Environ. Sci. Technol. 1999, 33, 1444-1447
Natural Denitrification in Drying Process of Dew N O R I M I C H I T A K E N A K A , * ,† TAKAHIKO SUZUE,† KINGO OHIRA,† TAZUKO MORIKAWA,‡ HIROSHI BANDOW,† AND YASUAKI MAEDA† College of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Sakai, Osaka 599-8531, Japan, and Japan Automobile Research Institute, 2530 Karima, Tsukuba-shi, Ibaraki 305, Japan
This report is the first observation of natural chemical denitrification in the drying process of dew. Nitrogen compounds released in the atmosphere are considered to be subjected to oxidation to form nitric acid by the chemical process so far. The results here show that nitrous acid is reduced to N2 by drying of dew. Dew was collected at Osaka Prefecture University in Sakai City, Japan from 1996 to 1997. Concentrations of ammonium and nitrite ions in dew were very high related to those in rain, and pHs of dew were relatively high, pH ca. 6.5. We found that when dew was dried, most nitrite and ammonium ions included in dew were decomposed. It is wellknown that concentrated ammonium nitrite aqueous solution is unstable and decomposes to N2 and H2O. During the drying process of dew, nitrite and ammonium would be concentrated and react to form N2, and as a result, nitrite and ammonia in the dried dew are lost, that is, natural denitrification occurs in the drying process of dew. We report here results of the natural denitrification.
Introduction Nitrogen oxides released into the environment are oxidized to nitric acid by various processes and are an important cause of acidification of the environment. Nitric acid dissolves into rainwater very much and is removed from the atmosphere by rainfall. In addition, nitric acid is removed by dry deposition and is reacted with sea salts and alkaline compounds. The fate of nitrogen oxides released into the environment has been considered as mentioned above so far. After cloud particles, ice nuclei, dew, and frost, which are condensed phases of water in the atmosphere, form in the atmosphere, they often experience a drying process. In the drying process, substances included in them are concentrated. As a result, rates of chemical reactions increase, and some reactions which could have been neglected due to slow reaction rate could occur within meaningful time scale. There are very few reports on dew chemistry (11-14). In dew, unstable compounds, such as nitrite and sulfite, are detected. Further, concentration of compounds such as ammonium which come from gaseous substances are very high related to those in rain, sand, and snow. It is well-known * Corresponding author phone: (81) 722-52-1161 ext 2397; fax: (81) 722-54-9321; e-mail:
[email protected]. † Osaka Prefecture University. ‡ Japan Automobile Research Institute. 1444
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that ammonium nitrite is very unstable at high concentration (1-10).
NH4NO2 f N2 + 2H2O
(1)
It is reported that this reaction in aqueous solution depends on first-order for ammonium and second-order for nitrite (3). Gushchin et al. investigated the decomposition of NH4NO2 aerosol in the gas phase, and they reported that 50.8-78.3% of NH4 NO2 was decomposed at 7-36 °C (7). Concentrations of nitrite and ammonium ions are very high in dew. After sunrise, dew dries with increasing temperature, and substances in dew are concentrated extremely. Nitrous acid in gas phase is decomposed by sunlight very quickly (15), while in solution it is very stable even under solar radiation (16) and rarely decomposes until dew dries completely. Hence, reaction 1 could occur. This is a natural denitrification by a chemical process, and this means that there is a chemical process in nature, which depresses the acidification of nature. We report here the natural denitrification by drying of dew. Further, we evaluate the effect of drying of dew, that is, natural denitrification on ozone buildup.
Experimental Section Dew was collected at Osaka Prefecture University in Sakai City, Japan from 1996 to 1997 with a Teflon coated stainless steel vat (W.200 mm × L.250 mm × H.50 mm) put on a foamed polystyrene block (17). After sunset, the six sets of dew samplers were put on a table of 1-m height on a roof of a building of which the height was ca. 15 m. Three of the dew samples are used for dew analysis and the other three for dried dew analysis. Before sunrise, dew that had formed and grown on the bottom surface of the vats at night was collected with a clean Teflon brush. The dew collected was filtrated with 0.22 µm membrane filter, and pH and ion concentrations were measured immediately. The dew collected in this manner contains not only gas-phase substances and particulate matter deposited with forming of dew but also dry deposition. To investigate the composition change in drying of dew, at the same time as the dew was collected, the other three vats on which dew were formed were transferred to a laboratory without any treatment. The vats transferred to the laboratory were covered loosely to prevent more deposition of particles, and the dew was dried in the dark in a laboratory at room temperature. Hereafter we call this sample dried dew. After the dew was dried, 5 cm3 of pure water (resistance >18.2 Mohm cm) was added to the each vat, and the residual was extracted carefully. The extracts were treated as the same method for dew. For natural dew and dried dew, the concentration is represented as eq/m2, which is an equivalent value per unit area of deposition, to compare the change in concentration. The ion concentrations were determined with an ion chromatography (Yokogawa Analytical Systems, Inc. IC-7000 type Ion Chromatographic Analyzer) with the ICS-A44 column for inorganic anions, with the ICS-C25 column for cations and with the CHA-E11 column for organic anions. To investigate whether nitrite reacts with ammonium or it escapes into the gas phase as HONO, an aliquot of NaNO2NH3‚H2O-H2SO4 (or NaOH) solution was dried at room temperature, and each concentration in the residuals and the gas phase was analyzed. The concentration of nitrite was adjusted to 5 mmol dm-3, and 0.1 cm3 of the sample solution was dried. It had already been found by the pre-experiment 10.1021/es980913h CCC: $18.00
1999 American Chemical Society Published on Web 03/27/1999
TABLE 1. Ion Concentrations of Dew and Dried Dew Obtained in Sakai City in 1996-1997a dew
volume, pH ion concn, µeq/m2
cm3 ForAceClNO2NO3SO32SO42Na+ NH4+ K+ Ca2+ Mg2+
dried dew
av
max.
min.
med.
no.
av
max.
min.
med.
no.
5.48 6.48 1.49 2.70 10.78 6.36 1.98 1.53 5.21 9.43 22.09 1.10 6.61 1.66
16.50 7.29 4.09 8.15 37.15 31.06 9.37 2.36 23.74 40.10 119.92 2.24 19.30 5.18
0.35 5.53 0.07 0.05 0.67 0.00 0.00 1.01 1.08 0.60 0.43 0.16 1.03 0.00
4.52 6.64 1.17 1.71 6.77 3.14 1.28 1.21 3.79 3.82 12.08 1.18 5.44 1.12
20 19 8 8 20 20 20 4 20 20 20 20 20 20
5.97 1.06 0.87 8.24 0.37 1.79 1.59 8.22 4.94 5.92 0.98 7.96 0.90
6.84 3.37 2.22 27.04 1.58 3.89 3.56 15.91 11.02 12.91 1.86 15.19 4.16
5.27 0.02 0.04 2.62 0.00 0.46 0.00 1.31 1.60 0.93 0.00 2.16 0.00
6.01 0.46 0.29 6.32 0.00 1.68 1.55 7.55 3.99 5.57 0.93 8.74 0.54
10 6 6 10 10 10 4 10 8 8 8 8 8
a The unit of ion concentration is an equivalent value per unit area of the dew sampler. For-, formate; Ace-, acetate. Med., median; no., number of samples.
TABLE 2. Nitrite, Ammonium, and Nitrate Ion Concentrations in Dew and Dried Dewb NO2- (µeq/m2)
NH4+ (µeq/m2)
NO3- (µeq/m2)
M/D/Y
dew
Ddewa
dew
Ddewa
dew
Ddewa
Feb27/97 Feb28/97 Nov06/97 Nov07/97 Nov10/97 Nov20/97 Dec01/97 Dec05/97 Dec16/97 Dec19/97
1.82 ND 12.42 4.14 0.96 0.43 3.23 10.08 6.24 21.35
ND ND 1.45 ND 0.70 ND ND ND 0.79 ND
24.20 7.60 24.85 15.28 3.03 0.43 8.89 16.75 22.74 54.55
12.91 5.21 0.98 3.24 1.64 7.55 10.72 7.98
2.28 1.32 2.31 1.45 0.28 0.25 0.84 2.06 2.19 6.66
0.63 1.58 2.44 1.80 1.38 2.69 0.46 2.43 1.43 3.89
a Ddew: dried dew. The dried dew samples were dried in dark and then extracted with pure water. b ND, below detection limit; -, no data. The unit of ion concentration is an equivalent value per unit area of the dew sampler.
that the change in composition in the residue occurred when almost all water was evaporated. Therefore, the initial concentration is not important. The residuals was extracted with 5 cm3 of pure water, and the extract was treated in the same way as mentioned above. Nitrous acid in the gas phase was flushed with synthesized air at 10 cm3/min of flow rate and introduced in alkaline solution. For ammonia in the gas phase, the flushing air was introduced in boric acid solution in the separate experiment.
Results and Discussion Table 1 shows average, maximum, minimum, and median concentrations of ions in dew and dried dew. The pHs of dew were high compared to those of rain and snow at Sakai (18) and usually over pH 6. Concentrations of ions that are speculated to come from gaseous substances (19), such as formate, acetate, nitrite, sulfite, and ammonium ions, were much higher than those in rain or snow (18). The same results are also reported in the literature (11-14). The concentration of ammonium ion was very high, and this could be the main cause of high pH and high concentrations of weak acid salts. The concentrations of nitrite, which ranged from several tens to more than a hundred µmol dm-3, were usually higher than those of nitrate. Table 2 shows nitrite, ammonium, and nitrate ion concentrations in each dew and dried dew. It is apparent from Table 2 that nitrite and ammonium ions which had contained in dew lost from dried dew samples. Most of the lost nitrite
was not oxidized to nitrate as shown in Table 2, and nitrite must escape to gas phase even at high pH or be converted to other nitrogen species. The fact that more ammonia was lost from the dried dew sample is probably due to differences in evaporation losses of ammonia and nitrous acid. Nitrous acid in the gas phase is decomposed immediately after sunrise (15). However, the results in Table 2 were obtained in dark. On the other hand, nitrite in an aqueous phase is very stable even under solar radiation. The lifetime of nitrite in aqueous phase in midday in summer at middle latitude is estimated as about 8 h (16). Then, the fate of nitrite in the drying of dew was investigated in laboratory experiments. The aqueous solution of known concentrations of sodium nitrite and ammonium ion was dried in dark, and concentrations of nitrite and ammonium ions in gas phase and residuals were measured. The results are shown in Table 3. Most nitrite and ammonium were missing from the residual and the gas phase when the solution was alkaline and NH4+ was contained (case 1 in Table 3). This is the same situation as drying of most natural dew. On the other hand, all nitrite was detected in the residual when the solution was alkaline and NH4+ was not contained (case 2 in Table 3). In the case of acidic conditions (case 3 in Table 3), some nitrite was oxidized to nitrate, 45% of nitrite was detected in the gas phase as HONO, and about 44% of nitrite was missing. The formation of nitrate is due to oxidation by dissolved oxygen, and the evaporation of HONO is due to HONO as a weak acid (pKa ) 3.45 at 25 °C). Figure 1 shows what factor dominates the amounts of nitrite in the residue. This figure implies that the loss of nitrite from the residual was governed not by pH but amounts of Na+ and SO42-. The solubility of Na2SO4 in water, 0.068 mol/(100 g saturated solution), is much lower than NaNO2, 0.66 mol/(100 g saturated solution) at 25 °C (20). While drying, sodium ion combines with sulfate to form precipitation first. The precipitation of Na2SO4 continues until the concentrations of sodium ion and sulfate ion in the solution reach to saturated concentration of other species. Nitrite ion has to combine with a cation at the end of the drying. In case 1 in Table 1, the possible situations are the following. (i) Nitrite combines with NH4+. When the solution is basic and contains high ammonium, NH4NO2 is formed. However, it is well-known that concentrated NH4NO2 is unstable and decomposes into N2 and H2O. This is the result in case 1 in Table 3.
NH4NO2 f N2 + 2H2O
(1)
Also, the nitrite losses from the dried dew in Tables 1 and 2 are due to reaction 1, in other words those are due to the VOL. 33, NO. 9, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 3. Fate of Nitrite by Drying of Aqueous Solution after dryingc original
solnc
residuea
gas phaseb
no.
composition of the original soln
NO2-
NH4+
NO2-
NH4+
NO3-
NO2-
NH4+
NO3-
1 2 3
100 µM NaNO2 + 50 µM (NH4)2SO4 100 µM NaNO2 + 50 µM Na2SO4 100 µM NaNO2 + 50 µM H2SO4
100.0 100.0 100.0
100.0 0 0
15.9 100.0 nd
15.3
nd nd 10.9
10.1 nd 44.9
5.6
nd nd nd
a Dried samples were extracted with pure water of which volume was the same as the original volume. b In the experiment no. 1, NO in the 2 gas phase (the gas passed through the absorption solution for HONO) was absorbed with the Saltzman reagent and obtained to be equivalent 3 c 3 to 5 µmol/dm nitrite. µmol/dm .
FIGURE 1. Nitrite concentration in the residue versus the ratio of equivalent concentration difference of Na+ and SO42-. The subscripts i and f denote initial and final concentrations, respectively. The results in three series of compositions are included in the figure, that is, NaNO2-Na2SO4-NaOH, NaNO2-Na2SO4-(NH4)2SO4, and NaNO2-(NH4)2SO4-NH3(aq). Initial concentration: NaNO2; 2.5 mmol dm-3 in three series, Na2SO4; 0-1.25 mmol dm-3, (NH4)2SO4; 0-1.25 mmol dm-3, NaOH; 0-2.5 mmol dm-3, and NH3(aq); 0-2.5 mmol dm-3. natural denitrification. (ii) Nitrite combines with Na+. When the solution contains low or no ammonium at high pH, NaNO2 salt is formed in the residual (case 2 in Table 3).
Na+ + NO2- f NaNO2V
(2)
(iii) Nitrite combines with H+ (acidic condition). When the solution is acidic, HONO is formed, and it is oxidized to nitrate (reaction 3), decomposes into NO and NO2 (reaction 4), or evaporates as HNO2 (reaction 5).
2HNO2 + O2 f 2HNO3
(3)
2HNO2 f NO + NO2 + H2O
(4)
HNO2(aq) f HNO2(g)
(5)
The ratio of the three paths (reactions 3-5) would depend on the drying speed of dew. As a result, the fate of nitrite deeply depends on the composition and drying speed of dew. The source of HONO has not been clarified yet but is probably direct emission from vehicles or heterogeneous formation. It is well-known that ozone concentration is influenced by the presence of HONO and other nitrogen oxides in the gas phase (21). The dew plays in part as a sink for HONO, but also in part as a source for HONO when it disappears at sunrise. Then, the effects of drying of dew on the ozone concentration, that is, evaporation of HONO 1446
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FIGURE 2. Calculation results of time profile of ozone concentrations in late autumn with CBM-IV box model. Solid line: (A) The case of dew formation and the denitrification. Some of HONO in the gas phase is absorbed into the dew. Nitrite concentration in dew and HONO in the gas phase are assumed to be 10 µeq/m2 and 0.2 ppbv, respectively. These values are commonly observed ones in this season. Further, it is assumed that the dew dries between 7 and 8 o’clock, and then all nitrite reacts with ammonium to form N2. Broken line: (B) The case of dew formation but no denitrification. The same assumption as (A) is adapted except that nitrite in dew released into the gas phase as HONO. Dotted line: (C) The case of no dew formation, that is, all HONO exists only in the gas phase. In the three cases, total amounts of HONO plus nitrite are the same in the initial condition. and denitrification by dew formation were evaluated with the CBM-IV box model calculations (22). Figure 2 shows the calculation results. The calculations were performed on late October at Sakai City. We postulated three situations, that is, (A) the formation of dew occurs, and denitrification occurs; (B) the formation of dew occurs, and nitrite is released into the gas phase as HONO by drying of dew; and (C) no dew is formed. Further we assumed that the dew dries between 7 and 8 o’clock, and total amounts of HONO plus nitrite are the same in the three calculations. The results are shown in Figure 2. In case (B), ozone concentration increases due to the dew formation. On the other hand, in case (A), it decreases due to the natural denitrification by drying of dew. As mentioned here, ozone concentration is affected by dew formation. Especially, the natural denitrification of dew is very important to evaluate global tropospheric ozone concentration.
Conclusion In the drying process of dew containing high concentration of ammonium and nitrite at relatively high pH, it is clarified that the natural denitrification can occur via a chemical process. In fog and cloud droplets, high concentrations of nitrite and ammonium have also been observed (23-26). These droplets also experience drying in nature. In these
cases also, the natural denitrification would occur. The natural denitrification via chemical reaction could play an important role in nitrogen chemistry in the environment.
Acknowledgments This study was supported in part by Grant-in-Aid for Scientific Research (B) from The Ministry of Education, Science, Sports and Culture, Japan.
Literature Cited (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14)
Mayer, J.; Trutzer, E. Z. Electrochem. 1911, 14, 69. Ray, P. C. J. Chem. Soc. 1912, 95, 345. Abel, E.; Schmid, H.; Schafranik, J. Z. Physik. Chem. 1931, 510. Ghosh, S. K. Z. Physik. Chem. 1957, 206, 321. Itkina, D. Ya.; Miniovich, M. A.; Nazarova, T. I. Zh. Prikl. Khim. 1962, 35, 43. Kolarov, N.; Popyankov, B.; Angelov, Sp. Monatsh. Chem. 1965, 96, 949. Gushchin, B. N.; Borodavko, P. P. Kim. Prom. 1968, 44, 356. Dusenbury, J. H.; Powell, R. E. J. Am. Chem. Soc. 1951, 73, 3266. Rubin, M. B.; Noyes, R. M.; Smith, K. W. J. Phys. Chem. 1987, 91, 1618. Mebel, A. M.; Lin, M. C.; Morokuma, K.; Melius, C. F. J. Phys. Chem. 1995, 99, 6842. Okochi, H.; Kajimoto, T.; Arai, Y.; Igawa, M. Bull. Chem. Soc. Jpn. 1996, 69, 3355. Foster, J. R.; Pribush, R. A.; Carter, B. H. Atmos. Environ. 1990, 24A, 2229. Mulawa, P. A.; Cadle, S. H.; Lipari, F.; Ang, C. C.; Vandervennet, R. T. Atmos. Environ. 1986, 20, 1389. Wagner, G. H.; Steele, K. F.; Peden, M. E. J. Geophys. Res. 1992, 97, 20591.
(15) Seinfeld, J. H.; Pandis, S. N. Atmospheric Chemistry and Physics; John Wiley and Sons: New York, 1998; p 252. (16) Fischer, M.; Werneck, P. J. Phys. Chem. 1996, 100, 18749. (17) Takenaka, N.; Suzue, T.; Sato, K.; Taguchi, K.; Bandow, H.; Maeda, Y. Proceedings of the 3rd International Joint Seminar on the Regional Deposition Processes in the Atmosphere; 1997; p 16. (18) Sakai City Government Office, Sakai-no-Kankyo, 1995, 75. (19) Dry deposition was also collected with a Teflon coated stainless steel vat without polystyrene foam block (17). The speculation of gaseous substances is based on the comparison of concentrations of ions in the dew and the dry deposition collected in the same period. (20) Kagaku-Binran (Handbook of Chemistry); The Chemical Society of Japan: Maruzen, 1975; pp 777-791. (21) Harris, G. W.; Carter, W. P.; Winer, L. A. M.; Pitts, J. N., Jr.; Platt, U.; Perner, D. Environ. Sci. Technol. 1982, 16, 414. (22) Gery M. W.; Whitten, G. Z.; Killus, J. P.; Dodge, M. C. J. Geophys. Res. 1989, 94, 12925. (23) Sigg, L.; Stumm, W.; Zobrist, J.; Zurcher, F. Chimia 1987, 41, 159. (24) Fuzzi, S.; Orsi, G.; Nardini, G.; Facchini, M. C.; McLaren, S.; McLaren, E.; Mariotti, M. J. Geophys. Res. 1988, 93, 11141. (25) Miller, D. R.; Byrd, J. E.; Perona, M. J. Water Air Soil Pollut. 1991, 32, 329. (26) Takenaka, N.; Daimon, T.; Ueda, A.; Sato, K.; Kitano, M.; Bandow, H.; Maeda, Y. J. Atmos. Chem. 1998, 29, 135.
Received for review September 7, 1998. Revised manuscript received February 1, 1999. Accepted February 8, 1999. ES980913H
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