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Alpine permafrost could account for a quarter of thawed carbon based on Plio-Pleistocene paleoclimate analogue
No full textEstimates of the permafrost-climate feedback vary in magnitude and sign, partly because permafrost carbon stability in warmer-than-present conditions is not well constrained. Here we use a Plio-Pleistocene lacustrine reconstruction of mean annual air temperature (MAAT) from the Tibetan Plateau, the largest alpine permafrost region on the Earth, to constrain past and future changes in permafrost carbon storage. Clumped isotope-temperatures (Delta(47)-T) indicate warmer MAAT (similar to 1.2 degrees C) prior to 2.7 Ma, and support a permafrost-free environment on the northern Tibetan Plateau in a warmer-than-present climate. Delta(47)-T indicate similar to 8.1 degrees C cooling from 2.7 Ma, coincident with Northern Hemisphere glacial intensification. Combined with climate models and global permafrost distribution, these results indicate, under conditions similar to mid-Pliocene Warm period (3.3-3.0 Ma), similar to 60% of alpine permafrost containing similar to 85 petagrams of carbon may be vulnerable to thawing compared to similar to 20% of circumarctic permafrost. This estimate highlights similar to 25% of permafrost carbon and the permafrost-climate feedback could originate in alpine areas
Alpine permafrost could account for a quarter of thawed carbon based on Plio-Pleistocene paleoclimate analogue
No full textEstimates of the permafrost-climate feedback vary in magnitude and sign, partly because permafrost carbon stability in warmer-than-present conditions is not well constrained. Here we use a Plio-Pleistocene lacustrine reconstruction of mean annual air temperature (MAAT) from the Tibetan Plateau, the largest alpine permafrost region on the Earth, to constrain past and future changes in permafrost carbon storage. Clumped isotope-temperatures (Delta(47)-T) indicate warmer MAAT (similar to 1.2 degrees C) prior to 2.7 Ma, and support a permafrost-free environment on the northern Tibetan Plateau in a warmer-than-present climate. Delta(47)-T indicate similar to 8.1 degrees C cooling from 2.7 Ma, coincident with Northern Hemisphere glacial intensification. Combined with climate models and global permafrost distribution, these results indicate, under conditions similar to mid-Pliocene Warm period (3.3-3.0 Ma), similar to 60% of alpine permafrost containing similar to 85 petagrams of carbon may be vulnerable to thawing compared to similar to 20% of circumarctic permafrost. This estimate highlights similar to 25% of permafrost carbon and the permafrost-climate feedback could originate in alpine areas
Chemical characteristics and sources of nitrogen-containing organic compounds at a regional site in the North China Plain during the transition period of autumn and winter
No full textOrganic nitrogen constitutes a significant fraction of the nitrogen budget in particulate matter (PM). However, the composition and sources of nitrogen-containing organic compounds (NOCs) in PM remain unclear currently in North China Plain (NCP), China. Rare local or regional studies on NOCs were conducted. In this study, ambient fine particles (PM2.5) were collected in Xianghe, a regional background site in NCP, from 26 October to 26 December 2017. The insights from this study include NOC molecule identification, concentration level, and NOC sources and origins. Specifically, we have identified and quantified >90 NOC species, with urea being the most abundant, accounting for 39.7 +/- 4.7% of the total NOC followed by free amino acids (FAAs; 21.9 +/- 1.5%), cyclic NOCs (15.3 +/- 4.5%), amines (14.8 +/- 1.5%), alkyl amides (5.8 +/- 0.5%), isocyanates (1.7 +/- 0.2%), and nitriles (1.1 +/- 0.2%). The time series of FAAs was well correlated (r = 0.51-0.68, p < 0.01) with the organic marker of levoglucosan and was moderately correlated with O-x (r = 0.29-0.41, p < 0.01), suggesting biomass burning and secondary formation were important FAAs sources. We also show that amines can be oxidized and/or reacted by aqueous-phase processing to form secondary aerosols, which are further enhanced by the involvement of iron in the catalytic process. Using the receptor model of positive matrix factorization (PMF), six factors were identified including coal combustion, crustal sources, biomass burning, industry-related sources, traffic emissions, and secondary aerosols. Source apportionment of NOC shows biomass burning was the dominant factor, accounting for 31.8% of the total NOCs. This study provides a unique dataset of NOCs at this regional background site in the NCP, with the insights of NOC chemical composition and sources gained in this study being important for future NOC modeling as well as NOC health effects studies
Initial soil formation by biocrusts: Nitrogen demand and clay protection control microbial necromass accrual and recycling
No full textMicrobial biomass is increasingly considered to be the main source of organic carbon (C) sequestration in soils. Quantitative information on the contribution of microbial necromass to soil organic carbon (SOC) formation and the factors driving necromass accumulation, decomposition and stabilization during the initial soil formation in biological crusts (biocrusts) is absent. To address this knowledge gap, we investigated the composition of microbial necromass and its contributions to SOC sequestration in a biocrust formation sequence consisting of five stages: bare sand, cyanobacteria stage, cyanobacteria-moss stage, moss-cyanobacteria stage, and moss stage on sandy parent material on the Loess Plateau. The fungal and bacterial necromass C content in soil was analyzed based on amino sugars -the cell wall biomarker. Microbial necromass was an important source of SOC, and was incorporated into the particulate and mineral-associated organic C (MAOC). Because bacteria have smaller and thinner cell wall fragments as well as more proteins than fungi, bacterial necromass mainly contributed to the MAOC pool, while fungal residues remained more in the particulate organic C (POC). MAOC pool was saturated fast with the increase of microbial necromass, and POC more rapid accumulation than MAOC suggests that the clay content was the limiting factor for stable C accumulation in this sandy soil. The necromass exceeding the MAOC stabilization level was stored in the labile POC pool (especially necromass from fungi). Activities of four enzymes (i.e., beta-1,4-glucosidase, beta-1,4-N-acetyl-glucosaminidase, leucine aminopeptidase, and alkaline phosphatase) increasing with fungal and bacterial necromass suggest that the raised activity of living microorganisms accelerated the turnover and formation of necromass. Microbial N limitation raised the production of N acquisition enzymes (e.g., beta-1,4-N-acetyl-glucosaminidase and leucine aminopeptidase) to break down necromass compounds, leading to further increase of the nutrient pool in soil solution. The decrease of microbial N limitation along the biocrusts formation chronosequence is an important factor for the necromass accumulation during initial soil development. High microbial N demands and insufficient clay protection lead to fast necromass reutilization by microorganisms and thus, result in a low necromass accumulation coefficient, that is, the ratio of microbial necromass to living microbial biomass (on average, 9.6). Consequently, microbial necromass contribution to SOC during initial soil formation by biocrust is lower (12-25%) than in fully developed soils (33%- 60%, literature data). Nitrogen (N) limitation of microorganisms and an increased ratio between N-acquiring enzyme activities and microbial N, as well as limited clay protection, resulted in a low contribution of microbial necromass to SOC by initial formation of biocrust-covered sandy soil. Summarizing, soil development leads not only to SOC accumulation, but also to increased contribution of microbial necromass to SOC, whereas the plant litter contribution decreases
Characterization of PM2.5-bound parent and oxygenated PAHs in three cities under the implementation of Clean Air Action in Northern China
No full textAmbient polycyclic aromatic hydrocarbons (PAHs) and oxygenated PAHs (oPAHs) could act as carcinogenic substances and cause severe health outcome on human beings through inhalation exposure. To protect human health, China government implemented strict control actions (Air Pollution Prevention and Control Action Plan, APPC) to reduce air pollutants emission. Notable achievement of PM2.5 pollution reduction was reported in several studies, while the pollution characteristics of non-routine monitoring PAHs and oPAHs under the impact of APCC were poorly understood. In this study, daily PM2.5 samples in three Chinese cites (Xingtai, Liaocheng, and Heze) were collected for one month in four seasons form 2018-2019 and determined for the PM2.5-bound PAHs and oPAHs. The results showed that the annual PAHs and oPAHs were 58.3 & PLUSMN; 86.1 and 62.7 & PLUSMN; 75.2 ng/m(3) in Xingtai, 36.2 & PLUSMN; 55.6 and 35.9 & PLUSMN; 28.3 ng/m(3) in Liaocheng, and 24.5 & PLUSMN; 37.7 and 35.1 & PLUSMN; 24.7 ng/m(3)& nbsp;in Heze, respectively, suggesting that the PAHs pollution in the measured cities was still considerable and more solid control measures are needed. Significant variations in seasonal concentrations were observed in all the cities with the highest occurring in winter, which is mainly due to solid fuel burning for house heating. Meanwhile, composition profiles of PAHs and oPAHs also showed significant seasonal variations due to the source differences in different seasons. The results of PMF model showed both vehicle exhausts emission and solid fuels burning dominated the PAHs pollution. In summer, vehicles were the major contributor to the total PAHs pollution, while in winter, the contribution of coal combustion emission increased sharply. The correlations analysis between PAHs (oPAHs) and K+ and hopanes also showed a stronger impact of coal combustion in winter time. The result of this study confirmed the goal of clean air in China was calling for continuous actions
PM2.5 composition and sources in the San Joaquin Valley of California: A long-term study using ToF-ACSM with the capture vaporizer
No full textThe San Joaquin Valley (SJV) of California has suffered persistent particulate matter (PM) pollution despite many years of control efforts. To further understand the chemical drivers of this problem and to support the development of State Implementation Plan for PM, a time-of-flight aerosol chemical speciation monitor (ToF-ACSM) outfitted with a PM2.5 lens and a capture vaporizer has been deployed at the Fresno-Garland air monitoring site of the California Air Resource Board (CARB) since Oct. 2018. The instrument measured non-refractory species in PM2.5 continuously at 10-min resolution. In this study, the data acquired from Oct. 2018 to May 2019 were analyzed to investigate the chemical characteristics, sources and atmospheric processes of PM2.5 in the SJV. Comparisons of the ToF-ACSM measurement with various co-located aerosol instruments show good agreements. The inter-comparisons indicated that PM2.5 in Fresno was dominated by submicron particles during the winter whereas refractory species accounted for a major fraction of PM2.5 mass during the autumn associated with elevated PM10 loadings. A rolling window positive matrix factorization analysis was applied to the organic aerosol (OA) mass spectra using the Multilinear Engine (ME-2) algorithm. Three distinct OA sources were identified, including vehicle emissions, local and regional biomass burning, and formation of oxygenated species. There were significant seasonal variations in PM2.5 composition and sources. During the winter, residential wood burning and oxidation of nitrogen oxides were major contributors to the occurrence of haze episodes with PM2.5 dominated by biomass burning OA and nitrate. In autumn, agricultural activities and wildfires were found to be the main cause of PM pollution. PM2.5 concentrations decreased significantly after spring and were dominated by oxygenated OA during March to May. Our results highlight the importance of using seasonally dependent control strategies to mitigate PM pollution in the SJV
Effects of vegetation restoration types on soil nutrients and soil erodibility regulated by slope positions on the Loess Plateau
No full textSoil degradation is significantly increased driven by soil nutrient loss and soil erodibility, thus, hampering the sustainable development of the ecological environment and agricultural production. Vegetation restoration has been widely adopted to prevent soil degradation given its role in improving soil nutrients and soil erodibility. However, it is unclear which vegetation type has the best improving capacity from soil nutrient and soil erodibility perspectives. This study selected three vegetation restoration types of grasslands (GL), shrublands (SL), and forestlands (FL) along the five slope positions (i.e., top, upper, middle, lower, and foot slope), to investigate the effects of vegetation restoration types on soil nutrients and soil erodibility. All vegetation restoration types were restored for 20 years from croplands (CL). We used comprehensive soil nutrient index (CSNI) and comprehensive soil erodibility index (CSEI) formed by a weighted summation method to reflect the effect of vegetation restoration on the improving capacity of soil nutrient and erodibility. The results showed the vegetation types with the highest comprehensive soil quality index (CSQI) at the top, upper, middle, lower and foot slope were FL (1.92), FL (1.98), SL (2.15), FL (2.37) and GL (3.93), respectively. When only one vegetation type was considered on the entire slope, SL (0.59) and FL (0.59) had the highest CSNI, the SL had the lowest CSEI (0.34) and the highest CSQI (1.89). The CSNI was mainly influenced by soil structure stability index (SSSI), sand content, silt + clay particles, and CSEI was controlled by soil organic matter (SOM), macroaggregates and microaggregates. Moreover, the CSQI was influenced by pH, silt and clay content, and biome coverage (BC). The study suggested the SL were advised as the best vegetation restoration type on the whole slope from improving soil nutrients and soil erodibility
Seasonal and diurnal variation of PM2.5 HULIS over Xi'an in Northwest China: Optical properties, chemical functional group, and relationship with reactive oxygen species (ROS)
No full textHumic-like substances (HULIS) in particulate matter (PM) play critical roles in the atmospheric changes in our environment. In this study, high time resolution PM2.5 samples were collected to insight the abundances, spectroscopic characters, chemical groups, and oxidative potential of HULIS in Xi'an, China. The average mass concentrations of HULIS in term of carbon (HULIS-C) was 11.55 +/- 5.85 and 8.28 +/- 2.23 mu g C m(-3) in winter and summer, respectively. The diurnal variations of HULIS displayed three peaks (03:00-07:00, 10:00-14:00, and 18:00-21:00 LT) in winter, but a single peak (08:00-12:00 LT) in summer. The optical parameters show obvious difference between winter and summer. Fourier Transform infrared (FT-IR) spectra revealed that the HULIS mainly consisted of aliphatic chains, aromatic rings, and carboxylic groups. The FT-IR spectra proved that the aromatic compounds and carboxylic acids dominated the diurnal variation in winter, while carboxylic acids were responsible for that in summer. The normalized oxidative potential of HULIS exhibited an inverse trend with the HULIS-C concentration measured by the DCFH assay. The diurnal reactive oxygen species (ROS) productions from HULIS presents three valleys (03:00-07:00, 10:00-14:00, and 18:00-21:00 LT) in winter but only a single valley (08:00-12:00 LT) in summer. In addition, a positive correlation (R-2 approximate to 0.6, p<0.01) between the oxidative potential and specific ultraviolet absorbance at a wavelength of 254 nm (SUVA254) in winter suggested that the oxidative power of HULIS could be more related to the compounds with high light absorbance, high aromaticity,and molecular weight. The results of this study offer more solid knowledge on the spectral and chemical char- acteristics and oxidative potential of the HULIS in PM2.5 of the typical city in northwest China
Optical properties of mountain primary and secondary brown carbon aerosols in summertime
No full textBrown carbon (BrC) can affect atmospheric radiation due to its strong absorption ability from the near ultraviolet to the visible range, thereby influencing global climate. However, given the complexity of BrC's chemical composition, its optical properties are still poorly understood, especially in mountainous areas. In this study, the black carbon (BC) tracer method is used to explore the light-absorbing properties of primary and secondary BrC at Mount Hua, China during the 2018 summer period. The primary BrC absorption contributes to 10-15% of the total BrC absorption at a wavelength of 370 nm. From the positive matrix factorization analysis, traffic emissions are found to be a major source of primary BrC absorption (44%), followed by industry and biomass-burning emissions (29%). The secondary BrC accounts for 87% of the total BrC absorption at a wavelength of 370 nm, indicating that BrC is dominated by secondary formation. The observation of a higher secondary BrC absorption diurnal pattern at Mount Hua can be affected by secondary BrC in the residual layer after sunrise and the formation of light-absorbing chromophores by photochemical oxidation in the afternoon. The estimated average mass absorption efficiencies of primary and secondary BrC (MAE(_pri) and MAE(_sec), respectively) are 0.4 m(2)/g and 2.1 m(2)/g at wavelengths of 370 nm, respectively, indicating a stronger light-absorbing ability for secondary BrC than for primary BrC. There is no significant difference in MAE(_pri) within a daily variation, but the daytime MAE(_sec) value is higher than that during the night. Our study shows that secondary BrC is important to light absorption in mountainous areas. (C) 2021 Elsevier B.V. All rights reserved
An automated method for thermal-optical separation of aerosol organic/elemental carbon for C-13 analysis at the sub-mu gC level: A comprehensive assessment
No full textWe describe and thoroughly evaluate a method for C-13 analysis in different fractions of carbonaceous aerosols, especially elemental carbon (EC). This method combines a Sunset thermal-optical analyzer and an isotope ratio mass spectrometer (IRMS) via a custom-built automated separation, purification, and injection system. Organic carbon (OC), EC, and other specific fractions from aerosol filter samples can be separated and analyzed automatically for C-13 based on thermal-optical protocols (EUSAAR_2 in this study) at sub-pgC levels. The main challenges in isolating EC for C-13 analysis are the possible artifacts during OC/EC separation, including the premature loss of EC and the formation of pyrolyzed OC (pOC) that is difficult to separate from EC. Since those artifacts can be accompanied with isotope fractionation, their influence on the stable isotopic composition of EC was comprehensively investigated with various test compounds. The results show that the thermal-optical method is relatively successful in OC/EC separation for C-13 analysis. The method was further tested on real aerosols samples. For biomass-burning source samples, (partial) inclusion of pOC into CC has negligible influence on the C-13 signature of CC. However, for ambient samples, the influence of pOC on the C-13 signature of CC can be significant, if it is not well separated from CC, which is true for many current methods for measuring C-13 on EC. A case study in Xi'an, China, where pOC is enriched in C-13 compared to EC, shows that this can lead to an overestimate of coal and an underestimate of traffic emissions in isotope-based source apportionment. (C) 2021 Elsevier B.V. All rights reserved