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Nitric acid, particulate nitrate and ammonium profiles at the bayerischer wald: evidence for large deposition rates of total nitrate
Pergamon
Atmospheric Environment Vol. 28, No. 2, pp. 311-315, 1994 Elsevier Science Ltd Printed in Great Britain. All rishts reserved 1352 2310/94 $6.00+0.00
NITRIC ACID, PARTICULATE NITRATE A N D A M M O N I U M PROFILES AT THE BAYERISCHER WALD: EVIDENCE FOR LARGE DEPOSITION RATES OF TOTAL NITRATE H. SIEVERING,*~"G. ENDERS,~L. KINS,* G. KRAMM,§K. RUOSS,* G. ROIDER,* M. ZELGER,* L. ANDERSON," a n d R. DLUGI* *Meteorologisches Institut der Universitfit Mfinchen, 8000 Miinchen 2, Germany; "t'Center for Environmental Sciences, University of Colorado at Denver, Campus Box 136, P.O. Box 173364, Denver, CO 80217-3364, U.S.A.; :~Lehrstuhl fiir Bioklimatologie und Angevandte, Meteorologie der Universit/it Miinchen; § Fraunhofer Institute fiir Atmospharisches Umweltforschung, Germany (First received 31 December 1992 and in final form 21 May 1993)
Almtract--Chemical measurements at five levels within and above a predominantly spruce forest at the Bayerischer Wald (Bavarian forest) National Park, Germany, site show that particulate nitrate, as well as nitric acid, is rapidly removed to the forest canopy. The rate of dry removal to the forest canopy for particulate nitrate was nearly as large as that for nitric acid. Cascade impactor data indicate a major reason for the large particulate nitrate deposition rates may have been its 2-2.5 gm mean diameter. Dry removal of total nitrate may be sufficient to cause nitrogen saturation at this forest site. In combination with nitrogen wet deposition, the nitrogen available for biotic demand appears to be in sufficient supply to cause ammonia emission from this spruce forest canopy. At some other European forest sites, where ambient air nitrate concentrations are higher, the dry plus wet removal of total nitrate may be sufficient to cause nitrogen availability to be substantially greater than its biotic demand. Key word index: Nitrogen, nitrate, nitric acid, spruce forest, Germany, dry deposition.
INTRODUCTION Nitrogen (N) fluxes at the Earth's forested regions play a dominant role in global nitrogen cycling (Schlesinger, 1991). Net, or above-ground, primary production in some temperate zone forests appears to show a correlation with N inputs in dry deposition and precipitation (Cole and Rapp, 1981; Kauppi et al., 1992). Gunderson (1991) has estimated long-term critical N loads for forest ecosystems to be 2-20 kg N h a - 1 y r - 1. Wet deposition alone, in parts of Asia, Europe, and North America, borders on or falls in this critical N loading range (Warneck, 1988). The term "critical N loading" is defined to be that amount of added nutrient N from atmospheric deposition to cause nitrogen saturation (N availability in excess of biotic demand--Aber et al., 1991). Thus, the 2-20 kg N h a - l y r -1 is felt to encompass the atmospheric loading of N at which this saturation occurs. To obtain a better understanding of nitrogen budgets and especially to quantify pollutant dry fluxes between the atmosphere and biosphere, the European Biosphere Atmosphere Exchange of Pollutants (BIATEX) program was instituted. The BIATEX experimental forest Schachtenau site (Enders et al., 1992), located in the Bayerischer Wald National Park at 48°57'N, 13°25'E in northeastern Bavaria, is an 86% spruce, 14% beech, 80-100 yearold forest site with a fairly even but slightly sloping
canopy. The mean canopy height is 29 m with its base somewhat uneven and beginning in the 10-14m height range; the leaf area to ground surface area ratio is 5 to 6. The Institute for Bioclimatology and Applied Meteorology of the University of Miinchen maintains meteorological and gaseous species monitoring at a 51-m tower, with wind speed, temperature, and humidity data obtained at 51, 46, 44, 36, 31, 26, 21, 16, 11, 6, and 2-m heights, and several gaseous species data obtained at 51, 41, 31, 26, 16, and 6-m heights above the mean forest floor elevation of 807 m a.s.I (Enders et al., 1992). The Meteorologisches Institute has established a program for measurements of growing-season particulate matter ion concentrations, including ammonium (NH2), nitrate (NO3), gaseous nitric acid (HNO3), and hydrogen peroxide (H202) using filter pack sampiing (Miiller and Rudolph, 1991) at 51, 32, 22, 18, and 2-m heights. A second filter pack, for gaseous ammonia (NH3) concentrations, was occasionally deployed at each of the three heights, 51, 29, and 3 m. Finally, five-stage, 80 l.p.m. Berner cascade impactors (Hillman and Kauppinen, 1991) were deployed at the 34- and 3-m heights to provide size distributions of particulate matter ions. The purpose of this paper is the assessment of vertical profiles for HNO3, NH~, and NO~, and, when available, NH 3, obtained at the Bayerischer Wald site as they relate to the particulate NH~ and NO~ size distribution data. Subsequent 311
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Fig. I. Height-averaged concentration plots of HNO 3, NO~, and NH~; 17-19 June 1990, Fig. la, and 25-27 June, 1990, Fig. lb. Averages are based on 16 2-hr long filter pack samples for each of the 17-19 and 25-27 June periods at the Bayerischer Wald experimental forest Schachtenau site in northeastern Bavaria, Germany. Horizontal bars are 90% confidence intervals in the ratios of 2- to 51-m and 32- to 51-m air concentrations which are given in Table I.
Nitric acid, particulate nitrate and ammonium profiles papers will more fully assess the N budget and also the behavior of 0 3 and H 2 0 2 at this predominantly spruce forest site.
EXPERIMENTAL Data sets of maximum filter pack sampling intensity (six times per day, 0600-0800, 0900-1100, 1200-1400, 1500-1700, 1800-2000, and 2100-2300 Middle European Standard I-MES] time) and maximum impactor sampling (four times per day) were obtained for the periods 17-19 June and 25-27 June 1990. Vertical profile data (i.e. when at least four of the 51, 32, 22, 18, and 2-m heights provided data) were obtained from the filter packs for the species HNOa, NH~, NO~, and SO42-. These 90-mm filter packs consist of a Teflon filter (Sartorius No. 11807 0.2/~m pore size) for particulate matter collection followed by two nylon filters for the absorption of H N O 3. Nylon filter mats (Lutravil Spinulies) are cut to size, cleaned by triple-washing in deionized water and dried at 60°C. Filter samples are collected for 2 h at ~60 1.p.m.; samples are then stored at -30°C. Analysis of 10ml deionized water extracts was performed by ion chromatography (Dionex 2020i with AS4A anion and fast cat-I cation columns). The efficiency of the HNO 3 collection is determined by consideration of the percentage of HNO 3 found on the second nylon filter. Generally, only during very high relative humidity conditions did the HNO3 collection efficiency decrease below 90%. Three-level vertical profile data were obtained for NH3 on 26 June at 50, 29, and 2-m heights using 47-mm filter packs. These used a 1.0 pm Teflon front filter to remove particulate matter from the airstream and a citric acid impregnated backup filter to collect NH 3. Metal, five-stage cascade impactors were also deployed at each of the 34- and 3-m heights throughout the 17-19 June and 25-27 June 1990 sampling periods. Given that sunrise occurred at 0400 MES and sunset at 2030 MES, five of the six filter packs (excluding only the 2100-2300 MES filter sampling time) provided daytime samples. However, during this period of June 1990 solar radiation data (Reuder, 199 l) show that only the 0900-1100, 1200-1400, and 1500-1700 MES samples from above the canopy may be declared to be daytime samples; beneath the canopy too much variability in solar intensity was observed to designate sampling times as daytime or otherwise. Thus, all sampling times will be aggregated to consider heightaveraged differences (averages of the differences in concentration between any two sampling heights for a given sampling period).
313
RESULTS A N D DISCUSSION
Figure 1 shows the height-averaged concentrations, along with representative 2- and 32-m height uncertainties therein, for the 17-19 June 1990 (Fig. la) and 25-27 June 1990 (Fig. lb) periods separately. Mean concentrations at the 51-m, ambient air sampiing level for H N O 3 , N O ~ , and N H ~ were 2.1, 0,49, and 2.4/zgm -3, 17-19 June, and 1.5, 0.33, and 1.2 #g m - 3 , 25-27 June, respectively. Trends of dry flux to the canopy are evident in the vertical profiles for both H N O 3 and N O ~ and, less so, due to uncertainty, for NH,~. It should be noted that the total nitrate (TNO3) concentration of 1.5-3.0 #g m - 3 , is substantially less than that found at some European sites, e.g., G6ttingen with 12 #g m - 3 , and is similar to that observed at other sites (Lindberg et al., 1990; Andersen et al., 1993). The percentage of T N O 3 as N O ~ at the Bayerischer Wald site during June 1990 was 15-20%, which is more than the ~ 6 % at the O a k Ridge (U.S.A.) site, but much less than the 5 0 - 5 5 % at G6ttingen, Germany. The mean diameter of N O 3 found at the Bayerischer Wald site was 2-2.5 gm, about the same as that found at G6ttingen, but somewhat less than what was found at Oak Ridge (Lindberg et al., 1990). Ratios of 2- to 51-m and 32- to 51-m concentrations of H N O 3 , N O ~ , N H ~ , and SO 2- are presented (along with 90% confidence intervals) in Table 1. N o precipitation occurred during the two periods considered, so the ratios, if statistically significant and less than 1, represent dry deposition contributions to species loss from the atmosphere. Table 1 results indicate a statisticaily significant dry removal of H N O 3 and N O ~ to the forest canopy and also true for SO 2-, although the rate of dry removal is less rapid. N o significant trend is evident for N H ~ ; uncertainties are too large in this case. The metal, H N O 3 denuding inlet five-stage cascade impactor results indicate the geometric median mass diameter ( G M M D ) of N O ~ to be 2.24-1-0.85 #m; no difference in G M M D for N O ~ was observed between the 34- and 3-m measurement heights. The G M M D
Table 1. Ratios of 2- to 51-m and 32- to 51-m bihourly air concentrations of HNO3, NO~, NH~, and SO~- at the Bayerischer Wald predominantly spruce forest canopy 17-19 June 1990 2 m/51 m HNO 3 NO~ NH~ SO 2-
0.15 0.51 0.81 0.81
(-I-.05)* (_+.11) (_+.17) (_+.08)
25-27 June 1990
32 m/51 m 0.56 0.72 0.87 0.80
(_+.12) (5-.17) (_+.24) (:t:.16)
2 m/51 m 0.17 0.42 0.72 0.73
(__+.05) (_+.08) (+.14) (+.14)
32 m/51 m 0.53 (___.09) 0.62 (+.10) 0.90 (+.26) 0.78 (_+.11)
* Numbers in parentheses are 90% confidence interval values, n = 16 for the 2 m/51 m ratios for both time periods; n = 14 for the 32 m/51 m ratios (except the 17-19 June HNO 3 data which had one poor nylon filter collection efficiency). Extreme outlier ratios were not included for two 32 m/51 m ratios: 1200-1400, 19 June had a four-species average ratio of 5.3 and 0900-1100, 27 June had a fourspecies average ratio of 2.6. Sample handling and analytical procedure errors may have led to these extreme outlier results.
314
H. SIEVERINGet al.
for all other ions observed (NH2, SO~-, K ÷, Na ÷, and C1-) were <0.9/tm with the G M M D of NH2 the largest of these. Vegetational ground cover on the floor of the Bayerischer Waid site is essentially 100%; thus, no fresh soil particle surfaces are available for HNO3 conversion, and the source of the coarse particle NO 3 is not apparent. The predominantly spruce forest of the Bayerischer Wald extends for some distance in all directions from the Schachtenau site. Nocturnal fog and low clouds in the vicinity of the site are common. Fog and cloud droplets do produce coarse particle NO3 upon evaporation of the droplets. Results of modeling aerosol particle dry deposition at a similar German spruce canopy (Peters and Eiden, 1992) show, for 2-2.5/~m diameter particles, that Vd of 1-5 cm s- 1 may be expected; for particles larger than 4-5 tim Vd > 10 cm s- ~ are possible. Considering the impactor-observed NO~- size distribution and the nonlinear trend in Vd VS size found by Peters and Eiden (1992), Vd of 2-8 c m s - t may be applicable to NO~ dry deposition at the Bayerischer Wald site. H6fken et al. (1983) had estimated Vd for NO 3 in the range of 1-4 cm s- ~ with an NO 3 G m M D of 2.4/~m above a spruce forest; however, assumptions made led to large uncertainties in the lid values. The Table 1 profile results, although indicating rapid dry removal for both HNO 3 and NO3, represent the combined influence of dry deposition and chemical reactions. Kramm et al. (1992) and Kramm and Dlugi (1993) have shown that HNO 3 dry deposition is generally overestimated and NO3 dry deposition underestimated without consideration of the chemical equilibrium reactions HNO 3 + NH3~--~ NH4NO3; also, the constant flux layer assumption does not generally hold for HNO3 or NO 3 alone. Unfortunately, NH 3 concentration data, which may elucidate the extent of chemical reaction influence, were available only on 26 June 1992. Nevertheless, the change in slope of the NH~ profile within the canopy (Fig. 1), along with a near-constant NO3 concentration within the canopy, suggests that these chemical reactions may be occurring. There is one combination of N species concentrations for which the constant flux layer assumption holds and for which its concentration is not plagued by large uncertainty, i.e. the HNO 3 + NH4NO3 system. If all the NO~ is in the form of NH4NO 3, then the profiles for TNO 3 may be considered to be unaffected by chemical reactions and a constant flux layer would prevail, at least approximately, for TNO 3 above the canopy. No direct evidence is available to support all particulate NO3 above the canopy being NH4NO3. However, given ion balance data for 26 June showing that, stoichiometrically, SO 2- is present as NH4HSO, and given the ratio [NH,~]/([NO3] + [SO2-]) of 1.2 at both the 32- and 5t-m heights, it is reasonable to assume that most of the NO~ was as NH4NO3. (Note also that both the NH,~ and NH 3 molar concentrations were an order of magnitude higher than that of NO3. )
Assuming that the [HNO3]+[NH4NO3] profile data reflect a combined N species for which the constant flux layer assumption holds, one can calculate height-independent, above-canopy dry fluxes using the 51- and 32-m height data along with the 51, 46, 41, 36, and 31-m height meteorological data using procedures described in Kramm (1989) and Kramm et al. (1992). Uncertainty in these calculated fluxes is + 50-60%, due mainly to the + 6-11% uncertainty in concentration data at each of the two heights above the forest canopy. Variability in the calculated TNO3 fluxes is + 7 5 - 8 0 % given the 32m/51m data on the 17-19 June and 25-27 June (Table 1). The TNO3 nitrogen dry flux to the forest canopy is found to fall in the range from 0.7 to 5.2 mg N m -2 d -1. Given the TNO 3 concentration data at 5 l-m height, this TNO 3 flux to the canopy implies a point-estimate Vd for the combined 17-19 and 25-27 June data of 5.5 cm s- ~. However, uncertainty in these Vd values is quite large, about +65% (mostly due to the -+ 50-60% uncertainty in concentration differences). Thus, the mean Vd for TNO 3 across the 17-19 and 25-27 June period fall in the 2-9 cm s- 1 range. The mean Vds for NO3 and HNO 3 separately are approximately equal. The critical N loading of 2-20 kg N ha- t yr- 1, or 0.5-5.5 mg N m -2 d - 1, referred to in the Introduction may be compared with the range found for TNO 3 dry flux of 0.7-5.2 mg N m 2 d - l . The chronic (daily) loading of TNO 3 in the upper half of this 17-19 and 25-27 June range is, very likely, sufficient to saturate the Bayerischer Wald spruce forest. The combination of wet N deposition (roughly equal to the TNO 3 dry deposition at this site) and the dry TNO 3 flux may cause N availability to exceed its biotic demand. If the Va for TNO 3 apply at other German forest sites, dry deposition alone appears to cause N to be in excess. For example, the 12 #g TNO 3 m-3 found above the G6ttingen site implies a dry loading of 3.8-28 m g N m -2 d -t. In response to this N saturation or, perhaps, N excess status the NH 3 profile data on 26 June 1990 indicate that the forest canopy may be a source of NH 3. Table 2 shows the five profiles obtained on that day; the 29-m height NH 3 concentrations are, on average, twice as high as the 50-m height concentra-
Table 2. Ammonia (NH3) concentration data from l-h-long citric acid impregnated filter pack sampling for 26 June 1990 at the Bayerischer Wald Schachtenau site. (Uncertainty in individual NH a values is + 30%) Time, MES
07-08 09-10 12-13 15-16 18-19
NH 3 (nmolm -3) on 26 June 1990, at each of three heights above the forest floor 50 m 31.8 67.5 47.6 47.6 95.3
29 m 115. I 91.3 71.5 51.6 150.9
2m 55.6 63.5 75.4 35.7 19.9
Nitric acid, particulate nitrate and ammonium profiles tions. The forest canopy may lose NH3 directly to the atmosphere following proteolysis (Farquhar et al., 1983); this is especially likely when the ammonia partial pressure is below the compensation point (Farquhar et al., 1980). Since the aerosol particles at 29- and 50-m heights are about equally neutralized, the higher NH 3 concentrations at the 29-m height are, presumably, due to NH3 emissions from the forest canopy. Comparison of these data with a.number of other NH3, NH~, HNO3, NO3, and SO 2- data sets presented in Langford et al. (1992--see especially their fig. 5) shows that enhanced unneutralized NH 3 concentrations prevailed at the 29-m height on 26 June. This supports the notion that the 0.7-5.2mg N m - 2 d -1 added by TNO 3 dry deposition may actually cause the forest canopy to be in a state of N excess with the result that NH 3 is emitted by the forest canopy elements.
CONCLUSIONS Total nitrate dry flux to the Bayerischer Wald spruce forest appears to be sufficient to cause N saturation. At this site HNO3 probably contributes more to TNO3 dry flux than NO3 due to its higher air concentration (1-3/~g H N O 3 m -3 vs 0.3-0.6/~g NO~ m-a). At other forest canopies, such as the G6ttingen site in northern Germany, NO3 and HNO3 may contribute more equally since their abovecanopy concentrations are about equal. Given the higher concentrations of both NO~ and HNO3 above many European sites than was observed during June 1990 at the Bayerischer Wald site, it is possible that N excess conditions exist at many European locations due to TNO3 dry flux. The limited data on NHs at the Bayerischer Wald site indicate that NH3 is emitted from the spruce forest canopy, thus supporting the view that N saturation has occurred and release of N may also be occurring. Acknowledgements--We want to thank D. K6hler, E. K6hler, H. Meyer, J. Reuder, and K. Reusswigfor their assistance in the field and with data analysis. This work was supported by the Bundesminister fiir Forschung und Technologic Grant No. 07VND09 and also by the BayerischesStaatsministerium fiir Landesentwicklung und Umwelffragen Grant No. 6495-953-23800.
REFERENCES
Aber J. D., Melillo J. M., Nadelhoffer K. J., Pastor J. and Boone R. D. (1991) Factors controlling nitrogen cycling and nitrogen saturation in northern temperate forest ecosystems. Ecol. Applic. 1, 303-315. Anderson H. V., Hovmand M. F., Hummelshoj P. and Jensen N. O. (1993) Measurements of ammonia flux to a spruce stand in Denmark. Atmospheric Environment 27A, 189-202.
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Cole D. W. and Rapp M. (1981) Element cycling in forest ecosystem. In Dynamic Properties of Forest Ecosystems (edited by Reichle D. E.), pp. 341-409. Cambridge University Press, London. Enders G., Dlugi R., Steinbrecher R., Clement B., Daiber B., Eijk J. V., Hazziza M., Helas G., Herrmann U., Kessel M., Kesselmeier J., Kotzias D., Kourtidis K., Kurth H. H., McMillen R. T., Roider G., Schiirmann W., Teichmann U. and Torres L. (1992) Biosphere/atmosphere interactions: integrated research in a European coniferousforest ecosystem. Atmospheric Environment 26A, 171-189. Farquhar G. D., Firth P. M., Wetselaar R. and Weir B. (1980) On the gaseous exchange of ammonia between leaves and the environment: determination of the ammonia compensation point. Plant Physiol. 66, 710-714. Farquhar G. D., Whitehead D. C. and Lockyer O. R. (1983) Gaseous nitrogen losses from plants. In Gaseous Losses of Nitrogen from Plant-Soil systems (edited by Nijhoff M.), pp. 159-180. W. Junk, The Hague, Netherlands. Gunderson P. (1991) Nitrogen deposition and the forest nitrogen cycle. Forest Ecol. Man. 44, 15-28. Hillman R. E. and Kauppinen E. I. (1991) On the performance of the Berner low pressure impactor. Aerosol Sci. Technol. 14, 33-47. H6fken K. D., Meixner F. X. and Ehhalt D. H. (1983) Deposition of atmospheric trace constituents onto different natural surfaces. In Precipitation Scavenging,, Dry Deposition, and Resuspension (edited by Pruppacher H., Semonin R. and Slinn W.), pp. 825-836 Proc. of 4th Int. Confer., Santa Monica, CA. Elsevier, New York Kauppi P. E., Mielik/iinenK. and Kuusela K. (1992)Biomass and carbon budget of European forests, 1971-1990. Science 256, 70-74. Kramm G. (1989) A numerical method for determining the dry deposition of atmospheric trace gases. Boundary-Layer Met. 48, 157-176. Kramm G. and Dlugi R. (1993) Modelling of the vertical fluxes of nitric acid, ammonia, and ammonium nitrate in the atmospheric surface layer. J. atmos. Chem. (submitted). Kramm G., Dlugi R., Mfiller H., Schaller E. and Seiler W. (1992) Modellingof the vertical transport of NO, NO2, 0 3, HNO 3, NH 3 and NH 4 NO 3 in the atmospheric surface layer. 7th EUMAC Workshop, Porto Canas, Greece, 28 September-2 October, 1992. Langford A. O., Fehsenfeld F. C., Zachariassen J. and Schimei D. S. (1992) Gaseous ammonia fluxes and background concentrations in terrestrial ecosystems of the United States. Global biogeochem. Cycles 6, 459-483. Lindberg S. E., Bredemeier M., Schaefer S. A. and Qi L. (1990) Atmospheric concentrations and deposition of nitrogen and major ions in conifer forests in the United States and Federal Republic of Germany. Atmospheric Environment 24A, 2207-2220. Miiiler K. P. and Rudolph J. (1991) Application of an improved method for measurements of gaseous nitric acid in the nonurban atmosphere. J. analyt. Chem. 339, 661-663. Peters K. and Eiden R. (1992) Modeling the dry deposition velocityof aerosol particles to a spruce forest. Atmospheric Environment 26A, 2555-2564. Reuder J. (1991) Experimentelle Untersuchungen zum EinfluBatmosph/irischer Parameter auf die Photolyse von Ozon in der bodennahen Troposphfire, Diplomarbeit fiir der Universit/it Miinchen, D8000 M/inchen 2, Germany. Schlesinger W. H. (1991) Biogeochemistry'. an Analysis of Global Chanoe. Academic Press, San Diego, CA. Slinn W. G. N. (1982) Predictions for particle deposition to vegetation. Atmospheric Environment 16, 1316-1339. Warneck P. (1988) Chemistry of the Natural Atmosphere. Academic Press, San Diego, CA.
Atmospheric Environment Vol. 28, No. 2, pp. 311-315, 1994 Elsevier Science Ltd Printed in Great Britain. All rishts reserved 1352 2310/94 $6.00+0.00
NITRIC ACID, PARTICULATE NITRATE A N D A M M O N I U M PROFILES AT THE BAYERISCHER WALD: EVIDENCE FOR LARGE DEPOSITION RATES OF TOTAL NITRATE H. SIEVERING,*~"G. ENDERS,~L. KINS,* G. KRAMM,§K. RUOSS,* G. ROIDER,* M. ZELGER,* L. ANDERSON," a n d R. DLUGI* *Meteorologisches Institut der Universitfit Mfinchen, 8000 Miinchen 2, Germany; "t'Center for Environmental Sciences, University of Colorado at Denver, Campus Box 136, P.O. Box 173364, Denver, CO 80217-3364, U.S.A.; :~Lehrstuhl fiir Bioklimatologie und Angevandte, Meteorologie der Universit/it Miinchen; § Fraunhofer Institute fiir Atmospharisches Umweltforschung, Germany (First received 31 December 1992 and in final form 21 May 1993)
Almtract--Chemical measurements at five levels within and above a predominantly spruce forest at the Bayerischer Wald (Bavarian forest) National Park, Germany, site show that particulate nitrate, as well as nitric acid, is rapidly removed to the forest canopy. The rate of dry removal to the forest canopy for particulate nitrate was nearly as large as that for nitric acid. Cascade impactor data indicate a major reason for the large particulate nitrate deposition rates may have been its 2-2.5 gm mean diameter. Dry removal of total nitrate may be sufficient to cause nitrogen saturation at this forest site. In combination with nitrogen wet deposition, the nitrogen available for biotic demand appears to be in sufficient supply to cause ammonia emission from this spruce forest canopy. At some other European forest sites, where ambient air nitrate concentrations are higher, the dry plus wet removal of total nitrate may be sufficient to cause nitrogen availability to be substantially greater than its biotic demand. Key word index: Nitrogen, nitrate, nitric acid, spruce forest, Germany, dry deposition.
INTRODUCTION Nitrogen (N) fluxes at the Earth's forested regions play a dominant role in global nitrogen cycling (Schlesinger, 1991). Net, or above-ground, primary production in some temperate zone forests appears to show a correlation with N inputs in dry deposition and precipitation (Cole and Rapp, 1981; Kauppi et al., 1992). Gunderson (1991) has estimated long-term critical N loads for forest ecosystems to be 2-20 kg N h a - 1 y r - 1. Wet deposition alone, in parts of Asia, Europe, and North America, borders on or falls in this critical N loading range (Warneck, 1988). The term "critical N loading" is defined to be that amount of added nutrient N from atmospheric deposition to cause nitrogen saturation (N availability in excess of biotic demand--Aber et al., 1991). Thus, the 2-20 kg N h a - l y r -1 is felt to encompass the atmospheric loading of N at which this saturation occurs. To obtain a better understanding of nitrogen budgets and especially to quantify pollutant dry fluxes between the atmosphere and biosphere, the European Biosphere Atmosphere Exchange of Pollutants (BIATEX) program was instituted. The BIATEX experimental forest Schachtenau site (Enders et al., 1992), located in the Bayerischer Wald National Park at 48°57'N, 13°25'E in northeastern Bavaria, is an 86% spruce, 14% beech, 80-100 yearold forest site with a fairly even but slightly sloping
canopy. The mean canopy height is 29 m with its base somewhat uneven and beginning in the 10-14m height range; the leaf area to ground surface area ratio is 5 to 6. The Institute for Bioclimatology and Applied Meteorology of the University of Miinchen maintains meteorological and gaseous species monitoring at a 51-m tower, with wind speed, temperature, and humidity data obtained at 51, 46, 44, 36, 31, 26, 21, 16, 11, 6, and 2-m heights, and several gaseous species data obtained at 51, 41, 31, 26, 16, and 6-m heights above the mean forest floor elevation of 807 m a.s.I (Enders et al., 1992). The Meteorologisches Institute has established a program for measurements of growing-season particulate matter ion concentrations, including ammonium (NH2), nitrate (NO3), gaseous nitric acid (HNO3), and hydrogen peroxide (H202) using filter pack sampiing (Miiller and Rudolph, 1991) at 51, 32, 22, 18, and 2-m heights. A second filter pack, for gaseous ammonia (NH3) concentrations, was occasionally deployed at each of the three heights, 51, 29, and 3 m. Finally, five-stage, 80 l.p.m. Berner cascade impactors (Hillman and Kauppinen, 1991) were deployed at the 34- and 3-m heights to provide size distributions of particulate matter ions. The purpose of this paper is the assessment of vertical profiles for HNO3, NH~, and NO~, and, when available, NH 3, obtained at the Bayerischer Wald site as they relate to the particulate NH~ and NO~ size distribution data. Subsequent 311
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Fig. I. Height-averaged concentration plots of HNO 3, NO~, and NH~; 17-19 June 1990, Fig. la, and 25-27 June, 1990, Fig. lb. Averages are based on 16 2-hr long filter pack samples for each of the 17-19 and 25-27 June periods at the Bayerischer Wald experimental forest Schachtenau site in northeastern Bavaria, Germany. Horizontal bars are 90% confidence intervals in the ratios of 2- to 51-m and 32- to 51-m air concentrations which are given in Table I.
Nitric acid, particulate nitrate and ammonium profiles papers will more fully assess the N budget and also the behavior of 0 3 and H 2 0 2 at this predominantly spruce forest site.
EXPERIMENTAL Data sets of maximum filter pack sampling intensity (six times per day, 0600-0800, 0900-1100, 1200-1400, 1500-1700, 1800-2000, and 2100-2300 Middle European Standard I-MES] time) and maximum impactor sampling (four times per day) were obtained for the periods 17-19 June and 25-27 June 1990. Vertical profile data (i.e. when at least four of the 51, 32, 22, 18, and 2-m heights provided data) were obtained from the filter packs for the species HNOa, NH~, NO~, and SO42-. These 90-mm filter packs consist of a Teflon filter (Sartorius No. 11807 0.2/~m pore size) for particulate matter collection followed by two nylon filters for the absorption of H N O 3. Nylon filter mats (Lutravil Spinulies) are cut to size, cleaned by triple-washing in deionized water and dried at 60°C. Filter samples are collected for 2 h at ~60 1.p.m.; samples are then stored at -30°C. Analysis of 10ml deionized water extracts was performed by ion chromatography (Dionex 2020i with AS4A anion and fast cat-I cation columns). The efficiency of the HNO 3 collection is determined by consideration of the percentage of HNO 3 found on the second nylon filter. Generally, only during very high relative humidity conditions did the HNO3 collection efficiency decrease below 90%. Three-level vertical profile data were obtained for NH3 on 26 June at 50, 29, and 2-m heights using 47-mm filter packs. These used a 1.0 pm Teflon front filter to remove particulate matter from the airstream and a citric acid impregnated backup filter to collect NH 3. Metal, five-stage cascade impactors were also deployed at each of the 34- and 3-m heights throughout the 17-19 June and 25-27 June 1990 sampling periods. Given that sunrise occurred at 0400 MES and sunset at 2030 MES, five of the six filter packs (excluding only the 2100-2300 MES filter sampling time) provided daytime samples. However, during this period of June 1990 solar radiation data (Reuder, 199 l) show that only the 0900-1100, 1200-1400, and 1500-1700 MES samples from above the canopy may be declared to be daytime samples; beneath the canopy too much variability in solar intensity was observed to designate sampling times as daytime or otherwise. Thus, all sampling times will be aggregated to consider heightaveraged differences (averages of the differences in concentration between any two sampling heights for a given sampling period).
313
RESULTS A N D DISCUSSION
Figure 1 shows the height-averaged concentrations, along with representative 2- and 32-m height uncertainties therein, for the 17-19 June 1990 (Fig. la) and 25-27 June 1990 (Fig. lb) periods separately. Mean concentrations at the 51-m, ambient air sampiing level for H N O 3 , N O ~ , and N H ~ were 2.1, 0,49, and 2.4/zgm -3, 17-19 June, and 1.5, 0.33, and 1.2 #g m - 3 , 25-27 June, respectively. Trends of dry flux to the canopy are evident in the vertical profiles for both H N O 3 and N O ~ and, less so, due to uncertainty, for NH,~. It should be noted that the total nitrate (TNO3) concentration of 1.5-3.0 #g m - 3 , is substantially less than that found at some European sites, e.g., G6ttingen with 12 #g m - 3 , and is similar to that observed at other sites (Lindberg et al., 1990; Andersen et al., 1993). The percentage of T N O 3 as N O ~ at the Bayerischer Wald site during June 1990 was 15-20%, which is more than the ~ 6 % at the O a k Ridge (U.S.A.) site, but much less than the 5 0 - 5 5 % at G6ttingen, Germany. The mean diameter of N O 3 found at the Bayerischer Wald site was 2-2.5 gm, about the same as that found at G6ttingen, but somewhat less than what was found at Oak Ridge (Lindberg et al., 1990). Ratios of 2- to 51-m and 32- to 51-m concentrations of H N O 3 , N O ~ , N H ~ , and SO 2- are presented (along with 90% confidence intervals) in Table 1. N o precipitation occurred during the two periods considered, so the ratios, if statistically significant and less than 1, represent dry deposition contributions to species loss from the atmosphere. Table 1 results indicate a statisticaily significant dry removal of H N O 3 and N O ~ to the forest canopy and also true for SO 2-, although the rate of dry removal is less rapid. N o significant trend is evident for N H ~ ; uncertainties are too large in this case. The metal, H N O 3 denuding inlet five-stage cascade impactor results indicate the geometric median mass diameter ( G M M D ) of N O ~ to be 2.24-1-0.85 #m; no difference in G M M D for N O ~ was observed between the 34- and 3-m measurement heights. The G M M D
Table 1. Ratios of 2- to 51-m and 32- to 51-m bihourly air concentrations of HNO3, NO~, NH~, and SO~- at the Bayerischer Wald predominantly spruce forest canopy 17-19 June 1990 2 m/51 m HNO 3 NO~ NH~ SO 2-
0.15 0.51 0.81 0.81
(-I-.05)* (_+.11) (_+.17) (_+.08)
25-27 June 1990
32 m/51 m 0.56 0.72 0.87 0.80
(_+.12) (5-.17) (_+.24) (:t:.16)
2 m/51 m 0.17 0.42 0.72 0.73
(__+.05) (_+.08) (+.14) (+.14)
32 m/51 m 0.53 (___.09) 0.62 (+.10) 0.90 (+.26) 0.78 (_+.11)
* Numbers in parentheses are 90% confidence interval values, n = 16 for the 2 m/51 m ratios for both time periods; n = 14 for the 32 m/51 m ratios (except the 17-19 June HNO 3 data which had one poor nylon filter collection efficiency). Extreme outlier ratios were not included for two 32 m/51 m ratios: 1200-1400, 19 June had a four-species average ratio of 5.3 and 0900-1100, 27 June had a fourspecies average ratio of 2.6. Sample handling and analytical procedure errors may have led to these extreme outlier results.
314
H. SIEVERINGet al.
for all other ions observed (NH2, SO~-, K ÷, Na ÷, and C1-) were <0.9/tm with the G M M D of NH2 the largest of these. Vegetational ground cover on the floor of the Bayerischer Waid site is essentially 100%; thus, no fresh soil particle surfaces are available for HNO3 conversion, and the source of the coarse particle NO 3 is not apparent. The predominantly spruce forest of the Bayerischer Wald extends for some distance in all directions from the Schachtenau site. Nocturnal fog and low clouds in the vicinity of the site are common. Fog and cloud droplets do produce coarse particle NO3 upon evaporation of the droplets. Results of modeling aerosol particle dry deposition at a similar German spruce canopy (Peters and Eiden, 1992) show, for 2-2.5/~m diameter particles, that Vd of 1-5 cm s- 1 may be expected; for particles larger than 4-5 tim Vd > 10 cm s- ~ are possible. Considering the impactor-observed NO~- size distribution and the nonlinear trend in Vd VS size found by Peters and Eiden (1992), Vd of 2-8 c m s - t may be applicable to NO~ dry deposition at the Bayerischer Wald site. H6fken et al. (1983) had estimated Vd for NO 3 in the range of 1-4 cm s- ~ with an NO 3 G m M D of 2.4/~m above a spruce forest; however, assumptions made led to large uncertainties in the lid values. The Table 1 profile results, although indicating rapid dry removal for both HNO 3 and NO3, represent the combined influence of dry deposition and chemical reactions. Kramm et al. (1992) and Kramm and Dlugi (1993) have shown that HNO 3 dry deposition is generally overestimated and NO3 dry deposition underestimated without consideration of the chemical equilibrium reactions HNO 3 + NH3~--~ NH4NO3; also, the constant flux layer assumption does not generally hold for HNO3 or NO 3 alone. Unfortunately, NH 3 concentration data, which may elucidate the extent of chemical reaction influence, were available only on 26 June 1992. Nevertheless, the change in slope of the NH~ profile within the canopy (Fig. 1), along with a near-constant NO3 concentration within the canopy, suggests that these chemical reactions may be occurring. There is one combination of N species concentrations for which the constant flux layer assumption holds and for which its concentration is not plagued by large uncertainty, i.e. the HNO 3 + NH4NO3 system. If all the NO~ is in the form of NH4NO 3, then the profiles for TNO 3 may be considered to be unaffected by chemical reactions and a constant flux layer would prevail, at least approximately, for TNO 3 above the canopy. No direct evidence is available to support all particulate NO3 above the canopy being NH4NO3. However, given ion balance data for 26 June showing that, stoichiometrically, SO 2- is present as NH4HSO, and given the ratio [NH,~]/([NO3] + [SO2-]) of 1.2 at both the 32- and 5t-m heights, it is reasonable to assume that most of the NO~ was as NH4NO3. (Note also that both the NH,~ and NH 3 molar concentrations were an order of magnitude higher than that of NO3. )
Assuming that the [HNO3]+[NH4NO3] profile data reflect a combined N species for which the constant flux layer assumption holds, one can calculate height-independent, above-canopy dry fluxes using the 51- and 32-m height data along with the 51, 46, 41, 36, and 31-m height meteorological data using procedures described in Kramm (1989) and Kramm et al. (1992). Uncertainty in these calculated fluxes is + 50-60%, due mainly to the + 6-11% uncertainty in concentration data at each of the two heights above the forest canopy. Variability in the calculated TNO3 fluxes is + 7 5 - 8 0 % given the 32m/51m data on the 17-19 June and 25-27 June (Table 1). The TNO3 nitrogen dry flux to the forest canopy is found to fall in the range from 0.7 to 5.2 mg N m -2 d -1. Given the TNO 3 concentration data at 5 l-m height, this TNO 3 flux to the canopy implies a point-estimate Vd for the combined 17-19 and 25-27 June data of 5.5 cm s- ~. However, uncertainty in these Vd values is quite large, about +65% (mostly due to the -+ 50-60% uncertainty in concentration differences). Thus, the mean Vd for TNO 3 across the 17-19 and 25-27 June period fall in the 2-9 cm s- 1 range. The mean Vds for NO3 and HNO 3 separately are approximately equal. The critical N loading of 2-20 kg N ha- t yr- 1, or 0.5-5.5 mg N m -2 d - 1, referred to in the Introduction may be compared with the range found for TNO 3 dry flux of 0.7-5.2 mg N m 2 d - l . The chronic (daily) loading of TNO 3 in the upper half of this 17-19 and 25-27 June range is, very likely, sufficient to saturate the Bayerischer Wald spruce forest. The combination of wet N deposition (roughly equal to the TNO 3 dry deposition at this site) and the dry TNO 3 flux may cause N availability to exceed its biotic demand. If the Va for TNO 3 apply at other German forest sites, dry deposition alone appears to cause N to be in excess. For example, the 12 #g TNO 3 m-3 found above the G6ttingen site implies a dry loading of 3.8-28 m g N m -2 d -t. In response to this N saturation or, perhaps, N excess status the NH 3 profile data on 26 June 1990 indicate that the forest canopy may be a source of NH 3. Table 2 shows the five profiles obtained on that day; the 29-m height NH 3 concentrations are, on average, twice as high as the 50-m height concentra-
Table 2. Ammonia (NH3) concentration data from l-h-long citric acid impregnated filter pack sampling for 26 June 1990 at the Bayerischer Wald Schachtenau site. (Uncertainty in individual NH a values is + 30%) Time, MES
07-08 09-10 12-13 15-16 18-19
NH 3 (nmolm -3) on 26 June 1990, at each of three heights above the forest floor 50 m 31.8 67.5 47.6 47.6 95.3
29 m 115. I 91.3 71.5 51.6 150.9
2m 55.6 63.5 75.4 35.7 19.9
Nitric acid, particulate nitrate and ammonium profiles tions. The forest canopy may lose NH3 directly to the atmosphere following proteolysis (Farquhar et al., 1983); this is especially likely when the ammonia partial pressure is below the compensation point (Farquhar et al., 1980). Since the aerosol particles at 29- and 50-m heights are about equally neutralized, the higher NH 3 concentrations at the 29-m height are, presumably, due to NH3 emissions from the forest canopy. Comparison of these data with a.number of other NH3, NH~, HNO3, NO3, and SO 2- data sets presented in Langford et al. (1992--see especially their fig. 5) shows that enhanced unneutralized NH 3 concentrations prevailed at the 29-m height on 26 June. This supports the notion that the 0.7-5.2mg N m - 2 d -1 added by TNO 3 dry deposition may actually cause the forest canopy to be in a state of N excess with the result that NH 3 is emitted by the forest canopy elements.
CONCLUSIONS Total nitrate dry flux to the Bayerischer Wald spruce forest appears to be sufficient to cause N saturation. At this site HNO3 probably contributes more to TNO3 dry flux than NO3 due to its higher air concentration (1-3/~g H N O 3 m -3 vs 0.3-0.6/~g NO~ m-a). At other forest canopies, such as the G6ttingen site in northern Germany, NO3 and HNO3 may contribute more equally since their abovecanopy concentrations are about equal. Given the higher concentrations of both NO~ and HNO3 above many European sites than was observed during June 1990 at the Bayerischer Wald site, it is possible that N excess conditions exist at many European locations due to TNO3 dry flux. The limited data on NHs at the Bayerischer Wald site indicate that NH3 is emitted from the spruce forest canopy, thus supporting the view that N saturation has occurred and release of N may also be occurring. Acknowledgements--We want to thank D. K6hler, E. K6hler, H. Meyer, J. Reuder, and K. Reusswigfor their assistance in the field and with data analysis. This work was supported by the Bundesminister fiir Forschung und Technologic Grant No. 07VND09 and also by the BayerischesStaatsministerium fiir Landesentwicklung und Umwelffragen Grant No. 6495-953-23800.
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