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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 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). 39441
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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. 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. Figure 1. Schematic diagram of the nitric acid fumigation system. Components are described in detailin the Materials and Methods Section. The system has three subsystems: (1) The volatilization anddelivery system, which is outlined by the dotted line. This is housed in the headhouse adjacent to thegreenhouse; (2) the fumigation chambers or continuously stirred tank reactors which are housed in agreenhouse and; (3) the monitoring system, housed an adjacent headhouse.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).