Environ. Sci. Technol. 1988, 22, 517-522
Atmospheric Speciation and Wet Deposition of Alkyllead Compounds Andrew 0. Allen, Mlroslav RadoJevIc,+* end Roy M. Harrison*
Institute of Aerosol Science, Department of Chemistry, University of Essex, Colchester C04 3SQ, Essex, U.K. Concentrations of individual tetra-, tri-, and dialkyllead compounds have been measured in the gas phase and in aerosol and rainwater samples at two sites in eastern England and at one site in western Ireland. The results show a predominance of gas-phase over aerosol-associated species in the air at all sites, and in general a greater abundance of tetraalkyllead than trialkyllead. Washout factors for total alkyllead are lower than for inorganic lead and correlate highly between adjacent urban and semirural sites.
Introduction Alkyllead compounds resulting from the use of tetraalkyllead (R4Pb) as a gasoline additive have been determined in a variety of environmental samples, including air, water, sediment, and fish (1). The major anthropogenic source involves the release of volatile R4Pb species into the atmosphere. About 1% of the organolead content of gasoline is emitted into the atmosphere via the exhaust unchanged, and further emissions include evaporative losses of fuel from fuel tanks and carburetors, evaporation at gasoline stations, and accidental spillages of gasoline as well as other minor sources that involve the use of leaded gaaoline (e.g., chain saws, lawnmowers, and motor boats). A possible environmental source involving alkylation of inorganic lead is also suspected (2). In the environment, R,Pb compounds decompose eventually to inorganic lead via trialkyllead (R3Pb+)and dialkyllead (R2Pb2+)species as intermediates. The mechanisms of decomposition and alkylation reactions are still not fully understood and some of these may be biologically mediated. Environmental pathways of organic lead have recently been reviewed (I), and atmospheric transformations play a prominent role in the biogeochemical cycle of organic lead. The primary pollutant R4Pb species may decompose in the atmosphere to produce vapor-phase R3Pb+and R2Pb2+ species and inorganic lead aerosol, the dominant mechanism being reaction with hydroxyl OH radical (3). R3Pb+ compounds also react with OH radical, but the reaction rates are slower than for the R4Pb - OH reaction, resulting in longer atmospheric lifetimes of R,Pb+ (4).R4Pb,R3Pb+, and R2Pb2+compounds may be scavenged in cloud and raindrops or may react with atmospheric aerosols. Recent measurements of alkyllead compounds in rainwater suggest that wet deposition processes (rainout/washout) may be an important removal mechanism of these pollutants from the atmosphere and a source of these species in surface waters (5-8). Alkyllead species released into the atmosphere may therefore enter into other environmental reservoirs via the hydrological cycle and have been identified at very low levels in potable water (9). Damage to German forests has recently been attributed to R3Pb+ compounds in rainwater (IO),but high concentrations measured in the latter study with a nonspecific biochemical technique have been disputed on the basis of measurements made by using species-specific gas chromatograPresent address: Department of Chemistry, UMIST, Manchester, U.K. 0013-936X/88/0922-0517$01.50/0
phy/atomic absorption spectroscopy (GC/AAS) (7). GC/AAS techniques have been applied in the determination of alkyllead compounds in air, atmospheric aerosol, and rainwater (5,6, 8, 11-14) in an attempt to improve our understanding of the atmospheric cycle of organic lead. R4Pb compounds in air and R4Pb,R3Pb+, and R2Pb2+species in atmospheric aerosol have been determined at urban and rural sites (13),and concentrations of vapor-phase R3Pb+and R2Pb2+compounds have been inferred by difference from total vapor-phase concentrations measured with a nonspecific extraction/graphite furnace atomic absorption spectroscopy (GFAAS) technique. Recently, a GCIAAS technique for gas-phase R3Pb+ and R2Pb2+compounds has been developed and successfully applied to atmospheric samples (14). Here we report simultaneous measurements of R4Pb, R3Pb+,and R2Pb2+compounds in the atmospheric gaseous and aerosol phases and in bulk deposition. Measurements were carried out simultaneously at an urban and a semirural site, and at a remote rural site on the west coast of Ireland for a brief period. In addition to the quantitative physicochemical speciation study of alkyllead in the atmosphere a t these sites, data on the deposition of these species were also obtained.
Experimental Section Alkyllead compounds in the gas phase, atmospheric aerosol, and bulk deposition were determined at urban (Colchester, U.K.) and semirural (Essex University, U.K.) sites and at a rural site, distant from major anthropogenic sources, on the west coast of Ireland (Adrigole, Republic of Ireland). A map of the U.K. and Ireland showing the location of the sampling sites is shown in Figure 1. Samplers were placed on the roofs of Colchester and Essex University libraries, both of which were the highest buildings in the locality. These sites were separated by a distance of 3.6 km. At the rural site, samples were collected in an open field with no major obstructions in the vicinity. Gas-phase R4Pb compounds were sampled at 100 mL m i d with stainless steel tubes (3.25 in. long X 0.2 in. id.) packed with Poropak Q. A filter for removing atmospheric ozone, consisting of a piece of Teflon tubing (2 in. long X 0.2 in. i.d.) packed with 0.25 g of iron(I1) sulfate crystals, was attached to the upstream end of the sampling tube, and a 0.45-km filter was placed upstream of this in order to remove atmospheric aerosol. A GN Instruments twostage concentrator was used to thermally desorb R4Pb compounds into the GC/AAS system. The sampling tube was heated to 150 OC with the carrier gas flowing through at a rate of 140 mL min-l, and the R4Pb species were transferred into a glass-lined stainless steel U tube packed with 0.05 g of ground glass (2501500 pm), which was found to perform better than several other packings that were tested. This U tube was kept at liquid nitrogen temperature during the transfer. After completion of this procedure, the tube was flash-heated to 140 OC, and the R4Pb compounds were swept into the GC/AAS system for analysis. Prior to sampling, each tube was thermally cleaned by flushing with helium for 10 min at 170 OC. A breakthrough volume of 800 L at 20 OC was found for this
0 1988 American Chemical Society
Environ. Scl. Technol., Vol. 22, No. 5, 1988 517
Table I. Detection Limits and Recoveries for Alkyllead Compounds in Environmental Samples Using the GC/AAS Method Employed in This Study air (vapor)n det lim, ng of Pb m-a recovery, %
compd Me4Pb MeaEtPb MezEtzPb MeEt,Pb Et,Pb MesPbt EtsPb’ MezPbZt EtzPb2+
0.14 0.14 0.28 0.49 0.83 0.34 0.76 0.53 1.96
Based on 24-hsample. *Based on
aerosoln det lim,” pg of Pb m-s recovery, %
78 73 76 56 53 78 70 66 46
I 2 3 4
u
LION WALK PRECINCT COLCH E STER UNIVERSITY OF ESSEX WIVENHOE PARK COLCHESTER miles
km
5
I--+-++
10
10 15 20 20
@
30
AORIGOLE COUNTY CORK, IRELAND
Figure 1. Map indicating locations.
sampling method by extrapolation from elevated-temperature measurements made in the laboratory and was confirmed by parallel sampling of the atmosphere over a sebtime interval with absorption tubes pumped at differing flow rates. The technique employed for vapor-phase R4Pb compounds is similar to that originally described by Hewitt and Harrison (11),who reported a collection efficiency of 100% and a breakthrough volume of 89 L on the basis of very conservative assumptions. Gas-phase trialkyllead and dialkyllead compounds were sampled by bubbling filtered (0.45 pm) air at 1L min-l into two bubblers in series, each containing 80 mL of Milli-Q purified water. After sampling was complete, the contents of the two bubblers were combined, transferred into a glass bottle, and extracted into n-hexane in the presence of NaCI and sodium diethyldithiocarbamate (NaDDTC). Further details of this method have recently been published (14). Atmospheric aerosols were sampled at 1 m3 min-l onto GF/A filter papers in a standard Hi-Vol sampler. After sample collection, the filter papers were rolled up, placed 518
93 93 93 93 93 89 100 101 104
-
0.2 0.1 0.2 0.3 0.3 0.3 0.6 0.4 1.0
60 60 60 60 60 102 100 98 102
1-Lsample. miles
@
0.19 0.13 0.22 0.30 0.30 0.27 0.54 0.32 0.86
rainwater det lim,b ng of Pb L-’ recovery, %
Envlron. Scl. Technol., Vol. 22, No. 5, 1988
inside glass tubes together with 30 mL of Milli-Q water, and extracted into n-hexane after addition of NaCl and NaDDTC in a manner reported previously (12, 13). Rainwater samples were collected with glass bulk “funnel in bottle” samplers, which were left in the field until a sufficient sample was collected to enable analysis. These samplers also collect dry-deposited alkyllead vapors and aerosols in addition to the webdeposited species. Sampling bottles were kept in the dark in the field so as to minimize any photochemical decomposition. In the laboratory, samples were extracted into n-hexane in the presence of NaCl and NaDDTC. Extraction was carried out in the sampling bottles because of possible adsorption of R4Pb compounds onto the walls of glass vessels (12),and samples were not filtered because of the possible adsorption of these species on suspended particles (15). Solvent extracts of atmospheric aerosols, rainwater, and vapor-phase trialkyl- and dialkyllead species were separated and alkylated by the addition of propyl magnesium chloride in diethyl ether so as to convert R3Pb+and &Pb2+ to the more volatile R,Pb form suitable for GC/AAS analysis. Details of the propylation technique have been given elsewhere (16). It was also possible to further concentrate the extracts prior to analysis by purging with N2 (16), and this was particularly useful when analyzing samples from less polluted sites. The GC/AAS system, consisting of a specialized silica furnace detector cell that was electrothermally heated, has been outlined in a separate publication (16),and identical instrumentation, operating conditions, and methods of analysis were employed in this work. Typical detection’limits and recovery efficiencies for the individual alkyllead compounds are given in Table I. Correction factors for incomplete recoveries have been used in calculating the environmental concentrations reported in this paper. Repeatabilities between duplicate environmental samples were typically on the order of 7-15% relative standard deviation (rsd) dependent upon compound and concentration. Inorganic lead was determined by extracting the 0.45-pm filter placed upstream of the vapor-phase ionic alkyllead sampling device into nitric acid and analyzing by graphite furnace atomic absorption spectrometry (GFAAS). Inorganic lead in rainwater was determined by GFAAS after filtering the solution through a 0,45-pm filter to remove the large particulate fraction. Detection limits for alkyllead species in the rural samples were lower than those in urban and semirural samples because of greater sample volumes and/or greater concentration of the extract. Deuterium arc background correction was always employed when analyzing environmental samples. Where results are reported for total alkyllead, these have been calculated assuming compounds below the analytical
Table 11. Results of Measurements of Atmospheric Alkyllead Compounds at the Urban Site (Colchester, U.K.) 7 January to 19 February 1986 gas compd MelPb MeaEtPb MezEtzPb MeEt,Pb Et4Pb Me,Pb+ Me2EtPb+ MeEtzPb+ Et,Pb+ MezPbz+ EtzPb" RPb3+ total alkyl lead" inorganic lead alkyl/inorganic Pb, %
aerosol
no. of samples with alkyllead compd detectable*
concn range, ng of Pb m-,
24 9 2 3 5 8 1 0 1 7 3 1 24
0.7-10.9 0.9 3.41 6.70 1.24 0.68 >0.5 1.40 2.80 0.84 0.28 0.92 >0.2 0.60 2.03 0.23 >LO 2.17 3.72 4.96 2.04 1280 0.20 0.20 0.08 0.10 0.12 0.06
Urban, Colchester, U.K.; semirural, Essex University, U.K.; rural, Adrigole, Republic of Ireland. Me4Pb, Me3EtPb, MepEtzPb, MeEt3Pb, and Et4Pb. CBelowdetection limits. dDetection limits for urban and semirural samples are the same as those cited in Table 11. 'NA = not available. 'Tentative assignment, see text.
Table V. Deposition Rates o f Alkyllead and Inorganic Lead
site urban semirural
deposition rate, ng of Pb cm-2 month-' alkyllead inorganic lead 1.1 0.7
~
217 98.7
=Average of measurements in the period 7 January to 3 February 1986.
alkyllead species in deposition may be responsible for the observed damage to forests (IO). The authors, however, employed a nonspecific biochemical technique, and their results have been disputed (7). Ionic alkyllead compounds observed in the deposition samples may result from washout and rainout of water-soluble gas-phase R3Pb+and RzPb2+,scavenging of atmospheric aerosols containing alkyllead species, and the aqueous decomposition in rainwater of scavenged R,Pb species (6). The same ionic alkyllead species were also found in deposition collected at the remote rural site on the west coast of Ireland although the concentrations were considerably lower than those measured in the U.K. deposition. Deposition rates estimated for the urban and semirural sites are compared in Table V. Washout factors for total alkyllead and inorganic lead were evaluated for the two sites, and the results appear in Table VI. Since earlier work had shown rapid conversion of tetraalkyllead in freshly collected rainwater (5, 6 ) , it was not possible to calculate washout factors for individual alkyllead compounds, and thus a single value for total alkyllead is presented for each sampling period. This value should be useful in predicting total alkyllead behavior but does not assist the comprehension of the behavior of individual species. In line with other studies (24,25),washout factors are very variable, although the values for igorganic lead agree well with those from an earlier study in northwestern England (24),which reported a range of 53-830 and a mean of 320 for lead. The generally lower values of washout factor for alkyllead than for inorganic lead are indicative of less efficient scavenging of the former species. It is quite possible, however, that the water-soluble trialkyllead compounds make up a disproportionate component of the scavenged alkyllead relative to the far less soluble tetraalkyllead species. It should be noted that the washout factors have been calculated from samples collected over
Table VI. Values of Washout Factor, W , for Total Alkyllead and Inorganic Lead at an Urban and Semirural Site
date, 1986 7-8.1 8-21.1 21-28.1 28.1-3.2 mean
washout factor, Wa urban semirural inorganic inorganic alkyllead lead alkyllead lead 12 51 8 79 38
187 93 633 304
30 104 33 99 66
141 24 47 1141 338
a W = lead in rainwater, ng of Pb kg-'/lead in air, ng of Pb kg-'. 1 m3 of air = 1.226 kg at 15 ' C . 1 atm.
several days, and thus represent a composite of different air masses and include periods during which no rain fell but air-sample collection continued. Because of this fact and the use of total deposition samplers, the washout factors must be treated with some caution as they are not strictly comparable with data collected over short-time intervals using wet-only collectors. They are, however, comparable with many other data sets that have been collected in a similar manner (e.g., ref 24) and provide a time-averaged representation of alkyllead behavior. They also serve to provide a useful comparison of the behavior of alkyllead and inorganic lead. Total alkyllead deposition fluxes for the urban and semirural tiites are plotted against one another in Figure 3. These are very highly correlated and show a ratio of approximately 1.45 in favor of the urban site, far lower than. the ratio of gas-phase alkyllead concentrations, confirming the minor importance of dry deposition to the collector. This behavior is indicative of the major proportion of alkyllead scavenging taking place at a considerable altitude, such that intersite differences are minimized on a small geographic scale.
Conclusions Total dkyllead correlates.strong1y with inorganic lead at the urban site, indicating a common vehicular source. At a semirural site 3.6 km distant, the correlation is far weaker, suggesting the influence of air masses of differing origins in which sink processes have removed alkyllead and Environ. Sci. Technol., VoI. 22, No. 5, 1988
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1
0
1
01
1
1
1
02
1
03
alkyl lead deposition (ng Pb cm-*)Essex University Figure 3. Plot of total alkyllead dewsttion during each sampling interval at urban (Colchester) and semlrural (University) sites.
inorganic lead at different rates. Concentrations of alkyllead at the remote Adrigole site on the west coast of Ireland are mostly below the detection limit of the analytical procedure. Washout of total alkyllead is less efficient than for inorganic lead. This is probably due to the predominant occurrence of alkyllead as gas-phase tetraalkyllead, which leads to less effective scavenging than for aerosol inorganic lead. If alkyllead is indeed removed from the atmosphere more slowly than inorganic lead (assuming inefficient dry deposition), the elevated ratios of total alkyllead to inorganic lead in maritime air relative to those typical of continental air (26) may arise partly as a result of this influence, although the generally lower rainfall at sea than over land would have a contrary influence. Registry No. Me4Pb, 75-74-1;Me3EtPb, 1762-26-1;Me2EhPb, 1762-27-2;MeEt3Pb, 1762-28-3;Et4Pb, 78-00-2; Me3Pb+,1457016-2; Me2EtPb+,103730-90-1; MeEt2Pb+,105956-70-5; Et3Pb+, 14570-15-1; Me2Pb2+, 21774-13-0; Et2Pb2+, 24952-65-6; Pb, 7439-92-1.
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Literature Cited (1) RadojeviC, M.; Harrison, R. M. Sci. Total Enuiron. 1987, 59, 157-180. (2) Harrison, R. M.; Laxen, D. P. H. Nature (London)1978, 275,130-740. (3) Harrison, R. M.; Laxen, D. P. H. Environ. Sci. Technol. 1978, 12, 1384-1392. (4) Hewitt, C. N.; Harrison, R. M. Environ. Sci. Technol. 1986, 20,797-802. (5) Harrison, R. M.; RadojeviE, M.; Wilson, S. J. Sci. Total Environ. 1986, 50, 129-137. (6) RadojeviE, M.; Harrison, R. M. Atmos. Environ. 1987,21, 2403-241 1. (7) Unsworth, M. H.; Harrison, R. M. Nature (London) 1985, 31 7, 674. (8) Van Cleuvenbergen, R. J. A,; Chakraborti, D.; Adams, F. C. Enuiron. Sci. Technol. 1986, 20, 589-593. (9) RadojeviE, M.; Harrison, R. M. Enuiron. Technol.Lett. 1986, 7, 519-524. (10) Faulstich, H.; Stournaras, C. Nature (London) 1985,314, 7 14-7 15. (11) Hewitt, C. N.; Harrison, R. M. Anal. Chim. Acta 1985,167, 271-287. (12) Harrison, R. M.; RadojeviE, M. Environ. Technol. Lett. 1985, 6, 129-136. (13) Harrison, R. M.; RadojeviE, M.; Hewitt, C. N. Sci. Total Environ. 1985, 44, 235-244. (14) Hewitt, C. N.; Harrison, R. M.; RadojeviE, M. Anal. Chim. Acta 1986,188, 229-238. (15) Potter, H. R.; Jarvie, A. W. P.; Markall, R. N. WaterPollut. Control (Maidstone,Engl.) 1977, 76, 123-128. (16) RadojeviE, M.; Allen, A.; Rapsomanikis, S.; Harrison, R. M. Anal. Chem. 1986,58,658-661. (17) RadojeviE, M.; Harrison, R. M. Environ. Technol. Lett. 1986, 7,525-530. (18) RadojeviE, M.; Allen, A.; Harrison, R. M. University of Essex, unpublished data, 1985. (19) Van Cleuvenbergen, R. J. A.; Chakraborti, D.; Adams, F. C. Anal. Chim. Acta 1986,182, 239-244. (20) Harrison, R. M.; Perry, R. Atmos. Environ. 1977, 11, 847-852. (21) De Jonghe, W. R. A.; Adams, F. C. Talanta 1982, 29, 1057-1067. (22) Nielsen, T. In Biological Effects of Organolead Compounds; CRC: Boca Raton, F1, 1984; pp 43-62. (23) Hewitt, C. N.; Harrison, R. M. Proceedings of the Inter-
national Conferenceon Heavy Metals in the Environment; C.E.P. Consultants: Edinburgh, 1985; pp 171-173. (24) Harrison, R. M.; Williams, C. R. Atmos. Environ. 1982,16, 2669-2681. (25) Cawse, P. A. U.K. Atomic Energy Authority Report AERE-R9164; HMSO: London, 1978. (26) Hewitt, C. N.; Harrison, R. M. Environ. Sci. Technol. 1987, 21, 260-266.
Received for review February 24,1987. Accepted October 6,1987.
