Conversion of Fogwater and Aerosol Organic Nitrogen to Ammonium, Nitrate, and Nox During Exposure to Simulated Sunlight and Ozon
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Environ. Sci. Technol. 2003, 37, 3522-3530 Past studies have shown that ON compounds are subject Conversion of Fogwater and Aerosol to chemical and photochemical transformations in the Organic Nitrogen to Ammonium, troposphere, forming products that might potentially influ- ence the properties of atmospheric condensed phases and Nitrate, and NOx during Exposure to the bioavailability of N in deposition (1, 17-21). For instance, the more bioavailable, lower molecular weight compounds, Simulated Sunlight and Ozone such as amino acids and urea, usually account for less than 20% of the atmospheric ON pool (8, 9, 11, 22, 23), while QI ZHANG² AND CORT ANASTASIO* larger and less bioavailable species, such as humic substances, appear to be more abundant (24-26). Although biologically Atmospheric Science Program, Department of Land, Air, and refractory (27), humic substances are photochemically reac- Water Resources, University of California, One Shields Avenue, tive (28, 29), and exposure to sunlight might cause them to Davis, California 95616-8627 decompose into smaller and more bioavailable molecules. Indeed, sunlight illumination of organic matter from surface waters leads to the formation of amino acids and ammonium - Although organic nitrogen (ON) compounds are apparently (30 33). While similar reactions might occur in atmospheric ubiquitous in the troposphere, very little is known about drops and particles, no such studies have been performed, and little is known about the transformation rates and fates their fate and transformations. As one step in addressing of atmospheric organic nitrogen. this issue, we have studied the transformations of bulk The photochemical transformations of specific ON com- (uncharacterized) organic nitrogen in fogwaters and aerosol pounds, such as amino acids, nitrogen heterocycles, and aqueous extracts during exposure to simulated sunlight nitroaromatics, have been studied in atmospheric samples and O3. Our results show that over the course of several or under atmospherically relevant conditions (1, 19-21). hours of exposure a significant portion of condensed-phase However, studies of individual compounds are limited by organic nitrogen is transformed into ammonium, nitrite, the fact that the bulk of ON in atmospheric drops and particles nitrate, and NOx. For nitrite, there was both photochemical is uncharacterized (8, 9, 11, 12). Furthermore, because the formation and destruction, resulting in a slow net loss. reactivity of individual ON compounds varies widely (1, 19), Ammonium and nitrate were formed at initial rates on the it is currently infeasible to extrapolate from single-compound order of a few micromolar per hour in the bulk fogwaters, studies to transformation rates of bulk (and largely unchar- acterized) organic nitrogen in the atmosphere. Therefore, ∼ -3 -1 corresponding to formation rates of 10 and 40 ng m h , we initiated this study to examine photochemical transfor- respectively, in ambient fog. The average initial formation mations of bulk organic N in atmospheric fogwaters and -3 -1 + rate (expressed as ng (m of air) h )ofNH4 in the aerosol particles. We describe here a preliminary set of aqueous extracts of fine particles (PM2.5) was approximately experiments to examine whether bulk atmospheric organic one-half of the corresponding fogwater value. Initial N is transformed during exposure to sunlight and, if so, formation rates of NOx (i.e., NO + NO2) were equivalent whether these reactions form inorganic N. In addition, to ∼2-11 pptv h-1 in the three fogwaters tested. Although because ozone can be a major sink for some atmospheric the formation rates of ammonium and nitrate were organic nitrogen compounds (19, 22), the influence of ozone relatively small as compared to their initial concentrations on organic nitrogen transformations and inorganic N forma- tion was also investigated. in fogwaters (∼200-2000 µM) and aerosol particles (∼400- - 1500 ng m 3), this photochemical mineralization and 2. Experimental Methods ªrenoxificationº from condensed-phase organic N is a 2.1. Samples, Materials, and Analyses. Since details of our previously uncharacterized source of inorganic N in the sampling dates and times, analytical methods, and sample atmosphere. This conversion also represents a new characteristics have been presented in prior reports (8, 9, component in the biogeochemical cycle of nitrogen that 22), only the main points are given here. Fogwaters were might have significant influences on atmospheric composition, collected at the Davis, CA, NADP site (CA88; 38°33′ N, 121°38′ condensed-phase properties, and the ecological impacts W) during winters from 1997 to 2001 using a Caltech Active of N deposition. Strand Cloudwater Collector (CASCC2). Immediately after collection, samples were filtered (0.22 µm Teflon) and stored frozen (-20 °C) in HDPE bottles. PM2.5 samples were collected 1. Introduction at the same location during August 1997-July 1998. Water- Organic nitrogen (ON) has been measured in precipitation soluble particle components were extracted into Milli-Q water (1-5), dry deposition (1, 5, 6), cloud waters (7), fogwaters (8), (g18.2 MΩ-cm) by sonication, followed by filtration (0.22 and atmospheric particles (4, 9-11). The ubiquity of these µm Teflon) and frozen storage (-20 °C) in HDPE bottles. + - - compounds suggests that they might play important roles in Concentrations of NH4 ,NO3 , and NO2 were analyzed atmospheric chemistry and in the biogeochemical cycling of using a Dionex DX-120 ion chromatograph (IC) with con- N. In addition, since organic forms often represent a ductivity detection. Dissolved organic nitrogen (DON) was significant and bioavailable portion of the total N in determined as the difference in inorganic N concentrations deposition (3-7, 12), atmospheric ON can be a nutrient in a given fogwater before and after adjustment to pH ≈3 burden to aquatic and terrestrial ecosystems (5, 13-16). and 24 h of illumination with 254 nm of light (to convert + - DON to inorganic forms): [DON] ) ([NH4 ] + [NO3 ] + * Corresponding author e-mail: [email protected]; tele- [NO -]) - ([NH +] + [NO -] + [NO -]) . phone: (530)754-6095; fax: (530)752-1552. 2 after 254 nm hν 4 3 2 before 254 nm hν ² Present address: Cooperative Institute for Research in Envi- Concentrations of each N species in all samples were ronmental Sciences (CIRES), 216 UCB, University of Colorado, considerably larger than the corresponding detection limits + - - Boulder, CO 80309-0216. (i.e., ∼0.1 µM for NH4 ,NO3 , and NO2 and 1.0 µM for DON). 3522 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 37, NO. 16, 2003 10.1021/es034114x CCC: $25.00 2003 American Chemical Society Published on Web 07/19/2003 into the quartz cuvette (Figure 1). O3 from the ªhigh- concentrationº tube (∼1 ppmv) was used to oxidize NO to NO2 (see below). The air/ozone flow exiting the quartz cuvette was first filtered (5-µm unlaminated Teflon, Pall-Gelman) to remove any particles or drops generated during bubbling and then passed through a denuder coated with citric acid to collect FIGURE 1. Equipment used to measure the photoformation of NH3(g) and then a denuder coated with Na2CO3 to collect inorganic nitrogen in experiments with ozone purging. Chemical HNO2(g) and HNO3(g). Downstream of these two denuders formulas (e.g., NH and NO ) indicate the species collected on each was a 1-L Pyrex reaction flask where NO was oxidized to NO2 3 x ∼ component. (and more oxidized forms) by mixing with 1 ppmv O3 at a flow rate of 0.5 L min-1. The reaction flask was kept at ambient temperature and pressure and was covered in aluminum Concentrations of total N (TN) were determined from the foil to keep it dark. NO2 and other oxidized forms were + - sum of N in the sample solution (([NH4 ] + [NO3 ] + collected downstream of the reaction flask by two denuders - [NO2 ])after 254 nm hν, see section 2.2) plus, in the experiments coated with a solution of 10% guaiacol and 5% NaOH in + with ozone exposure, the concentrations of gas-derived NH4 , methanol (36). - - NO3 , and NO2 found on the denuders and reaction flask At measured time intervals, aliquots of the fog sample (section 2.3). Note that in the samples exposed to ozone, the were taken from the quartz cuvette; diluted 10-50 times + - - sum of nitrate and nitrite includes photoproduced NOx that with Milli-Q; and measured for NH4 ,NO3 ,NO2 , and DON. - - was converted to NO3 and NO2 in the reaction flask. At the same time, the exposed denuders and reaction flask 2.2. Dissolved Inorganic N (DIN) Formation during were replaced by a new set and were each extracted with 2.0 Simulated Sunlight Illumination. 2.2.1. Lyophilized Fog and mL of Milli-Q water. The aqueous extract from the citric + + PM2.5 Samples. Because concentrations of NH4 were gener- acid-coated denuder was analyzed for NH4 ; extracts from - - ally high in our samples (8, 9), we were initially concerned the other components were analyzed for NO3 and NO2 . + - - that it would be difficult to distinguish between the photo- Concentrations of NH4 ,NO2 , and NO3 formed in the formation of DIN and analytical variations. To increase our fogwaters were calculated by combining the amounts in the + ability to see the photochemical formation of NH4 , the first quartz cuvette with those collected on the citric acid-coated group of fogwaters and aerosol extracts were lyophilized (i.e., and Na2CO3-coated denuder, respectively. Concentrations + - - freeze-dried; 34) to reduce NH4 levels prior to illumination. of NOx were calculated from the sum of NO3 and NO2 To do this, NaOH (1 M in Milli-Q) was added to form NH4OH measured on the two guaiacol-coated denuders and in the stoichiometrically (based on the initial measurement of reaction flask. Controls were run in the same way as those + NH4 ); the sample was rapidly frozen in liquid nitrogen; and in section 2.2 (i.e., without O3 bubbling through the sample).