Environ. Sci. Technol. 1985, 19, 258-261
Griffith, E. J. Nature (London) 1974,248, 458-460. Varde, K. S.; Lewis, D. K. J.Air Pollut. Control Assoc. 1977, 27, 678-679.
Mader, P. M. J. Am. Chem. SOC.1958, 80, 2634-2639. Hoffman, M. R.; Edwards, J. 0. J. Phys. Chem. 1975, 79, 2096-2098.
Penkett, S. A.; Jones, B. M. R.; Brice, K. A.; Eggleton, A. E. J. Atmos. Environ. 1979, 13, 123-137. Dasgupta, P. K. Atmos. Environ. 1980, 14, 272-275. Martin, L. R.; Damschen, D. E. Atmos. Environ. 1981,15, 1615-1621.
Kunen, S. M.; Lazrus, A. L.; Kok, G. L.; Heikes, B. G. J. Geophys. Res. 1983, 88, 3671-3674.
Middleton, P.; Kiang, C. S.; Mohnen, V. A. Atmos. Enuiron. 1980,14,463-472.
Scatchard, G.; Kavanagh, G. M.; Ticknor, L. B. J. Am. Chem. SOC.1952, 74, 3715-3720.
Schumb,W. C. Satterfield, C. N.; Wentworth, R. L. ACS Monogr. 1955, No. 128. Yoshizumi, K.; Aoki, K.; Nouchi, I.; Okita, T.; Kobayashi, T.; Kamakura, S.; Tajima, M. Atmos Environ. 1984, 18, 395-401. Tanner, R. L. Brookhaven National Laboratory, personal communication, Sept, 1984.
(22) Vogel, A. I. "A Textbook of Quantitative Inorganic Analysis"; Longmans Green: London, 1961; p 363. (23) Sellers, R. M. Analyst (London) 1980, 105, 950-954. (24) Guilbault, G. G.; Brignac, R. J., Jr.; Juneau, M. Anal. Chem. 1968, 40, 1256-1263. (25) Lazrus, A. L.; Kok, G. L.; Lind, J. A,; Sperry, P. D. Eos 1984, 64, 670. (26) Kok, G. L.; Wilson, K. Ext. Abstr. Conf. Gas-Lip. Chem. Nut. Waters 1984, BNL51757, 19-1. (27) Scaringelli, F. P.; O'Keeffe, A. E. Rosenberg, E.; Bell, J. P. Anal. Chem. 1970,42, 871-876. (28) Schiff, H. I.; MacKay, G. I. "Development of a Method for
Measuring HzOzin Real Air Using a Tunable Diode Laser Absorption Spectrometer". Electric Power Research Institute, 1984, RP 2023-5. (29) Okita, T.; Ohta, S.; Abe, J.; Nakajima, K. Taika Osen Gakkaishi 1983,18,491-495. Received for review May 25,1984. Accepted October 9,1984. This research was supported by the U.S. Environmental Protection Agency through Grant R 810894-010. This report has not been subject to review by the Environmental Protection Agency and threfore does not necessarily reflect the views of the Agency and no official endorsement should be inferred.
Size Distributions of Ammonium Nitrate and Sodium Nitrate in Atmospheric Aerosols Kunio Yoshizumi* Tokyo Metropolitan Research Institute for Environmental Protection, 2-7-1 Yuraku-cho, Chiyoda-ku, Tokyo 100, Japan
Atsushi Hoshi Department of Applled Science, Tokyo Denki Universlty, 2-2 Kandanishiki-cho, Chlyoda-ku, Tokyo 101, Japan
An analytical method was developed to determine separately ",NO3 and NaN03 in atmospheric aerosols by the difference of their thermal stability. Volatile ",NO:, was recovered in a cold trap by heating a filter sample on which ambient aerosols were collected in an electric furnace under a gas flow system. The heating condition of 160 "C for 1 h under 1 L/min N2 was concluded to be optimum. On the other hand, nonvolatile NaN0, was evaluated as the difference between the total nitrate measured without the heating treatment and volatile nitrate. It was directly observed by using an Andersen sampler that the bimodal total nitrate size distribution consisted of ",NOB size distribution with a peak in the fine particle size range and NaN0, size distribution with a peak in the coarse particle size range. In addition, their distributions showed the very distinguishable variation by season which can be explained by the temperature dependence of the ",NO3 vapor pressure. W
Introduction
Nitrates play important roles as intermediate species in sink processes of oxides of nitrogen in the atmosphere. It was discussed on the basis of the seasonal and regional variations of their size distributions by Kadowaki (1)and Moskowitz (21, respectively, that they consist mainly of ",NO3 and NaNO, in the atmosphere. Cronn et al. tried to separate the nitrates by the difference of their thermal stability but obtained poor consistency with a wet chemical method (3). The presence of atmospheric ",NO3 has been investigated (4-9). On the other hand, Stelson et al. (10-12) showed the equilibrium constant between NH,, 258
Environ. Sci. Technol., Vol. 19, No. 3, 1985
"OB, and ",NO3 to give a high vapor pressure at ambient temperature. Quantitative determination of ",NO, and NaN03 in ambient aerosols, however, has not been conducted so far. Recently, Yoshizumi et al. (13)showed that it was possible to evaluate ",NO3 and NaN0, separately on high volume filters stored for 15 months after sampling in light of ",NO3 volatility. The purpose of this study is to develop a method to determine ambient aerosol ",NO3 and NaN03 by applying the above principle to a filter sample to be heated in a furnace under a gas flow system. Experimental Section
A filter sample was heated in an electric furnace as shown in Figure 1. Volatile nitrate was collected in a cold trap which contained 10 mL of distilled water by using a 1L/min high purity nitrogen carrier gas flow. Nitrate on a filter was extracted into 10 mL of distilled water during a 1-h shaking. In the case of a Teflon filter, it was impregnated with 0.5 mL of ethanol before the addition of water. Nitrate in a solution was determined by the modified Cox method (14, 15) to analyze nitrate by a chemiluminescent NO, analyzer after the reduction of nitrate to NO gas in an acidic aqueous solution containing 4% ferrous ammonium sulfate and 2% ammonium molybdate. Detailed instrumentation was given in a previous paper (15). Ammonium was determined by the indophenol blue method (16). Atmospheric aerosols were collected on a filter by using an Andersen sampler (2000 INC Model 21-000) and a high volume sampler (KIMOTO Model 120). Quartz (Pallflex 2500 QAST) and Teflon (Toyo PF-2,3)
0013-936X/85/0919-0258$01.50/0
0 1985 American Chemical Society
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8
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Flgure 1. Schematic diagram of apparatus for separatlon of volatile and nonvolatile nitrates. 1.5 I
I
5- 1.08 0
- 0 0
1 Time, hour
2
Figure 3. Time dependence of volatile nitrate recovery. Filter samples from a same hlgh volume filter were heated at 160 OC in a nitrogen flow.
2
-0
8
8
a
0.5 -
'0
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= 1.023 X - 0.002 = 0.9913)
(r
1.0 Supplied NO;, pmol
0.5
.E
Residual NO;
.-0
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1
1.5
Flgure 2. Recovery characteristics of NO3- on reagent ",NO, evaporation under the heating condition at 160 O C for 30 min In a nitrogen flow.
50-
b 0
B
8
Evapolated NO;
E
were used in winter and summer, respectively, as the filter medium. The sampling was made for several days in winter and summer on the roof of the Tokyo Metropolitan Research Institute for Environmental Protection which is located in downtown Tokyo about 5 km from the Tokyo Bay.
Flgure 4. Temperature dependence of volatile nitrate recovery. Filter samples from a same hlgh volume fllter were heated in a nitrogen flow for 1 h.
Results and Discussion Analytical Method of Volatile and Nonvolatile Nitrates. Figure 2 shows a result of the recovery check by using standard nitrate samples prepared by dropping a certain amount of aqueous solution of reagent ",NO3 on a Teflon filter and drying it a t room temperature. The abscissa corresponds to the amount of NH4N03supplied on a filter in terms of NO3-. The ordinate corresponds to the amount of NO3- collected in a cold trap when the sample was heated in a furnace shown in Figure 1a t 160 "C for 30 min under a 1L/min N2 flow. The result show a good linear correlation through the origin with 1:l slope and a high correlation coefficient. Therefore, the recovery ratio of nitrate can be considered to be about 100%. The similar relation was also observed as for NH4+. Moreover, an equimolar consistency between NO3- and NH4+ in a cold trap was observed satisfactorily. It is thought that the dissociation of ",NO3 to NH3 and HN03 is a dominant reaction and its decomposition to NO is negligible under the heating condition of reagent NH4N03. The relation between volatile nitrate recovery and heating time a t 160 OC is shown in Figure 3 by using a Quarz filter on which ambient aerosols were collected by a high volume sampler for 24 h during Oct 2-3, 1982. Round filter samples with 1.5 cm of diameter were prepared from the same high volume filter. It is realized that a sufficient amount of nitrate could be recovered by heating for 1h. The vapor pressure of ",NO3 a t 160 OC is calculated to be 604 ppm according to its equilibrium constant (10). Therefore, all of ",NO3 would be ex-
pected to complete the evaporation instantaneously under a 1L/min nitrogen gas flow from the equilibrium theory. But the heating for 30 min seemed to be insufficient. This result suggests that the evaporation of ambient ",NO3 was controlled by a diffusion process (17)because it was surrounded by other aerosol substances. In the case of ambient particulate NH4+,excess amount of NH4+to NO3was collected in a cold trap on the molar basis because NH4Cl and (NH4)&304in the aerosol could also release NH3 under the heating condition. Figure 4 shows the relation between volatile nitrate recovery and heating temperature by using samples cut from the same high volume filter as used in Figure 3. The sample was heated for 1 h. Closed circles show the evaporated nitrate collected in a cold trap. Open circles show the sum of residual nitrate on a filter after the heating treatment and evaporated nitrate. Although a small amount of nitrate was evaporated at 50 "C, the evaporation was increased with the increase of temperature with a maximum recovery of volatile nitrate at 160 O C . The evaporation of ",NO3 in the sample would have been completed a t more than 58 "C from the standpoint of ",NO3 equilibrium under the condition of a 1 L/min nitrogen flow and heating for 1 h. Nevertheless, the recovery a t low temperature was observed to be very small. As discussed before, these results suggest that ambient ",NO3 does not evaporate easily because of the diffusion control condition due to its structure in ambient aerosols. On the other hand, the summed total nitrate recovery is kept to be about 100% until about 120 "C. But it de-
n v
0
I
100
200
300
Temperature, 'C
Envlron. Scl. Technol., Vol. 19, No. 3, 1985
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l - l l
Table I. Meteorological Conditions over the Sampling Periods
10
1000 on Dec 8 to 1000 on
1O:OO on Aug 5 to 1O:OO on Aug 9, 1983
Dec 10. 1982 nighttime daytime (1800(6:00-1800)
temperature, 9.0 f 2.7"
600) 6.9 f 1.5
daytime (600-
nighttime (18:00-
1800)
6:OO)
33.0
"C
* 1.6
29.4 f 1.1
humidity, % 63.3 f 13.3 68.7 f 11.5 65.9 f 6.6 81.1 f 6.7 wind speed, 1.7 f 0.5 1.6 0.8 1.8 f 0.7 1.1 f 0.5 m/s wind approximate S approximate NW direction Standard deviation.
*
Table 11. Recovery Ratios of Nitrate by the Andersen Sampler Stage
stage no. 0
10.
NaN03 5-
1 2 3 4
.
recovery ratio, %" 81.2 93.7 95.5 87.8 87.7
stage no. 5 6
7 backup filter
recovery ratio, %" 94.9 97.0 99.9 98.1
Recovery ratio = (sum of vcutile and residur. nitrates)/(total nitrate). The total nitrate was measured without the heating treatment. Sampling was made for 96 h during Dec 8-10, 1982.
creased gradually with the increase of temperature. Finally, the residual nitrate on a filter did not appear at 280 OC. These results suggest that the decomposition of nitrate to NO took place competitively with the evaporation in ambient aerosols. It has been shown by Moskowitz (2) and Spicer et al. (18)to analyze nitrate that the thermal decomposition takes place quantitatively to NO a t a high temperature of more than 500 O C . Heating at a lower temperature for a longer time might give a better nitrate balance but would result in consuming too much time in the sample treatment. In conclusion, the temperature of 160 "C is considered to be optimum from the above results in this study, a t which the highest amount of evaporated nitrate was recovered after 1h. In addition, nonvolatile nitrate should be evaluated by the difference between the total nitrate on a filter analyzed without the heating treatment and volatile nitrate to minimize the influence of the decomposition to NO on the measurement. The evaporation seems to be rather rapid compared with the thermal decomposition. Therefore, the error by the decomposition is thought to be not so large as the evaporation. Moreover, the influence of the decomposition might depend on the physical conditions of volatile nitrate in aerosols partially according to sampling conditions to accumulate aerosols on a filter. Measurements of Nitrate Size Distribution in Ambient Aerosols. Figure 5 shows the size distributions of nitrates collected by an Andersen sampler in downtown Tokyo for 48 h during Dec &10,1982. Total nitrate shown in Figure 5a was obtained by using a half of a whole filter by stage without the heating treatment. Some meteorological data are given in Table I. The distribution seems to be bimodal, fine particle nitrate being in higher concentration than coarse particle nitrate. This is considered 260
Environ. Sci. Technol., Voi. 19, No. 3, 1985
to be one of the typical nitrate size distributions in winter as shown by Kadowaki (1). Volatile nitrates from respective filters were shown in Figure 5b by using the remainder of a whole filter. In Figure 5c, the difference between total and volatile nitrates was shown as an amount of nonvolatile nitrate which should be consistent, in principle, with the residual nitrate on a filter after the heating treatment. Table I1 shows the material balance between the total nitrate and the sum of volatiitle residual nitrates with respect to the above data. The recovery ratio in this method falls in the range 81.2-99.9%, which shows that a sufficient amount of nitrate was recovered except for the first stage of an Andersen sampler on which the smallest amount of aerosol was collected resulting in a large experimental error. In this case, the reduction of nitrate to NO seems to be substantially negligible. Volatile nitrate can be ascribed to ",NO3 because it has high volatility at ambient temperature as Kadowaki (1)and Stelson et al. (10) discussed. It shows a monomodal size distribution with a sharp peak in a fine particle size range. This result suggests that the formation of ",NO3 depends on a gas-phase reaction between NH3 and HN03. On the other hand, nonvolatile nitrate shows a peak in the coarse particle size range. It has been pointed out that the nitrate in the coarse particle would be NaN03 (might also include K+, Ca2+,Mg2+, etc. as a countercation) whose vapor pressure is negligibly low a t ambient temperature, which is derived from sea salt particles (1, 8, 9, 19). Size distributions of nitrates in summer are shown in Figure 6, which were collected for 96 h during Aug 5-9, 1983. In contrast to the distribution in winter shown in Figure 5, total nitrate shown in Figure 6a seems to be bimodal, the coarse particle nitrate being in higher concentration than the fine particle nitrate, which is considered to be one of the typical nitrate distributions in summer as shown by Kadowaki (1). Volatile and nonvolatile nitrates are shown in parts b and c of Figure 6 which can be ascribed to ",NO3 and NaN03, respectively, as discussed with respect to Figure 5. The significant difference
formation mechanisms and have sharp monomodal peaks in fine and coarse particle ranges, respectively. As a result, the total nitrate is distributed in a bimodal form by overlapping both nitrates. Registry No. NH4N03,6484-52-2; NaN03, 7631-99-4.
Literature Cited (1) Kadowaki, S. Atmos. Environ. 1977, 11, 671. (2) Moskowitz, A. H. “Particle Size Distribution of Nitrate Aerosols in the Los Angeles Air Baisn”. U.S. Environmental
i “4N4
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-
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Flgure 6. Size distributions of atmospherlc nitrates in downtown Tokyo in summer.
between the levels of NH4N03in winter and summer is observed although the NaN03 levels seem to be similar. It is considered that a large part of “,NO3 exists as HN03 and NH3 gases in summer due to its high vapor pressure. That is, the aerosol NH4N03concentration in summer becomes smaller compared with in winter as shown experimentally in this study. This result is consistent with the observations in previous papers, in which it is shown that gaseous HN03 concentration is sometimes higher than particulate nitrate in summer (15,20-25). The present NH4N03levels collected with a filter base technique may be influenced by the artifact nitrate formation due to its volatilization and condensation (10-12) and its reaction with sulfuric acid (26). However, the artifact would not be so serious that the distinguishable seasonal trend was invalid. In conclusion, ambient nitrates are considered to consist of NH4N03and NaN03 which are derived from different
Protection Agency, 1977, EPA-60013-77-053. (3) Cronn, D. R.; Charlson, R. J.; Knight, R. L.; Crittenden, A. L.; Appel, B. R. Atmos. Environ. 1977,11, 929. (4) Lundgren, D. A. J. Air. Pollut. Control Assoc. 1970,20,603. (5) Gordon, R. J.; Bryan, R. J. Environ. Sci. Technol. 1973, 7, 645. (6) Grosjean, D.; Friedlander, S. K. J. Air Pollut. Control Assoc. 1975,25,1038. (7) Mamane, Y.; Pueschel, R. F. Atmos. Environ. 1979,14,629. (8) Isawa, Y.; Ono, A. J . Meteorol. Soc. Jpn. 1979, 57, 599. (9) Harrison, R. M.; Pio, C. A. Atmos. Environ. 1983,17,1733. (10) Stelson, A. W.; Friedlander, S. K.; Seinfeld, J. H. Atmos. Environ. 1979, 13, 369. (11) Stelson, A. W.; Seinfeld, J. H. Atmos. Environ. 1982, 16, 983. (12) Stelson, A. W.; Seinfeld, J. H. Atmos. Environ. 1982,16, 993. (13) Yoshizumi, K.; Okita, T. J. Air Pollut. Control Assoc. 1983, 33, 224. (14) Cox, R. D. Anal. Chem. 1980, 52, 332. (15) Yoshizumi, K.; Aoki, K.; Matsuoka, T.; Asakura, S. Anal. Chem., in press. (16) Weatherburn, M. W. Anal. Chem. 1967, 39, 971. (17) Larson, T. V.; Taylor, G. S. Atmos. Environ. 1983,17,2489. (18) Spicer, C. W.; Schumacher, P. M.; Kouyoumjian, J. A.; Joesph, D. W. “Sampling and Analytical Methodology for Atmospheric Particulate Nitrates”. U.S. Environmental Protection Agency, 1978, EPA-60012-78-067. (19) Robbins, R. C.; Cadle, R. D.; Eckhardt, D. L. J. Meteorol. 1959, 16, 53. (20) Forrest, J.; Tanner, R. L.; Spandau, D.; D’Ottavio, T.; Newman, L. Atmos. Environ. 1980, 14, 137. (21) Grennfelt, P. Atmos. Environ. 1980, 14, 311. (22) Appel, B. R.; Tokiwa, Y.; Haik, M. Atmos. Environ. 1981, 15, 283. (23) Shaw, R. W., Jr.; Stevens, R. KO;Bowermaster, J. Atmos. Environ. 1982, 16, 845. (24) Forrest, J.; Spandau, D. J.; Tanner, R. L.; Newman, L. Atmos. Environ. 1982, 16, 1473. (25) Cadle, S. H.; Countess, R. J.; Kelly, N. A. Atmos. Environ. 1982,16, 2501. (26) Harker, A. B.; Richards, L. W.; Clark, W. E. Atmos. Environ. 1977, 11, 87.
Received for review June 8,1984. Revised manuscript received October 9, 1984. Accepted October 16, 1984.
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