Environ. Sci. Technol. 1988, 22, 1305-1311
Thermal Speciation of Atmospheric Nitrate and Chloride: A Critical Evaluation William 1. Sturges and Roy M. Harrison” Institute of Aerosol Science, University of Essex, Wivenhoe Park, Colchester, Essex, C04 3SQ
rn A thorough investigation was made of the thermal speciation method of Yoshizumi and Hoshi for separating ammonium nitrate from sodium nitrate in atmospheric aerosol by their differences in volatility and of the possibility of extending the method to analogous chloride salts. The method was found to be seriously prone to a number of interferences. Nitric acid evolved from the decomposition of ammonium nitrate was found to react with sodium chloride, causing displacement of the supposedly nonvolatile chloride (as HCl) and retention of “volatile” nitrate as sodium nitrate. Ammonium sulfate promoted the volatilization of the sodium salts. Further problems occurred where samples absorbed moisture and recrystallized as more volatile species. We also demonstrated differences in thermal stability between ammonium nitrate and ammonium sulfate/nitrate double salts. We believe that similar criticisms may also apply to thermal denuder sampling systems, in which aerosols are thermally decomposed and separated during sampling.
Introduction When investigating the atmospheric chemistry of strong acids and their neutralization products, it is important to be able to distinguish the various species involved. Sampling methods, such as filter packs and denuder tubes, have been developed (I) to separate the gaseous acidic phase from the neutralized particulate compounds in the case of nitrate, but no information is provided on the actual composition of the particulate phase by these means. Yoshizumi and Hoshi (2)suggested that it is possible to distinguish two known atmospheric nitrate species, ammonium nitrate and sodium nitrate, by their different temperatures of volatilization. Their procedure involved taking a portion of a filter on which atmospheric particulate had been collected (by filtration or impaction) and placing it in a furnace at a temperature at which only ammonium nitrate should evaporate. The evaporated nitrate was collected by bubbling the nitrogen carrier gas through water. Yoshizumi and Okita (3) likewise inferred the concentrations of the sodium and ammonium salts of nitrate and chloride by measuring NO3- and C1- losses during long-term storage of atmospheric particulate filters. A similar principle has been employed in thermal denuder designs ( 4 , 5 ) . Here the thermal speciation takes place as the sample is collected, by passing the air stream through heated tubes, which are coated with a chemical to absorb evaporated nitrate or sulfate. Both the above methodologies were only tested for a limited range of artificial, single-component aerosols. No account was made of the possible effect of mixtures of compounds, or the presence of other atmospheric components. In the study by Yoshizumi and Hoshi, the only compound tested was ammonium nitrate. It was assumed that the “nonvolatile”-phase nitrate consisted of sodium nitrate and that none of this would volatilize at the optimum temperature for ammonium nitrate volatilization (160 “C). We set out to validate the method of Yoshizumi and Hoshi and to extend it to a method for examining the 0013-936X/88/0922-1305$01.50/0
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speciation of chloride into ammonium and sodium salts. In the course of the study, it became clear that interreactions between different components in atmospheric aerosols make it impossible to separate either the nitrate or chloride salts by preferential volatilization. By analogy this implies that the thermal denuder method may also be prone to error.
Experimental Section A simple tube furnace for heating filter samples was constructed along similar lines to that used by Yoshizumi and Hoshi. It consisted of a glass tube wrapped with electrical heating tape, connected to a temperature controller. Argon gas, cleaned by passage through molecular sieve, activated carbon, glass fiber, and nylon filters, was used to sweep the volatile products out of the furnace, through a heated PTFE tube, to a midget impinger. The impinger contained 15 mL of distilled deionized water and was cooled by immersion in an ice bath. The collection efficiency was high; there was less than 2% breakthrough of any of the measured ions to a second impinger. Samples containing mixtures of ammonium nitrate, ammonium chloride, ammonium sulfate, sodium chloride, and sodium nitrate were prepared either by drying solutions onto filters or by mixing powdered dry reagents. Aliquots (50-200 pL) of either separate or mixed solutions containing the required compounds were spotted onto Whatman PTFE filters (1.0-pm pore size, 47-mm diameter), which had first been wetted by the addition of 50 pL of propan-2-01. These were then dried in a desiccator. Powder samples were investigated to avoid possible redistributions of ions into different compounds during crystallization. They were prepared by finely grinding mixtures of the compounds with an agate mortar and pestle. Initially, the precautions taken to avoid hydration of the powder samples consisted of thorough drying of the reagents, desiccator storage, and minimal exposure to ambient air. It became evident that this was not adequate to prevent some chemical alterations of the mixtures. An improved procedure involved oven drying all the reagents (except “,NO3) at 130 “C, followed by further drying in a vacuum desiccator, and storage at all times over P206. All sample handling, including grinding, mixing, loa’ding of the furnace, and X-ray specimen preparation, was carried out under dry nitrogen in a glovebox. Filter samples were rolled up before insertion into the tube furnace. For the powder samples, a PTFE filter was first formed into a thimble at the outlet end of the tube to contain the powder and prevent it from being blown through into the impinger. The samples were heated for periods of between 30 and 360 min, 60 min being normal. A t the end of this time, 5 mL of water was flushed through the PTFE transfer tube to carry any condensed compounds into the impinger. The same procedure was followed with the second impinger, where used. Failure to do this resulted in losses of up to 20% of the volatile species. Good recovery was obtained with a flow of 2 L min-l, although for the powder samples the flow was reduced to 1.2 L min-l to minimize the chance of powder being carried through to the impinger. An experiment with
0 1988 American Chemical Society
Environ. Sci. Technol., Vol. 22, No. 11, 1988 1305
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Flgure 1. Loss of NO,- and CI- from dried solution filter samples containing ammonium and sodium salts of nitrate and chlorlde, at elevated temperatures (70-250 “0. O w n bars represent the amount of volatilized nitrate and chloride trapped in the impinger solution, and the shaded bars represent the amount remaining in the furnace.
powders of the nonvolatile sodium salts of nitrate and chloride showed an apparent penetration of less than 1% through to the impinger at this flow rate. The thermally treated filters were extracted into 10-25 mL of deionized distilled water, depending on expected concentrations, with the aid of a mechanical shaker, after first wetting the filter with 0.5 mL of propan-2-01. Filter extracts and impinger solutions were analyzed by ion chromatography for nitrate, chloride, and sulfate and by a fluorescence method (6) for ammonium. Recovery efficiencies of all the ionic species from unheated dried solution spiked filters ranged from 98 to 100%. Duplicate analysis of the unheated spiked filters gave relative standard deviations of -0.5%. Duplicate analysis of spiked filters after heating for 30 min at 130 “C gave standard deviations of 1-4 pg for volatilized, nonvolatilized, and total nitrate and chloride, compared with the total sample loadings of 300 pg of NO3- and 50 pg of C1-. Similarly, duplicate analysis of powder samples gave standard deviations of 1-4% in the percentage mass distributions of NO< and C1-. Sdme of the powder samples were analyzed by X-ray powder diffractometry (XRD). A silica gel bag was placed in the sample chamber, and a relatively fast scan rate of 0 . 5 O 219 m i d was used, to minimize hydration of the sample.
Results and Discussion An assumption inherent in the use of thermal speciation techniques is that atmospheric aerosol particles contain simple discrete compounds that behave in a manner similar to the bulk pure compound. In fact, in the free troposphere many particles have been processed through cloud systems and comprise complex internal mixtures as a result. There is, however, evidence from X-ray powder diffraction studies (7-9) that aerosols sampled at ground level do contain discrete crystalline salts. This study will assume that such compounds exist as individual particles in an externally mixed aerosol. 1306
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1. Mixtures of NH4N03,NaN03,NH,Cl, and NaCl. Figure 1 shows the results from experiments conducted with filter samples containing mixtures of the ammonium and sodium salts of nitrate and chloride. The first pair of bars on the left represent the amounts of nitrate and chloride added to the filters (200 pg of each nitrate compound and 40 pg of each chloride compound). The supposedly nonvolatile sodium nitrate and chloride component is shown as the lower shaded part of the bar and the “volatile” ammonium nitrate and chloride as the upper open part of the bar. In the case of the actual sample results, the shaded part of the bar represents the amount of ion remaining on the filter, while the open part represents that found in the impinger solution. The next two sets of results to the right in Figure 1 are quality checks on the preparation of the filter samples. Analysis of the solution used to impregnate the filters showed excellent agreement. Direct analysis of a filter sample, without any heating in the furnace, showed a very small loss of material, possibly due to loss of solution to the watch glass on which the filters were dried or evaporative loss from the filter. The remaining results in Figure 1are from filter samples heated in the furnace. In all cases the total recovery of nitrate and chloride was 100%. Recoveries above and below 100% could be due to absorption losses in the system, or subsequent desorption, as well as possible contamination and analytical errors. Runs 1-4 were conducted at 130 “C for varying times. This temperature, according to Yoshizumi and Hoshi, is the maximum that nitrate can be heated to without detectable decomposition to NO, which is not trapped by the impinger solution. The results show good precision and little, if any, dependence on duration of heating. The most notable feature of these results is the large volatilization of supposedly nonvolatile sodium chloride, together with a marked increase in the amount of “nonvolatile” nitrate. Runs 5-10 were made at different temperatures and a fixed duration of 60 min. At 70 “C only partial volatilN
Table I. Comparison of Nonvolatile C1- “Lost”(Cl- as NaCl Added Minus C1- Remaining on Filter) and Nonvolatile NO, “Gained”(NO, Remaining on Filter Minus NO, as NaNOa Added) for Filter Samples Heated to Different Temperatures (See Figure 1) temp, OC 100 6
run no. nonvolatile C1- lost, pmol nonvolatile NO; gained, pmol
130 7 0.54 0.46
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Flaure 2. Loss of NO,- and CI- from dried solution filter samdes containing ammonium and sodium salts of nitrate and chloride, at temperatures u p t o 70 O C .
ization had taken place. HoweverSincreasing the temperature to 100 OC resulted in the large loss of chloride already observed in the previous samples. Further increases in temperature up to 250 “C produced little change in amounts of volatilization of either ion. There was also no evidence of the pronounced loss of total nitrate, presumed to be as NO, reported by Yoshizumi and Hoshi for these higher temperatures. This discrepancy may be due to reactions with other constituents of the ambient aerosol samples for which they observed this effect. Table I compares the amount of ”nonvolatile” chloride lost (expressed in moles) with the gain in “nonvolatile” nitrate. The amounts are approximately equal. This suggests that the ammonium nitrate decomposed to ammonia and nitric acid and that -50% of the nitric acid reacted with the sodium chloride displacing HC1: HN03 + NaCl HC1+ NaN03 (1)
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The sodium nitrate so produced accounts for the increase in nonvolatile nitrate. The shortfall in nonvolatile NO3gained may be due to some of the sodium nitrate formed being carried over into the impinger. Figure 2 shows the results from attempts to find conditions under which ammonium chloride could be efficiently volatilized without also volatilizing ammonium nitrate, and so causing the loss of chloride from sodium chloride. The furnace was operated at temperatures below 100 “C for varying times. Even at temperatures as low as 30 OC, both nitrate and chloride began to evaporate. Over
longer periods of time at low temperature, only a little more chloride was evaporated and was accompanied by further volatilization of nitrate. The optimum conditions for chloride volatilization with minimum nitrate loss was at 40 “C, but the efficiency for chloride volatilization was below 50%. There were in fact no conditions under which ammonium chloride could be efficiently recovered without inducing significant nitrate volatilization. There is some possibility of circumventing the problem of interferences from the presence of sodium chloride if size-fractionated samples are collected. Sodium chloride is derived from sea salt aerosols and road deicing salt, which are concentrated in the coarser particle fractions. Ammonium nitrate is found in finer size fractions (IO); thus, separate analysis of particles in different size ranges should allow ammonium nitrate to be determined in the absence of significant amounts of sodium chloride. However, the results of Yoshizumi and Hoshi for cascade impactor samples showed a considerable overlap in the size distributions of volatile and involatile nitrate. Furthermore, this approach cannot be used in the case of thermal denuders, where total air particulate is sampled. 2. Mixtures with (NH4)#04 Added. There are a number of other potential interferents to thermal analysis present in the atmosphere. One possible candidate is ammonium sulfate, which is found ubiquitously in concentrations comparable to that of ammonium nitrate. We therefore also tested the effect of mixtures with this compound. Figure 3 shows the results from heating dried Environ. Sci. Technol., Vol. 22, No. 11, 1988
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Figure 4. Loss of NO3- and CI- from various mixtures of solid (finely ground) sodium nitrate, sodium chloride, and ammonium sulfate at elevated temperatures. No S O : was detected in the impinger solution.
solution filter samples containing mixtures with 200 bg of ammonium sulfate at temperatures from 70 to 160 "C. At a temperature of 100 O C , both the ammonium and sodium salts of chloride had completely volatilized. Increasing the temperature to 160 O C resulted in a nearly complete volatilization of the nitrate compounds as well. Experiments with mixtures of ammonium sulfate with either sodium chloride or sodium nitrate showed the same effect. In all cases, at least 97% of the sulfate added was found to remain on the filter. One possible explanation for the above effect is that the solutions crystallized out with all the chloride and nitrate in the form of ammonium salts and the ammonium sulfate 1308
Environ. Scl. Technol., Vol. 22, No. 11, 1988
converted to sodium sulfate. If this is so, then we would not expect to see any loss of chloride or nitrate from mixtures of dry powders of sodium chloride and sodium nitrate with ammonium sulfate. Figure 4 shows the results of experiments to test this. Since it was not possible to accurately determine the total amount of compounds added, the results are given as percentages of the total measured; thus, the amount of volatile ions may be somewhat underestimated. The column on the left is a control run with sodium nitrate and sodium chloride at 160 "C. Only 1% of the ions were found in the impinger, probably due to some of the powder escaping past the filter "thimble". The next column to the right is a check to see
Table 11. Comparison of the Amounts of Volatilized C1and N 0 f with Volatilized NH4+Found in the Impinger Solution from Heating a Mixture of NaNOa, NaCl, and (NH&S04 (See Figure 4)
run no.
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+
C1- NO< in impinger, pmol 11.7 13.7 17.9 13.8 20.0 12.0 14.6 17.6 13.2 18.0 NH4+ in impinger, pmol
if we could reproduce, with a powder sample, the reaction between heated ammonium nitrate and sodium chloride to volatilize chloride; 22% of the chloride did indeed evaporate. The remaining data are for mixtures of the sodium salts of chloride and nitrate with ammonium sulfate. At 100 "C, there was a small volatile loss of nitrate and chloride. At 160 "C, the temperature used by Yoshizumi and Hoshi for their experiments, this loss became pronounced. Here too, virtually none of the sulfate was found in the impinger solution. Due to the hygroscopic nature of the sodium salts, there is the possibility that the powder mixtures had rapidly absorbed water vapor from the atmosphere, producing some ammonium salts of nitrate and chloride by recrystallization. To counter this, further powder samples were prepared and loaded into the furnace under a dry nitrogen atmosphere in a glovebox. The argon carrier gas was immediately introduced into the furnace tube and the experiment carried out. With these samples too (Figure 4) there was a large proportion of chloride and nitrate volatilized. In another experiment, a portion of a desiccatordry mixture was first tested immediately and then a second portion tested after exposure to water vapor from breath. There was only a small increase in the amount of volatilization from the latter sample. However, after the same mixture had been left out in the laboratory atmosphere for several hours, there was a distinct increase in the amount of both nitrate and chloride evaporated. Thermal speciation analysis is therefore further devalued where samples are not stored desiccated. Furthermore, atmospheric samples collected under humid conditions will very likely be subject to recrystallization effects, obliterating the original distribution between volatile and nonvolatile species. The process involved in the volatilization of the sodium salts in the presence of ammonium sulfate is presumably a solid-phasereaction. When ammonium sulfate alone was heated at temperatures of 70-200 "C, none of the sulfate and less than 8% of the ammonium appeared in the impinger solution, insufficient to account for a process involving the production and reaction with sulfuric acid. Table I1 gives a comparison of the moles of ammonium with the total moles of nitrate and chloride determined in the impinger solution from the experiments in Figure 4. These quantities are approximately equal, indicating that ammonium nitrate and ammonium chloride were the species lost from the powder mixtures. 3. Ammonium Nitrate/Sulfate Double Salts. Another consideration in thermal speciation is the consequences of the simplistic assumptions that, for instance, all nonvolatile nitrate is the sodium salt and all volatile nitrate is ammonium nitrate. There is, in fact, growing evidence that this is not so. There are reports that nitrate may be present as mixed ammonium sulfate nitrate double salts (7, 9, 11). These may have a different thermal behavior than ammonium nitrate. To test this we investigated the thermal decomposition of (NH4)2S04.2NH4N03 and (NH4)2S04.3NH4N03.
The ammonium sulfate nitrate compounds were synthesized according to the method of Coates and Woodard (12). Successful synthesis of the compounds was confirmed by XRD. Approximately 5-mg portions of the dry, finely ground compounds were tested. Mixtures of ammonium sulfate and ammonium nitrate in the same molar proportions were also tested under the same conditions. XRD analysis of these latter samples revealed that a certain amount of conversion to the respective double salts had already taken place. Caution should therefore be taken in intepreting XRD studies of atmospheric particulate in which these compounds have been reported, since there is the possibility of artifact formation of the double salts. The results are shown in Figure 5. There was a slight loss of nitrate at 70 "C, increasing to a nearly complete loss at 200 "C. There were differences in the degree of volatilization between the double salts and their corresponding mixtures of nitrate and sulfate salts. The former appeared to give a smaller volatile loss of nitrate than the latter, thus the amount of nitrate associated with ammonium would tend to be underestimated if the double salts were present in an aerosol sample. 4. X-ray Diffraction Analysis of Powder Samples. It was hoped that some insight would be gained into the reactions involved during heating of the mixtures described in the sections above by examining the solid crystalline compounds remaining in the tube furnace after thermal treatment of powder mixtures. In the initial stages of the study, when only normal precautions were taken against sample hydration, it was noted that unheated powder mixtures exhibited substantial alteration. For example, mixtures of NaN03, NaCl, and (NH4)2S04formed significant amounts of NH4N03and NaN03.Na2S04.H20,with lesser amounts of NH4Cl and possibly NH4NaS04.2H20,presumably as a result of absorption of water vapor and recrystallization. Mixtures of NH4N03and NaCl were found to form some NH&l and NaN03. There is, as previously observed, the potential for serious errors to arise in thermal analysis methods if the samples are not kept scrupulously dry. A more stringent sample-handlingprocedure (see Experimental Section) was implemented and found to result in a negligible degree of such hydration effects. A mixture of ",NO3 and NaC1, in a molar ratio of 0.79:1.0, was heated to 160 "C for 2 h. XRD revealed that some of the original constituents remained in the sample, although some of the ammonium nitrate had apparently converted to a higher temperature form that occurs at 155 "C (JCPDS diffraction file 9-74). Large amounts of NaN03 were produced, which is further evidence that reaction 1 could explain the loss of C1- and retention of NOf in this system. Several diffraction peaks could not be conclusively assigned; some might be attributed to NaN03.2NH4N03 and NH4C103(the presence of a chlorate group was confirmed by infrared spectroscopy), although the reactions involved are not clear. Ionic analysis of the heated sample showed an NH4+:N03-:C1-molar ratio of 1.01.2:0.24, which confirms that loss of ammonia had occurred from the sample, with a larger loss of chloride than nitrate. A mixture of NaC1, NaN03, and (NH4)2S04with a relative molar composition of 0.53:2.4:1.0 was also heated to 160 "C for 2 h. No (NH4)$04 or NaCl remained in the heated sample, although some NaN03 persisted, consistent with our earlier observation of greater C1- than NO3- loss during heating. Ionic analysis showed that the molar ratios of C1-, NO3-, and NH4+ to S042- had all declined to 0.07:1.2:0.87:1.0. The principal product was Na2S04(the high-temperature form metathenardite). There were Environ. Sci. Technol., Vol. 22, No. 11, 1988
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Figure 5. Loss of NO3- and NH,' from ammonium nitrate/sulfate double salts (CMPD., compound) and molar equivalent mixtures of ammonium nitrate and ammonium sulfate (MIX., mixture) at elevated temperatures. In all cases, at least 97% of the sulfate remained in the tube furnace after heating.
smaller amounts of NH4Cland ",NO3 present, although presumably most of the mass of these two compounds had already evaporated. This would seem to confirm that C1and NO3- losses occur by formation of these volatile species. There was also evidence for the presence of some (NHJ2S04.3NH4N03. Conclusions There are a number of serious errors that can arise in sampling or postsampling techniques that aim to produce a separation of nitrate and chloride species according to their relative volatilities. This has been demonstrated with reference to the postsampling filter heating method of Yoshizumi and Hoshi, but may also cause problems with thermal denuder sampling techniques. In the development of both the above methods, the possible interactions that could take place with other compounds commonly present in the atmosphere were ignored. We have shown that in the case of two ubiquitous atmospheric compounds, sodium chloride and ammonium sulfate, interactions do indeed take place that affect the volatilization of ammonium nitrate and ammonium chloride. When sodium chloride is present, the nitric acid evolved from thermal decomposition of ammonium nitrate displaces chloride, producing volatile HC1 and nonvolatile NaNO,. The net result is that volatile nitrate is underestimated and volatile chloride is overestimated. When it is considered that one version of the thermal denuder actually uses a sodium chloride coating as a highly efficient trap for thermally evolved nitric acid, then it is obvious that the nitric acid may also react with sodium chloride aerosols in the air stream. In the presence of ammonium sulfate, both sodium chloride and sodium nitrate are volatilized, resulting in an overestimate of volatile ammonium chloride and nitrate. The effect is further exacerbated if the samples are allowed to absorb water vapor out of the atmosphere, leading to particulate dissolution and recrystallization as more volatile compounds. Incidently, it should be noted that such hydration effects could 1310
Environ. Sci. Technol., Vol. 22, No. 11, 1988
cause serious artifacts in X-ray diffraction studies of atmospheric particulate composition. It is likely that other atmospheric constituents could also exert an influence on the thermal decomposition of nitrates and chlorides. It is, for example, highly probable that the acidic sulfates known to be present in the atmosphere, such as ammonium bisulfate, letovicite, and sulfuric acid (8,9), would react with "nonvolatile" nitrate and chloride. We also note that further problems of interpretation arise where there is no a priori knowledge of the actual composition of the atmospheric particulate sample, It is not possible to simply divide nitrates and chlorides into volatile ammonium compounds and nonvolatile sodium compounds. Both ions may be present in a number of different forms, with different thermal decomposition characteristics. For example we have found some differences in thermal behavior between ammonium sulfate/ nitrate double salts and ammonium nitrate. We might also speculate on the implications for the thermal denuder techniques used to investigate the speciation of atmospheric sulfate (4,13). It is highly probable that these too suffer from similar problems to those described here, as has previously been suggested by Newman (14). In these methods, sulfuric acid droplets are evaporated, and ammonium sulfate compounds decomposed, by heating to produce gaseous H2S04,which is either analyzed directly or trapped by a denuder. In a manner analogous to the effect of nitric acid, this will probably cause reactions with sodium chloride in the aerosol producing sodium sulfate, which is stable at the temperatures used ( 4 ) . As a result, the amount of volatile sulfate will be underestimated. There is also the problem of the different forms of sulfate that can occur in the atmosphere, apparently more numerous than those of nitrate (9). The conclusion is that thermal methods to speciate atmospheric nitrate, chloride, and sulfate do not appear to be workable. The results from studies in which these techniques are employed must be treated with suspicion, unless it can be proved that the methods do in fact produce realistic and interpretable results. Acknowledgments We thank N. Hewitt and J. Bowman of the Department of Environmental Sciences, University of Lancaster for performing the X-ray diffraction analyses. Registry No. NH4N03,6484-52-2;NH4Cl,12125-02-9; NaN03, 7631-99-4; NaC1, 7647-14-5.
Literature Cited (1) Spicer, C. W.; Howes, J. E., Jr.; Bishop, T. A.; Arnold, L. H.; Stevens, R. K. Atmos. Environ. 1982, 16(6), 1487. (2) Yoshizumi, K.; Hoshi, A. Environ. Sci. Technol. 1985,19(3), 258.
(3) Yoshizumi, K.; Okita, T. J . Air Pollut. Contr. Assoc. 1983, 33(3), 224. (4) Niessner, R.; Klockow, D. Int. J. Environ. Anal. Chem. 1980, 8, 163. (5) Slanina, J.; v. Lamoen-Doornenbal, L.; Lingerak, W. A.;
(6) (7)
(8) (9)
Meilof, W.; Klockow, D.; Niessner, R. Int. J. Enuiron. Anal. Chem. 1981,9,59. Rapsomanikis, S.;Wake, M.; Kitto, A.-M. N.; Harrison, R. M. Environ. Sci. Technol. 1988, 22, 948. Harrison, R. M.; Sturges, W. T. Atmos. Environ. 1984,18(9), 1829. O'Connor, B. H.; Jaklevic, J. M. Atmos. Enuiron. 1981, 15(9), 1681. Tani, B.; Siegel, S.; Johnson, S. A.; Kumar, R. Atmos. Environ. 1983, 17(11), 2277.
Environ. Sci. Techno/. 1988, 22, 1311-1319
(13) Slanina,J.; Schoonebeek,C. A. M.; Klockow, D.; Niessner, R. Anal. Chem. 1985,57, 1955. (14) Newman, L. Atmos. Enuiron. 1978, 12, 113.
Milford, J. B.; Davidson, C. I. J . Air Pollut. Contr. Assoc. 1987, 37(2), 125.
Fukasawa, T.; Iwatsuki, M.; Tillekeratne, S. P. Environ. Sci. Technol. 1983, 17, 596. Coates, R. V.; Woodard, G. D. J . Sci. Food Agric. 1963,14, 398.
Received for review November 12,1987. Accepted May 23,1988.
Photolysis of Polycyclic Aromatic Hydrocarbons Adsorbed on Fly Ash Thomas D. Behymer and Ronald A. Hites” School of Public and Environmental Affairs and Department of Chemistry, Indiana University, Bloomington, Indiana 47405
rn A rotary photoreactor was used to simulate the environmental conditions encountered by particle-bound polycyclic aromatic hydrocarbons (PAH) in the atmosphere. Eighteen PAH adsorbed on carbon black and 15 coal fly ash samples of varying physical and chemical compositions were photolyzed. Photolytic half-lives were found to be highly dependent on the substrate. On low-carbon fly ash samples, PAH showed a wide range of half-lives, indicating a relationship between PAH structure and photochemical reactivity. On fly ash samples with a carbon content greater than -590, PAH showed half-lives similat to one another. This indicates that the photolytic process is independent of PAH structure and dependent on the physical and chemical nature of the fly ash. Substrates that stabilize reactive PAH are black or gray in color; these dark substrates adsorb the most light and prevent the light from getting to the PAH. Introduction Polycyclic aromatic hydrocarbons (PAH), formed by the combustion of almost any fuel under oxygen-deficient conditions, are transported through the atmosphere in the vapor phase and adsorbed on particulate matter. Because some PAH are carcinogenic, there is considerable interest in their atmospheric fate (1-9). Currently, two schools of thought exist concerning the ultimate environmental fate of PAH. One school, which we will call the “Fast School”, says that PAH degrade quickly in the atmosphere. For example, some laboratory studies have found that PAH degrade with lifetimes as short as a few hours (20-22). The other school, which we will call the “Slow School”, says that PAH degrade slowly, if at all, in the atmosphere and eventually deposit on soil or water. This idea is supported by studies of marine and lacustrine sediments, the ultimate environmental sinks of PAH. These studies have shown that the relative abundances of PAH, even at the most remote locations, are similar to those in combustion sources (23-25); this suggests that PAH are stable in the atmosphere. To determine which school of thought is correct, it is useful to simulate the atmospheric degradation of PAH in the laboratory. To do so, one must simulate environmental photolytic conditions, and one must simulate the phase conditions of the reactants. The latter is tricky. PAH in the atmosphere are present in both the vapor phase and adsorbed to atmospheric particulates; the amount in each phase is a substrate-dependent equilibrium process. This paper addresses the photolytic degradation of PAH adsorbed on atmospheric particulates. We have selected experimental conditions to simulate the environment encountered by particle-bound PAH, and we have studied a wide range of PAH from acenaphthylene to coronene. 0013-936X/88/0922-1311$01.50/0
These PAH have been adsorbed onto several fly ash samples to elucidate relationships between PAH structure and photochemical reactivity. Dramatically different substrates such as silica gel and alumina were also studied to aid in determining the effect of the physical and chemical characteristics of a substrate on PAH stability. Because we have studied several PAH, we are able to make statistical comparisons between degradation half-lives and substrate composition. This experimental design tells us if fly ash can stabilize PAH and what properties of the fly ash are important. Experimental Section Eighteen common PAH have been chosen to represent combustion-generated PAH. These are acenaphthylene, acenaphthene, fluorene, dibenzothiophene, phenanthrene, anthracene, 4H-cyclopenta[deflphenanthrene,fluoranthene, pyrene, benz [a]anthracene, chrysene, benzo [e]pyrene, benzo[a]pyrene, indeno[l,2,3-~d]pyrene,benzo[ghi]perylene, anthanthrene, and coronene. Isomeric pairs were chosen, when possible, to study the effect of structure on reactivity. These PAH have all been adsorbed on silica gel, alumina, carbon black, and 15 different fly ash samples. We have used fluidized-bed and rotary photoreactors to simulate environmental conditions encountered by particle-bound PAH. The experimental procedures have been described elsewhere (26,27). In these studies, the rotary reactor was found to be better suited for long-term photolysis experiments because particulate matter could be sampled easily, small particles could be used, and the irradiation time could be lengthy. Although the rotary reactor does not simulate the environment as well as the fluidized-bed photoreactor, half-lives for PAH determined in both reactors under identical conditions were found to be equivalent. Additionally, half-lives were found to be reproducible in the rotary reactor. Thus, this photoreactor was used in our studies to simulate environmental processes. The light source was a 450-W medium-pressure mercury vapor lamp with a measured irradiance of 17.6 f 1.4 W/m2 at the photoreactor. Equivalent amounts of each substrate were used in all experiments. Although sticking problems occurred with substrates of very small particle sizes, continual monitoring and tapping of the reactor reduced this problem. To ensure experimental reproducibility, several replicates were done for each experiment. Along with PAH losses due to photodegradation, it was important to determine if reactions occurred in the absence of light. Therefore, dark experiments were carried out. Due to slight heating in the photoreactor from the light source and the rotary motor, volatilization of PAH adsorbed onto a substrate could have been possible. If this
0 1988 American Chemical Society
Environ. Sci. Technol., Vol. 22, No. 11, 1988
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