DESIGN, EVALUATION AND APPLICATION OF A CONTINUOUSLY STIRRED TANK REACTOR SYSTEM FOR USE IN NITRIC ACID AIR POLLUTANT STUDIES PAMELA E. PADGETT1∗ , ANDRZEJ BYTNEROWICZ1 , PHILIP J. DAWSON1 , GEORGE H. RIECHERS1 and DENNIS R. FITZ2 1 Pacific Southwest Forest and Range Experiment Station, USDA-FS, Forest Fire Laboratory, Riverside, CA, U.S.A.; 2 Center for Environmental Research and Technology, University of
California, Riverside, CA, U.S.A. (∗ author for correspondence, e-mail:
[email protected], Fax: (909) 680 1501)
(Received 22 June 2001; accepted 27 June 2003)
Abstract. Nitric acid (HNO3 ) vapor is a significant component of air pollution. Dry deposition of HNO3 is thought to be a major contributor to terrestrial loading of anthropogenically-derived nitrogen (N), but many questions remain regarding the physico-chemical process of deposition and the biological responses to accumulation of dry-deposited HNO3 on surfaces. To examine these processes experimentally, a continuously stirred tank reactor (CSTR) fumigation system has been constructed. This system enables simultaneous fumigation at several concentrations in working volumes 1.3 m dia by 1.3 m ht, allowing for simultaneous fumigation of many experimental units. Evaluation of the system indicates that it is appropriate for long-term exposures of several months duration and capable of mimicking patterns of diurnal atmospheric HNO3 concentrations representative of areas with different levels of pollution. Keywords: air pollution, dry deposition, fumigation studies, nitrogen deposition
1. Introduction Nitric acid vapor is produced naturally in the stratosphere by chain reactions starting with N2 O as the nitrogen source. In the troposphere, closer to terrestrial ecosystems, HNO3 is a pollution by-product formed by the photochemical reaction of NO2 and hydroxyl radicals via chain reactions with ozone (O3 ) (Seinfeld and Pandis, 1998). Deposition of HNO3 occurs in both wet and dry forms. Nitric acid vapor deposits directly onto exposed surfaces. It may also react with ammonia vapor to form dry particulate ammonium nitrate or dissolve in rainwater to be deposited in rainfall. All of these reactions lead to a relatively short resident time in the atmosphere and fairly rapid transfer to terrestrial, marine and aquatic ecosystems (Ganzeveld and Lelieveld, 1995; Seinfeld and Pandis, 1998). The deposition of HNO3 to terrestrial and aquatic ecosystems is thought to be a significant con∗ Mention of trade names or product is for information only and does not imply endorsement by
the U.S. Department of Agriculture. Water, Air, and Soil Pollution 151: 35–51, 2004. © 2004 Kluwer Academic Publishers. Printed in the Netherlands.
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tributor to acidification and eutrophication, particularly in areas adjacent to dense urban populations (Hanson and Lindberg, 1991; Bytnerowicz et al., 1998). Acidification and transport of nitrate following wet deposition has been widely studied; less is known about the physical, chemical and biological consequences of dry deposition (Bytnerowicz and Fenn, 1996). The dry-deposition flux and the fate of HNO3 vapor depend upon the characteristics of the contact surface, the micro-meteorological conditions, and the presence of biological activity (Lovett and Lindberg, 1993; Ganzeveld and Lelieveld, 1995; Bytnerowicz and Fenn, 1996). However, the experimental evidence that would allow for developing predictive models of HNO3 vapor deposition behavior is lacking (Ganzeveld and Lelieveld, 1995). In order to study the physico-chemistry of HNO3 vapor deposition and its effects on biotic and abiotic surfaces, a controlled fumigation system has been developed. Unlike other HNO3 fumigation systems previously reported (Norby et al., 1989; Krywult et al., 1996), this system allows for simultaneous exposure at five different concentrations and yields large working volumes in which replication of experimental units is possible. Here we describe the development, evaluation and application of this HNO3 fumigation system.
2. Materials and Methods 2.1. D ESCRIPTION OF THE SYSTEM The system consists of 3 parts: (i) HNO3 volatilization and delivery system, (ii) continuously stirred tank reactors (CSTR) and (iii) monitoring system (Figure 1). It was designed to accommodate five to 10 CSTRs. The delivery system maintains HNO3 concentrations between 0 and 200 µg – HNO3 m−3 , which encompasses ambient concentrations typical of the northern hemisphere, including the heavily polluted Los Angeles region (Bytnerowicz and Fenn, 1996; Fenn et al., 1998). Much higher concentrations can be achieved, however. The chambers are housed in a charcoal-filtered, temperature-controlled greenhouse and no supplemental light is used. The ambient photosynthetically active radiation in the chambers is typically one-half to two-thirds that of full sunlight for the season, typical of greenhouse conditions. The HNO3 vapor delivery and monitoring equipment are housed in the headhouse adjacent to the greenhouse. 2.1.1. Volatilization and Delivery System Gaseous HNO3 is difficult to handle and control; it readily adsorbs to surfaces, particularly in the presence of water. Teflon and glass were used in all components directly in contact with aqueous or gaseous forms of the acid. Contact with metal surfaces, even stainless steel, results in rapid corrosion and failure of those components. Where there was no other alternative and the concentrations were relatively low, non-Teflon or non-glass components were sealed with Teflon tape.
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Figure 1. Schematic diagram of the nitric acid fumigation system. Components are described in detail in the Materials and Methods Section. The system has three subsystems: (1) The volatilization and delivery system, which is outlined by the dotted line. This is housed in the headhouse adjacent to the greenhouse; (2) the fumigation chambers or continuously stirred tank reactors which are housed in a greenhouse and; (3) the monitoring system, housed an adjacent headhouse.
For safety, the delivery system was installed in a fume hood. In order to reduce differences in backpressure among the chambers, equal lengths of delivery tubing were used for all chambers. The volatilization system operates on the principle that HNO3 volatilizes at 83 ◦ C (Weast, 1988). An aqueous solution of HNO3 is introduced drop wise into the volatilization chamber (Figure 2). The chamber is filled with glass beads and heated to 85 to 90 ◦ C using a water bath. Dry air is passed through the volatilization chamber, constructed from standard glass components, and the air stream carries the volatilized HNO3 and water vapor to the CSTRs. The concentrations of aqueous HNO3 solutions used varied between 10:1 (v/v) and 50:1 (v/v) deionized distilled water: concentrated HNO3 . One-liter batches were large enough to provide for up to two month-long exposures in four CSTRs at 25 to 150 µg – HNO3 m−3 without replacement. The solution pump used in the HNO3 delivery system was a piston-type manufactured by Fluid Metering Inc.
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Figure 2. Diagram of the volatilization chamber. The volatilization chamber is immersed in a water bath at 92 ◦ C. Dilute aqueous solutions of HNO3 are dripped into the chamber onto a bed of glass beads. Scrubbed, dry air is pumped through the chamber, picking up the volatilized HNO3 and delivering it to the CSTRs. All components of the chamber are made from glass.
(Oyster Bay, N.Y., U.S.A.,) model QG6, which has a range of 1.0 to 25 mL per hour. In general, very low flow rates of 2.2 to 3.0 mL hr−1 were used in this system. Ambient air was dried to a relative humidity less that 1%, with a heatless air drier (Purgas Heatless Air Dryer model HF200-12-143, General Cable Corp., Westminster CO, U.S.A.). Before being introduced into the HNO3 volatilization chamber, the air stream passed through an activated charcoal canister and a HEPA filter capsule (model 12144 Gelman Sciences, Ann Arbor, MI, U.S.A.). Because the air compressor and dryer are a serious noise hazard, they were installed in a protective structure outside the greenhouse and headhouse. Air was delivered from
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the air dryer at 65 psi but reduced by a pressure-reducing valve to about 3 psi before entering the volatilization chamber. The purified air stream is introduced into the bottom of the HNO3 volatilization chamber (Figure 2). As the air flows upward to the exit port at the top of the chamber, the air stream scavenges the volatilized HNO3 . Once out of the volatilization chamber the HNO3 vapor is distributed to the individual CSTRs through a glass manifold, fabricated from a standard borosilicate 20 × 150 mm screw cap glass test tube. A professional glassblower added ten screw-type fittings to the side of the tube to enable connection of the Teflon delivery tubes. One of the ports is connected to a needle valve that regulates direct evacuation of excess HNO3 vapor. The concentrations of HNO3 delivered to the chambers were regulated by (i) varying the concentration of the aqueous solution, (ii) adjusting the evacuation needle valve, releasing HNO3 prior to delivery into the CSTRs, (iii) changing solution pump speed or (iv) restricting flows with a needle valve installed in-line of each delivery tube at the CSTRs. The strength of the aqueous HNO3 solutions has significant effects on the efficiency of vapor delivery; solutions that are too dilute tend to saturate the air stream with H2 O causing condensation along the walls of the glass and Teflon tubing. This is a particular problem during cool, cloudy weather. Once water droplets form, HNO3 dissolves in the condensate and little is delivered to the CSTRs. Solutions that are too concentrated require ultra low pump speeds and volatilization tends to occur in pulses resulting in inconsistent delivery. The addition of the evacuation valve enabled higher concentrations of solutions to be used. Automatic timers were added to the HNO3 delivery system to control the diurnal pattern of HNO3 concentrations. 2.1.2. Continuously Stirred Tank Reactors (CSTRs) The CSTRs are housed in a multiple use greenhouse equipped with particulate and charcoal filtration of incoming air. An independent blower installed in the greenhouse coupled with an exhaust blower provides the chambers with 1.5 air exchanges per minute under slightly negative pressure. Air supplied to the CSTRs from the greenhouse is further purified by permanganate embedded chemisorbent/absorbent filters installed on the intake duct of the blower (model: 4-inch Purafilter B-850-4404, Purolator Products Air Filtration Co., Henderson NC, U.S.A.). Permanganate traps HNO3 , a potential contaminant due to normal greenhouse operations. All chambers were connected independently to the same blower; there are no connections between CSTRs. Nitric acid is introduced into the CSTRs through a port in the air duct 0.5 m upstream of the CSTRs (Figure 3). The CSTRs are similar to those originally described by Heagle and Philbeck (1979) (Figure 3). They are constructed of wood and metal covered with Teflon film. All exposed surfaces inside the tanks are also coated in Teflon and the bottom of the tank is lined with a 2 mm Teflon sheet. The tanks are 1.35 m dia and 1.35 m ht. Each is fitted with a 0.6 m by 1.2 m hinged door. Internal air circulation is
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Figure 3. Continuously stirred tank reactors. The super structure is made of wood and metal. The external coatings and all exposed surfaces are made of Teflon. The components are identified and described in the Material and Methods Section.
provided by continuous speed impellers (Dayton, Model 22811A) mounted in the top of the tanks. 2.1.3. Monitoring System The monitoring system provides continuous sampling from each of the CSTRs. A sampling port was installed approximately one-third of the way up the wall of the chamber, opposite the air-supply vent. The air sample from the CSTR is fed directly into a molybdenum converter (‘Molycon’ Monitor Labs Inc., Englewood, CO, U.S.A.) mounted next to the chambers (Figure 3). The reduction of HNO3 to NO before transport to the monitoring instruments in the headhouse is critical for monitoring stability because of the very high deposition velocity of the pollutant. As with the delivery lines, all sample lines were of equal length. Nitric acid vapor concentrations in the individual chambers are monitored with a nitrogen oxide monitor model 8840 (Monitor Labs Inc., Englewood, CO, U.S.A.) using a chemiluminescence method. Nitrogen oxide monitors measure bulk NOx and can only separate and identify NO from most other forms of oxidized N. Therefore, under ambient conditions nitrogen oxide monitors cannot be used for monitoring HNO3 directly. However, because of the air purification systems in the bulk air supply, ambient levels of NO, NO2 , HNO3 , are very low and there-
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fore oxidized N compounds detected within the CSTRs as NO are presumed to be HNO3 . Simultaneous monitoring of the bulk greenhouse air with a separate system aided in identifying species and attribution of the NO readings. Additional verification was accomplished by monitoring air delivered to the 0-HNO3 , control chambers. A modified scanivalve (Scanivalve Corp., San Diego CA, U.S.A.) directs the incoming sample to the nitrogen analyzer. Air in the samples lines is measured sequentially as the scanivalve rotates through the chambers and the bulk greenhouse air. A Campbell CR21X datalogger (Campbell Scientific, Inc. Logan Utah, U.S.A.) controls the sampling sequence and duration. Continuously stirred tank reactors and the ambient greenhouse air are normally monitored for 6 to 11 min per sample line, however all sample lines are continuously purged. After a 1 min equilibration period, concentrations are recorded every minute and then averaged. As each chamber is monitored, the concentrations are recorded on a strip chart and a Campbell CR21X datalogger. The scanivalve also allows for manual control over the chamber sequence, or for extended monitoring of single CSTRs. Manual controls were used for determination of monitoring consistency and the effects of altering condition such as opening chamber doors or changes in HNO3 concentrations. 2.2. C OMPARISON OF THE REAL TIME NITROGEN MONITOR SYSTEM TO HONEYCOMB DENUDERS
For evaluation of the efficiency of the NOx monitoring system in determining HNO3 concentrations, recorded concentrations were compared to concentrations measured by honeycomb denuder (Koutrakis et al., 1993). Denuders systems are still the best-accepted method of determining HNO3 and NO− 3 concentrations independent of the other oxide forms (Sickles, 1992; Slanina et al., 1992). The denuders were mounted outside of the chambers with short sample tubes extended through a sampling port on the door. Denuders were exchanged twice daily in coordination with the delivery system: just before the delivery system began producing HNO3 vapor in the morning and just after the delivery system was turned off in the evening. The denuders were extracted and atmospheric concentrations calculated as described by Ogawa & Company (1995). The comparison study was conducted for 5 days and tested 3 CSTRs set for high, moderate and zero pollutant concentrations as well as the bulk greenhouse air. Evaluations have been repeated several times. Total exposures (cumulative doses) were calculated by integration under the concentration curves for each method. The use of a cumulative dose allowed for accurate comparisons of exposure intensity among the different treatment chambers and across independent experiments. 2.3. D EPOSITION STUDIES Deposition rates can be quantified by several means (Seinfeld and Pandis, 1998). Because of the large volume of air (2.65 m−3 ) and the rapid air exchange (1.5
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Figure 4. Diurnal patterns of HNO3 concentrations from a high treatment CSTR as determined by nitrogen oxide monitor () and honeycomb denuder (). Cumulative doses were calculated by integration under the curves. The values are shown as ‘area’ in the legends.
exchanges min−1 ), the common inlet/outlet method is inappropriate for the CSTRs. Therefore, deposition is typically measured by surface accumulation. Two examples are displayed here: (i) inert soil surfaces and (ii) biologically active plant material. In both cases the approach is similar. Samples are placed in the chambers and subsampled periodically during the experiment. For soils, samples are weighed into aluminum weigh boats having an exposed surface area of 20 cm2 . At each sampling date, 3 replicate weigh boats were removed and the samples extracted by standard method (Maynard and Kalra, 1993). For plant tissue studies, four shrub species native to southern California were tested, Artemisia (Artemisia californica), Brittlebush (Encelia farinose), buckwheat ((Eriogonum fasciculatum), and white sage (Salvia apiana). Whole leaves were removed and placed into 50 mL plastic centrifuge tubes with 20 mL nano-pure water. The contents are shaken for 30 sec and the extract or wash solution is analyzed for NO− 3 by continuous flow analyzer. The leaf area for each sample is measured and deposition is calculated on an area basis. Both studies were run for 4 weeks with samples collected weekly. Two chambers were employed. The data shown are the washable NO− 3 concentrations from each surface at a calculated dose.
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TABLE I Comparison of cumulative HNO3 values using honoycomb denuders and nitrogen oxide monitors. Data were calculated by integration under the concentration curve Chamber
High Moderate Control
Cumulative dose Curve area (µg-N m−3 ) N-Monitor
Denuder
21.0 14.8 5.5
28.6 15.2 7.6
3. Results and Discussion Atmospheric concentrations as determined by honeycomb denuder were compared to values determined by the nitrogen oxide monitors from a high HNO3 treatment chamber are shown in Figure 4. The apparent square waves generated by the denuders are an artifact of the 12 hr air sampling period. The calculated concentrations are hourly averages over the exposure time. The actual, real-time values determined by the Nitrogen oxide monitor were much less consistent as demonstrated by the nitrogen monitor-derived data. However, when each curve was integrated and the areas compared, the total exposures, or doses, were similar, 21 vs. 29 µgN m−3 × hr, (Figure 4, legend and Table I). The integrated concentrations for the high chamber were 36% higher by denuder method (Table I). In the moderate dose CSTR the denuder value was only 3% higher, and 38% higher in the control chamber. The source of higher denuder value in the high dose CSTR appears to have been caused by a single day’s sample at day 30 (Figure 4). The cumulative dose was recalculated without day 30, the difference between methods was negligible (data not shown). The HNO3 concentrations from the denuder method shown in Figure 4 are the combination of HNO3 vapor and NO− 3 fine particles. When these two components are examined separately, HNO3 vapor was present only during the daylight hours when the volatilization system was producing HNO3 vapor, but the particles were present at all times (Figure 5). The pattern of NO− 3 particulate concentration in the two treatment chambers was very similar, even though the vapor HNO3 concentrations were about twice as high in the high chamber (Figures 5a and b). Particulate NO− 3 was also present in the control chambers (Figure 5c), although at lower concentrations than in the treatment chambers. This information suggests a portion of the particulate fraction in each of the HNO3 treatment chambers was from general greenhouse environment that had not been removed from the air by the filtration
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Figure 5. Concentrations of particulate nitrate (NO− 3 ) and nitric acid vapor (HNO3 ) in two treatment chambers and the control chamber. The data were generated from honeycomb denuders exchanged every 12 hr in conjunction with on/off cycle for nitric acid vapor production. The top panel (A) are data from the high concentration chamber, panel B are from the moderate concentration panel and the data in panel C are from the control chamber.
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system. Another portion of the particles in the HNO3 treatment chambers appear to be the result of HNO3 vapor reacting with ambient NH3 vapor to form NH4 NO3 aerosols (details are provided later in this section). The two methods of measuring HNO3 vapor seem to be disagreement on the presence of HNO3 vapor in the CSTRs during the evening hours once the volatilization system has been turned off. Without the benefit of the denuder measurements, the detection of NO by the nitrogen oxide monitor during the off-hours was originally hypothesized to be due to remobilization and reemission of HNO3 that had deposited on the chamber walls and tubing during the on-hours. However, data acquired from the honeycomb denuders indicate that the vapor phase HNO3 was not present during the evening, but particulates containing NO− 3 were. This information suggests that the NO detected by the on-line nitrogen monitoring system during the off-hours may be due to particulates, rather that vapor HNO3. The behavior of the molycons and the nitrogen oxide monitors in the presence of fine particles is unknown, but in the catalytic process the air sample is heated to over 150 ◦ C before passing it over the molybdenum catalyst. It is reasonable to speculate that this could cause thermal decomposition of NO− 3 particles into some nitrogen-oxide containing vapor. The second molycon integral to the nitrogen analyzer would insure that any non-NO decomposition product released from the first molycon associated with the individual chambers would be measured as NO once it reached the detector. Attempts to deconstruct concentration data from the nitrogen analyzer and correlate it with honeycomb data by diurnal cycles were not successful, but deductive reasoning suggest that the most likely the source of the NO reading form the Nitrogen oxide analyzers during off hours are the result of thermal decomposition of the NOx -containing particles rather than the result of remobilization and reemission of HNO3 vapor deposited on the walls of the chamber or delivery lines. Further indication of the complexity of the HNO3 fumigation systems was revealed by investigation of other nitrogenous atmospheric components (Table II). Of all the nitrogenous pollutants, ammonia vapor dominated the nitrogen profile. On average 81% of the measured nitrogen was contributed by NH3 in the control chamber, 66% in the moderate HNO3 chamber and 53% in the high HNO3 chamber. Unlike HNO2 , which was also present in small amounts (Table II), the NH3 concentrations indicated diurnal fluctuations. Ammonia fluctuations in the treatment chambers were in opposition to the fluctuations recorded for HNO3 . The honeycomb denuder evaluations were repeated several times and each time NH3 concentrations were similarly high. Ammonia is assumed to be from the bulk greenhouse air and therefore they would be expected to be identical in all chambers. However, the NH3 concentrations in the control chambers were, on average, 1.5 times higher than in the moderate HNO3 chamber and 3 times higher that in the high HNO3 chamber. A regression analysis of NH3 levels across HNO3 levels suggests that HNO3 acts as a titrant for NH3 , resulting in reduced atmospheric concentrations of NH3 in the presence of high HNO3 and supporting the notion of titration
Period
Day Night Day Night Day Night
Treatment
High High Low Low Control Control
5.61 0.11 1.87 0.07 0.05 0.06
(2.11) (0.03) (0.66) (0.01) (0.01) (0.03)
1.34 1.40 1.42 1.54 1.25 1.11
Mean
Mean
(SEM)
HNO2
HNO3
(0.14) (0.10) (0.19) (0.10) (0.16) (0.23)
(SEM)
4.64 11.47 8.77 13.84 16.33 12.93
Mean
NH3 Mean
(0.95) (1.19) (1.56) (1.62) (1.00) (2.60)
2.78 1.28 2.15 1.04 1.03 1.40
(µg N m−3 )
(SEM)
NO− 3
(0.73) (0.47) (0.61) (0.39) (0.31) (0.98)
(SEM)
2.29 1.00 1.94 1.24 0.85 0.71
Mean
NH+ 4
(0.68) (0.41) (0.59) (0.37) (0.32) (0.37)
(SEM)
16.66 15.26 16.14 17.73 19.51 16.21
Mean
Total N
(2.19) (0.66) (1.08) (1.45) (1.38) (2.70)
(SEM)
TABLE II Honeycomb denuder determinations of the average hourly concentrations of three vapor species and two particulates in the treatment chambers. Data shown are the means of two chambers per treatment level. The denuders were exchanged at the beginning and end of the HNO3 volatilization cycle
46 P. E. PADGETT ET AL.
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Figure 6. Regression analysis of gaseous ammonia concentrations against nitric acid vapor as determined by honeycomb denuders. The data were taken from both moderate and high chambers.
of NH3 gases by HNO3 to form NH4 NO3 particles (Figure 6). The stoichiometry of ammonia vapor in comparison to ammonium nitrate particles has been difficult to reconstruct because of the differences in measurement methodologies (real-time as opposed to 12 hr collection periods), but an operational hypothesis of spontaneous particulate formation under the chamber conditions described seems reasonable. These data suggests that in the treatment chambers, the NO− 3 particles measured with the honeycomb denuders were from two sources: spontaneous formation of NH4 NO3 in the chambers via titration processes (Figure 6) and dust from the general greenhouse environment. The presence of NO− 3 particulates, but no HNO3 vapor in the control chambers indicates that the source of the contamination was from the greenhouse air. Particulate concentrations in the two treatment chambers appeared to have similar patterns of high daytime levels and low nighttime concentrations. The notion is also supported by higher NH+ 4 particles in the treatment − chambers (Figure 5 and Table II). The effect of airborne NH+ 4 , NO3 or NH4 NO3 particles on biological systems has not been fully explored, however, should not be ignored. In other fumigation systems, e.g. Marshall and Cadle (1989), Norby et al. (1989), Hanson and Garten (1992), or Krywult et al. (1996), complete assessments of the atmospheric environment of the exposure chambers were not performed, so it is unknown whether this is a typical phenomenon. Two examples of experimental results are included to demonstrate utility of the system. Figure 7a shows the relationship between HNO3 dose and deposition as measured by extractable NO− 3 in clay-sized particles. The relationship is highly
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Figure 7. A comparison of nitric acid deposition (as measured by surface accumulation and subsequent extraction) on biologically inert and biologically active materials. The top panel indicates washable nitrate form clay-sized soil particles. The lower panel indicates a similar experiment using actively growing plant material. The concentrations of soluble nitrate are plotted against the dose as measured by the nitrogen oxide monitor. Error bars indicate ± 1 SE.
linear and non-saturating with respect to increasing dose, as might be expected from a purely physico-chemical processes occurring between a reactive surface such as clay particles and a highly reactive compound such HNO3. It also clearly indicates a non-biological aspect of atmospheric deposition (detailed analysis of atmospheric deposition to soil surfaces can be found in Padgett and Bytnerowicz, 2001). In contrast to the abiotic interactions of HNO3 deposition, Figure 7b shows the results of a similar experiment using growing plant material exposed to similar doses, for similar durations – about 4 weeks. Four different shrub species were evaluated for apparent deposition using a leaf wash method similar to that described by Bytnerowicz and Riechers (1995). In the plant experiment, not only was
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deposition saturating at relatively low doses, but a significant difference existed in ‘washable’ NO− 3 among the four species investigated. The differences in apparent deposition velocity of HNO3 to different plant surfaces have been recognized for a decade or more (Hanson and Lindberg, 1991) although the mechanisms are not well understood. It has been hypothesized that differences in cuticle chemistry or differences in leaf boundary conditions drive interspecific differences in washable deposition, but the means for testing these hypotheses have not been available until now. The tendency of plant surfaces to saturate with time or exposure has also been noted by others (Cadle et al. 1991). It is has been reported as a change in deposition velocity with time, but again the mechanisms are not understood. Perhaps decreases in deposition velocity are related to surface chemical phenomenon or perhaps uptake and assimilation contribute to the apparent saturation of leaves over time. This system has enabled the testing of these types of hypothesis not previously testable and is now leading to new interpretations of deposition values and deposition effects (e.g. Padgett and Bytnerowicz, 2001; Parry, 2001).
4. Conclusions Understanding dry deposition of HNO3 and its resultant effects on ecosystems has been hampered by the lack of accurate, reproducible exposures where the N species can be quantified. Many questions about the physico-chemistry of deposition, biological assimilation and measurement of HNO3 deposition remain unanswered. The fumigation system described represents a new tool for advancing our understanding of atmospheric pollution effects on terrestrial systems. Results presented here indicate that HNO3 concentrations and diurnal patterns that mimic ambient patterns can be achieved consistently and reproducibly. The evaluations also highlighted the complexity of atmospheric reactions, even in simple systems such as the CSTRs where relatively high concentrations of NH3 due to greenhouse activities interacted with HNO3 to form NH4 NO3 aerosols. The authors welcome direct inquiries for more specific answers to the more specific technical questions.
Acknowledgements Funding for this project was provided by USDA-NRI Grant #9701063. We thank the University of California, Riverside and the State Air Pollution Research Center for facilities cooperation.
References Bytnerowicz, A. and Fenn, M. E.: 1996, ‘Nitrogen deposition in California forests: A review’, Environ. Pollut. 92, 127–146.
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