Seasonal variability and long term trends of chlorofluorocarbon mixing ratios in the unsaturated zone

To investigate processes that might affect chlorofluorocarbon (CFC) mixing ratios at the water table, a time series was obtained of unsaturated zone soil gas CFCs to depths of ca. 4 m at a site near New York City (NYC). Observed CFC 11, 12, and 113 mixing ratios were lower in winter than expected from either a local, high-resolution time series or remote atmospheric mixing ratios. A diffusion model, which includes seasonal changes in soil temperature, moisture, and CFC solubility, reproduces to first order the observed soil gas mixing ratios for CFC 11 and 12. Underestimation by the model of the seasonal cycle of CFC 11 points to changing levels of sorption to soils due to seasonal changes in temperature as an additional cause of the cycle seen in CFC 11 mixing ratios in soil air. In the case of spring recharge, low CFC mixing ratios in soil air caused by increased solubility may result in low CFC 11 concentrations in groundwater and, when dating groundwater recharged before the 1990s with CFCs, older apparent ages by up to 4 years. Attempts to observe average atmospheric CFC levels from soil gas are also significantly hindered by these seasonal fluctuations. Our results indicate the importance of considering seasonal changes in soil temperature when making precise observations of even very moderately soluble gases in the unsaturated zone and shallow groundwater.     

Regional modeling of the atmospheric fate and transport of benzene and diesel particles

The Community Multiscale Air Quality model (CMAQ) was modified to simulate the atmospheric fate and transport of benzene and diesel particles. We simulated the July 11-15, 1995 period over a domain covering the eastern United States with a 12-km horizontal resolution and a finer (4 km) resolution over a part of the northeastern United States that includes Washington, DC and New York City. The meteorological fields were obtained from a simulation conducted earlier with the mesoscale model MM5. Gridded emission files for benzene and diesel particles were developed using the SMOKE modeling system. The results of the model simulations showed that benzene concentrations were commensurate with available measurements. Over the 4-km resolution domain, a comparison between simulated and measured 24-h average concentrations showed a fractional error of 0.46, a fractional bias of 0.14, and a coefficient of determination (r2) of 0.25. A comparison between simulated benzene hourly concentrations in New York City and in the Brigantine Wilderness Area, NJ, showed that urban concentrations were greater than the remote area concentrations by a factor of 2-5. The results of the diesel particle simulations showed spatial and temporal patterns that were similar to those obtained for benzene. However, because of the lesser contribution of on-road mobile sources to diesel particle emissions compared to benzene emissions, diesel particle concentrations showed stronger gradients between urban areas and remote areas. A comparison between diesel particle concentrations in New York City and in the Brigantine Wilderness Area, NJ, showed that the urban concentrations were greater than the remote area concentrations by a factor of 2-10. Assuming that diesel particles consist of 50% "elemental" carbon (EC), the simulated EC concentrations were in close agreement (within 10%) with the measured concentration in the urban area (Washington, DC) but were significantly lower than the measured EC concentrations in the remote area (Brigantine Wilderness Area). This result suggests that other sources beside diesel fuel engines contribute to atmospheric EC concentrations and that EC may not be a good surrogate for diesel particles. A comparison of both benzene and diesel particle simulated concentrations between an urban area (New York City) and a remote area (Brigantine Wilderness Area) shows that, at a spatial resolution of 4 km, the regional background may contribute from 10 to 20% to the peak concentrations. These results suggest that the regional background may not be negligible and should be taken into account in urban air toxics studies.     

Effect of tides on solute flushing from a strait: imaging flow and transport in the East River with SF6

In June 2003, two injections of approximately 3.9 mol of sulfur hexafluoride (SF6) were made 8 days apart in the East River, a 25 km tidal strait, to observe solute mixing and dissipation. The first injection occurred at slack before flood, and the second at slack before ebb (flood = northward flow). Tidally synchronized surveys of the SF6 tracer patch, supplemented by vertical profiles, were conducted by boat for 6 and 4 days following the flood and ebb injections, respectively. Residence times for the tracer-tagged water mass in the East River were estimated to be 3.3 +/- 0.7 days and 1.7 +/- 0.5 days for the flood and ebb injections, respectively, after correcting SF6 inventories for losses of SF6 from the water column by air-water gas exchange. The data indicate that the majority of East River solutes are transported to New York Harbor and that tidal mixing dominates subtidal circulation with respectto solute transport. Surveys of the adjacent lower Hudson River revealed a northward-moving, intermediate layer of East River water. Our results suggest that tidal phasing of contaminant discharges in the East River could reduce environmental impacts, by increasing flushing rates and directing a greater fraction of material away from Long Island Sound.     

Determination of longitudinal dispersion coefficient and net advection in the tidal hudson river with a large-scale,high resolution SF6 tracer release experiment

Physical processes such as advection, dispersion, and air-water gas exchange play important roles in determining the movement and change in concentration of contaminants discharged into rivers. In the following, we report results from a large-scale SF6 tracer release experiment conducted in the tidal Hudson Riverto examine longitudinal dispersion and net advection. SF6 was injected into the Hudson River near Newburgh, NY, and surveyed for 13 days using a new, fully automated, high-resolution SF6 sampling and analysis system. Net down river advection of the water body originally tagged with SF6 was slow, averaging mean displacement rates of about 0.5 +/- 0.2 km d(-1). In contrast, spreading of the tracer was driven by tidal movement, causing rapid mixing of the water up and down river. By examining the change in the second moment of the tracer distribution with time, we determined the mean longitudinal dispersion coefficient to be 70.1 +/- 4.3 m2 s(-1). Temporal evolution of the SF6 inventory indicates an average gas transfer velocity over the period of the experiment of 6.5 +/- 0.5 cm h(-1) (1.56 +/- 0.12 m d(-1)). Vertical profiles show that mixing into the bottom layers of the river, in places reaching more than 53 m, seemed to be rapid.     

Atmospheric trends and radiative forcings of CF4 and C2F6 inferred from firn air

The atmospheric histories of two potent greenhouse gases, tetrafluoromethane (CF4) and hexafluoroethane (C2F6), have been reconstructed for the 20th century based on firn air measurements from both hemispheres. The reconstructed atmospheric trends show that the mixing ratios of both CF4 and C2F6 have increased during the 20th century by factors of approximately 2 and approximately 10, respectively. Initially, the increasing mixing ratios coincided with the rise in primary aluminum production. However, a slower atmospheric growth rate for CF4 appears to be evident during the 1990s, which supports recent aluminum industry reports of reduced CF4 emissions. This work illustrates the changing relationship between CF4 and C2F6 that is likely to be largely the result of both reduced emissions from the aluminum industry and faster growing emissions of C2F6 from the semiconductor industry. Measurements of C2F6 in the older firn air indicate a natural background mixing ratio of <0.3 parts per trillion (ppt), demonstrating that natural sources of this gas are negligible. However, CF4 was deduced to have a preindustrial mixing ratio of 34 -1 ppt (-50% of contemporary levels). This is in good agreement with the previous work of Harnisch et al. (18) and provides independent confirmation of their results. As a result of the large global warming potentials of CF4 and C2F6, these results have important implications for radiative forcing calculations. The radiative forcings of CF4 and C2F6 are shown to have increased over the past 50 years to values in 2001 of 4.1 x 10(-3) Wm(-2) and 7.5 x 10(-4) Wm(-2), respectively, relative to preindustrial concentrations. These forcings are small compared to present day forcings due to the major greenhouse gases but, if the current trends continue, they will continue to increase since both gases have essentially infinite lifetimes. There is, therefore, a large incentive to reduce perfluorocarbon emissions such that through the implementation of the Kyoto Protocol, the atmospheric growth rates may decline in the future.     

Semivolatile organic compounds in window films from lower Manhattan after the September 11th World Trade Center attacks

The September 11th World Trade Center (WTC) terrorist attacks resulted in the large-scale release of contaminants that were deposited on the environment of New York City (NYC). Six weeks after the attacks, samples of an organic film on window surfaces were collected and analyzed for polybrominated diphenyl ethers (PBDE), polychlorinated biphenyls (PCB), polychlorinated naphthalenes (PCN), polycyclic aromatic hydrocarbons (PAH), and organochlorine pesticides (OCPs). Concentrations dropped by an order of magnitude within 1 km of the WTC and reached background concentrations by 3.5 km. Concentrations within 1 km of the WTC averaged 3280 ng/m2 for sigmaPBDE, 900 ng/m2 for sigmaPCB, 33 ng/m2 for sigmaPCN, and 77100 ng/m2 for sigmaPAH. Congener profiles of the sites nearest the WTC suggested a combination of combustion and evaporative sources of all compounds, whereas the background sites exhibited profiles consistent with evaporative sources. PBDE profiles showed enrichment in lower molecular weight congeners near the WTC, suggesting that these congeners were formed as a result of the combustion conditions. Homologue fractions of PCN combustion markers were approximately 2-9 times greater at near WTC sites compared to background NYC. Gas-phase air concentrations were back-calculated from measured film concentrations using the film-air partition coefficient (KFA), and calculated air concentrations followed spatial trends observed in films.     

Atmospheric chlorine chemistry in southeast Texas: impacts on ozone formation and control

Recent evidence has demonstrated that chlorine radical chemistry can enhance tropospheric hydrocarbon oxidation and has the potential to enhance ozone formation in urban atmospheres. To assess these effects quantitatively, an August-September 2000 photochemical episode in southeast Texas was simulated using the comprehensive air quality model, with extensions (CAMx). During this episode, ambient measurements of a unique marker of atmospheric chlorine chemistry, 1-chloro-3-methyl-3butene-2-one (CMBO), were made and model performance was assessed by comparing modeled and observed CMBO mixing ratios. The model predicted ambient CMBO mixing ratios within the uncertainty limits associated with the emissions inventory, so the model was used to assess the impacts of chlorine chemistry on ozone formation. Based on the current emissions inventory, chlorine emissions have the potential to enhance 1-h-averaged ozone mixing ratios by 70 ppb, in very localized areas, during morning hours. Over wider areas, and at times of day when peak ozone concentrations are observed, the impacts of chlorine emissions on ozone concentrations are typically less than 10 ppb. Chlorine emissions also influenced changes in ozone concentrations due to hydrocarbon and NOx emission controls.     

Summertime ambient formaldehyde in five U.S. metropolitan areas: Nashville, Atlanta, Houston, Philadelphia, and Tampa

First, we briefly review the atmospheric chemistry and previous intercomparison measurements for HCHO, with special reference to the diffusion scrubber Hantzsch reaction based fluorescence instrument used in the field studies reported herein. Then we discuss summertime HCHO levels in five major U.S. cities measured over 1999-2002, primarily from ground-based measurements. Land-sea breeze circulations play a major role in observed concentrations in coastal cities. Very high HCHO peak mixing ratios were observed in Houston (>47 ppb) where the overall median mixing ratio was 3.3 ppb; the corresponding values in Atlanta were approximately >18 and 7.9 ppb, respectively. The peak and median mixing ratios (9.3 and 2.3 ppb) were the lowest for Tampa, where the land-sea breeze also played an important role. In several cities, replicate HCHO measurements were made by direct spectroscopic instruments; the instruments were located kilometers from each other and addressed very different heights (e.g., 106 vs 10 m). Even under these conditions, there was remarkable qualitative and often quantitative agreement between the different instruments, when they were all sampling the same air mass within a short period of each other. Local chemistry dominates how HCHO is formed and dissipated. The high concentrations in Houston resulted from emissions near the ship channel; the same formaldehyde plume was measured at two sites and clearly ranged over tens of kilometers. Local micrometeorology is another factor. HCHO patterns measured at a high-rise site in downtown Nashville were very much in synchrony with other ground sites 12 km away until July 4 celebrations whence HCHO concentrations at the downtown site remained elevated for several days and nights. The formation and dissipation of HCHO in the different cities are discussed in terms of other concurrently measured species and meteorological vectors. The vertical profiles of HCHO in and around Tampa under several different atmospheric conditions are presented. The extensive data set represented in this paper underscores that urban HCHO measurements can now be made easily; the agreement between disparate instruments (that are independently calibrated or rely on the absolute absorption cross section) further indicates that such measurements can be done reliably and accurately for this very important atmospheric species. The data set presented here can be used as a benchmark for future measurements if the use of formaldehyde precursors such as methanol or methyl tert-butyl ether (MTBE) as oxygenated fuel additives increases in the future.     

Transport dynamics in a sheltered estuary and connecting tidal straits: SF6 tracer study in New York Harbor

In July 2002, approximately 0.9 mol of sulfur hexafluoride (SF6) was injected into Newark Bay, NJ, a 14 km2 estuary that forms part of New York Harbor, to investigate circulation, mixing, and the transport and fate of solutes. The SF6 tracer was observed over 11 consecutive days using a high-resolution measurement system. Total tracer mass in the sheltered waters declined quasiexponentially at a rate of 0.29 +/- 0.03 d(-1). Air-water gas exchange was estimated to account for 56% of tracer mass loss, upon the basis of wind speed/gas exchange parametrizations. Large-scale tidal transfer of solutes through the Kill van Kull strait (7 km long) caused net seaward flushing contrary to the apparent residual circulation. Seaward transport via the Arthur Kill strait (20 km long) appeared to depend on longitudinal dispersion, residual circulation, and freshwater discharge and was approximately 1 order of magnitude lower. The loss rate due to flushing alone was 0.13 +/- 0.02 d(-1), indicating a mean residence time for water and solutes in Newark Bay of approximately 8 days (without gas exchange). The experiment provides direct visualization of the transport of a released contaminant, and suggests a relationship between the length and configuration of tidal straits and related transport of solutes.     

Polychlorinated dioxins and furans from the World Trade Center attacks in exterior window films from lower Manhattan in New York City

Samples of ambient organic films deposited on exterior window surfaces from lower Manhattan and Brooklyn in New York City were collected six weeks after the terrorist attacks at the World Trade Center (WTC) on September 11, 2001 and analyzed for polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs). Total tetra- through octa-CDD/F concentrations in window films within 1 km of the WTC site in lower Manhattan ranged up to 630,000 pg/m2 (estimated as a mass concentration of ca. 1,300,000 pg/ g) and a maximum toxic equivalent (TEQ) concentration of 4700 TEQ/m2 (ca. 10 000 pg TEQ/g). Measurements at a background site 3.5 km away in Brooklyn showed lower concentrations at 130 pg TEQ/m2 (260 pg TEQ/g). Ambient gas-phase PCDD/F concentrations estimated for each site using an equilibrium partitioning model suggested concentrations ranging from ca. 2700 fg-TEQ/m3 near the WTC site to the more typical urban concentration of 20 fg-TEQ/m3 atthe Brooklyn site. Multivariate analyses of 2,3,7,8-substitued congeners and homologue group profiles suggested unique patterns in films near the WTC site compared to that observed at background sites in the study area and in other literature-derived combustion source profiles. Homologue profiles near the WTC site were dominated by tetra-, penta-, and Hexa-CDD/Fs, and 2,3,7,8-substituted profiles contained mostly octa- and hexachlorinated congeners. In comparison, profiles in Brooklyn and near mid-Manhattan exhibited congener and homologue patterns comprised mainly of hepta- and octa-CDDs, similar to that commonly reported in background air and soil.     

The costs, air quality, and human health effects of meeting peak electricity demand with installed backup generators

Existing generators installed for backup during blackouts could be operated during periods of peak electricity demand, increasing grid reliability and supporting electricity delivery. Many generators, however, have non-negligible air emissions and may potentially damage air quality and harm human health. To evaluate using these generators, we compare the levelized private and social (health) costs of diesel internal combustion engines (ICE) with and without diesel particulate filters (DPF), natural gas ICEs, and microturbines to a new peaking plant in New York, NY. To estimate the social cost, first we calculate the upper range emissions for each generator option from producing 36,000 megawatt-hours (MWh) of electricity over 3 days. We then convert the emissions into ambient concentrations with a 3-D chemical transport model, PMCAMx, and Gaussian dispersion plumes. Using a Monte Carlo approach to incorporate the uncertainties, we calculate the health endpoints using concentration-response functions and multiply the response by its economic value. While uncontrolled diesel ICEs would harm air quality and health, a generator with a DPF has a social cost, comparable to natural gas options. We conclude on a full cost basis that backup generators, including controlled diesel ICEs, are a cost-effective method of meeting peak demand.     

Tedlar bag sampling technique for vertical profiling of carbon dioxide through the atmospheric boundary layer with high precision and accuracy

Carbon dioxide is the most important greenhouse gas other than water vapor, and its modulation by the biosphere is of fundamental importance to our understanding of global climate change. We have developed a new technique for vertical profiling of CO2 and meteorological parameters through the atmospheric boundary layer and well into the free troposphere. Vertical profiling of CO2 mixing ratios allows estimates of landscape-scale fluxes characteristic of approximately100 km2 of an ecosystem. The method makes use of a powered parachute as a platform and a new Tedlar bag air sampling technique. Air samples are returned to the ground where measurements of CO2 mixing ratios are made with high precision (< or =0.1%) and accuracy (< or =0.1%) using a conventional nondispersive infrared analyzer. Laboratory studies are described that characterize the accuracy and precision of the bag sampling technique and that measure the diffusion coefficient of CO2 through the Tedlar bag wall. The technique has been applied in field studies in the proximity of two AmeriFlux sites, and results are compared with tower measurements of CO2.     

Future emissions and atmospheric fate of HFC-1234yf from mobile air conditioners in Europe

HFC-1234yf (2,3,3,3-tetrafluoropropene) is under discussion for replacing HFC-134a (1,1,1,2-tetrafluoroethane) as a cooling agent in mobile air conditioners (MACs) in the European vehicle fleet. Some HFC-1234yf will be released into the atmosphere, where it is almost completely transformed to the persistent trifluoroacetic acid (TFA). Future emissions of HFC-1234yf after a complete conversion of the European vehicle fleet were assessed. Taking current day leakage rates and predicted vehicle numbers for the year 2020 into account, European total HFC-1234yf emissions from MACs were predicted to range between 11.0 and 19.2 Gg yr(-1). Resulting TFA deposition rates and rainwater concentrations over Europe were assessed with two Lagrangian chemistry transport models. Mean European summer-time TFA mixing ratios of about 0.15 ppt (high emission scenario) will surpass previously measured levels in background air in Germany and Switzerland by more than a factor of 10. Mean deposition rates (wet + dry) of TFA were estimated to be 0.65-0.76 kg km(-2) yr(-1), with a maxium of ～2.0 kg km(-2) yr(-1) occurring in Northern Italy. About 30-40% of the European HFC-1234yf emissions were deposited as TFA within Europe, while the remaining fraction was exported toward the Atlantic Ocean, Central Asia, Northern, and Tropical Africa. Largest annual mean TFA concentrations in rainwater were simulated over the Mediterranean and Northern Africa, reaching up to 2500 ng L(-1), while maxima over the continent of about 2000 ng L(-1) occurred in the Czech Republic and Southern Germany. These highest annual mean concentrations are at least 60 times lower than previously determined to be a safe level for the most sensitive aquatic life-forms. Rainwater concentrations during individual rain events would still be 1 order of magnitude lower than the no effect level. To verify these results future occasional sampling of TFA in the atmospheric environment should be considered. If future HFC-1234yf emissions surpass amounts used here studies of TFA accumulation in endorheic basins and other sensitive areas should be aspired.     

Dynamic air-water exchange of polychlorinated biphenyls in the New York-New Jersey Harbor Estuary

Simultaneous measurements of polychlorinated biphenyls (PCBs) in the air and water over Raritan Bay and New York Harbor were taken in July 1998, allowing the first determinations of air-water exchange fluxes for this heavily impacted system. Average gas-phase concentrations of sigmaPCBs were 1.0 ng m(-3) above Raritan Bay and 3.1 ng m(-3) above New York Harbor. A similar gradient was observed for dissolved water concentrations (1.6 and 3.8 ng L(-1), respectively). Shallow slopes of log K(oc) vs log K(ow) plots indicated a colloidal contribution to the dissolved concentrations, and a three-phase partitioning model was therefore applied. PCBs associated with colloids ranged from 6% to 93% for trichloro- to nonachlorobiphenyls, respectively. Air-water gas exchange fluxes of sigmaPCBs exhibited net volatilization for both Raritan Bay at +400 ng m(-2) day(-1) and New York Harbor at +2100 ng m(-2) day(-1). The correction for the colloidal interactions decreased the volatilization flux of sigmaPCBs by about 15%. Net air-water exchange fluxes of PCBs are expected to remain positive throughout the year due to the large water-air fugacity gradient and relatively constant seasonal water concentrations. The volatilization fluxes are approximately 40 times greater than atmospheric deposition of PCBs via both wet and dry particle deposition, suggesting that the estuary acts as a net source of PCBs to the atmosphere year-round.     

New field method: gas push-pull test for the in-situ quantification of microbial activities in the vadose zone

Quantitative information on microbial processes in the field is important. Here we propose a new field method, the "gas push-pull test" (GPPT) for the in-situ quantification of microbial activities in the vadose zone. To evaluate the new method, we studied microbial methane oxidation above an anaerobic, petroleum-contaminated aquifer. A GPPT consists of the injection of a gas mixture of reactants (e.g., methane, oxygen) and nonreactive tracer gases (e.g., neon, argon) into the vadose zone and the subsequent extraction of the injection gas mixture together with soil air from the same location. Rate constants of gas conversion are calculated from breakthrough curves of extracted reactants and tracers. In agreement with expectations from previously measured gas profiles, we determined first-order rate constants of 0.68 h(-1) at 1.1 m below soil surface and 2.19 h(-1) at 2.7 m, close to the groundwater table. Co-injection of a specific inhibitor (acetylene) for methanotrophs showed that the observed methane consumption was microbially mediated. This was confirmed by increases of stable carbon isotope ratios in methane by up to 42.6 %. In the future, GPPTs should provide useful quantitative information on a range of microbial processes in the vadose zone.     

Airborne emissions from 1961 to 2004 of benzo[a]pyrene from U.S. vehicles per km of travel based on tunnel studies

We identified 13 historical measurements of polycyclic aromatic hydrocarbons (PAHs) in U.S. vehicular traffic tunnels that were either directly presented as tailpipe emission factors in microg per vehicle-kilometer or convertible to such a form. Tunnel measurements capture fleet cruise emissions. Emission factors for benzo[a]pyrene (BaP) for a tunnel fleet operating under cruise conditions were highest prior to the 1980s and fell from more than 30-microg per vehicle-km to approximately 2-microg/km in the 1990s, an approximately 15-fold decline. Total annual U.S. (cruise) emissions of BaP dropped by a lesser factor, because total annual km driven increased by a factor of 2.7 during the period. Other PAH compounds measured in tunnels over the 40-year period (e.g., benzo[ghi]perylene, coronene) showed comparable reduction factors in emissions. PAH declines were comparable to those measured in tunnels for carbon monoxide, volatile organic compounds, and particulate organic carbon. The historical PAH "source terms" determined from the data are relevant to quantifying the benefits of emissions control technology and can be used in epidemiological studies evaluating the health effects of exposure, such as those undertaken with breast cancer in New York State.     

Comparison between back-trajectory based modeling and Lagrangian backward dispersion modeling for locating sources of reactive gaseous mercury

Reactive gaseous mercury (RGM) was measured using an annular denuder coated with potassium chloride at three rural sites (Potsdam, Stockton, and Sterling) in New York State from April 2002 to April 2003. Concentrations of RGM ranged from 0.1 to 84.6 pg m(-3) with large spatial and temporal variation. Potential source contribution function (PSCF), a common receptor modeling tool, was used with these measurements, and source-receptor relationships were calculated using back-dispersion and deposition as well as back-trajectories. Modeling results were compared with the RGM emissions inventory, and Spearman rank-order correlation coefficients were calculated. PSCF results incorporating backward dispersion and deposition were better correlated with the emissions inventory than PSCF based on back-trajectories alone. This difference was determined to be mainly due to the inclusion of dispersion rather than deposition. The main sources of RGM were suggested to be coal-fired power plants in New York and Pennsylvania, the large copper smelter in Quebec, and the taconite mining areas around the Great Lakes.     

Modeling semivolatile organic aerosol mass emissions from combustion systems

Experimental measurements of gas-particle partitioning and organic aerosol mass in diluted diesel and wood combustion exhaust are interpreted using a two-component absorptive-partitioning model. The model parameters are determined by fitting the experimental data. The changes in partitioning with dilution of both wood smoke and diesel exhaust can be described by two lumped compounds in roughly equal abundance with effective saturation concentrations of approximately 1600 microg m(-3) and approximately 20 microg m(-3). The model is used to investigate gas-particle partitioning of emissions across a wide range of atmospheric conditions. Under the highly dilute conditions found in the atmosphere, the partitioning of the emissions is strongly influenced by the ambient temperature and the background organic aerosol concentration. The model predicts large changes in primary organic aerosol mass with varying atmospheric conditions, indicating that it is not possible to specify a single value for the organic aerosol emissions. Since atmospheric conditions vary in both space and time, air quality models need to treat primary organic aerosol emissions as semivolatile. Dilution samplers provide useful information about organic aerosol emissions; however, the measurements can be biased relative to atmospheric conditions and constraining predictions of absorptive-partitioning models requires emissions data across the entire range of atmospherically relevant concentrations.     

Twentieth Century Atmospheric Metal Fluxes into Central Park Lake, New York City

It is generally assumed that declining atmospheric lead concentrations in urban centers during the 1970s and 1980s were due almost entirely to the progressive introduction of unleaded gasoline. However, most environmental data are from monitoring programs that began only two to three decades ago, which limits their usefulness. Here, trace metal and radionuclide data from sediment cores in Central Park Lake provide a record of atmospheric pollutant deposition in New York City through the 20th century, which suggests that leaded gasoline combustion was not the dominant source of atmospheric lead for NYC. Lead deposition rates, normalized to known Pb-210 atmospheric influxes, were extremely high, reaching maximum values (>70 μg cm(-2) yr(-1)) from the late 1930s to early 1960s, decades before maximum emissions from combustion of leaded gasoline. Temporal trends of lead, zinc, and tin deposition derived from the lake sediments closely resemble the history of solid waste incineration in New York City. Furthermore, widespread use of solid waste incinerators in the United States and Europe over the last century suggests that solid waste incineration may have provided the dominant source of atmospheric lead and several other metals to many urban centers.     

Elemental and molecular evidence of soot- and char-derived black carbon inputs to New York City's atmosphere during the 20th century

Soot black carbon (here expressed as GBC) is present in sediments of Central Park and Prospect Park Lakes, New York City (NYC), and peaks in the middle of the 20th Century at the highest values (1-3% dry weight) ever reported in urban lakes. During that period (approximately 1940-1970), the GBC represents up to 28% of the total organic carbon (OC). Radionuclide-normalized whole core inventories of accumulated GBC are similar in the two lakes which are separated by approximately 15 km, suggesting that emissions of fine soot particles may have accumulated homogeneously over at least the urban center of NYC. The distribution of polycyclic aromatic hydrocarbons (PAHs) in the sediments is decoupled from that of GBC. The highest levels of total PAHs correspond to peak coal use for space heating in NYC in the early 1900s. In contrast, GBC concentrations were highest in the mid 1900s, a period when oil combustion dominated local fossil fuel use and incineration of municipal solid waste (MSW) was common practice in NYC. Decreases in GBC levels observed in more recently deposited sediments are consistent with improvements in particle emissions control systems. Non-soot BC (char) was identified by a high carbon to nitrogen (C/N) ratio that persisted after correction for GBC. This likely tracer of MSW incineration was estimated to contribute an additional '35% of total organic carbon found in the sediments deposited during the peak period of combustion. The temporal trends of soot-BC observed in our lake cores do not agree with published historical reconstructions based on fuel consumption and estimated emission factors.