Stable isotopes are increasingly being used as tracers in environmental studies. One application is to use isotopic ratios to quantitatively determine the proportional contribution of several sources to a mixture, such as the proportion of various pollution sources in a waste stream. In general, the proportional contributions of n+1 different sources can be uniquely determined by the use of n different isotope system tracers (e.g., δ13C, δ15N, δ18O) with linear mixing models based on mass balance equations. Often, however, the number of potential sources exceeds n+1, which prevents finding a unique solution of source proportions. What can be done in these situations? While no definitive solution exists, we propose a method that is informative in determining bounds for the contributions of each source. In this method, all possible combinations of each source contribution (0–100%) are examined in small increments (e.g., 1%). Combinations that sum to the observed mixture isotopic signatures within a small tolerance (e.g., ±0.1‰) are considered to be feasible solutions, from which the frequency and range of potential source contributions can be determined. To avoid misrepresenting the results, users of this procedure should report the distribution of feasible solutions rather than focusing on a single value such as the mean. We applied this method to a variety of environmental studies in which stable isotope tracers were used to quantify the relative magnitude of multiple sources, including (1) plant water use, (2) geochemistry, (3) air pollution, and (4) dietary analysis. This method gives the range of isotopically determined source contributions; additional non-isotopic constraints specific to each study may be used to further restrict this range. The breadth of the isotopically determined ranges depends on the geometry of the mixing space and the similarity of source and mixture isotopic signatures. A sensitivity analysis indicated that the estimated ranges vary only modestly with different choices of source increment and mass balance tolerance parameter values. A computer program (IsoSource) to perform these calculations for user-specified data is available at http://www.epa.gov/wed/pages/models.htm
 .

Source partitioning using stable isotopes: coping with too many sources.

The global distribution of carbonaceous aerosols is simulated online in the Goddard Institute for Space Studies General Circulation Model II-prime (GISS GCM II-prime). Prognostic tracers include black carbon (BC), primary organic aerosol (POA), five groups of biogenic volatile organic compounds (BVOCs), and 14 semivolatile products of BVOC oxidation by O_3, OH, and NO_3, which condense to form secondary organic aerosols (SOA) based on an equilibrium partitioning model and experimental observations. Estimated global burdens of BC, organic carbon (OC), and SOA are 0.22, 1.2, and 0.19 Tg with lifetimes of 6.4, 5.3, and 6.2 days, respectively. The predicted global production of SOA is 11.2 Tg yr^(−1), with 91% due to O_3 and OH oxidation. Globally averaged, top of the atmosphere (TOA) radiative forcing by anthropogenic BC is predicted as +0.51 to +0.8 W m^(−2), the former being for BC in an external mixture and the latter for BC in an internal mixture of sulfate, OC, and BC. Globally averaged, anthropogenic BC, OC, and sulfate are predicted to exert a TOA radiative forcing of −0.39 to −0.78 W m^(−2), depending on the exact assumptions of aerosol mixing and water uptake by OC. Forcing estimates are compared with those published previously.

/pdf/global-distribution-and-climate-forcing-of-carbonaceous-288icw2yrh.pdf

Global distribution and climate forcing of carbonaceous aerosols

Arctic contaminants : sources, occurrence and pathways

. During springtime in the polar regions, unique photochemistry converts inert halide salt ions (e.g. Br−) into reactive halogen species (e.g. Br atoms and BrO) that deplete ozone in the boundary layer to near zero levels. Since their discovery in the late 1980s, research on ozone depletion events (ODEs) has made great advances; however many key processes remain poorly understood. In this article we review the history, chemistry, dependence on environmental conditions, and impacts of ODEs. This research has shown the central role of bromine photochemistry, but how salts are transported from the ocean and are oxidized to become reactive halogen species in the air is still not fully understood. Halogens other than bromine (chlorine and iodine) are also activated through incompletely understood mechanisms that are probably coupled to bromine chemistry. The main consequence of halogen activation is chemical destruction of ozone, which removes the primary precursor of atmospheric oxidation, and generation of reactive halogen atoms/oxides that become the primary oxidizing species. The different reactivity of halogens as compared to OH and ozone has broad impacts on atmospheric chemistry, including near complete removal and deposition of mercury, alteration of oxidation fates for organic gases, and export of bromine into the free troposphere. Recent changes in the climate of the Arctic and state of the Arctic sea ice cover are likely to have strong effects on halogen activation and ODEs; however, more research is needed to make meaningful predictions of these changes.

/pdf/halogens-and-their-role-in-polar-boundary-layer-ozone-3v9c7c6k0c.pdf

Halogens and their role in polar boundary-layer ozone depletion

Isotopic source signatures for atmospheric lead: the Northern Hemisphere

Pb 206 /Pb 207 and Pb 208 /Pb 207 isotope ratios were measured in a series of daily atmospheric aerosol samples collected in southern Sweden between 9 February and 30 May 1988. The ratios were observed to vary considerably, depending on the origin of air. 5-day back-trajectories were used to classify the samples according to source region: Northwest, Western Europe, East, Eastern Europe, or a combination of two regions. Significant differences in the isotope ratios were found. Unique signatures based on the Pb 206 /Pb 207 and Pb 208 /Pb 207 ratios could be assigned for each region. The signatures were generally consistent with isotope ratios of Swedish and other European gasolines, and with literature values of the isotope ratios of economically significant lead ores used in Europe. A comparison was made between the regional signatures and the Pb 206 /Pb 207 ratios observed in Arctic aerosol samples. Excellent agreement was observed between the Arctic Pb 206 /Pb 207 ratios (1.160) and an average source signature (1.156) calculated from observations in this study using equal contributions from Western Europe, Eastern Europe, and the East as predicted by a chemical transport model. DOI: 10.1034/j.1600-0889.1991.00004.x

https://www.tandfonline.com/doi/pdf/10.3402/tellusb.v43i1.15245

Regional source discrimination of atmospheric aerosols in Europe using the isotopic composition of lead

In situ measurements of carbon dioxide, methane, black carbon, peroxyacetylnitrate, condensation nuclei, and radon were made from the Canadian Baseline Atmospheric Monitoring Observatory at Alert, Northwest Territories, Canada (82°28′N, 62°30′W′) during the “Polar Sunrise Experiment” (January 16 to April 20, 1992). The time series of methane, carbon dioxide, peroxyacetylnitrate, and black carbon were frequently highly correlated during January and February during well-defined episodes lasting from 2 to 5 days. This is consistent with data from earlier years. Shortly after polar sunrise, the temporal variability in both trace gases and aerosols diminished. Using a definition of black carbon concentrations exceeding 100 ng m−3, 11 long-range transport episodes were defined. Lagrangian 5-day back trajectories along with the concentration data were classified into six geographical sectors to characterize the major episodes. The winter variability is related to synoptic meteorology, weak vertical mixing, and rapid air mass transport originating from Siberian and/or European source regions. Measurements of the radon daughter (222Rn) activity in the atmosphere were used to further explore the transport of continental material across the Arctic basin.

Analysis of long‐range transport events at Alert, Northwest Territories, during the Polar Sunrise Experiment

Observations of ozone mixing ratios in the lower troposphere of Arctic Canada in April 1994 are summarized. Except during a brief period of anomalous transport from high altitude, air depleted in ozone was always observed above sea ice and snow throughout the northern Ellesmere Island/Lincoln Sea region, or whenever air was sampled which had been in recent contact with sea ice and snow. Ozone mixing ratios observed at a camp on the sea ice north of Alert were consistent with a 1992 study. Observations at the ice camp confirmed that ozone was depleted more frequently (74% of all observations <5 ppbv) than at coastal and inland sites near Alert (10% of observations <5 ppbv). Mixing ratios briefly attained a maximum of 36 ppbv at the ice camp but were normally completely depleted or below typical free tropospheric levels of 35–45 ppbv observed elsewhere in the region. Aircraft measurements and vertical profiles of ozone and meteorological parameters from balloon sondes confirmed that ozone depletion existed in a layer above the sea ice, from the surface up to heights of 200–400 m. Some observations showed a very abrupt transition between depleted and nondepleted conditions at the upper boundary of the layer. The occurrence of a layer of fully depleted ozone was well correlated with surface high-pressure systems. At Alert the appearance of air that was fully depleted in ozone was driven by advection from ocean areas to the north. A fortuitous set of meteorological conditions allowed the use of a simple model for testing general aspects of some proposed hypotheses for ozone destruction, during a period when it is believed the depleted layer was forming. It was found that the observed rate of ozone destruction would require levels of HO and Cl atoms much higher than would be expected for the prevailing conditions but reasonable concentrations of Br atoms. It was also found that an effective ozone deposition velocity of 0.1–0.2 cm s−1 could account for the observed depletion rate during this period. That is, the observed rate of ozone depiction during formation of a depleted layer was consistent with either a volume sink or a surface sink for ozone.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/97JD02888

Ozone and meteorology during the 1994 Polar Sunrise Experiment

A 7-year record (1990–1996) of continuous atmospheric methane (CH4) measurements is presented from a remote midcontinental monitoring station at Fraserdale, Ontario (49°53′N, 81°34′W). Ninety-six air samples per day were measured with a fully automated gas chromatograph with flame ionization detection. Five-day Lagrangian back trajectories over the 7-year period were used to establish a climatology in the region of the station. The site is predominantly influenced by air flow from northern and high-latitude regions and therefore uniquely positioned to monitor wetland emissions. During winter, CH4 concentration time series from Fraserdale often match the short-term variability observed at the high Arctic monitoring station at Alert, Northwest Territories (82°27′N, 61°31′W). During summer, due to diurnal changes of vertical mixing in the boundary layer, large diurnal cycles in CH4 mixing ratio up to 150 ppb are observed. The data selected for the afternoon, when the boundary layer is well-mixed, are representative of a larger spatial scale. The mean annual cycle of CH4 at Fraserdale determined using these selected data is significantly different from annual cycles at other mid- and high-northern latitude sites thus providing key information for global atmospheric CH4 models. In late summer the annual cycle at Fraserdale shows a distinct secondary maximum in CH4. This is the result of advection of air with enhanced CH4 due to emissions from the extensive wetland areas to the north and northwest. The average growth rate (using selected data) for the period was 5.6 ppb yr−1 with a growth rate pattern that is slightly different and out of phase with growth rate changes observed at other high-latitude observing sites by 2 to 6 months.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/98JD00925

J. F. Hopper

Papers

Regional source discrimination of atmospheric aerosols in Europe using the isotopic composition of lead

Analysis of long‐range transport events at Alert, Northwest Territories, during the Polar Sunrise Experiment

Ozone and meteorology during the 1994 Polar Sunrise Experiment

Seven years of continuous methane observations at a remote boreal site in Ontario, Canada

Atmospheric observations of aerosol black carbon, carbon dioxide and methane in the high arctic