Use of stable isotopes for tracing sources of atmospheric nitrate to aquatic ecosystems Carol Kendall and Emily Elliott U.S. Geological Survey Menlo Park, CA Other collaborators: Beth Boyer (SUNY-Syracuse), Doug Burns (USGS-NY), Rick Carlton (EPRI-CA), Greg Michalski (UC-San Diego), Tom Butler (IES- NY), Scott Wankel (USGS-CA), Karen Harlin (NADP), Greg Lawrence (USGS- NY), Mark Nilles (USGS-CO). Where does atmospheric nitrate come from? Natural atmospheric processes Agricultural emissions Power plant exhaust Vehicle emissions NO3) for 2000 Nitrate concentrations in mg/L (as NO3) for 2000 Can we use nitrate isotopes to determine the main sources of nitrate at NADP sites? What are isotopes? Isotopes are part of an element that have different numbers of neutrons (but all their isotopes have the same number of electrons and protons). What are stable isotopes? Stable isotopes are ones that are NOT radioactive. Example: The element N has 7 electrons and 7 protons. It can have various numbers of neutrons. The two stable isotopes have 7 and 8 neutrons, respectively (14N, 15N). How isotopes are used to trace sources of nitrate: There are 2 stable nitrogen isotopes (15N,14N). ratios of 15N/14N are reported as 15N. (“ values”, in units of permil = ppt = ‰). There are 3 stable oxygen isotopes (18O,17O,16O). ratios of 18O/16O are reported as 18O. ratios of 17O/16O are reported as 17O. Therefore, if different sources of nitrate have different 15N, 18O, or 17O values, we can sometimes determine how much nitrate comes from the different sources Ranges of 18O and 15N values of nitrate from different sources Note: good 18O separation of atmospheric and microbial sources microbial Several examples of studies where nitrate 18O has been useful for determining sources of nitrate in streamwater: Use of 18O of nitrate to quantify relative proportions of microbial vs atmospheric nitrate during snowmelt in streams in small forested watersheds atmospheric microbial These data are from Loch Vale (CO) and show that <50% of the nitrate in streamwater is derived from the melting snowpack in 1995. From Kendall (1998); Campbell et al. (2002) Seasonal contributions of atmospheric NO3 (from overland flow) and sewage in urban streams in Austin TX These data show that most of the nitrate in these urban streams during storms is derived from atmospheric nitrate. (from Silva et al., 2002; Kendall, 1998) Surface water nitrate in the Mississippi River Basin has higher 18O and lower 15N values than nitrate in the San Joaquin Basin Mississippi River Basin San Joaquin River Basin NE USA rivers, Mayer et al. (2002) atmospheric nitrate appears to be a significant source in urban and forested catchments, for small and large watersheds What is causing the large range of 18O and 15N values in precipitation? Seasonal and spatial variation in sources of nitrate? Seasonal and spatial variations in atmospheric processes? Both? Other factors? What is the current state of knowledge? The 18O of atmospheric nitrate appears to be slightly bimodel, perhaps because of mixing of sources with different values Seasonal shifts in the 18O of atmospheric NO3 from 3 sites in the NE Winter Summer Winter (modified from Williard, 1999) The seasonal patterns may suggest temporal variability in sources. Kendall (1998) speculated that the seasonality might be caused by the relative contributions from power plant exhaust vs vehicle emissions. 15N values for NOx from vehicle and power plant exhaust exhaust Vehicle Power plant exhaust Modified from Heaton (1990) Can the isotopic composition of atmospheric nitrate be used to determine whether the nitrate is derived from vehicle exhaust vs power plant emissions? Source of the 15N 18O Δ17O nitrate Vehicle exhaust -2 to -11‰ +20 to +25‰ ??? Power plants +6 to +13‰ +50 to +80‰ ??? Terrestrial -5 to +23‰ ??? 0‰ Atmospheric >50‰ +14 to +30‰ ??? ??? Relations between the 18O and 17O of terrestrial and atmospheric materials (caused by mass dependent and independent fractionations) Atmospheric NO3 is labeled by its 17O and 18O values Modified from Michalski et al. (2002) Origin of the high 18O and 17O values of atmospherically derived NO3 NO generated by combustion, biomass burning, or biological emissions is eventually oxidized to NO2 by several reactions. One such reaction is: NO + O3 NO2 + O2 (this O3 has high 18O and 17O values) The NO2 is then photolyzed back to NO, and is the primary source of troposphere O3: NO2 + hv NO + O O + O2 O3 The NO is then re-oxidized to NO2 etc etc. Eventually NO2 is removed by several “sink” reactions. For example: NO2 + OH HNO3 Nitric acid can then react to form nitrate: HNO3 + NaCl NaNO3 + HCl Updated ranges of 18O and 15N values of nitrate from different sources high atmosphere sources ? low atmosphere sources ? High nitrate 18O values are caused by the high ozone 18O values, and should be associated with high 17O and 17O values Conceptual model for how nitrate derived from car exhaust might have a different isotopic signature than nitrate derived from power plants. Our hypothesis is that NOx from car exhaust gets most of its O from photosynthetic O2 (with low 18O and 17O) whereas NOx from power plants gets most of its O from tropospheric ozone (with high 18O and 17O). We will be testing our conceptual model with NADP samples Alternative explanations for some of the 18O and 17O variability: (1) The spatial variations could be caused by changes in atmospheric oxidation capacity or changes in relative proportion of high atmosphere vs low atmosphere penetration of pollutants. (2) The seasonal variations could be caused by seasonal differences in reactions in the upper atmospheres. The 18O of atmospheric NOx, regardless of source, might be partially controlled by the relative contributions of the two primary NOx sink reactions. During the winter when there is less daylight (hv), reactions that involve exchange with O3 are the main source of nitrate, and the resulting 18O of NO3 is higher. In the summer when photolysis produces maximum OH concentrations, reactions with water vapor (where the oxygen in OH is -30 to +2 ‰) are a main source of nitrate, and the resulting 18O of NO3 is lower. Study design: analyze 2-month composites of archived 2000 samples from ~150 sites for nitrate isotopes Site selection criteria: All NY sites (because of our NY- funded study); all CO sites (because of USGS studies). All sites in National Parks, LTER sites, and in important USGS- studied watersheds. Tom Butler (IES) helped pick a reasonable spatial distribution of sites (for later atmospheric source models). NADP composite sampling • Archived precipitation from 2000 • Samples composited over 6 2-month periods • Composites volume-weighted within each 2- month period • Samples with insufficient rainfall for complete chemical analysis not included in composite • Analytical requirements for isotopic analysis: ~30 nmoles NO3- Analytical Methods 18O and 15N: Use the Sigman-Casciotti microbial denitrification method to convert ~20 nanomole aliquots of nitrate quantitatively to N2O. Analyze the N2O for 18O and 15N using an automated headspace sampler on a continuous flow stable isotope mass spectrometer. 17O: We will convert a split of the N2O to O2 and analyze this for 17O to calculate 17O; this will also be used to correct the 15N values for 17O effects. Nafion Water Trap Pneumatic Arm CO Trap 2 Valves GC Column Double Heating Needle Sleeve Sample Rack Liquid Nitrogen Trap Flow and Temperature Controls An important preliminary test: is the nitrate concentration in archived 2000 samples affected by storage? Another preliminary test: how well do our composites represent the average concentration for the two months sampled? First results for AL10: Our NY study: Quantifying Atmospheric Nitrogen Sources with New Stable Isotope Techniques Carol Kendall Elizabeth W. Boyer Douglas A. Burns U.S. Geological Survey State University of New York U.S. Geological Survey WRD-Nat’l Research Program Coll. of Envir. Sci. & Forestry, WRD-NY District Menlo Park, California Syracuse, New York Troy, New York Richard G. Carlton Emily M. Elliott Greg Michalski Electric Power Research U.S. Geological Survey University of California Institute WRD-Nat’l Research Program Dept. of Chem. & Biochem. Palo Alto, California Menlo Park, California La Jolla, California Other collaborators: Tom Butler (IES), Scott Wankel (USGS), Karen Harlin (NADP), Greg Lawrence (USGS), Mark Nilles (USGS). Our study design is to assess spatial/seasonal isotopic variations in wet precipitation at NADP sites and dry precipitation at selected sites, and see if we can model the seasonal contributions of nitrate sources in a few representative watersheds with good land-use data. We also will analyze tree rings from selected locations in NY to look for spatial changes in N sources 15N of trees at different distances from a roadway thru a forest From Saurer, M., Cherubini, P., et al. 2004, Submitted to Atmospheric Environment FUTURE PLANS: We want to test the spatial and temporal predictions of mass balance models such as SPARROW with nitrate isotope data from large rivers (to compare with nitrate isotope data from precipitation). SPARROW predictions about sources of N in streams of major Isotope data are watersheds particularly valuable Fertilizer Atmospheric because they can provide information on: (1) seasonality of source contributions (2) comparison of input Wastewater Animal agriculture contributions versus export contributions. This information is critical for successful remediation of the actual critical loads. Smith and Alexander, 2000 Final philosophical point: Value of isotopes for water resources management At a recent IAEA meeting, Andy Herczeg suggested that one main value of isotope techniques is: To tell us things about water resources that we didn’t know before. I propose that an even more important value is: To tell us things about water resources that CONTRADICT what we thought we knew before.
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