H52B-01 INVITED 10:20h
Stable Isotope Constraints on N Deposition and Cycling in Lake Tahoe
Oligotrophic Lake Tahoe has seen a decrease in opacity over the past 5 decades, which has been attributed to particulate matter (shoreline development) and algal growth (nutrients). The lake has also seen a shift from being nitrogen limited to phosphorous limited in the same time frame. Identifying the source of the increased nitrogen loading is essential for mitigation strategies to keep Lake Tahoe Blue - a 10 billion dollar watershed restoration campaign. Atmospheric deposition of nitric acid and nitrate aerosols (NO$_{3}^{-}_{atm}$) is thought to be a significant source of new N to the lake surface. Quantifying the flux and fate of NO$_{3}^{-}_{atm}$ is limited by modeling estimates of deposition, utilization and re nitrification of organic N. Stable isotope tracers can help resolve these limitations. $\Delta$$^{17}$O measurements, the $\delta$$^{17}$O enrichment over the expected .52 $\delta$$^{18}$O enrichment, have been shown to be a sensitive tracer of NO$_{3}^{-}_{atm}$. Oxygen isotopic analysis of NO$_{3}^{-}_{atm}$ from the basin have shown $\Delta$$^{17}$O values of ~ 22$\permil$. Lake water nitrate have $\Delta$$^{17}$O values of 1-4$\permil$ depending on depth and season, indicating that up to 20% of the lake nitrate is retained from the atmosphere. The $\delta$$^{18}$O values (-2.0 to 12$\permil$) cannot be used to estimate of the NO$_{3}^{-}_{atm}$ loading because of the wide range of $\delta$$^{18}$O values associated with nitrification. Variations of $\Delta$$^{17}$O with season can provide estimates of the flux of the nitrification of organic N. Balancing the isotopic budget with $\delta$$^{15}$N and $\delta$$^{18}$O measurements further constrains the N cycling dynamic within the lake. From these data a nutrient flux/utilization model can be developed.
H52B-02 INVITED 10:40h
Mapping the Spatial and Temporal Distribution of N and O Isotopes in Precipitation Nitrate Across the Northeastern and Mid-Atlantic United States
Atmospheric deposition of N is a universally important pathway by which ecosystems receive fixed, bioavailable N. Since the 1880s, atmospheric deposition of N has become increasingly important, as NO$_{x}$ emissions from fossil fuel combustion have steadily increased. In particular, the Northeastern and Mid-Atlantic U.S. receive some of the highest rates of nitrate wet deposition in the country, causing a cascade of detrimental effects. In order to effectively mediate the impacts of nitrate deposition, it is critical to understand the dynamics among NO$_{x}$ sources, atmospheric chemical transformations and transport, and the characteristics of the nitrate that is ultimately deposited. To address this need, this research takes advantage of recent methodological improvements, coupled with national networks (NADP, AIRMoN) of archived precipitation, to characterize N and O isotopic composition of nitrate in precipitation across the Northeastern and Mid-Atlantic U.S. We investigate the critical question of whether variations in \delta$^{15}$N and \delta$^{18}$O of nitrate wet deposition are mainly a function of atmospheric processes (e.g., seasonal variations in reaction pathways) or variable NO$_{x}$ source contributions (e.g., power plant emissions, vehicle exhaust). Spatial and seasonal variability of \delta$^{15}$N and \delta$^{18}$O is investigated using bimonthly archived samples from 2000. Furthermore, a high resolution record of daily precipitation from a single site is used to highlight within-season isotopic variability. Potential correlations between isotopic values and major NO$_{x}$ sources are explored using EPA datasets for monthly county-level emissions from two major NO$_{x}$ sources, electric generating units and on-road vehicles. Analysis of samples for \Delta$^{17}$O is in progress. A key concern regarding analysis of archived samples is nitrate preservation. We tested the stability of nitrate concentrations, and hence potential isotopic fractionations, by reanalyzing filtered, refrigerated, archived NADP samples with a range of nitrate and ammonium concentrations. We found highly significant correlations (R$^{2}$=0.9995, p$<$0.001, n=28) between nitrate concentrations measured in 2000 and 2003, indicating that no major alterations had occurred. With regard to spatial patterns, preliminary isotopic analyses indicate that \delta$^{15}$N of precipitation nitrate varies considerably among states. Moreover, initial data corroborate previously reported seasonal trends in both \delta$^{15}$N and \delta$^{18}$O, with higher values in the colder months. Seasonal trends in \delta$^{15}$N are remarkably consistent, with up to an 8 \permil difference between winter and summer months. \delta$^{18}$O values of nitrate are generally higher and have a smaller range than previously reported for precipitation, with values ranging from +60 to +90 \permil. In addition, archived daily precipitation collected during 2000 from a single AIRMoN site give insight into the seasonal and within-season variability of \delta$^{15}$N and \delta$^{18}$O of precipitation nitrate. Back-trajectory analyses are used to examine the geographic source of air masses for individual events, and seasonal frontal patterns are discussed.
H52B-03 11:00h
Tracing the Importance of Atmospheric Nitrate Deposition in Watersheds with Triple Oxygen Isotopes
Measuring the flux of atmospherically deposited nitrate in watersheds is important because increasing nitrogen loads in many rivers are degrading aquatic ecosystems. Atmospheric nitrate is enriched in $^{17}$O and $^{18}$O. These isotopes can be used to quantify the flux of atmospheric N through watersheds. In the Neuse River Basin, NC it is estimated that up to 50% of the externally supplied "new" nitrogen flux that enters the estuary is from atmospheric sources. The $^{15}$N / $^{18}$O relationship of nitrate in urban and agricultural watersheds indicates that atmospheric nitrogen may be an important part of the river nitrogen flux in these areas. This system is light limited by sediment turbidity, so phytoplankton uptake in the main stem is not important. The $^{15}$N / $^{18}$O relationship of nitrate in the river main stem indicates that denitrification also is not important. Yet the concentration of riverine nitrate deceases down basin. Dissolved phosphate and HD / $^{18}$O values of river and groundwater indicate that decreased nitrate concentrations in the lower basin result from significant deep groundwater inputs. $^{17}$O of rainfall varies from 10 to 25 per mil over the year, with the lightest values occurring in the winter months. Over a 7 year period, the flux of nitrate N is slightly higher than ammonium N, and organic N deposition rates are half the DIN deposition rates in wet deposition. Dry N deposition is one third the wet N deposition on an annual basis. $^{17}$O of nitrate in surface and ground waters varies between different watersheds with different land use. The heaviest values are found in forested watersheds indicating the importance of atmospheric deposition in these environments. Urban creeks have the greatest $^{17}$O nitrate variation, but the average $^{17}$O nitrate of urban, agricultural and main stem samples are below 1 per mil indicating that atmospheric nitrogen is not important in river nitrogen flux. Two hydrographic events were sampled on an hourly basis in an urban watershed over 3-5 day periods to see if discharge variations biased the discrete sample $^{17}$O results. Large changes were observed in the $^{15}$N, $^{18}$O, and $^{17}$O of nitrate as well as nitrate, ammonium and chloride concentrations. $^{17}$O of nitrate varied from 0.3 to 11 per mil in a 12 hour period. The flux of atmospheric N can be calculated using the rates of deposition of DIN, DON, wet and dry N deposition, and the $^{17}$O composition of rainfall nitrate. The greatest flux of atmospheric nitrogen occurs in this urban watershed at the end of the falling discharge hydrograph. On an event basis atmospheric N can account for over 20 % of the DIN flux. Discharge variations are important when calculating atmospheric N fluxes from $^{17}$O data, and the importance of atmospheric N deposition can be underestimated if these variations are not taken into account.
H52B-04 11:15h
High-resolution nitrate and sulfate stable isotope profiles for soils in the Antarctic Dry Valleys
The Antarctic Dry Valleys (ADV), being hyperarid and frigid, provide an ideal surface for the accumulation of atmospheric salts (aerosols and residues of snow sublimation). At any site, salt composition and flux, and degree of post-depositional ion migration can be attributed to the duration of exposure (age of the surface), distance to the open ocean, direction of prevailing wind, snow cover, brightness of the surface (albedo), elevation, and if the deposit is a glacial till, the nature of the till (basal or sublimation till). We expect to encounter, therefore, a variety of profiles in the distribution of salts and their stable isotope compositions in Antarctic soils. This exclusive salt repository offers us two unique opportunities. First, the polar atmospheric process associated with the origin of salts can be studied in soils by examining the abundant atmospheric salt accumulated over the time, an alternative to the use of expensive atmospheric sampling techniques. Second, ion migration has never been quantified for cold and hyper-arid desert soils, due partly to the difficulty of obtaining realistic parameters through laboratory experiments. Chemical and stable isotope profiles from a variety of soils in the ADV place critical constrains to any model that combines the deposition and migration of atmospheric salts with the vapor flux and with the formation of subsurface ice cement in this unique surface environment. We have analyzed more than 20 soil profiles with the highest vertical sampling resolution physically attainable in dry, non-cohesive soils of the ADV. Nitrate and sulfate are two of the major salt components in soils of the ADV. We predict that there should be only one nitrate source, i.e., all secondary, due to the lack of local primary source. Our results show that the $\Delta$$^{17}$O values of nitrate in the ADV are quite uniform ($\sim$ +31.0 $\pm$ 1.0$\permil$) both in space and in depth, supporting our prediction. However, accompanied $\delta$$^{15}$N values are not uniform, ranging from $\sim$ -13.0 to $\sim$ -25.0$\permil$, and are significantly higher in sites that are at low elevations and close to the ocean. Some local anomalies in both $\Delta$$^{17}$O and $\delta$$^{15}$N for nitrate are observed in specific soil horizons. We suspect that there might be an unidentified nitrate process, either from the local atmospheric contribution or from post-depositional alteration (biological?). On the basis of observed variations in earlier $\Delta$$^{17}$O data, sulfate appears to have two sources; sea-salt sulfate and secondary sulfate. This hypothesis can be independently tested by accompanied $\delta$$^{34}$S values for sulfate, because sea-salt sulfate has a fixed $\delta$$^{34}$S value, whereas secondary sulfate should have a lower $\delta$$^{34}$S value. Our $\delta$$^{34}$S results are consistent with the two-end-member sulfate source scenario. Furthermore, we found that the sulfate $\delta$$^{34}$S values are decreasing with depth consistently in most of the soil profiles, except for those disturbed by ancient glacial movement or those near the base of sublimation tills. These high-resolution geochemical and multiple stable-isotope data for salts in the ADV provide a solid ground to test the theories on the origin and migration of atmospheric salts in the ADV and to quantify landscape stability and the long-term affects of soil re-working by cryoturbation in hyperarid and cold deserts.
H52B-05 INVITED 11:30h
Online Method for Oxygen Triple Isotope Analyses of Nitrate
Combined $^{17}$O/$^{16}$O and $^{18}$O/$^{16}$O isotope ratio analyses of nitrate in ground and surface waters help to understand the partitioning between atmospheric and terrestrial nitrate sources because only terrestrial nitrate shows mass-dependent relative enrichments in $^{17}$O and $^{18}$O, whereas atmospheric nitrate displays an anomalous enrichment in $^{17}$O. The $^{17}$O isotope anomaly of nitrate is therefore a sensitive tracer of fresh water pollution. Furthermore, isotope measurements of atmospheric nitrate in aerosols and precipitation provide insight into the partitioning between atmospheric NO$_{x}$ cycling pathways via ozone or hydroxy/peroxy radicals because only ozone has a significant non-mass dependent enrichment in $^{17}$O. Previous methods to analyze the oxygen triple isotope composition of nitrate rely on offline thermal decomposition of AgNO$_{3}$ amounts in the $\micro$mol range. We have recently developed an online (coupled gas chromatography-mass spectrometry) method that requires two to three orders of magnitude less material to achieve essentially the same analytical precision: 30 nmol of nitrate give a 1$\sigma$ uncertainty of 1.0 $\permil$ for the $\delta$^{18}$O value and 0.3 $\permil$ for the $^{17}$O anomaly ($\Delta$^{17}$O). The method uses a strain of bacterial denitrifiers to convert nitrate to N$_{2}$O [Casciotti et al., 2002], which is then quantitatively converted to elemental nitrogen and oxygen in a gold furnace at 800$\deg$C. Both gases are separated on a molecular sieve capillary column and introduced into the isotope ratio mass spectrometer. There is no significant memory effect, but calibration via nitrate or N$_{2}O$ standards is required for scale normalization. This novel method was used to analyze nitrate isotopes in rain water and streams and, thanks to the low sample size requirements, will also be suitable for ice core samples, which have very low nitrate concentrations. A tight correlation between $\Delta^{17}$O and $\delta^{18}O$ in rain water was found with a slope of about 0.3 (R$^2$ = 0.86), which reflects the average isotopic composition of hydroxy/peroxy radicals and tropospheric ozone [cf. contribution by Hastings et al. in session H38]. Nitrate isotope measurements in ice core samples offer opportunities for paleoatmospheric studies. The same method can also be used to analyze N$_2$O itself or other oxy compounds of nitrogen that can be converted to nitrate.
H52B-06 11:50h
Controls on the Nitrogen and Oxygen Isotopic Composition ($\delta^{15}$N, $\delta^{18}$O, $\delta^{17}$O) of Atmospheric Nitrate in Princeton, NJ
The oxygen isotopic composition of atmospheric nitrate reflects the oxidative mechanisms that convert NO$_{x}$ to HNO$_{3}$, while the nitrogen isotopic composition of atmospheric nitrate may reflect different NO$_{x}$ source signatures and/or fractionations related to NO$_{x}$ chemistry [{\it Michalski et al.}, 2003; {\it Hastings et al.}, 2003; {\it Freyer et al.}, 1993]. New analysis techniques are capable of determining the $^{15}$N/$^{14}$N, $^{18}$O/$^{16}$O and $^{17}$O/$^{16}$O isotope ratios in samples at the nanomolar level [{\it Sigman et al.}, 2001; {\it Casciotti et al.}, 2002; see {\it Kaiser et al.}, session H38]. This allows for the analysis of short-term variations in the isotopes of HNO$_{3}$ with the potential to diagnose causal relationships by comparing the isotopic data with other features of atmospheric deposition. The $^{15}$N/$^{14}$N, $^{18}$O/$^{16}$O and $^{17}$O/$^{16}$O of nitrate were analyzed from precipitation samples collected on an event-basis in Princeton, NJ between December 2002 and 2003. The nitrate concentration in Princeton rain ranges from 2.5 to 99.7 $\mu$M (mean=21.1 $\mu$M, n=61), similar to that found in other urban areas of New Jersey by the National Atmospheric Deposition Program. The isotopes of nitrate fall in the wide range reported for various environments with the $\delta^{15}$N ranging from -4.0 to 9.5$\permil$ (vs. air), and the $\delta^{18}$O and $\delta^{17}$O ranging from 57.2 to 90.5$\permil$ and 50.7 to 77.8$\permil$ (vs. VSMOW), respectively. The correlation between nitrate and sulfate concentration (R$^{2}$=0.66) and the lack of a relationship between these major ions and the isotopes of nitrate supports the conclusion that below cloud scavenging is not the dominant control on the isotopic variations observed. Seasonal variations are observed in both the nitrogen and oxygen isotopes of nitrate. Overall the $\delta^{15}$N is not correlated with either $\delta^{18}$O or $\delta^{17}$O, although both the $\delta^{15}$N and $\delta^{18}$O average lowest in the summer and highest in the winter. $\delta^{18}$O is highly correlated with $\delta^{17}$O of nitrate with anomalous enrichment in $^{17}$O relative to $^{18}$O ($\Delta^{17}$O ranges from 19.7 to 30.8$\permil$), as a result of the interaction of NO$_{x}$ and ozone in the atmosphere prior to HNO$_{3}$ deposition. Comparison of $\delta^{17}$O and $\delta^{18}$O of nitrate show the data falling along a mixing line between the oxygen isotopic composition of tropospheric ozone and that of hydroxy/peroxy radicals (i.e. water), which has also been observed in nitrate aerosol samples collected in La Jolla, CA [{\it Michalski et al.,} 2003]. Although there is considerable scatter in the isotopic time series, a distinctive increase in the oxygen isotope ratios occurs in mid-September and continues through the end of December 2003. This aspect of the time series will be discussed in the context of changes in atmospheric chemistry based on seasonal variations in atmospheric transport patterns, meteorology, and NO$_{x}$ and ozone concentrations. In addition to these parameters, the nitrogen isotopic variations will also be interpreted in the context of changes in source contributions, e.g. coal burning in the Midwest, based on multiple chemical analyses including trace metals, mercury, and major ion concentrations.