The amount of fixed nitrogen (N that has chemically combined with other

Transcription

1 Chapter 1: Introduction The cycle of N is unique in that it consists of a massive, well-mixed, and (to most organisms) wholly unavailable pool of nitrogen gas (N 2 ) in the atmosphere; a relatively small and almost wholly biologically mediated conversion of N 2 to chemical forms of N that are available to most organisms; and a pool of N that cycles among plants, animals, microorganisms, soils, solutions, and sediments, and between land, water, and the atmosphere. [Vitousek et al., 1997] The amount of fixed nitrogen (N that has chemically combined with other elements through biological, electrical [i.e. lightning], and industrial processes) is miniscule compared to the atmospheric reservoir of N 2 gas on the earth. The research presented in this thesis addresses the trace amounts of a special group of fixed nitrogen compounds and their rates of transfer between the atmosphere and biosphere. This family of gases, odd nitrogen (NO y ), consists of all oxidized nitrogen-containing compounds that act as sources or sinks of nitric oxide, NO, or nitrogen dioxide, NO 2 (NO+NO 2 =NO x ), on relatively short time scales. The functional definition of NO y demonstrates the central role of NO x in the chemistry of the atmosphere. NO x is a natural product of microbial activity in soils and oceans, but it is also released with fossil fuel combustion and biomass burning. NO 2 is one of the U.S. Environmental Protection Agency s six criteria pollutants [U.S. EPA, 22], largely because, in the presence of hydrocarbons, it is the direct precursor to ground-level ozone (O 3 ), or smog (another EPA criteria pollutant). One NO 2 molecule can produce many O 3 molecules through an efficient photochemical cycle. Elevated O 3 levels are associated with severe respiratory problems in humans and animals, reduced agricultural yields, and increased plant disease and mortality [U.S. EPA, 1997]. Large urban and semi-rural 1

2 areas of the United States, particularly in parts of California, Texas, the Great Lakes, and the entire Eastern urban corridor between Washington, D.C. and Maine are currently classified by the EPA as ozone nonattainment areas [U.S. EPA, 22] based on exceeding a one-hour average O 3 concentration of.12 ppm (reported in parts per million by volume mixing ratio by law, equivalent to µmol mol -1 in SI units) more than once per year. A revised.8 ppm, 8-hour average O 3 standard acknowledges that harmful physiological effects begin at much lower concentrations and that prolonged exposure causes greater risk, but implementation has been delayed due to legal action. The extent to which NO x is removed by deposition to the earth s surface has a direct impact on O 3 production in the troposphere, yet remains highly uncertain due to the very limited number and variety of measurements to date. Nitric acid (HNO 3 ), the oxidation product of NO 2, is second only to sulfuric acid as a source of acid precipitation over much of the United States [EPA, 21]. The abundance of sulfuric acid and sulfur deposition in general have declined since 199 due to regulation of sulfur dioxide emissions, largely from coal-burning power plants. Nitrogen deposition (total wet and dry), however, has remained approximately constant over the same period [Lavery et al., 22]. If these trends continue, HNO 3 may become a more important contributor to acid rain. Weekly HNO 3 dry deposition measurements from the Clean Air Status and Trends Network (CASTNet) indicate that dry and wet HNO 3 deposition contribute nearly equally to the total [U.S. EPA, 21]. Since these measurements are based on weekly average, non-species specific techniques, it is important to verify the large contribution of HNO 3 dry deposition to total N deposition. 2

3 Human activities perturb all parts of the N cycle. Bio-available N levels are increasing in the atmosphere, biosphere, and hydrosphere due to fertilizer use, fossil fuel combustion, cultivation of nitrogen-fixing crops, and biomass burning. Total anthropogenic N fixation now rivals or exceeds that of natural processes [Vitousek et al., 1997]. In unaltered terrestrial ecosystems, N is often a limiting nutrient; with large increases in available N, systems may approach N saturation, leading to changes in species composition, ecosystem productivity, soil chemistry, and water quality [Aber, 1992]. The N cycle is linked intimately to the earth s climate; directly via its control of nitrous oxide (N 2 O), a powerful greenhouse gas; and indirectly via its effect on the nutrient status of ecosystems and amount of carbon stored in the terrestrial biosphere, and therefore on carbon dioxide (CO 2 ), another important greenhouse gas. The research objectives of this thesis explicitly address the scientific and regulatory issues for both the atmosphere and the biosphere described above. We sought to quantify the relative and absolute contributions of HNO 3 and NO x to total reactive nitrogen deposition at a typical Northeastern U.S. forest under a wide range of seasonal, meteorological, and transport conditions. A new tunable diode laser absorption spectrometer (TDLAS) was designed, built, and deployed on top of a 22 m tower at Harvard Forest as part of this work. This sensitive, laboratory-quality device was outfitted for all-weather, automated operation and placed at canopy-height in order to minimize sample contact with inlet materials for these reactive gases, particularly HNO 3. New (PAN) and continuing (NO y, NO, O 3, temperature, humidity, pressure) measurements at the site completed the dataset. Figures 1.1 and 1.2 show summary plots of all concentration and flux data, April-November 2, presented and analyzed herein. 3

4 Chapters 2, 3, and 4 are presented as self-contained papers, each with its own abstract, text, figures, and references. Portions of Chapter 2 appeared as Horii et al. [1999]; the dual HNO 3 /NO 2 TDLAS is described in detail, including physical principles, construction methods, calibration procedures, data analysis sequences, and a preliminary field intercomparison of HNO 3. Chapter 3 takes up the question of NO x deposition. The new TDLAS made direct measurements of NO 2 fluxes using the eddy covariance technique, the first time this measurement has been carried out with a device free of significant interference. Simultaneous eddy covariance fluxes of NO, NO 2, and O 3 provide the first data to demonstrate conclusively that daytime downward NO and upward NO 2 fluxes above the trees arise from the canopy-induced light gradient. Other processes contribute to net daytime and nighttime NO x deposition in ways that differ substantially from the behavior postulated in current models of atmospheric chemistry. In Chapter 4, HNO 3 concentrations and inferred fluxes, combined with NO x, NO y, and PAN, reveal a reactive nitrogen concentration and flux budget that is essentially closed for the relatively clean northwest transport regime at Harvard Forest. In contrast, the more polluted southwesterly conditions are associated with significant concentrations and deposition rates of unidentified reactive nitrogen compounds. We also investigated potential bias arising from the dry deposition inferential method (DDIM) for calculating daily and weekly average HNO 3 fluxes (the standard practice for several national deposition monitoring networks) and found none. Chapter 5 summarizes the main findings of this work and suggests how the research should best be used and expanded. 4

5 3 Concentrations: nmol mol -1 [NO y ] 2 1 [NO 2 ] [HNO 3 ] [NO] [PAN] [O 3 ] Wind Dir E 27 N 18 W 9 S E Day of Year Figure 1.1. Summary of all concentration measurements, plus wind direction, presented and analyzed in this thesis. 5

6 FNO y FHNO Fluxes: µmol m -2 hr -1 FNO 2 FNO FO 3 T (C) R.H. (%) Day of Year Figure 1.2. Same as Figure 1.1 for flux data, plus temperature and relative humidity. 6

7 References Aber, J. D., Nitrogen Cycling and Nitrogen Saturation in Temperate Forest Ecosystems, TREE 7, , Horii, C. V., M. S. Zahniser, D. D. Nelson, J. B. McManus, S. C. Wofsy, Nitric Acid and Nitrogen Dioxide Flux Measurements: a New Application of Tunable Diode Laser Absorption Spectroscopy, Proceedings of SPIE, 3758, , Lavery, T, C. Rogers, H. Howell, M. Burnett, C. Wanta, Clean Air Status and Trends Network (CASTNet) 2 Annual Report, Vitousek, P. M., J. D. Aber, R. W. Lowarth, G. E. Likens, P. A. Matson, D. W. Schindler, W. H. Schlesinger, D. G. Tilman, Human alteration of the global nitrogen cycle: sources and consequences, Ecological Applications 7, , United States Environmental Protection Agency, Health Effects Notebook for Hazardous Air Pollutants, U.S. EPA, Atmospheric Deposition of Sulfur and Nitrogen Compounds, in National Air Quality and Emission Trends Report, 1999, EPS454/R-1-4, Office of Air Quality Planning and Standards, Research Triangle Park, NC, 21. U.S. EPA, Green Book,