976) and dimethylarsine is oxidized into dimethylarsinic acid. They are considered to exist as particulate matter in the atmosphere. The monomethyl form of As has not been found so far in the air despite its detection in aquatic environment (Braman and Foreback, 1973). If the monomethyl form of As is produced by the biological degradation of trimethyl or dimethyl forms of As, the monomethyl form of As may 185
EXPERIMENTAL (I) Slunpling Airborne particulate matter was collected with a high volume air sampler (IWO C min - ‘) for 1-3 days on a quartz fiber glter (Pallflex 25OOQAST) The filter was heated to 200°C in a vacuum drying oven for 1 day prior to sampling. A cascade impactor (Sierra Model 218) fitted with glassfiber filters was used to measure the size distribution of As compounds. These samplings were conducted in the summer (June-September 1985) on the roof of a building of the Institute at Tsukuba, surrounded by agricultural fields. (2) Ancrlysis The annlytical principk proposed by Braman et al. (1977) was used for the speciation of alkybrraenic compounds. As alkylarsenic compounds in airborne particulate matter are considered to be acid forms, they were extracted ultrasonitally from the filter with 0.05 k NaOH for 10min. The extract was centrifuged (300 rpm, 10 min). Five ml of the supematant were taken into a reaction vessel with 5 ml of
186
H~TOSHIMUKAI and YOSHINARI A~etz
saturated oxalic acid and 1 ml of 0.02 M EDTA solution as a masking agent of Fe and other elements (Howard and ArbabZavar, 1981). Alkylarsenic compounds were converted into their arsines by 8 ml of 2% NaBH, and the arsines were trapped by a half packed (60/8Omesh glass beads) U tube dipped in liquid nitrogen. Detection of As was made by the atomic absorption system proposed by Ebdon et al. (1982). Arsines released from the U tube by N, carrier (40 ml min _ ‘) were mixed with Hr (30 ml min - ‘) dnd introduced into the center of an alumina tube heated by an air-acetylene flame. The analytical wavelength was 193.7 nm and no background correction was made. A typical chromatogram is illus:rated in Fig. I. The separation between monomethyl- and dimethyl-arsine was good under these analytical conditions. The recovery of alkylarscnic compounds (disodium monomethylarsonate, dimethylarsinicacid)added to the sample filter, was over 92 % and thedetection limits of both compounds wereabout 0.5 ng As with S/N = 2. When trimethylarsine was present, the Dimothylarslne 1 Monomethylarelne AJ
Fig. 1. Typical chromatogram of monomethyl and di-methyl forms of arsenic. Each arsine corresponds to 5 ng As and 10 ng As, respaztively. Peaks a and b are due to inorganic arsenic blank and CO2 contained in reagents, respectively.
TIME
separation of dimethyl- from trimethyl-arsine was not good enough IO determine them individually. Therefore, their peak areas were summed and their amounts described as their summation,assuming the sensitivity of trimethylarscnic IO be the same as that of dimethylarsenic. Thus the measurement values of dimethyl- and trimethyl-arsenic were rathet qualitative. (3) Me)eoroloyicol dota Meteorological measurements (temperature, rain and solar radiation) were carried out every hour. Relative humidity was measured once a day (IO a.m.) near the experimental farm of the institute (Yamaguchi and Fuginuma, 1986). RESULTS AND DISCUSSION (1)
Yuriation
in concentration
o/
alkylarsenic
com-
pounds
Figure 2 shows the concentration of the monomethyl form of As and summation of dimethyl and trimethyl forms of As in airborne particulate matter for I month (June-July 1985) together with mcteorological conditions (rain, radiation and relative humidity). From June to early July, the alkylarsenic pesticide was said to be spread over rice fields, 20-30 km from the sampling site. The monomethyl form of As was detected in airborne particulate matter on sunny days with strong radiation and relatively low humidity over a period from the middle to end of July. A high concentration of monomethylarsenic (1.4 ng As m - ‘) was observed on 16 July and corresponded to 16.7 ppm in airborne particulate matter. As the reported value for all the alkylarsenic compounds in outdoor air by Johnson and Braman (1975) was 0.1-0.9 ng As m _ ‘, this observed value was considered to be relatively high. The monomethyl forms of As were detected at least 10 days after the pesticide was said to be spread. This may mean that the mono-
(DAY)
Fig. 2. Variation in concentrations of monomethyl and, di- and tri-methyl forms of arsenic, and changes of meteorological conditions (mean temperature, rain, solar radiation and relative humidity).
187
Airborne monomethylarsenic compounds methylarsenic compound observed was not that which drifted directly at the time of application, but this is uncertain since we have no information on the exact dates of application by each farmer. On the other hand, dimethyl and trimethyl forms of As were detected in the rainy season (early July) at a concentration up to 0.43 ng Asm-’ in air (about 5.6 ppm in particulate matter), increasing with the mean temperature during the sampling time. In contrast with the monomethyl form of arsenic, their concentrations became very low after the rainy season (15 July) when the relative humidity was low. These differences in behavior between monomethyl- and diand tri-methylarsenic compounds suggest that their origins are different, or that the di- and tri-methylarsenic compounds are decomposed to monomethyl form in the air by sunlight or other factors. (2) Decomposition sunlight
of
alkylursenic
compounds
by
To find out whether dimethylarsenic and monomethylarsenic are decomposed by sunlight, the following experiment was carried out. Ten ng As 10~1 of 1 ppm solutions) of each As compound, dimethylarsinic acid (DMAA) and disodium monomethylarsonate (DSMMA), were put on glass plates and exposed to sunlight for 3 h (daytime) and 48 h (day and night). The exposed As compounds were compared with those placed in the shade. The results are shown in Table 1. Each residual percent was almost 100% within the range of analytical error, suggesting that no degradation by sunlight occurred. Therefore, monomethyl As does not originate from the degradation product of dimethylarsinic acid by sunlight. Although an experiment on the trimethyl form of arsenic such as trimethylarsineoxide was not performed, the decomposition of this form is considered to stop at dimethylarsinic acid, as mentioned above, even if it decomposes in the air by some means. Thus neither of the di- nor tri-methylarsenic compounds appear to be the origin of the monomethyl form of arsenic. Further, monomethylarsenic acid was also stable in sunlight and did not decompose to inorganic As. This suggests that methylated As compounds produced naturally do not contribute very much to the inorganic As fraction in the airborne particulate matter in this case.
(3) Origin oj the monomethyl jorm ol arsenic The size distribution of alkylarsenic compounds was measured with a cascade impactor. Figure 3 shows the size distributions of alkylarsenic compounds in airborne particulate matter collected for two diITerent periods. The sample collected at the end of July had only the monomethyl form of As concentrated in the particles of mean diameter 2-4 pm, while another sample taken at the middle of September had mainly the di- and tri-methyl forms (trimethyl form was dominant) and a small amount of the monomethyl form. The mean diameter of the particle abundant with di- and &methyl forms of As was 0.2-0.5 pm. The reason why di- and tri-methyl forms of As in airborne particulate matter have a fine diameter is that gases of trimethylarsine and/or dimethylarsine produced by biological activity are converted into particles by oxidation in the air. Therefore, the monomethyl form of As, having a different size distribution from di- and tri-methyl form, does not originate from natural sources such as biological methylation, because it would have a similar size distribution to that of di- and tri-methylarsenic if it originated from the degradation products of alkylarsenic compounds or monomethylarsine gas. The coarse diameter of monomethylarsenic is similar to that of soil particles in airborne particulate 1.0
1
I
I
Monomethyl
Monomothyl
Dla.
8/24 9: 10-12: 10 (3 h) S/24 9 : 30-8/26 9: 30 (48 h)
Fig. 3. Difference of the size distributions between monomethyl form, and di- and tri-methyl forms of arsenic. (AW/y) log D, = concentration of arsenic compounds (ng As m -‘) at each diameter.
Residual percent (“/,) DMAA’ DSMMAt 99.1 106 95.6 107
Dimethylarsinic acid. Disodium monomethylarsonate. $Glass plate was covered with aluminium iuii.
l
t
form
(pm)
Table 1. Changes of dimethyl and monomethyl forms of arsenic by sunlight
Exposure time
1 form
101 108 101 101
Place in in in in
the the the the
sun shade2 sun shade$
188
HITOW
NUKAI
and YOSHINARI Alr(aE
matter Wadowaki, 1979), suggesting that it is blown up from the land by wind even though it is not clear whether the pesticide is blown up directly or the contaminated soil is blown up. As mentioned above, monomethyl arsenic was detected on a sunny day when the sunlight was strong and relative humidity low. Thus it is easy for organic As pesticide (iron methane arsonate)spread over fields to be blown up by the wind and contaminate the air when the surface of the land is dried by strong sunlight. Though un~lluted ordinary soil (Takamatsu er ol.. 19823and sea water (Andreae, 1979) also contain mono- and di-methylarsenic compounds, their contributions to airborne particulate matter were considered to be very small compared with the measured values here, because their concentrations in air, derived from the soil and sea water, were estimated to be 0.0005-0.001 ng m-’ at most. Figure 4 shows the relation between wind direction observed for the sampling period and the concentration ofeach Ascompound at that time. In thecase of di- and tri-methyf arsenic, there was no significant relation to wind direction. A high concentration of monomethylarsen~c was observed when the wind blew from the direction of the rice field where iron methane arsonate had been spread. This figure shows that the origin of the monomethyl form of As in airborne particulate matter is apparently alkylarsenic pesticide. The source of di- and tri-methylarsenic appears to
SUM. of Dl- and Trl-methyl forms N
0.4 ng-As/ma
be ubiquitous, in view of the fact that their concentrations do not have much relation to wind direction and are influenced rapidly by temperature, sunlight and relative humidity during the sampling period. The methylation of As in nature is considered to require not only warmth but also moisture (Woolson, 1977). The concentrations of these compounds thus became lower at the end of July even at su~cient~y high tem~rature, since strong sunlight and low humjdity caused the land surface to become dry. Their concentrations in early July were of the same order as those reported by Johnson and Braman (1975), despite the possible effect from the fact that alkylarsenic pesticide spread over rice fields causes an increase in the rate of production of volatile alkylarsines in that area.
CONCLUSION it was found that alkylarsenic
pesticide spread as a
fields 20-30 km from the sampling site was blown up by the wind when the surface of the land was dried by strong sunlight. The concentratjon of the monomethyl form of As in the air was similar to that of di- and tri-methylarsenic produced naturally, or greater, which means that the contribution of this pesticide to airborne particulate matter is important when the geochemical cycle ofarsenic is evaluated. It is not clear from this study whether the spread pesticide itself or contaminated soil is blown up, and thus further study on the mechanism of the movement of the pesticide is necessary. Regarding the methyfation of As in nature, meteorological condition, ~rticularly moisture and temperature were found to be important. sterilizer on rice
Acknowkfnements-The
authors are very grateful to Dr S. Uehiro and Dr T. Yamaguchi for supplyingihe meteoroiogicat data. The helpful suggestionsof Dr Y. Yokouchi, Dr T. Uehiro and Dr M. Morita are gratefully acknowledged. application flelda REFERENCKS Monomethyl form N
1.2 no-Asfrn*~
.
sppllcatlon flrldr
Fig. 4. Relation hc~wccn the concentrations of mcrhylarsenic compounds and rhe wind direction at that time. The wind directions of 24 data are plotted vs the mean concentration of methylarsenic compounds for 24 h. Their concentrations are given by the distance from the center of the circle to the plot. illustrated circles indicate the Iocations of notations of 0.4 and 1.2 ng As m-‘, respectively.The djrecf~onso~~lppli~~llionfields are from SEE to s.
Andreae M. 0. (1979) Arsenic speciation in seawater and interstitial water: the infioenccof hioiopi~illshcmical inlcractions on the chemistry of a lrace element. timnuf. ~c~~n~r. 24,44#52. Braman R. S. and Foreback C. C. (1973) Methy~?ed forms of arsenic in the environment. Science 21, 1247-1249. Braman R. S.. Johnson D. L., Foreback C. C., Ammons J. M. and Bricker J. L, (1977) Separation and determination of nanogram amounts of inorganic arsenic and methybtrsenic compounds. An4lyt. Chem. 49.621-625. Cox D. P. and Alexander M. (1973) Production of trimerhylarsenic gas from various arsenic compounds by three sewage fungi. &t/l. fkir. CoflIum. T0.r. 9. X4 XX. Ebdon L., Ward R. W. and Leathard D. A. (1982) Development and optimiiution of atom cells for sensitive coupled gas chro&tography-flame atomic-absorption soeclrometrv. Anulvsr 107. f29-f43. F&d V. H.: H&e it, .and Schmedding D. (1972) Vaporitation and environ~ntal contaminalion by DDT. ~~i~~~s~e~~‘ 1, 6 I -66. Howard A. CL and Arbab-Zavar M. H. (IYX 1) ~~erminalion
Airborne monomethylarsenic compounds of “Inorganic” arsenic(lll) and arsenic(V), “methyfarsenic” and ‘dimethylarsenic” species by selective hydrid evolution atomjc-absorption spectroscopy. AnoIyst 106, 2 13-220. Johnson D. L. and Braman R. S. (1975) Alkyl- and inorganic arsenic in air samples. Chemosphere 4, 333-338. _ Kadowaki S. (1979) Silicon and aluminum in urban aerosols for characterization of atmospheric soil particles in the Nagoya area. Enuir. Sci. Technof. 13, I130-1133. McBride B. C. and Wolfe R. S. (1971) Biosynthesis of dimethylarsine by Methano~cterjum. Biochem. IO, 4312 4317. ParrisG. E.and Brinckman F. E. (1976) Reaction which relate to environmental mobility of arsenic and antimony. II.
189
Oxidation of trimethylarsine and trimethylstibine. E&r. Sri. Technol. 10, I 128-I 134. Stanley C. W., Barney J. E, Helton M. R. and Yobs A. R. (1971) Measurement of atmospheric level of pesticides. Envir. Sci. Technol. 5, 430-435. Takamatsu T.. Aoki H. and Yoshida T. (1982) Determination of arsenate,.arsenite. monemethylarsonatc, and dimethylarsinate in soil oolluted with arsenic. Soil Sci. 133.239-246. Woolson E. A. (1977) Fate of arsenicals in different environmental substrates. Envir. ~euff~ Perspecr. 19, 73-81. Yamaguchi T. and Fuginuma Y. (1986)Table of meteorological data observed in experimental farm of National Institute for Environmental Studies in 1985 (unpublished data). Personal communication.
,%womm
Vol
21. No. I. pp. 185-W
ooo4-69al/87 t3.OO+o.M) F’qamon Journals Ltd.
1987.
Printed in Great Britain.
DETECTION OF MONOMETHYLARSENIC COMPOUNDS ORIGINATING FROM PESTICIDE IN AIRBORNE PARTICULATE MATTER SAMPLED IN AN AGRICULTURAL AREA IN JAPAN HITOSHI
MUKAI
and YOSHINARIAMBE
Chemistry and Physics Division, National Institute for Environmental Studies, Yatabe Tsukuba, Ibaraki, 305 Japan Abstract-Alkylarsenic speciesin airborne particulate matter sampled in an agricultural area in Japan were investigated.The monomethyl form of arsenic, which has not been found so far in the air, was detected in a concentration as much as 1.4 ngm-’ in a sample collected on a sunny summer day. It had a digermt size distribution from that of di- and tri-methyl forms of arsenic. The mean particle diameter containing monomethylarsenic compound was 2-4 pm, while those of the di- and/or t&methyl forms of arsenic were 0.2-0.5 pm. This monomethyl form is thought to originate from the alkylarsenic pesticide spread over rice fields, based on the relation between variation in its concentration and meteorological conditions. Alkylarsenic pesticide appears to be blown up by the wind when the land surface is dry. Further, the methylation of arsenic in nature was found to be influenced by humidity and temperature.
Kev word index: Airborne particulate matter, pesticide, methylarsenic compound, iron methane arsonate, bidmethylation.
be produced under the general conditions of the atmosphere, because the retention time of airborne particulate matter in atmosphere is not long enough for biological degradation to occur. Since iron methane arsonate used as a pesticide in Japan is a monomethylarsenic compound, if this pesticide contaminates the air, it should be detectable as the monomethyl form of As in airborne particulate matter. In this report, to identify the origin of the monomethyl form of As detected in airborne particulate matter, the relationship between its concentration and the meteorological conditions (temperature, sunlight, humidity and wind direction)and the characteristics of its size distribution are discussed. not
INTRODUCTION It is well known that air is contaminated with several volatile pesticides, e.g. DDT, BHC etc. (Stanley et 01.. 1971; Freed et al., 1972). Nonvolatile pesticides such as organic As pesticide may also contaminate the air by drift and/or wind erosion of the land where the pesticide has spread. The organic As pesticide, consisting mainly of iron methane arsonate ((CH,As0&Fe2), is used in Japan to prevent Sheath blight of rice. It is spread over rice fields as powder (0.4 % iron methane arsonate) having a mean particle diameter of 12-13 pm or 22-23 pm. In 1984, about 200 t of this pesticide (corresponding to 800 kg of iron methane arsonate) was spread over this district (Ibaraki prefecture). In this study, speciation of As compounds in airborne particulate matter was carried out and the contribution of alkylarsenic pesticide to As compounds in air was evaluated. Johnson and Braman (1975) reported that trimethyl and dimethyl forms of As were present in the air. They are considered to be produced as trimethylarsine and dimethylarsine gases by biological activity (McBride and Wolfe, 1971; Cox and Alexander, 1973). Trimcthylarsine is partially converted into trimethylarsineoxide and dimethylarsinic acid by oxygen (Parris and Brinckman, 1976) and dimethylarsine is oxidized into dimethylarsinic acid. They are considered to exist as particulate matter in the atmosphere. The monomethyl form of As has not been found so far in the air despite its detection in aquatic environment (Braman and Foreback, 1973). If the monomethyl form of As is produced by the biological degradation of trimethyl or dimethyl forms of As, the monomethyl form of As may 185
EXPERIMENTAL (I) Slunpling Airborne particulate matter was collected with a high volume air sampler (IWO C min - ‘) for 1-3 days on a quartz fiber glter (Pallflex 25OOQAST) The filter was heated to 200°C in a vacuum drying oven for 1 day prior to sampling. A cascade impactor (Sierra Model 218) fitted with glassfiber filters was used to measure the size distribution of As compounds. These samplings were conducted in the summer (June-September 1985) on the roof of a building of the Institute at Tsukuba, surrounded by agricultural fields. (2) Ancrlysis The annlytical principk proposed by Braman et al. (1977) was used for the speciation of alkybrraenic compounds. As alkylarsenic compounds in airborne particulate matter are considered to be acid forms, they were extracted ultrasonitally from the filter with 0.05 k NaOH for 10min. The extract was centrifuged (300 rpm, 10 min). Five ml of the supematant were taken into a reaction vessel with 5 ml of
186
H~TOSHIMUKAI and YOSHINARI A~etz
saturated oxalic acid and 1 ml of 0.02 M EDTA solution as a masking agent of Fe and other elements (Howard and ArbabZavar, 1981). Alkylarsenic compounds were converted into their arsines by 8 ml of 2% NaBH, and the arsines were trapped by a half packed (60/8Omesh glass beads) U tube dipped in liquid nitrogen. Detection of As was made by the atomic absorption system proposed by Ebdon et al. (1982). Arsines released from the U tube by N, carrier (40 ml min _ ‘) were mixed with Hr (30 ml min - ‘) dnd introduced into the center of an alumina tube heated by an air-acetylene flame. The analytical wavelength was 193.7 nm and no background correction was made. A typical chromatogram is illus:rated in Fig. I. The separation between monomethyl- and dimethyl-arsine was good under these analytical conditions. The recovery of alkylarscnic compounds (disodium monomethylarsonate, dimethylarsinicacid)added to the sample filter, was over 92 % and thedetection limits of both compounds wereabout 0.5 ng As with S/N = 2. When trimethylarsine was present, the Dimothylarslne 1 Monomethylarelne AJ
Fig. 1. Typical chromatogram of monomethyl and di-methyl forms of arsenic. Each arsine corresponds to 5 ng As and 10 ng As, respaztively. Peaks a and b are due to inorganic arsenic blank and CO2 contained in reagents, respectively.
TIME
separation of dimethyl- from trimethyl-arsine was not good enough IO determine them individually. Therefore, their peak areas were summed and their amounts described as their summation,assuming the sensitivity of trimethylarscnic IO be the same as that of dimethylarsenic. Thus the measurement values of dimethyl- and trimethyl-arsenic were rathet qualitative. (3) Me)eoroloyicol dota Meteorological measurements (temperature, rain and solar radiation) were carried out every hour. Relative humidity was measured once a day (IO a.m.) near the experimental farm of the institute (Yamaguchi and Fuginuma, 1986). RESULTS AND DISCUSSION (1)
Yuriation
in concentration
o/
alkylarsenic
com-
pounds
Figure 2 shows the concentration of the monomethyl form of As and summation of dimethyl and trimethyl forms of As in airborne particulate matter for I month (June-July 1985) together with mcteorological conditions (rain, radiation and relative humidity). From June to early July, the alkylarsenic pesticide was said to be spread over rice fields, 20-30 km from the sampling site. The monomethyl form of As was detected in airborne particulate matter on sunny days with strong radiation and relatively low humidity over a period from the middle to end of July. A high concentration of monomethylarsenic (1.4 ng As m - ‘) was observed on 16 July and corresponded to 16.7 ppm in airborne particulate matter. As the reported value for all the alkylarsenic compounds in outdoor air by Johnson and Braman (1975) was 0.1-0.9 ng As m _ ‘, this observed value was considered to be relatively high. The monomethyl forms of As were detected at least 10 days after the pesticide was said to be spread. This may mean that the mono-
(DAY)
Fig. 2. Variation in concentrations of monomethyl and, di- and tri-methyl forms of arsenic, and changes of meteorological conditions (mean temperature, rain, solar radiation and relative humidity).
187
Airborne monomethylarsenic compounds methylarsenic compound observed was not that which drifted directly at the time of application, but this is uncertain since we have no information on the exact dates of application by each farmer. On the other hand, dimethyl and trimethyl forms of As were detected in the rainy season (early July) at a concentration up to 0.43 ng Asm-’ in air (about 5.6 ppm in particulate matter), increasing with the mean temperature during the sampling time. In contrast with the monomethyl form of arsenic, their concentrations became very low after the rainy season (15 July) when the relative humidity was low. These differences in behavior between monomethyl- and diand tri-methylarsenic compounds suggest that their origins are different, or that the di- and tri-methylarsenic compounds are decomposed to monomethyl form in the air by sunlight or other factors. (2) Decomposition sunlight
of
alkylursenic
compounds
by
To find out whether dimethylarsenic and monomethylarsenic are decomposed by sunlight, the following experiment was carried out. Ten ng As 10~1 of 1 ppm solutions) of each As compound, dimethylarsinic acid (DMAA) and disodium monomethylarsonate (DSMMA), were put on glass plates and exposed to sunlight for 3 h (daytime) and 48 h (day and night). The exposed As compounds were compared with those placed in the shade. The results are shown in Table 1. Each residual percent was almost 100% within the range of analytical error, suggesting that no degradation by sunlight occurred. Therefore, monomethyl As does not originate from the degradation product of dimethylarsinic acid by sunlight. Although an experiment on the trimethyl form of arsenic such as trimethylarsineoxide was not performed, the decomposition of this form is considered to stop at dimethylarsinic acid, as mentioned above, even if it decomposes in the air by some means. Thus neither of the di- nor tri-methylarsenic compounds appear to be the origin of the monomethyl form of arsenic. Further, monomethylarsenic acid was also stable in sunlight and did not decompose to inorganic As. This suggests that methylated As compounds produced naturally do not contribute very much to the inorganic As fraction in the airborne particulate matter in this case.
(3) Origin oj the monomethyl jorm ol arsenic The size distribution of alkylarsenic compounds was measured with a cascade impactor. Figure 3 shows the size distributions of alkylarsenic compounds in airborne particulate matter collected for two diITerent periods. The sample collected at the end of July had only the monomethyl form of As concentrated in the particles of mean diameter 2-4 pm, while another sample taken at the middle of September had mainly the di- and tri-methyl forms (trimethyl form was dominant) and a small amount of the monomethyl form. The mean diameter of the particle abundant with di- and &methyl forms of As was 0.2-0.5 pm. The reason why di- and tri-methyl forms of As in airborne particulate matter have a fine diameter is that gases of trimethylarsine and/or dimethylarsine produced by biological activity are converted into particles by oxidation in the air. Therefore, the monomethyl form of As, having a different size distribution from di- and tri-methyl form, does not originate from natural sources such as biological methylation, because it would have a similar size distribution to that of di- and tri-methylarsenic if it originated from the degradation products of alkylarsenic compounds or monomethylarsine gas. The coarse diameter of monomethylarsenic is similar to that of soil particles in airborne particulate 1.0
1
I
I
Monomethyl
Monomothyl
Dla.
8/24 9: 10-12: 10 (3 h) S/24 9 : 30-8/26 9: 30 (48 h)
Fig. 3. Difference of the size distributions between monomethyl form, and di- and tri-methyl forms of arsenic. (AW/y) log D, = concentration of arsenic compounds (ng As m -‘) at each diameter.
Residual percent (“/,) DMAA’ DSMMAt 99.1 106 95.6 107
Dimethylarsinic acid. Disodium monomethylarsonate. $Glass plate was covered with aluminium iuii.
l
t
form
(pm)
Table 1. Changes of dimethyl and monomethyl forms of arsenic by sunlight
Exposure time
1 form
101 108 101 101
Place in in in in
the the the the
sun shade2 sun shade$
188
HITOW
NUKAI
and YOSHINARI Alr(aE
matter Wadowaki, 1979), suggesting that it is blown up from the land by wind even though it is not clear whether the pesticide is blown up directly or the contaminated soil is blown up. As mentioned above, monomethyl arsenic was detected on a sunny day when the sunlight was strong and relative humidity low. Thus it is easy for organic As pesticide (iron methane arsonate)spread over fields to be blown up by the wind and contaminate the air when the surface of the land is dried by strong sunlight. Though un~lluted ordinary soil (Takamatsu er ol.. 19823and sea water (Andreae, 1979) also contain mono- and di-methylarsenic compounds, their contributions to airborne particulate matter were considered to be very small compared with the measured values here, because their concentrations in air, derived from the soil and sea water, were estimated to be 0.0005-0.001 ng m-’ at most. Figure 4 shows the relation between wind direction observed for the sampling period and the concentration ofeach Ascompound at that time. In thecase of di- and tri-methyf arsenic, there was no significant relation to wind direction. A high concentration of monomethylarsen~c was observed when the wind blew from the direction of the rice field where iron methane arsonate had been spread. This figure shows that the origin of the monomethyl form of As in airborne particulate matter is apparently alkylarsenic pesticide. The source of di- and tri-methylarsenic appears to
SUM. of Dl- and Trl-methyl forms N
0.4 ng-As/ma
be ubiquitous, in view of the fact that their concentrations do not have much relation to wind direction and are influenced rapidly by temperature, sunlight and relative humidity during the sampling period. The methylation of As in nature is considered to require not only warmth but also moisture (Woolson, 1977). The concentrations of these compounds thus became lower at the end of July even at su~cient~y high tem~rature, since strong sunlight and low humjdity caused the land surface to become dry. Their concentrations in early July were of the same order as those reported by Johnson and Braman (1975), despite the possible effect from the fact that alkylarsenic pesticide spread over rice fields causes an increase in the rate of production of volatile alkylarsines in that area.
CONCLUSION it was found that alkylarsenic
pesticide spread as a
fields 20-30 km from the sampling site was blown up by the wind when the surface of the land was dried by strong sunlight. The concentratjon of the monomethyl form of As in the air was similar to that of di- and tri-methylarsenic produced naturally, or greater, which means that the contribution of this pesticide to airborne particulate matter is important when the geochemical cycle ofarsenic is evaluated. It is not clear from this study whether the spread pesticide itself or contaminated soil is blown up, and thus further study on the mechanism of the movement of the pesticide is necessary. Regarding the methyfation of As in nature, meteorological condition, ~rticularly moisture and temperature were found to be important. sterilizer on rice
Acknowkfnements-The
authors are very grateful to Dr S. Uehiro and Dr T. Yamaguchi for supplyingihe meteoroiogicat data. The helpful suggestionsof Dr Y. Yokouchi, Dr T. Uehiro and Dr M. Morita are gratefully acknowledged. application flelda REFERENCKS Monomethyl form N
1.2 no-Asfrn*~
.
sppllcatlon flrldr
Fig. 4. Relation hc~wccn the concentrations of mcrhylarsenic compounds and rhe wind direction at that time. The wind directions of 24 data are plotted vs the mean concentration of methylarsenic compounds for 24 h. Their concentrations are given by the distance from the center of the circle to the plot. illustrated circles indicate the Iocations of notations of 0.4 and 1.2 ng As m-‘, respectively.The djrecf~onso~~lppli~~llionfields are from SEE to s.
Andreae M. 0. (1979) Arsenic speciation in seawater and interstitial water: the infioenccof hioiopi~illshcmical inlcractions on the chemistry of a lrace element. timnuf. ~c~~n~r. 24,44#52. Braman R. S. and Foreback C. C. (1973) Methy~?ed forms of arsenic in the environment. Science 21, 1247-1249. Braman R. S.. Johnson D. L., Foreback C. C., Ammons J. M. and Bricker J. L, (1977) Separation and determination of nanogram amounts of inorganic arsenic and methybtrsenic compounds. An4lyt. Chem. 49.621-625. Cox D. P. and Alexander M. (1973) Production of trimerhylarsenic gas from various arsenic compounds by three sewage fungi. &t/l. fkir. CoflIum. T0.r. 9. X4 XX. Ebdon L., Ward R. W. and Leathard D. A. (1982) Development and optimiiution of atom cells for sensitive coupled gas chro&tography-flame atomic-absorption soeclrometrv. Anulvsr 107. f29-f43. F&d V. H.: H&e it, .and Schmedding D. (1972) Vaporitation and environ~ntal contaminalion by DDT. ~~i~~~s~e~~‘ 1, 6 I -66. Howard A. CL and Arbab-Zavar M. H. (IYX 1) ~~erminalion
Airborne monomethylarsenic compounds of “Inorganic” arsenic(lll) and arsenic(V), “methyfarsenic” and ‘dimethylarsenic” species by selective hydrid evolution atomjc-absorption spectroscopy. AnoIyst 106, 2 13-220. Johnson D. L. and Braman R. S. (1975) Alkyl- and inorganic arsenic in air samples. Chemosphere 4, 333-338. _ Kadowaki S. (1979) Silicon and aluminum in urban aerosols for characterization of atmospheric soil particles in the Nagoya area. Enuir. Sci. Technof. 13, I130-1133. McBride B. C. and Wolfe R. S. (1971) Biosynthesis of dimethylarsine by Methano~cterjum. Biochem. IO, 4312 4317. ParrisG. E.and Brinckman F. E. (1976) Reaction which relate to environmental mobility of arsenic and antimony. II.
189
Oxidation of trimethylarsine and trimethylstibine. E&r. Sri. Technol. 10, I 128-I 134. Stanley C. W., Barney J. E, Helton M. R. and Yobs A. R. (1971) Measurement of atmospheric level of pesticides. Envir. Sci. Technol. 5, 430-435. Takamatsu T.. Aoki H. and Yoshida T. (1982) Determination of arsenate,.arsenite. monemethylarsonatc, and dimethylarsinate in soil oolluted with arsenic. Soil Sci. 133.239-246. Woolson E. A. (1977) Fate of arsenicals in different environmental substrates. Envir. ~euff~ Perspecr. 19, 73-81. Yamaguchi T. and Fuginuma Y. (1986)Table of meteorological data observed in experimental farm of National Institute for Environmental Studies in 1985 (unpublished data). Personal communication.