Nitrogen Deposition Fluxes Research Articles

Uptake of acid pollutants by mineral dust and their effect on aerosol solubility

Abstract Due to the implications caused by mineral dust and sea-salt heterogeneous reactions with SO 2 , NO x and NH 3 derivatives, this study aims to understand the interaction between gaseous and particulate phases; PM10 and PM2.5, in dust-rich and dust-poor environments. During dust outbreaks, the increase in PM10 and PM2.5 mass concentrations by 80 and 75%, respectively, was accompanied with approximately 30% decrease in water soluble inorganic ions. However, nitrate ion concentration, which increased by 36% during dust-rich episodes, was correlated with a 96% increase in gaseous HONO concentration. This implies a significant impact of dust storms on the tropospheric NO 2 to HONO conversion and consequently the formation of nitrate in PMs. Products of the reaction between HONO and mineral dust render atmospheric aerosols more soluble and consequently higher nitrogen deposition fluxes were calculated.

Nitrogen species in rainwater and aerosols of the Yellow and East China seas: Effects of the East Asian monsoon and anthropogenic emissions and relevance for the NW Pacific Ocean

[1] Rainwater and aerosol samples were collected from a coastal urban area (Qingdao) and remote islands (Qianliyan and Shengsi) and along cruise tracks in the Yellow Sea and East China Sea from 1997 to 2005. The samples were analyzed for nitrogen species (NO3−, NO2−, NH4+, and organic nitrogen) and other important elements. The nitrogen species concentrations showed considerable temporal and spatial variations for wet as well as dry atmospheric depositions. In addition, there was a dramatic reduction in the influence of anthropogenic emissions on nitrogen species with increasing distance from coastal urban stations to remote areas across the Yellow Sea and East China Sea. The monsoon climate of East Asia also had prominent effects on the atmospheric composition of nitrogen, with higher loadings in northerly (i.e., winter) than southerly (i.e., summer) monsoon periods, owing to strong emissions from the East Asian landmass. Dust storms in spring dramatically reduced the periodically high concentrations of atmospheric pollutants (e.g., nitrogen species) across the NW Pacific Ocean, but this was accompanied by a twofold-to-fourfold increase in the temporal deposition flux, which showed broad spatial dimensions. Finally, our study identified a strong gradient of wet as well as dry nitrogen deposition fluxes from East Asia to the interior of the North Pacific Ocean. The gradient reflected changes in emission sources and chemical reactions (e.g., forming secondary aerosols), rainfall and scavenging, and change in air mass trajectory.

Atmospheric inorganic nitrogen in marine aerosol and precipitation and its deposition to the North and South Pacific Oceans

Aerosol and rain samples were collected between 48°N and 55°S during the KH-08-2 and MR08-06 cruises conducted over the North and South Pacific Ocean in 2008 and 2009, to estimate dry and wet deposition fluxes of atmospheric inorganic nitrogen (N). Inorganic N in aerosols was composed of ~68% NH 4 + and ~32% NO 3 – (median values for all data), with ~81% and ~45% of each species being present on fine mode aerosol, respectively. Concentrations of NH 4 + and NO 3 − in rainwater ranged from 1.7–55 μmol L−1 and 0.16–18 μmol L−1, respectively, accounting for ~87% by NH 4 + and ~13% by NO 3 − of total inorganic N (median values for all data). A significant correlation (r = 0.74, p 0.1, n = 6) and subtropical (r = 0.33, p > 0.5, n = 6) western North Pacific, suggesting that emissions of ammonia (NH3) by marine biological activity from the ocean could become a significant source of NH 4 + over the South Pacific. While NO 3 − was the dominant inorganic N species in dry deposition, inorganic N supplied to surface waters by wet deposition was predominantly by NH 4 + (42–99% of the wet deposition fluxes for total inorganic N). We estimated mean total (dry + wet) deposition fluxes of atmospheric total inorganic N in the Pacific Ocean to be 32–64 μmol m−2 d−1, with 66–99% of this by wet deposition, indicating that wet deposition plays a more important role in the supply of atmospheric inorganic N than dry deposition.

Wet deposition of nitrogen and sulfur in Guangzhou, a subtropical area in South China

With the aim of understanding the seasonal distribution of deposition fluxes of nitrogen (N) and sulfur (S) in South China, a main acid deposition region in China, precipitation samples were collected and analyzed from Guangzhou area, between March 2005 and February 2006. The estimated wet deposition of N (including ammonium nitrogen (NH(4)-N) and nitrate nitrogen (NO(3)-N)) and S (sulfate sulfur (SO(4)-S)) during the monitoring period were 40.47 kg N ha(-1) and 65.29 kg S ha(-1), respectively. The average deposition of NH(4)-N was ∼1.5 times of the NO(3)-N deposition, suggesting that the reduced and oxidized N depositions were comparable in the study area. The S and N depositions in the rainy season were greater than those in the dry season, showing great seasonal variation, which was consistent with both the distribution of precipitation and the period of fertilizer application for agriculture. The N and S wet deposition fluxes in Guangzhou were greater than those in Beijing and Zhengzhou, located in the northern China, but comparable to the level of Chongqing, located in the southwestern China, another major acid deposition region. The atmospheric N and S depositions in these cities from north to south were affected by both intensive agricultural and industrial activities.

Altitudinal difference of nitrogen deposition flux, Central Japan.

Altitudinal difference of nitrogen deposition flux, Central Japan.

Estimation of dry deposition fluxes of major inorganic species by canopy throughfall approach

Dry deposition in China has not yet been intensively studied even though it constitutes an important pathway for acid deposition. The total deposition was monitored at Tieshanping catchment, a regional remote site in Chongqing City. The dry deposition loads of sulphur, calcium and nitrogen are estimated by using a canopy throughfall approach. The results indicate that the annual dry deposition loads of sulphur and calcium at Tieshanping site are much higher than those measured in forestry areas in other countries, while the dry deposition flux of nitrogen is at a comparable level. The dry deposition loads of sulphur, calcium and nitrogen are all higher than the wet deposition loads. Applying a canopy budget model has proven that the routine, simplified canopy throughfall approach may underestimate the dry deposition flux of nitrogen-containing species.

Comparison of modelled and monitored deposition fluxes of sulphur and nitrogen to ICP-forest sites in Europe

Abstract. The EMEP MSC-W Eulerian chemical transport model, and its predictions of deposition of acidifying and eutrophying pollutants over Europe, play a key role in the development of emission control strategies for Europe. It is important that this model is tested against observational data. Here we compare the results of the EMEP model with measured data from 160 sites of the European Union/ICP Forest (Level II) monitoring network, for the years 1997 and 2000. This comparison comprises: (a) Precipitation amount, (b) Total deposition of SO2−4 to coniferous and deciduous forests, (c) Wet deposition of SO2−4, NO3− and NH4+ in open field sites, and (d) Concentrations of SO2−4, NO3− and NH4+ in precipitation. Concerning precipitation, the EMEP model and ICP network showed very similar overall levels (within 4% for 1997 and 11% for 2000). The correlation was, however, poor (r2=0.15–0.23). This can be attributed largely to the influence of a few outliers, combined with a small range of rainfall amounts for most points. Correlations between modelled and observed deposition values in this study were rather high (r2 values between 0.4–0.8 for most components and years), with mean values across all sites being within 30%. The EMEP model tends to give somewhat lower values for SO2−4, NO3− and NH4+ wet deposition to ICP, but differences in mean values were within 20% in 1997 and 30% in 2000. Modelled and observed concentrations of SO2−4, NO3− and NH4+ in precipitation are very similar on average (differences of 0–14%), with good correlation between modelled and observed data (r2=0.50–0.78). Differences between the EMEP model and ICP measurements are thought to arise from a mixture of problems with both the observations and model. However, the overall conclusion is that the EMEP model performs rather well in reproducing patterns of S and N deposition to European forests.

Effects of geographical location and land use on atmospheric deposition of nitrogen in the State of Connecticut

A network of eight monitoring stations was established to study the atmospheric nitrogen concentration and deposition in the State of Connecticut. The stations were classified into urban, rural, coastal and inland categories to represent the geographical location and land use characteristics surrounding the monitoring sites. Nitrogen species including nitrate, ammonium, nitric acid vapor and organic nitrogen in the air and precipitation were collected, analyzed and used to infer nitrogen concentrations and dry and wet deposition flux densities for the sampling period from 1997 through 1999, with independently collected meteorological data. Statistical analyses were conducted to evaluate the spatial variations of atmospheric concentration and deposition fluxes of total nitrogen in Connecticut. A slightly higher atmospheric concentration of total nitrogen was observed along the Connecticut coastline of Long Island Sound compared to inland areas, while the differences of nitrogen deposition fluxes were insignificant between coastal and inland sites. The land use characteristics surrounding the monitoring sites had profound effects on the atmospheric nitrogen concentration and dry deposition flux. The ambient nitrogen concentration over the four urban sites was averaged 38.9% higher than that over the rural sites, resulting a 58.0% higher dry deposition flux in these sites compared to their rural counterparts. The local industrial activities and traffic emissions of nitrogen at urban areas had significant effects on the spatial distribution of atmospheric nitrogen concentration and dry deposition flux in the State. Wet and total deposition fluxes appeared to be invariant between the monitoring sites, except for high flux densities measured at Old Greenwich, a monitoring station near to and downwind of the New York and New Jersey industrial complexes.

Trends in Sulphur and Nitrogen Deposition Fluxes in the GEOMON Network, Czech Republic, between 1994 and 2000

Deposition fluxes of sulphur and nitrogen in bulk and throughfall precipitation were monitored for the 1994–2000 period for seven small forest catchments in the GEOMON network, Czech Republic (CR). Four catchments are situated at similar elevations (roughly between 800 and 1000 m) and represent three areas: the Black Triangle near the Krusne hory Mts. coalfield (catchments JEZ and LYS), the Orlicke hory Mts., close to the Polish industrial regions (UDL), and the Sumava Mts., a relatively unpolluted area (LIZ). Three catments (GEM, POM, LES) lie at lower elevations (400–600 m) in Central Bohemia. A substantial decrease in the bulk and throughfall deposition of pollutants occurred as a result of the desulphurisation programme implemented in the Czech Republic between 1996 and 1998. A reduction has been described mainly in the Krusne hory Mts. (JEZ), in Slavkovský les (LYS) and also in Central Bohemia (GEM). The decrease in the throughfall to less than one half within a single year in POM (Central Bohemia) was an example of a direct response to the local emission reduction in the nearby Chvaletice power plant. However, in some areas, the throughfall deposition of sulphur, which includes wet and dry deposition, is still significant, especially at higher elevations. Recent forest degradation was observed in the Orlicke hory Mts., where, particularly in 1998, as much as 91.1 kg S was found in coniferous throughfall. The fraction of dry deposition in the coniferous forests of CR represents 30–70% of the total deposition. The difference between coniferous (higher) and deciduous (lower) throughfall fluxes is significant because of the larger surface area of conifers and year-round exposure to air-borne sulphur. At several of the GEOMON sites, the flux of nitrogen via throughfall increased during the observation period and, at the end of the studied period (2000), nitrogen became the main source of acidification, replacing sulphur compounds. The highest fluxes (81.7 kg N ha-1 yr-1)were measured in 2000 in the Orlicke hory Mts., which provide an example of multiple causes of forest decline – the direct impact of air pollution, abundance of nitrogen, acidification and secondary stressors (weather changes, insect pests, fungal infections). A comparison is given with data from other countries.

Estimates of the atmospheric deposition of sulfur and nitrogen species: Clean Air Status and Trends Network 1990-2000.

The Clean Air Status and Trends Network (CASTNet) was established by the U.S. EPA in response to the requirements of the 1990 Clean Air Act Amendments. To satisfy these requirements CASTNet was designed to assess and report on geographic patterns and long-term, temporal trends in ambient air pollution and acid deposition in order to gauge the effectiveness of current and future mandated emission reductions. This paper presents an analysis of the spatial patterns of deposition of sulfur and nitrogen pollutants for the period 1990-2000. Estimates of deposition are provided for two 4-yr periods: 1990-1993 and 1997-2000. These two periods were selected to contrast deposition before and after the large decrease in SO2 emissions that occurred in 1995. Estimates of dry deposition were obtained from measurements at CASTNet sites combined with deposition velocities that were modeled using the multilayer model, a 20-layer model that simulates the various atmospheric processes that contribute to dry deposition. Estimates of wet deposition were obtained from measurements at sites operated bythe National Atmospheric Deposition Program. The estimates of dry and wet deposition were combined to calculate total deposition of atmospheric sulfur (dry SO2, dry and wet SO4(2-)) and nitrogen (dry HNO3, dry and wet NO3-, dry and wet NH4+). An analysis of the deposition estimates showed a significant decline in sulfur deposition and no change in nitrogen deposition. The highest rates of sulfur deposition were observed in the Ohio River Valley and downwind states. This region also observed the largest decline in sulfur deposition. The highest rates of nitrogen deposition were observed in the Midwest from Illinois to southern New York State. Sulfur and nitrogen deposition fluxes were significantly higher in the eastern United States as compared to the western sites. Dry deposition contributed approximately 38% of total sulfur deposition and 30% of total nitrogen deposition in the eastern United States. Percentages are similar for the two 4-yr periods. Wet sulfate and dry SO2 depositions were the largest contributors to sulfur deposition. Wet nitrate, wet ammonium, and dry HNO3 depositions were the largest contributors to nitrogen deposition.

Estimation of wet, dry and bulk deposition of atmospheric nitrogen in Connecticut

Atmospheric nitrogen species including NO3-, NH4+ and total nitrogen in air and precipitation samples were collected with low-volume filter packs and wet deposition collectors from March 1999 through the end of December 2000 in seven sampling locations in Connecticut. Three sampling locations were chosen along the shores of Long Island Sound and four were chosen in interior sections of Connecticut. Sampling sites were chosen to represent both rural and urban sectors. Wet deposition flux of nitrogen species was calculated using wet concentrations, the volume of collected precipitation and the opening surface area of the Aerochemetrics wet deposition collector. The dry deposition flux of nitrogen species was estimated with the application of the dry deposition inferential model (DDIM). Bulk deposition of nitrogen was collected with the aid of a device based on the Swedish IVL Sampler. The dry deposition fluxes of NO3-, NH4+ and total nitrogen were found to be significantly higher in urban areas than the rural sampling locations. There was, however, no significant difference between the wet deposition fluxes of different nitrogen species in rural and urban sampling locations. When inland and coastal sites were compared, the dry deposition fluxes of NH4+ and total nitrogen were significantly higher in inland locations and there was no significant difference between coastal and inland sampling locations for wet deposition fluxes of nitrogen species. No significant difference was observed between the bulk deposition and the sum of the wet and dry deposition fluxes of total nitrogen at rural sampling locations. In urban sampling locations, the bulk deposition flux of total nitrogen was significantly lower than the sum of dry and wet deposition fluxes. There appears to be a similar seasonal trend in wet and dry deposition fluxes of total nitrogen in Connecticut with high and low deposition fluxes occurring in summer and winter periods, respectively.

Atmospheric Deposition to the Turkey Lakes Watershed: Temporal Variations and Characteristics

We investigated the atmospheric concentrations and deposition fluxes of major ions to the Turkey Lakes Watershed (TLW) between 1980 and 1996. During that time, daily SO4 2− concentrations in precipitation decreased markedly, while NO3 −, NH4 +, and H+ concentrations remained roughly constant. It appears that precipitation acidity did not decrease in spite of declining SO4 2− concentrations due to a concurrent and counterbalancing decrease in the concentrations of Ca2+, Mg2+, and K+ in precipitation. The reasons for the decline in base cations are unknown, but this decline is probably related to decreasing emissions of soil-derived particles from agricultural, industrial, and road sources. A similar situation was seen during the same period in other parts of Canada, the eastern United States, and Europe. Wet, dry, and total (wet + dry) deposition fluxes of sulphur (S) and nitrogen (N) were estimated annually for the years 1980–96. The 17-year mean annual total (wet + dry) deposition of S to the watershed was estimated at 38.5 mmol m−2 y−1 (range 24.3–50.3). Total S deposition decreased by 35% from the early 1980s (1982–84) to the mid-1990s (1994–96), a decline consistent with the 23% decline in annual SO2 emissions in eastern North America during the same period. In contrast, the annual total (wet + dry) deposition of oxidized N ranged from 39.8 to 60.4 mmol m−2 y−1, with a 15-year mean of 50.1 mmol m−2 y−1 and a net increase of 10% between the early 1980s (1983–85) and the mid-1990s (1994–96). This is in keeping with a 10% increase in NOx emissions in eastern North America during the same period. For both S and N (oxidized), wet deposition dominated over dry deposition as the major mechanism for atmospheric input to the watershed. Annually, wet deposition accounted for approximately two-thirds of the total atmospheric deposition of both S and N. Dry S deposition was due more to gaseous SO2 deposition (two-thirds of dry S deposition) than to particulate SO4 2− deposition (one-third of dry S deposition). Dry deposition of oxidized N, however, was dominated (95%) by gaseous HNO3 deposition, with minimal input from particulate NO3 − deposition. Compared to several selected watershed/forest sites in Canada, the United States, and Europe, the estimated total deposition of S and N at the TLW was relatively high during the measurement period.

Modeling the atmospheric deposition and stormwater washoff of nitrogen compounds

We investigated the suitability of integrating deterministic models to estimate the relative contributions of atmospheric dry and wet deposition onto an urban surface and the subsequent amounts removed by stormwater runoff. The CIT airshed model and the United States Environmental Protection Agency Storm Water Management Model (SWMM) were linked in order to simulate the fate and transport of nitrogen species through the atmosphere and storm drainage system in Los Angeles, California, USA. Coupling CIT and SWMM involved defining and resolving five critical issues: (1) reconciling the different modeling domain sizes, (2) accounting for dry deposition due to plant uptake, (3) estimating the fraction of deposited contaminant available for washoff, (4) defining wet deposition inputs to SWMM, and (5) parameterizing the SWMM washoff algorithm. The CIT–SWMM interface was demonstrated by simulating dry deposition, wet deposition, and stormwater runoff events to represent the time period from November 18, 1987 to December 4, 1987 for a heavily urbanized Los Angeles watershed discharging to Santa Monica Bay. From November 18th to December 3rd the simulated average dry deposition flux of nitrogen was 0.195 kg N/ha-day to the watershed and 0.016 kg N/ha-day to Santa Monica Bay. The simulated rainfall concentrations during the December 4th rainfall event ranged from 3.76 to 8.23 mg/l for nitrate and from 0.067 to 0.220 mg/l for ammonium. The simulated stormwater runoff event mean concentrations from the watershed were 4.86 mg/l and 0.12 mg/l for nitrate and ammonium, respectively. Considering the meteorology during the simulation period, the CIT and SWMM predictions compare well with observations in the Los Angeles area and in other urban areas in the United States.

Spatial and temporal variations of atmospheric deposition in interior and Coastal Connecticut

Both wet and dry deposition fluxes of sulfur, nitrogen, and phosphorus were estimated at three monitoring stations in Connecticut, one inland near the Connecticut-Massachusetts border and two along the Connecticut coastline of Long Island Sound. Three years of data, from 1991 through 1993, were analyzed and are reported in this paper. The total annual deposition ranged from 1.27 to 1.58 gm −2 for sulfur, 0.8 to 0.85 gm −2 for nitrogen, and 3.81 to 4.69 mgm −2 for phosphorus. More than 90% of the phosphorus and about two-thirds of the sulfur and nitrogen were deposited in wet form. The estimated wet deposition quantities were all higher in summer, indicating a higher scavenging efficiency of rainfalls. The dry deposition quantities were higher for particles but lower for gases during the summer seasons. While there were no significant differences in wet deposition among the three monitoring stations, spatial gradients in atmospheric concentration and dry deposition, especially for sulphur, were detected with higher values to the southwest industrial area. Although more sites might be needed to pinpoint the sources of the pollutants, one station is sufficient for a reliable estimation of the overall deposition. The strong episodic nature of the deposition quantities, however, demands more frequent than denser sampling in the area for long-term monitoring. Also, since the weekly deposition quantities were all strongly positively skewed, more descriptive statistics in addition to the mean values are recommended in data reports.

Sulfur and reactive nitrogen oxide fluxes in the North Atlantic atmosphere

This paper compiles atmospheric budgets for the large‐scale cycling of sulfur and reactive nitrogen oxides in the atmosphere of the North Atlantic Ocean. Using model results and measurements reported in the literature, estimates are made of the following large‐scale fluxes: eastward from North America into the North Atlantic atmosphere, northward into the Arctic, eastward into and westward out of Europe, westward from Africa, and westward at the Caribbean. Marine emissions into the North Atlantic atmosphere and deposition to the ocean are estimated. Atmospheric transport from continental sources contributes 4.4–7.5 Tg S/yr and 1.7–3.4 Tg N/yr to the North Atlantic Ocean atmosphere, while 0.6–3.6 Tg S/yr comes from marine emissions. Atmospheric transport removes 1.4–1.9 Tg S/yr and 0.5–0.8 Tg N/yr, and deposition to the ocean accounts for 5.2–15.7 Tg S/yr and 1.6–5.7 Tg N/yr. The main input terms for sulfur are flow from North America, followed by marine dimethylsulfide emissions and flow from Africa; for nitrogen, North American and African inputs are comparable. Although outputs exceed inputs for both sulfur and nitrogen, the ranges of estimates overlap by approximately 75%. The spatial distribution of total sulfur deposition flux is generally consistent with known sources and flow regimes except for high values which occur near the European west coast and off the east coast of the Americas south of 30°N. The spatial distribution of total nitrogen deposition flux is more consistent with current knowledge, with the exception of high values across the 0–10° latitude band. The molar sulfur to nitrogen ratio of total deposition exceeds 2 north of 30°N but is close to or below 1 to the south. Both sulfur and nitrogen outputs exceed inputs in the northeastern and southwestern parts of the region, indicating that either inputs from adjacent continents are underestimated or that the deposition is overestimated.

Interannual variability in acidic deposition on the Mt. Mitchell area forest

Recently the forest decline at high elevation mountains in the eastern U.S. has been suggested to be associated with the deposition of acidic substances on the forest canopy through dry, wet and cloud deposition pathways. To determine the relative importance of these deposition mechanisms, a field study was initiated in May 1986, at Mt. Mitchell, NC. Since Mt. Mitchell is frequently immersed in clouds (immersion time being in the range of 28–41%), our investigations were primarily focused on the collection of cloud water and the monitoring of meteorology and ambient air quality. The precipitation data and related chemistry were obtained from a nearby NADP (National Atmospheric Deposition Program) site (Clingman's Peak). To estimate the dry and cloud deposition, the deposition velocities for gases and the rates of cloud deposition for cloud droplets are calculated with the ATDD (Atmospheric Turbulence and Diffusion Division, National Oceanic and Atmospheric Administration) model and a cloud deposition model (CDM), respectively. The wet deposition is obtained from NADP annual reports. Computations show the deposition velocities for SO 2, NO 2 and O 3 to be in the range of tenths of cm s −1. The mean rate of cloud deposition is about 0.13–0.21 mm h −1. The rainfall ranged from 40 to 60 cm during the growing seasons (from mid-May to the end of September) of 1986–1988. Using these deposition parameters and the 3 year database, the deposition fluxes of sulfur (S) compounds are found primarily contributed through cloud capture mechanism (60%) followed by incident precipitation (25%) and dry deposition (15%). As to the deposition fluxes of nitrogen (N) compounds, cloud, wet and dry deposition contributedf about 50%, 25% and 25%, respectively. A comparison of deposition estimates at Mt. Mitchell with those at other sites shows that the sulfate deposition at sites exceeding 1200 m MSL in elevation in Bavaria, F.R.G., and the eastern U.S. is a almost identical within error limits. Reasons for large uncertainties in deposition estimates are also discussed as the mechanisms for redistribution of the deposited material on the forest canopy.

Tropospheric NOx and O3 budgets in the equatorial Pacific

By combining the results of model calculations and simultaneous measurements of O3, NO, HNO3, particulate NO3−, and meteorological parameters in the mid‐Pacific, we have investigated the budget and photochemistry of near surface atmospheric ozone and odd nitrogen. The observed decrease of O3 toward the Intertropical Convergence Zone is consistent with the transport of O3 from higher latitude and a photochemical sink for O3 by the reaction between H2O and O(1D). The total deposition flux of odd nitrogen deduced from the data is about 1.3×109 cm−2 s−1, or 5 MT(N)/yr averaged globally. Most of the deposition flux is due to HNO3 and particulate NO3−. This deposition flux is probably representative of values over most of the Pacific because the HNO3 and particulate NO3− densities measured over the Pacific during the GAMETAG experiments are consistent with our values. Since the residence time of odd nitrogen in the lower troposphere is short, a widely distributed, diffuse source of odd nitrogen is required to balance the deposition flux. It seems that the NO formed by cloud to cloud lightning in the tropical upper troposphere is the most likely source.