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Abstract. Nitric acid (HNO3) vapor is a significant component of air pollution. Dry depositionof HNO3 is thought to be a major contributor to terrestrial loading of anthropogenically-derivednitrogen (N), but many questions remain regarding the physico-chemical process of deposition andthe biological responses to accumulation of dry-deposited HNO3 on surfaces. To examine these pro-cesses experimentally, a continuously stirred tank reactor (CSTR) fumigation system has been con-structed. This system enables simultaneous fumigation at several concentrations in working volumes1.3 m dia by 1.3 m ht, allowing for simultaneous fumigation of many experimental units. Evaluationof the system indicates that it is appropriate for long-term exposures of several months duration andcapable of mimicking patterns of diurnal atmospheric HNO3 concentrations representative of areaswith different levels of pollution.Keywords: air pollution, dry deposition, fumigation studies, nitrogen deposition1. IntroductionNitric acid vapor is produced naturally in the stratosphere by chain reactions start-ing with 29308
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N2O as the nitrogen source. In the troposphere, closer to terrestrial eco-systems, HNO3 is a pollution by-product formed by the photochemical reactionof NO2 and hydroxyl radicals via chain reactions with ozone (O3) (Seinfeld andPandis, 1998). Deposition of HNO3 occurs in both wet and dry forms. Nitric acidvapor deposits directly onto exposed surfaces. It may also react with ammoniavapor to form dry particulate ammonium nitrate or dissolve in rainwater to bedeposited in rainfall. All of these reactions lead to a relatively short resident timein the atmosphere and fairly rapid transfer to terrestrial, marine and aquatic ecosys-tems (Ganzeveld and Lelieveld, 1995; Seinfeld and Pandis, 1998). The depositionof 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 bythe U.S. Department of Agriculture.Water,
Air, and Soil Pollution 151: 35–51, 2004.© 2004 Kluwer Academic Publishers. Printed in the Netherlands. tributor to acidification and eutrophication, particularly in areas adjacent to denseurban populations (Hanson and Lindberg, 1991; Bytnerowicz et al., 1998).Acidification and transport of nitrate following wet deposition has been widelystudied; less is known about the physical, chemical and biological consequencesof dry deposition (Bytnerowicz and Fenn, 1996). The dry-deposition flux and thefate of HNO3 vapor depend upon the characteristics of the contact surface, themicro-meteorological conditions, and the presence of biological activity (Lovettand Lindberg, 1993; Ganzeveld and Lelieveld, 1995; Bytnerowicz and Fenn, 1996).However, the experimental evidence that would allow for developing predictivemodels of HNO3 vapor deposition behavior is lacking (Ganzeveld and Lelieveld,1995). In order to study the physico-chemistry of HNO3 vapor deposition and itseffects on biotic and abiotic surfaces, a controlled fumigation system has beendeveloped. Unlike other HNO3 fumigation systems previously reported (Norby etal., 1989; Krywult et al., 1996), this system allows for simultaneous exposure atfive different concentrations and yields large working volumes in which replicationof experimental units is possible. Here we describe the development, evaluation andapplication of this HNO3 fumigation system.2. Materials and Methods2.1. DESCRIPTION OF THE SYSTEMThe system consists of 3 parts: (i) HNO3 volatilization and delivery system, (ii) con-tinuously stirred tank reactors (CSTR) and (iii) monitoring system (Figure 1). Itwas designed to accommodate five to 10 CSTRs. The delivery system maintainsHNO3 concentrations between 0 and 200 µg – HNO3 m−3, which encompassesambient concentrations typical of the northern hemisphere, including the heavilypolluted Los Angeles region (Bytnerowicz and Fenn, 1996; Fenn et al., 1998).Much higher concentrations can be achieved, however. The chambers are housed ina charcoal-filtered, temperature-controlled greenhouse and no supplemental light isused. The ambient photosynthetically active radiation in the chambers is typicallyone-half to two-thirds that of full sunlight for the season, typical of greenhouseconditions. The HNO3 vapor delivery and monitoring equipment are housed in theheadhouse adjacent to the greenhouse.2.1.1. Volatilization and Delivery SystemGaseous 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 compon-ents directly in contact with aqueous or gaseous forms of the acid. Contact withmetal surfaces, even stainless steel, results in rapid corrosion and failure of thosecomponents. Where there was no other alternative and the concentrations wererelatively low, non-Teflon or non-glass components were sealed with Teflon tape. For safety, the delivery system was installed in a fume hood. In order to reducedifferences in backpressure among the chambers, equal lengths of delivery tubingwere used for all chambers.The volatilization system operates on the principle that HNO3 volatilizes at83 ◦C (Weast, 1988). An aqueous solution of HNO3 is introduced drop wise intothe volatilization chamber (Figure 2). The chamber is filled with glass beads andheated to 85 to 90 ◦C using a water bath. Dry air is passed through the volatilizationchamber, constructed from standard glass components, and the air stream carriesthe 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 batcheswere large enough to provide for up to two month-long exposures in four CSTRsat 25 to 150 µg – HNO3 m−3without replacement. The solution pump used inthe HNO3 delivery system was a piston-type manufactured by Fluid Metering Inc. (Oyster Bay, N.Y., U.S.A.,) model QG6, which has a range of 1.0 to 25 mL perhour. In general, very low flow rates of 2.2 to 3.0 mL hr−1were used in this system.Ambient air was dried to a relative humidity less that 1%, with a heatlessair drier (Purgas Heatless Air Dryer model HF200-12-143, General Cable Corp.,Westminster CO, U.S.A.). Before being introduced into the HNO3 volatilizationchamber, the air stream passed through an activated charcoal canister and a HEPAfilter capsule (model 12144 Gelman Sciences, Ann Arbor, MI, U.S.A.). Becausethe air compressor and dryer are a serious noise hazard, they were installed in aprotective structure outside the greenhouse and headhouse. Air was delivered fromthe air dryer at 65 psi but reduced by a pressure-reducing valve to about 3 psi beforeentering the volatilization chamber.The purified air stream is introduced into the bottom of the HNO3 volatilizationchamber (Figure 2). As the air flows upward to the exit port at the top of the cham-ber, the air stream scavenges the volatilized HNO3. Once out of the volatilizationchamber the HNO3 vapor is distributed to the inpidual CSTRs through a glassmanifold, fabricated from a standard borosilicate 20×150 mm screw cap glasstest tube. A professional glassblower added ten screw-type fittings to the side ofthe tube to enable connection of the Teflon delivery tubes. One of the ports isconnected to a needle valve that regulates direct evacuation of excess HNO3 vapor.The concentrations of HNO3 delivered to the chambers were regulated by (i) vary-ing the concentration of the aqueous solution, (ii) adjusting the evacuation needlevalve, releasing HNO3 prior to delivery into the CSTRs, (iii) changing solutionpump speed or (iv) restricting flows with a needle valve installed in-line of eachdelivery tube at the CSTRs. The strength of the aqueous HNO3 solutions has signi-ficant effects on the efficiency of vapor delivery; solutions that are too dilute tendto saturate the air stream with H2O causing condensation along the walls of theglass and Teflon tubing. This is a particular problem during cool, cloudy weather.Once water droplets form, HNO3 dissolves in the condensate and little is deliveredto the CSTRs. Solutions that are too concentrated require ultra low pump speedsand volatilization tends to occur in pulses resulting in inconsistent delivery. Theaddition of the evacuation valve enabled higher concentrations of solutions to beused. Automatic timers were added to the HNO3 delivery system to control thediurnal pattern of HNO3 concentrations.2.1.2. Continuously Stirred Tank Reactors (CSTRs)The CSTRs are housed in a multiple use greenhouse equipped with particulateand charcoal filtration of incoming air. An independent blower installed in thegreenhouse coupled with an exhaust blower provides the chambers with 1.5 airexchanges per minute under slightly negative pressure.Air supplied to the CSTRs from the greenhouse is further purified by perman-ganate embedded chemisorbent/absorbent filters installed on the intake duct ofthe blower (model: 4-inch Purafilter B-850-4404, Purolator Products Air FiltrationCo., Henderson NC, U.S.A.). Permanganate traps HNO3, a potential contaminantdue to normal greenhouse operations. All chambers were connected independentlyto the same blower; there are no connections between CSTRs. Nitric acid is intro-duced 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 Teflonfilm. All exposed surfaces inside the tanks are also coated in Teflon and the bottomof the tank is lined with a 2 mm Teflon sheet. The tanks are 1.35 m dia and 1.35 mht. Each is fitted with a 0.6 m by 1.2 m hinged door. Internal air circulation is provided by continuous speed impellers (Dayton, Model 22811A) mounted in thetop of the tanks.2.1.3. Monitoring SystemThe monitoring system provides continuous sampling from each of the CSTRs.A sampling port was installed approximately one-third of the way up the wall ofthe chamber, opposite the air-supply vent. The air sample from the CSTR is feddirectly into a molybdenum converter (‘Molycon’ Monitor Labs Inc., Englewood,CO, U.S.A.) mounted next to the chambers (Figure 3). The reduction of HNO3 toNO before transport to the monitoring instruments in the headhouse is critical formonitoring 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 inpidual chambers are monitored witha nitrogen oxide monitor model 8840 (Monitor Labs Inc., Englewood, CO, U.S.A.)using a chemiluminescence method. Nitrogen oxide monitors measure bulk NOxand can only separate and identify NO from most other forms of oxidized N.Therefore, under ambient conditions nitrogen oxide monitors cannot be used formonitoring HNO3 directly. However, because of the air purification systems inthe bulk air supply, ambient levels of NO, NO2, HNO3, are very low and there-fore oxidized N compounds detected within the CSTRs as NO are presumed tobe HNO3. Simultaneous monitoring of the bulk greenhouse air with a separatesystem aided in identifying species and attribution of the NO readings. Additionalverification was accomplished by monitoring air delivered to the 0-HNO3, con-trol 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 ismeasured sequentially as the scanivalve rotates through the chambers and the bulkgreenhouse air. A Campbell CR21X datalogger (Campbell Scientific, Inc. LoganUtah, U.S.A.) controls the sampling sequence and duration. Continuously stirredtank reactors and the ambient greenhouse air are normally monitored for 6 to11 min per sample line, however all sample lines are continuously purged. Aftera 1 min equilibration period, concentrations are recorded every minute and thenaveraged. As each chamber is monitored, the concentrations are recorded on a stripchart and a Campbell CR21X datalogger. The scanivalve also allows for manualcontrol over the chamber sequence, or for extended monitoring of single CSTRs.Manual controls were used for determination of monitoring consistency and theeffects of altering condition such as opening chamber doors or changes in HNO3concentrations.2.2. COMPARISON OF THE REAL TIME NITROGEN MONITOR SYSTEM TOHONEYCOMB DENUDERSFor evaluation of the efficiency of the NOx monitoring system in determiningHNO3 concentrations, recorded concentrations were compared to concentrationsmeasured by honeycomb denuder (Koutrakis et al., 1993). Denuders systems arestill the best-accepted method of determining HNO3 and NO−3 concentrations inde-pendent of the other oxide forms (Sickles, 1992; Slanina et al., 1992). The denuderswere mounted outside of the chambers with short sample tubes extended througha sampling port on the door. Denuders were exchanged twice daily in coordin-ation with the delivery system: just before the delivery system began producingHNO3 vapor in the morning and just after the delivery system was turned off in theevening. The denuders were extracted and atmospheric concentrations calculatedas described by Ogawa & Company (1995). The comparison study was conductedfor 5 days and tested 3 CSTRs set for high, moderate and zero pollutant concen-trations as well as the bulk greenhouse air. Evaluations have been repeated severaltimes. Total exposures (cumulative doses) were calculated by integration under theconcentration curves for each method. The use of a cumulative dose allowed for ac-curate comparisons of exposure intensity among the different treatment chambersand across independent experiments.2.3. DEPOSITION STUDIESDeposition 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 (1.5exchanges min−1), the common inlet/outlet method is inappropriate for the CSTRs.Therefore, deposition is typically measured by surface accumulation. Two examplesare 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 andsubsampled periodically during the experiment. For soils, samples are weighed intoaluminum weigh boats having an exposed surface area of 20 cm2. At each samplingdate, 3 replicate weigh boats were removed and the samples extracted by standardmethod (Maynard and Kalra, 1993). For plant tissue studies, four shrub speciesnative to southern California were tested, Artemisia (Artemisia californica), Brit-tlebush (Encelia farinose), buckwheat ((Eriogonum fasciculatum), and white sage(Salvia apiana). Whole leaves were removed and placed into 50 mL plastic cent-rifuge tubes with 20 mL nano-pure water. The contents are shaken for 30 sec andthe extract or wash solution is analyzed for NO−3 by continuous flow analyzer. Theleaf 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 chamberswere employed. The data shown are the washable NO−3 concentrations from eachsurface at a calculated dose. Atmospheric concentrations as determined by honeycomb denuder were comparedto values determined by the nitrogen oxide monitors from a high HNO3 treat-ment chamber are shown in Figure 4. The apparent square waves generated by thedenuders are an artifact of the 12 hr air sampling period. The calculated concentra-tions are hourly averages over the exposure time. The actual, real-time values de-termined by the Nitrogen oxide monitor were much less consistent as demonstratedby the nitrogen monitor-derived data. However, when each curve was integratedand the areas compared, the total exposures, or doses, were similar, 21 vs. 29 µg-Nm−3×hr, (Figure 4, legend and Table I). The integrated concentrations for thehigh chamber were 36% higher by denuder method (Table I). In the moderate doseCSTR the denuder value was only 3% higher, and 38% higher in the control cham-ber. The source of higher denuder value in the high dose CSTR appears to havebeen caused by a single day’s sample at day 30 (Figure 4). The cumulative dosewas 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 thecombination of HNO3 vapor and NO−3 fine particles. When these two componentsare examined separately, HNO3 vapor was present only during the daylight hourswhen the volatilization system was producing HNO3 vapor, but the particles werepresent at all times (Figure 5). The pattern of NO−3 particulate concentration in thetwo treatment chambers was very similar, even though the vapor HNO3 concentra-tions were about twice as high in the high chamber (Figures 5a and b). ParticulateNO−3 was also present in the control chambers (Figure 5c), although at lower con-centrations than in the treatment chambers. This information suggests a portion ofthe particulate fraction in each of the HNO3 treatment chambers was from generalgreenhouse environment that had not been removed from the air by the filtration system. Another portion of the particles in the HNO3 treatment chambers appearto be the result of HNO3 vapor reacting with ambient NH3 vapor to form NH4NO3aerosols (details are provided later in this section).The two methods of measuring HNO3 vapor seem to be disagreement on thepresence of HNO3 vapor in the CSTRs during the evening hours once the volatiliz-ation 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 origin-ally hypothesized to be due to remobilization and reemission of HNO3 that haddeposited on the chamber walls and tubing during the on-hours. However, data ac-quired from the honeycomb denuders indicate that the vapor phase HNO3 was notpresent during the evening, but particulates containing NO−3 were.