Institute of Earth Environment
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Plutonium isotopes in the northwestern South China Sea: Level, distribution, source and deposition
No full textThe spatial distribution of plutonium isotopes (Pu-239, Pu-240) in the surface sediments collected from the northwestern South China Sea (SCS) in 2018 was investigated. The Pu-239,Pu-240 concentrations in surface sediments vary from 0.048 to 0.960 mBq/g (with mean of 0.282 +/- 0.242 mBq/g) depending on the geographical feature of the sampling location such as the river estuary, continental shelf, slope and deep basin. Higher Pu-240/Pu-239 atomic ratios (0.24-0.31) in the surface sediment of the SCS compared to the global fallout value of 0.18 were observed, this is attributed to the input of close-in fallout of the Pacific Proving Ground (PPG) transported by the North Equatorial Current and Kuroshio Current to the northern SCS. The contribution of the PPG derived plutonium in the SCS sediment was estimated to be 39%-78% using a simple two-end member mixing model based on the measured Pu-240/Pu-239 atomic ratios in the sediment. Besides the soluble Pu-239,Pu-240 level in seawater, load of suspended particulate matter from the river runoff and biological debris, hydrographic and hydrodynamic conditions are key parameters influencing the deposition process of plutonium to the sediment
Rapid determination of plutonium isotopes in small samples using single anion exchange separation and ICP-MS/MS measurement in NH3-He mode for sediment dating
No full textTo accurately determine ultra-trace Pu isotopes in small environmental samples, we explored ICP-MS/MS in NH3-He mode, and investigated mechanism of U-238 interference removal and measurement sensitivity improvement for plutonium isotopes. The interference of uranium and uranium hydrides was effectively eliminated using 0.4 mL/min NH3 as reaction gas by shifting them to U(NHm)(n)(+) and UH(NHm)(n)(+). The overall interference of uranium was reduced to <2.4 x 10 (-7), while remaining excellent Pu-239 sensitivity (13,900 Mcps/(mg/L)) mainly due to ion focusing effect of Pu by helium gas. On this basis, the purification of plutonium using a single AG1- x 4 column was proved to be sufficient for accurate determination of plutonium isotopes by the developed detection method, and the detection limits for the method were estimated to be 0.16 fg (0.4 mu Bq) for Pu-239, 0.046 fg (0.4 mu Bq) for Pu-240 and 0.039 fg (0.15 mBq) for Pu-241. The method was validated by analyzing plutonium isotopes in certificated reference materials and reported environmental samples of only 1-2 g. The analytical results of ultra-trace Pu isotopes in small amounts (similar to 1 g) of lake sediments obtained by the developed method were successfully applied to sediment dating
Substantial peak size effect on compound-specific delta D values analyzed on isotope ratio mass spectrometry
No full textLipid compound-specific hydrogen isotopes (delta D) have been widely applied in studies of biogeochemistry and paleoclimatology. delta D values of lipid homologues with different chain lengths from a single analysis are often utilized together in order to make full use of the delta D information. However, this approach could be potentially impacted by the large analytical error due to the inappropriate amount injected for some homologues. Here, we systematically investigate compound-specific delta D deviations with varying contents injected on the isotope ratio mass spectrometry, expressed as amplitudes, using both lab working standards and natural samples. The n-alkane delta D values of lab working standards (mixed C-21, C-25, C-27, C-29, C-31, and C-33) could vary by 40 parts per thousand to 70 parts per thousand when the amplitudes of n-alkanes change from 0.5 V to 10 V. For natural samples (27 n-alkane and 77 fatty acid samples), we have made repeated analyses of the same samples with different homologues targeted for the optimum range. The measured delta D values are higher by 20 parts per thousand to 40 parts per thousand with the amplitudes lower than the optimum range, and lower by 10 parts per thousand to 20 parts per thousand with higher amplitudes. All the results consistently show higher delta D values with decreasing amplitudes, and larger deviations occurring in low amplitude range, implying that special caution should be taken with the delta D values measured at low amplitude range. We have attempted amplitude-based correction of lipid delta D values, however, this approach should be cautious owing to their large residual errors. Hence, delta D values of different homologues in the same samples have to be measured in separate analysis if they could not fall within the optimum range at the same time
Substantial peak size effect on compound-specific delta D values analyzed on isotope ratio mass spectrometry
No full textLipid compound-specific hydrogen isotopes (delta D) have been widely applied in studies of biogeochemistry and paleoclimatology. delta D values of lipid homologues with different chain lengths from a single analysis are often utilized together in order to make full use of the delta D information. However, this approach could be potentially impacted by the large analytical error due to the inappropriate amount injected for some homologues. Here, we systematically investigate compound-specific delta D deviations with varying contents injected on the isotope ratio mass spectrometry, expressed as amplitudes, using both lab working standards and natural samples. The n-alkane delta D values of lab working standards (mixed C-21, C-25, C-27, C-29, C-31, and C-33) could vary by 40 parts per thousand to 70 parts per thousand when the amplitudes of n-alkanes change from 0.5 V to 10 V. For natural samples (27 n-alkane and 77 fatty acid samples), we have made repeated analyses of the same samples with different homologues targeted for the optimum range. The measured delta D values are higher by 20 parts per thousand to 40 parts per thousand with the amplitudes lower than the optimum range, and lower by 10 parts per thousand to 20 parts per thousand with higher amplitudes. All the results consistently show higher delta D values with decreasing amplitudes, and larger deviations occurring in low amplitude range, implying that special caution should be taken with the delta D values measured at low amplitude range. We have attempted amplitude-based correction of lipid delta D values, however, this approach should be cautious owing to their large residual errors. Hence, delta D values of different homologues in the same samples have to be measured in separate analysis if they could not fall within the optimum range at the same time
Effects of hydroperoxy radical heterogeneous loss on the summertime ozone formation in the North China Plain
No full textHydroperoxy radical (HO2) is a crucial oxidant participating in the oxidation of nitrogen oxide to nitrogen dioxide which constitutes one of the most important pathways for the ozone (O-3) photochemical formation in the troposphere. Laboratory experiments and field observations have revealed efficient HO2 heterogeneous uptake on wet aerosols, but its impact on the O-3 formation remains controversial. A severe and persistent O-3 pollution episode has been simulated using the WRF-Chem model to evaluate the impacts of the HO2 heterogeneous loss on the O-3 formation in the North China Plain (NCP) during the summertime of 2018. Comparisons between experimental simulations with the HO2 effective uptake coefficient of 0.2 and 0.0 shows that the HO2 heterogeneous loss decreases the daytime HO2 and maximum daily average 8-hour (MDA8) O-3 concentrations by about 5% and 1% in the NCP, respectively. Emission mitigation from 2013 to 2018 is found to contribute a 2.1 mu g m(-3) (5%) increase in the MDA8 O-3 concentration due to decreased aerosol sink for the HO2 heterogeneous loss in the NCP. Our results reveal that decreased HO2 heteroge-neous uptake does not constitute an important factor driving the O-3 trend since 2013 in the NCP
Seasonality of Aerosol Sources Calls for Distinct Air Quality Mitigation Strategies
No full textAn Aerosol Chemical Speciation Monitor (ACSM) was deployed to investigate the temporal variability of non-refractory particulate matter (NR-PM1) in the coastal city of Galway, Ireland, from February to July 2016. Source apportionment of the organic aerosol (OA) was performed using the newly developed rolling PMF strategy and was compared with the conventional seasonal PMF. Primary OA (POA) factors apportioned by rolling and seasonal PMF were similar. POA factors of hydrocarbon-like OA (HOA), peat, wood, and coal were associated with domestic heating, and with an increased contribution to the OA mass in winter. Even in summer, sporadic heating events occurred with similar diurnal patterns to that in winter. Two oxygenated OA (OOA) factors were resolved, including more-oxygenated OOA and less-oxygenated OOA (i.e., MO-OOA and LO-OOA, accordingly) which were found to be the dominant OA factors during summer. On average, MO-OOA accounted for 62% of OA and was associated with long-range transport in summer. In summer, compared to rolling PMF, the conventional seasonal PMF over-estimated LO-OOA by nearly 100% while it underestimated MO-OOA by 30%. The results from this study show residential heating and long-range transport alternately dominate the submicron aerosol concentrations in this coastal city, requiring different mitigation strategies in different seasons
Tuning the nitrogen contents in carbon matrix encapsulating Co nanoparticles for promoting formaldehyde removal through Mott-Schottky effect
No full textThe increment of N dopants in carbon matrix can effectively enrich the electron density of N-doped carbon encapsulating metal nanoparticles (NPs) via Mott-Schottky effect, thereby favoring the activation of molecular O2 with abundant free electrons on carbon surface. Herein, given the essential role of oxygen activation in formaldehyde (HCHO) oxidation, Co NPs-embedded nitrogen-doped carbon and carbon nanotubes (Co/NCNT) with controllable N contents (2.30 - 4.38 wt%) were prepared and utilized for catalytic oxidation of HCHO at room temperature. Electron transfer from the Co core to the N-doped carbon layer was modulated by the tailorable concentrations of N dopant through the Mott-Schottky effect at their interface, being validated by X-ray and ultraviolet photoelectron spectroscopy, as well as CO2 temperature-programmed desorption experiment. The catalytic activity increased gradually with the elevation of nitrogen content, achieving a HCHO removal efficiency of approximately 80% over Co/NCNT-5 with the maximum nitrogen content. Combined with the comprehensive characterizations, the mechanism underlying the catalytic activity improvement in HCHO oxidation induced by the Mott-Schottky effect was proposed. This work provides a new insight into optimizing the non-noble metal catalysts for HCHO oxidation at room temperature
Tuning the nitrogen contents in carbon matrix encapsulating Co nanoparticles for promoting formaldehyde removal through Mott-Schottky effect
No full textThe increment of N dopants in carbon matrix can effectively enrich the electron density of N-doped carbon encapsulating metal nanoparticles (NPs) via Mott-Schottky effect, thereby favoring the activation of molecular O2 with abundant free electrons on carbon surface. Herein, given the essential role of oxygen activation in formaldehyde (HCHO) oxidation, Co NPs-embedded nitrogen-doped carbon and carbon nanotubes (Co/NCNT) with controllable N contents (2.30 - 4.38 wt%) were prepared and utilized for catalytic oxidation of HCHO at room temperature. Electron transfer from the Co core to the N-doped carbon layer was modulated by the tailorable concentrations of N dopant through the Mott-Schottky effect at their interface, being validated by X-ray and ultraviolet photoelectron spectroscopy, as well as CO2 temperature-programmed desorption experiment. The catalytic activity increased gradually with the elevation of nitrogen content, achieving a HCHO removal efficiency of approximately 80% over Co/NCNT-5 with the maximum nitrogen content. Combined with the comprehensive characterizations, the mechanism underlying the catalytic activity improvement in HCHO oxidation induced by the Mott-Schottky effect was proposed. This work provides a new insight into optimizing the non-noble metal catalysts for HCHO oxidation at room temperature
Variations in monsoon precipitation over southwest China during the last 1500 years and possible driving forces
No full textUnderstanding hydroclimatic patterns and their possible driving mechanisms during distinct climate periods over the last 1500 years-such as the Medieval Warm Period (MWP), the Little Ice Age (LIA), and the Current Warm Period-is crucial for predicting future changes to monsoon precipitation in southwest China under global warming scenarios. In this study, based on Pb-210 and Cs-137 dating of surface sediments and AMS C-14 dating of terrestrial plant residues, we establish a robust age model that covers the last similar to 1500 years (AD 439-2012) at Lake Yihai in southwest China. We use analyses of multiple geochemical proxy indices, including loss on ignition at 550 degrees C, total organic carbon, total nitrogen, C/N ratios, and stable carbon isotopes of organic matter to reconstruct changes in summer monsoon precipitation at Lake Yihai during the last similar to 1500 years. The results show that, over southwest China, warm and dry climate conditions prevailed during the MWP (AD 1000-1400) and the past 200 years, whereas conditions during the LIA (AD 1400-1800) were cold and wet. This is consistent with evidence from other geological records over southwest China, such as stalagmite and lake sediment data. Similar hydroclimatic patterns have occurred over the last 1500 years in adjacent tropical/subtropical monsoon regions where the climate is similarly dominated by the Indian summer monsoon (e.g., South China, the South China Sea, Southeast Asia, Northeast India). We suggest that the meridional migration of the mean position of the Inter-tropical Convergence Zone, and El Nino/Southern Oscillation conditions which are linked to tropical Pacific sea surface temperature, are responsible for centennial-scale hydroclimatic patterns over southwest China and adjacent areas during the last 1500 years
Heterogeneous HONO formation deteriorates the wintertime particulate pollution in the Guanzhong Basin, China
No full textDespite implementation of strict emission mitigation measures since 2013, heavy haze with high levels of secondary aerosols still frequently engulfs the Guanzhong Basin (GZB), China, during wintertime, remarkably impairing visibility and potentially causing severe health issues. Although the observed low ozone (O3) concentrations do not facilitate the photochemical formation of secondary aerosols, the measured high nitrous acid (HONO) level provides an alternate pathway in the GZB. The impact of heterogeneous HONO sources on the wintertime particulate pollution and atmospheric oxidizing capability (AOC) is evaluated in the GZB. Simulations by the Weather Research and Forecast model coupled with Chemistry (WRF-Chem) reveal that the observed high levels of nitrate and secondary organic aerosols (SOA) are reproduced when both homogeneous and heterogeneous HONO sources are considered. The heterogeneous sources (HET-sources) contribute about 98% of the near-surface HONO concentration in the GZB, increasing the hydroxyl radical (OH) and O3 concentration by 39.4% and 22.0%, respectively. The average contribution of the HET-sources to SOA, nitrate, ammonium, and sulfate in the GZB is 35.6%, 20.6%, 12.1%, and 6.0% during the particulate pollution episode, respectively, enhancing the mass concentration of fine particulate matters (PM2.5) by around 12.2%. Our results suggest that decreasing HONO level or the AOC becomes an effective pathway to alleviate the wintertime particulate pollution in the GZB