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Measurement report: PM2:5-bound nitrated aromatic compounds in Xi'an, Northwest China - seasonal variations and contributions to optical properties of brown carbon
No full textNitrated aromatic compounds (NACs) are a group of key chromophores for brown carbon (light-absorbing organic carbon, i.e., BrC) aerosol, which affects radiative forcing. The chemical composition and sources of NACs and their contributions to BrC absorption, however, are still not well understood. In this study, PM2:5-bound NACs in Xi'an, Northwest China, were investigated for 112 daily PM2:5 filter samples from 2015 to 2016. Both the total concentrations and contributions from individual species of NACs show distinct seasonal variations. The seasonally averaged concentrations of NACs are 2.1 (spring), 1.1 (summer), 12.9 (fall), and 56 ng m(-3) (winter). Thereinto, 4-nitrophenol is the major NAC component in spring (58 %). The concentrations of 5-nitrosalicylic acid and 4-nitrophenol dominate in summer (70 %), and the concentrations of 4-nitrocatechol and 4-nitrophenol dominate in fall (58 %) and winter (55 %). The NAC species show different seasonal patterns in concentrations, indicating differences in emissions and formation pathways. Source apportionment results using positive matrix factorization (PMF) further show large seasonal differences in the sources of NACs. Specifically, in summer, NACs were highly influenced by secondary formation and vehicle emissions (similar to 80 %), while in winter, biomass burning and coal combustion contributed the most (similar to 75 %). Furthermore, the light absorption contributions of NACs to BrC are wavelength-dependent and vary greatly by season, with maximum contributions at similar to 330 nm in winter and fall and similar to 320 nm in summer and spring. The differences in the contribution to light absorption are associated with the higher mass fractions of 4-nitrocatechol ((lambda)max = 345 nm) and 4-nitrophenol ((lambda)max = 310 nm) in fall and winter, 4-nitrophenol in spring, and 5-nitrosalicylic acid ((lambda)max = 315 nm) and 4-nitrophenol in summer. The mean contributions of NACs to BrC light absorption at a wavelength of 365 nm in different seasons are 0.14% (spring), 0.09% (summer), 0.36% (fall), and 0.91% (winter), which are about 6-9 times higher than their mass fractional contributions of carbon in total organic carbon. Our results indicate that the composition and sources of NACs have profound impacts on the BrC light absorption
Two-stage evolution of glacial-period Asian monsoon circulation by shifts of westerly jet streams and changes of North American ice sheets
No full textCompilation of North American loess, dune, lacustrine, and glacial moraine morphological records at places along the retreating Laurentide Ice Sheet shows intra D-O/Heinrich-scale climate oscillations between 28,000 and 10,000 calibrated years before present (calBP). Grayscale variations of the loess-paleosol record that represents the composite stratigraphy reflect the poleward-equatorward displacement of westerly jet stream, associated with advance-retreat of the Laurentide Ice Sheet, both of which superpose on a 2-stage trend of drastic changes. Compilation of paleo-Asian monsoon records from NE, East Central, and SW China also shows a 2-stage evolution of Asian monsoon rainbelt distribution with patterns on intra D-O/Heinrich scales between 28,000 and 10,000 calBP. In stage 1, Asian monsoon rainfall intensity decreases over NE/SW but increases over EC, then drastically fluctuates over NE/SW and keeps constant over EC China. In stage 2, Asian monsoon rainbelts distribute a modern-synoptic-style tripole and reversed tripole mode during boreal interstadials and stadials. Additional compilation of more paleo-Asian monsoon rainfall records and state-of-the-science model precipitation records confirms these patterns over large geographic region and Asian continent during this period. The comparison of global climate forcing factors excludes Atlantic Meridional Overturning Circulation but suggests poleward-equatorward displacements of North American westerly jet streams coupled with ice sheet margin fluctuations at middle to upper midlatitudes, which altered the structures of Eurasian jet streams, to modulate the 2-stage intra D-O/Heinrich-scale changes on Asian monsoon evolution. The drastic retreating of the Laurentide Ice sheet during the second stage made the modern-synoptic-style monsoon rainbelt distribution with seesaw patterns on intra D-O/Heinrich warm and cold phases prominent at 16,000 calBP
Speleothem record of mild and wet mid-Pleistocene climate in northeast Greenland
No full textThe five interglacials before the Mid-Brunhes Event (MBE) [c.430 thousand years (ka) ago] are generally considered to be globally cooler than those post-MBE. Inhomogeneities exist regionally, however, which suggest that the Arctic was warmer than present during Marine Isotope Stage (MIS) 15a. Using the first speleothem record for the High Arctic, we investigate the climatic response of northeast Greenland between c.588 and c.549 ka ago. Our results indicate an enhanced warmth of at least +3.5 degrees C relative to the present, leading to permafrost thaw and increased precipitation. We find that delta O-18 of precipitation was at least 3 parts per thousand higher than today and recognize two local cooling events (c.571 and c.594 ka ago) thought to be caused by freshwater forcing. Our results are important for improving understanding of the regional climatic response leading up to the MBE and specifically provide insights into the climatic response of a warmer Arctic
The genesis, development, and evolution of original vertical joints in loess
No full textOriginal vertical joints (OVJs), which are highly correlated with loess anisotropy, have been considered a non-tectonic source of extensive erosion, gully development, and landform fragmentation in loess regions such as the Chinese Loess Plateau. The existing literature discussing the origin of OVJs lacks a systematic overview of the current theories related to OVJ genetic mechanisms, development patterns, theoretical models, and relationships between OVJ morphologies and loess historical sedimentary processes (LHSP) since the Quaternary. By reviewing the key factors governing the formation and evolution of OVJs in loess, we present both a conceptual framework and a case study to demonstrate the genesis and dynamics of OVJs in the loess landscape. We propose that when the driving force produced by the hydro-mechanical coupling regime exceeds the resistance force due to tensile strength, OVJs appear along the boundaries between different structural units within the loess soils. Influenced by the loess sedimentary effects in the later period (i.e., LHSP), such as loess pile thickening, drying-wetting cycles, and weathering, OVJs formed in historical loess frequently degrade and expand. Based on the knowledge of OVJ dynamics, we reconstruct the evolutionary trajectory of OVJs in the loess profile from the LHSP perspective, which is further confirmed by field observations on OVJ morphologies under different local microtopographical and geological conditions. The new perspective on the genesis, development, and evolution of OVJs enhances theoretical understanding of the formation of anisotropy and structural evolution in loess, which helps describe and model subsurface hydrology and geohazard risks in loess regions
Synchronous changes in the East Asian-Australian summer monsoons around 7.2 ka
No full textPrevious evidence has identified the presence of numerous abrupt changes in the East Asian-Australian summer monsoons (EAASMs) on centennial to millennial timescales during the Holocene, which are tightly coupled with high northern latitude cooling anomalies. However, less attention has been paid to spatial characteristics in the EAASMs during the 7.2 ka event. Here a series of hydrological records from the EAASMs region are systematically integrated to improve understanding of the spatial precipitation distributions in the EAASMs region around 7.2 ka. The results show significant dry conditions in northern China, but relatively dry conditions or dry-to-wet shifts in southern tropical China around 7.2 ka, distinctly indicating an abrupt collapse of the East Asian summer monsoon during the 7.2 ka event. A similar pattern is also observed in the Australian summer monsoon region. The precipitation in northern Australia and its adjacent seas abruptly decreases around 7.2 ka, whereas it takes on a slight increase with a dry-to-wet shift near the center of the Indo-Pacific warm pool region. Therefore, it seems that synchronous collapse in the EAASMs occurs during the 7.2 ka event, which is different from a see-saw in the EAASMs during the 8.2 ka event. And it is inferred that synchronous changes in the EAASMs at the 7.2 ka event is probably a response to a low solar irradiance superimposed with influence of the volcanic eruption and ENSO activities
Improved estimation of PM2.5 brown carbon contributions to filter light attenuation
No full textMultiwavelength light attenuation measurements have been acquired as part of thermal/optical carbon analysis in the U.S. Chemical Speciation Network (CSN) and the Interagency Monitoring of PROtected Visual Environments (IMPROVE) network beginning in 2016. These are used to estimate PM2.5 brown carbon (BrC) contributions to light absorption at various wavelengths, a useful method for separating biomass burning contributions from other sources. Attenuation of light transmitted through the filter deviates from Beers Law as the mass of light absorbing materials increase. This study estimates the effects of these deviations with empirical adjustment factors applied to samples for CSN from 2016 to 2017 and for IMPROVE from 2016 to 2019. Accounting for the filter loading effect results in an annual average increase of similar to 6-7% BrC contribution to light attenuation: from 3.6% to 10.7% for the urban, more heavily loaded CSN samples; and from 23.7% to 29.5% for the non-urban IMPROVE samples. An alternative method is examined for BrC and black carbon (BC) adjustments by calculating the Absorption Angstrom Exponent (AAE) for BC (i.e., AAE(BC)) based on the ratios of 635 nm/780 nm light attenuation rather than assuming AAE(BC) of unity. These paired-wavelength calculations result in a median AAE(BC) of 0.76 for CSN and 0.8 for IMPROVE, with the majority of samples (i.e., 91% of CSN and 70% of IMPROVE) showing AAE(BC) < 1. By assuming negligible contributions from BrC to AAE at longer wavelengths, the amount of light attenuation at shorter wavelengths (e.g., 405 nm) where BrC is dominant can be calculated. The paired-wavelength method applied to the filter loading adjusted data has a greater effect on urban (fresh) than on non-urban (aged) aerosols, resulting in a factor of two increase in annual averaged BrC light attenuation (from 10.7% to 21.6%) for CSN and by a factor of 1.11 (from 29.5% to 32.7%) for IMPROVE samples. This result demonstrates the importance of particle loading and AAE correction on quantifying BrC light attenuation from multi-wavelength thermal/optical analysis. Published by Elsevier B.V. on behalf of Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences
Global, continental, and national variation in PM2.5, O-3, and NO2 concentrations during the early 2020 COVID-19 lockdown
No full textLockdowns implemented in response to COVID-19 have caused an unprecedented reduction in global economic and transport activity. In this study, variation in the concentration of health-threatening air pollutants (PM2.5, NO2, and O-3) pre- and post-lockdown was investigated at global, continental, and national scales. We analyzed ground-based data from >10,000 monitoring stations in 380 cities across the globe. Global-scale results during lockdown (March to May 2020) showed that concentrations of PM2.5 and NO2 decreased by 16.1% and 45.8%, respectively, compared to the baseline period (2015-2019). However, O-3 concentration increased by 5.4%. At the continental scale, concentrations of PM2.5 and NO2 substantially dropped in 2020 across all continents during lockdown compared to the baseline, with a maximum reduction of 20.4% for PM2.5 in East Asia and 42.5% for NO2 in Europe. The maximum reduction in O-3 was observed in North America (7.8%), followed by Asia (0.7%), while small increases were found in other continents. At the national scale, PM2.5 and NO2 concentrations decreased significantly during lockdown, but O-3 concentration showed varying patterns among countries. We found maximum reductions of 50.8% for PM2.5 in India and 103.5% for NO2 in Spain. The maximum reduction in O-3 (22.5%) was found in India. Improvements in air quality were temporary as pollution levels increased in cities since lockdowns were lifted. We posit that these unprecedented changes in air pollutants were mainly attributable to reductions in traffic and industrial activities. Column reductions could also be explained by meteorological variability and a decline in emissions caused by environmental policy regulations. Our results have implications for the continued implementation of strict air quality policies and emission control strategies to improve environmental and human health
Genomic insights into the formation of human populations in East Asia
No full textThe deep population history of East Asia remains poorly understood owing to a lack of ancient DNA data and sparse sampling of present-day people(1,2). Here we report genome-wide data from 166 East Asian individuals dating to between 6000 BC and AD 1000 and 46 present-day groups. Hunter-gatherers from Japan, the Amur River Basin, and people of Neolithic and Iron Age Taiwan and the Tibetan Plateau are linked by a deeply splitting lineage that probably reflects a coastal migration during the Late Pleistocene epoch. We also follow expansions during the subsequent Holocene epoch from four regions. First, hunter-gatherers from Mongolia and the Amur River Basin have ancestry shared by individuals who speak Mongolic and Tungusic languages, but do not carry ancestry characteristic of farmers from the West Liao River region (around 3000 BC), which contradicts theories that the expansion of these farmers spread the Mongolic and Tungusic proto-languages. Second, farmers from the Yellow River Basin (around 3000 BC) probably spread Sino-Tibetan languages, as their ancestry dispersed both to Tibet-where it forms approximately 84% of the gene pool in some groups-and to the Central Plain, where it has contributed around 59-84% to modern Han Chinese groups. Third, people from Taiwan from around 1300 BC to AD 800 derived approximately 75% of their ancestry from a lineage that is widespread in modern individuals who speak Austronesian, Tai-Kadai and Austroasiatic languages, and that we hypothesize derives from farmers of the Yangtze River Valley. Ancient people from Taiwan also derived about 25% of their ancestry from a northern lineage that is related to, but different from, farmers of the Yellow River Basin, which suggests an additional north-to-south expansion. Fourth, ancestry from Yamnaya Steppe pastoralists arrived in western Mongolia after around 3000 BC but was displaced by previously established lineages even while it persisted in western China, as would be expected if this ancestry was associated with the spread of proto-Tocharian Indo-European languages. Two later gene flows affected western Mongolia: migrants after around 2000 BC with Yamnaya and European farmer ancestry, and episodic influences of later groups with ancestry from Turan
The impacts of volcanic eruptions and climate changes on the development of Hani peatland in northeastern China during the Holocene
No full textPeatland development is closely correlated with regional environment and climate changes. In this study, two sediment cores covering the past 13.8 cal kyr BP were collected from Hani peatland in northeastern China. The organic content, carbonate content, Rb/Sr and Zr/Rb ratios were obtained to discuss the peat development process and associated environment and climate changes. The results showed that there were two deposition events in Hani peatland during 11.2-9.3 cal kyr BP and 2-1.4 cal kyr BP, when the organic content and Rb/Sr ratio decreased sharply, the carbonate content and Zr/Rb ratio increased dramatically. The synchronous occurrence of these two events and nearby volcanic eruptions, combining with tephra observed in sediments, indicated that volcanic activities have some impacts on the peat development. Except for the two events, the organic content of peat sediments increased gradually from the early to mid Holocene, and was highest at 8-6 cal kyr BP, then decreased gradually from the mid to late Holocene. The comparison with regional vegetation and paleoclimate records, together with the chemical weathering intensity obtained from the Rb/Sr, suggested that the orbital scale peat development in Hani during the Holocene was closely coupled with regional climate changes, including both temperature and precipitation. In addition, the 0.5-2 ka band filtering results of organic content showed similar oscillations with the drift ice indices of the North Atlantic Ocean and the atmospheric Delta C-14 record, implying that the North Atlantic climate and solar activities could also leave footprints on peat development of Hani on millennial timescale
Potential health risk assessment of HFRs, PCBs, and OCPs in the Yellow River basin
No full textThe concentrations of PBDEs, NBFRs, DP, PCBs, and OCPs were analyzed in water samples of the Yellow River Basin (YRB) and in soil and maize samples collected from basin irrigation areas to understand the status of POPs and associated health risks. The results showed: (1) the congeners of eight PBDEs and seven NBFRs were detected in 10 tributaries, with average concentrations of 1575 and 4288 pg. L-1. Thirty-three congeners of PCBs were detected, and the average concentration of PCB was 232 pg. L-1. Five HCHs were the primary congeners among twenty-three congeners of OCPs in the ten tributaries, accounting for 79% of the total. The average concentration of OCPs was 8287 pg. L-1. (2) Similar congeners of HFRs, PCBs, and OCPs were found in the trunk water. The ranking based on the HFR concentration was upstream > downstream > midstream, and that of the PCB and OCP concentration was downstream > upstream > midstream. (3) PCBs and OCPs in the trunk water of the YRB and in the soil and maize irrigated with river water pose potential carcinogenic and non-carcinogenic risks. The results indicate considerable organic pollution in the YRB, suggesting that national emission standards for POPs should be implemented soon. (C) 2021 Elsevier Ltd. All rights reserved