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Chemical characteristics and sources of organosulfates, organosulfonates, and carboxylic acids in aerosols in urban Xi'an, Northwest China
No full textWe investigated speciation and levels of organosulfates, organosulfonates as well as carboxylic acids in aerosol samples collected during summer (2014) and winter (2014/15) in Xi'an, Northwest China, to improve understanding of composition and sources of organic aerosols in this region heavily affected by air pollution. Organosulfates are formed from reactive gas-phase organic compounds and acidic sulfate aerosols, contributing to secondary organic aerosols, SOA. The aerosol samples show a large diversity in organosulfur species in line with other regions of China, reflecting the high levels and complexity of SOA precursors. In summer samples, organosulfates from isoprene are prevalent due to transport of air masses from southern regions with isoprene-emitting mountain forests. During winter, air masses are local or from areas north of the city with low population density and very low temperatures. The estimated levels of organosulfates and organosulfonates in summer (768 +/- 346 ng m(-3)) and winter samples (938 +/- 374 ng m(-3)) are more similar than expected given the high levels of sulfate and organic carbon in winter, indicating the complexity of organosulfur formation processes. We observed an organosulfonate with molecular weight 214 (C6H14O6S) at high estimated levels (254 +/- 232 ng m(-3)) in winter, but much lower concentrations (12 +/- 13 ng m(-3)) in summer. High levels of organosulfur compounds were mainly observed at aerosol pH below about 2.5. Concentrations of carboxylic adds from oxidation of monoterpenes were low (5.2 +/- 2.7 ng m(-3) in summer). Phthalic acid was as high as 90 +/- 29 ng m(-3) during winter and correlated highly with organic carbon, chloride and potassium, indicating a common origin, most likely burning of biomass and plastic-containing waste. Further research is needed to elucidate formation and sources of organosulfates and organosulfonates, as well as the impact on aerosol properties affecting e.g. health effects. (C) 2021 The Authors. Published by Elsevier B.V
Chemical characteristics and sources of organosulfates, organosulfonates, and carboxylic acids in aerosols in urban Xi'an, Northwest China
No full textWe investigated speciation and levels of organosulfates, organosulfonates as well as carboxylic acids in aerosol samples collected during summer (2014) and winter (2014/15) in Xi'an, Northwest China, to improve understanding of composition and sources of organic aerosols in this region heavily affected by air pollution. Organosulfates are formed from reactive gas-phase organic compounds and acidic sulfate aerosols, contributing to secondary organic aerosols, SOA. The aerosol samples show a large diversity in organosulfur species in line with other regions of China, reflecting the high levels and complexity of SOA precursors. In summer samples, organosulfates from isoprene are prevalent due to transport of air masses from southern regions with isoprene-emitting mountain forests. During winter, air masses are local or from areas north of the city with low population density and very low temperatures. The estimated levels of organosulfates and organosulfonates in summer (768 +/- 346 ng m(-3)) and winter samples (938 +/- 374 ng m(-3)) are more similar than expected given the high levels of sulfate and organic carbon in winter, indicating the complexity of organosulfur formation processes. We observed an organosulfonate with molecular weight 214 (C6H14O6S) at high estimated levels (254 +/- 232 ng m(-3)) in winter, but much lower concentrations (12 +/- 13 ng m(-3)) in summer. High levels of organosulfur compounds were mainly observed at aerosol pH below about 2.5. Concentrations of carboxylic adds from oxidation of monoterpenes were low (5.2 +/- 2.7 ng m(-3) in summer). Phthalic acid was as high as 90 +/- 29 ng m(-3) during winter and correlated highly with organic carbon, chloride and potassium, indicating a common origin, most likely burning of biomass and plastic-containing waste. Further research is needed to elucidate formation and sources of organosulfates and organosulfonates, as well as the impact on aerosol properties affecting e.g. health effects. (C) 2021 The Authors. Published by Elsevier B.V
Tree-Ring-Based Drought Reconstruction in Northern North China over the Past Century
No full textA tree-ring width chronology was developed from the Chinese pine (Pinus tabuliformis) in northern North China. To acquire a long-term perspective on the history of droughts in this region, the Standardized Precipitation Evapotranspiration Index (SPEI) from August of the previous year to February of the current year was reconstructed for the period of 1903-2012 AD. The reconstruction explained 46.6% of the instrumental records over the calibration period of 1952-2012. Five dry periods (1916-1927, 1962-1973, 1978-1991, 1994-1999 and 2002-2005) and three wet periods (1908-1915, 1928-1961 and 1974-1977) were found in the reconstructed period, and most of the dry years (periods) in the reconstruction were supported by historical records. Comparisons between the reconstruction and other nearby dryness/wetness indices and precipitation reconstructions demonstrated a good repeatability and high reliability in our reconstruction. Spatial correlation implied that the reconstruction could represent regional hydroclimatic characteristics on a larger regional scale. Significant periodicities and correlations were observed between the reconstructed data and the quasi-biennial oscillation (QBO), El Nino-Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO), which suggested that the hydroclimatic variation in northern North China may be closely connected to remote oceans. The significant and high correlation between the reconstructed series and sea surface temperatures (SSTs) in the eastern equatorial and Southeast Pacific Ocean indicated that ENSO may be the main factor influencing the regional climate
Direct Radiative Effect (DRE) of Dust Aerosols on West African and East Asian Monsoon: The Role of Ocean-Atmosphere Interactions
No full textThe direct radiative effect (DRE) of dust aerosols on the West African and East Asian monsoons is analyzed using the Community Atmosphere Model version 4 containing the dust aerosol parameterization module with and without the coupling of ocean-atmosphere processes, respectively. The atmospheric model (AM) and the ocean-atmosphere coupled model (CM) can both capture the major spatial characteristics of dust aerosols and summer monsoons over West Africa and East Asia. Our results show that the DREs on the West African (WASM) and East Asian summer monsoon (EASM) are more significant in the CM simulations compared to the AM simulations, causing the strengthening of the WASM and weakening of the EASM. The DRE of dust aerosols induces a negative radiative forcing leading to a surface cooling of the downstream ocean of the dust source region when the coupled ocean feedback is added. The distinct DRE of dust aerosols on the WASM and EASM are resulted from the relative locations of the ocean cooling regions to the respective monsoon regions. This study shows the importance of the coupled ocean-atmosphere processes in the DRE and its influences on the WASM and EASM systems
Radiocarbon in the Atmosphere and Seawater in the South China Sea: Flux, Inventory and Air-Sea CO2 Exchange Rate Tracing
No full textDelta C-14 values of the atmosphere and seawater dissolved inorganic carbon (DIC) were measured during a cruise in the South China Sea (SCS) in September 2015, in order to determine the C-14 flux and bomb C-14-based air-sea CO2 exchange rates for this region. The background atmospheric Delta C-14 value (13.8 +/- 5.0 parts per thousand) for the SCS during that period was lower than that (35.4 +/- 3.4 parts per thousand) of surface seawater (5 m) DIC, and a net transfer of C-14 from the sea to the atmosphere (7.4 +/- 5.0 x 10(11) atoms m(-2) yr(-1)) was determined at the wind speed of 5.2 +/- 1.7 m s(-1). Seawater DIC Delta C-14 profiles showed the highest value (37.9 +/- 3.7 parts per thousand) at a depth of 100 m, a rapid decrease below that depth to -220.3 +/- 3.2 parts per thousand at 1,500 m, and nearly constant values below 1,500 m. The average mean penetration depth of bomb C-14 was 585.5 +/- 99.2 m, and a value of 8.2 +/- 1.0 x 10(9) atoms cm(-2) was obtained for the bomb C-14 inventory in this region. Based on this inventory, a long-term (1954-2015) average air-sea CO2 exchange rate of 20.2 +/- 2.8 mol m(-2) yr(-1) was traced for the SCS. Combined with the pCO(2) measurements in this region, a net CO2 flux rate of 0.54 +/- 0.08 mol m(-2) yr(-1) was yielded for the SCS, which is comparable to the cruise measured flux (0.44 +/- 0.62 mol m(-2) yr(-1)) obtained from a synthesis study (Li et al., 2020, https://doi.org/10.1016/j.pocean.2020.102272). Our study highlights the importance of continued atmospheric and seawater C-14 observations on determining the air-sea flux in this region
Organic aerosol formation and aging processes in Beijing constrained by size-resolved measurements of radiocarbon and stable isotopic C-13
No full textThis study investigates the sources and atmospheric processes of size-resolved carbonaceous aerosols in winter 2018 in urban Beijing, based on analysis of dual-carbon isotopes (i.e., radiocarbon and the stable isotope C-13). We found a size dependence of fossil source contributions to elemental carbon (EC), but no clear size dependence for organic carbon (OC). Comparable fossil source contributions to water-insoluble OC (WIOC; 55 +/- 3%) and to water-soluble OC (WSOC; 54 +/- 4%) highlight the importance of secondary aerosol formation, considering that fossil sources emit only small amounts of primary WSOC. OC concentrations increased during high PM2.5 pollution events, with increased fossil and non-fossil WSOC concentrating at larger particles (0.44-2.5 mu m) than WIOC (0.25-2.5 mu m), highlighting the aqueous-phase chemistry as an important pathway for OC production. The ratio of C-13/C-12 (expressed as delta C-13) of total carbon (-27.0 parts per thousand to -23.3 parts per thousand) fell in the range of anthropogenic aerosol, reflecting small biogenic influence. delta C-13 of OC increased with desorption temperature steps (200 degrees C, 350 degrees C and 650 degrees C). The strongly enriched delta C-13(OC, 650) (-26.9 parts per thousand to -20.3 parts per thousand) and large mass fraction of OC650 degrees C in total desorbed OC, both increasing with the increase of particle sizes, were caused by photochemical aging, especially during low and moderate PM2.5 pollution events, when regional, aged aerosol played an important role. During low pollution events, higher delta C-13(OC, 650) and WSOC/OC ratios reflect a larger contribution and more extensive chemical processing of aged aerosol. In contrast, relatively low delta C-13(OC, 200) (-27.2 parts per thousand to -25.7 parts per thousand) suggests the influence of secondary OC formation on the more volatile OC desorbed at 200 degrees C. delta(13)C(OC, 20)0 was similar for all particle sizes and for different pollution events, pointing to an internal mixture of local and aged regional OC. Our results show that the organic aerosol in Beijing arises from a mixture of various sources and complex formation processes, spanning local to regional scales. Particle sizes < 250 nm show strong contribution from local secondary OC formation, whereas refractory OC in particles around 1 mu m shows strong evidence for regional aging processes. In summary, primary emission, secondary and aqueous-phase formation, and (photo-) chemical aging all need to be considered to understand organic aerosol in this region and their importance varies with particle size
Plant and soil elemental C:N:P ratios are linked to soil microbial diversity during grassland restoration on the Loess Plateau, China
No full textPlant and soil elemental ratios of carbon (C), nitrogen (N) and phosphorus (P) play a central role in shaping the composition and structure of microbial communities. However, the relationships between plant and soil elemental C:N:P ratios and microbial diversity are still poorly understood. Here, we evaluated the effects of C:N:P ratios in plant-soil systems on microbial diversity in a chronosequence of restored grasslands (1, 5,10,15, 25, and 30 years since restoration) on the Loess Plateau. We found that C and N concentrations, C:N and C:P ratios in leaf, root, soil and microbial biomass, bacterial and fungal diversity (Shannon-Wiener index) gradually increased with year since grassland restoration. Microbial C:N:P ratios ranged from 17.8:4.5:1 to 24.3:6.6:1, and C:P ratio increased from 17.8:1 at the 1-year site to 24.3:1 at the 30-year site, indicating the increasing P limitation for soil microorganisms during grassland development. Soil microbial diversity increased with root, soil, and microbial C and N concentrations, and decreased with P concentration (p < 0.05). Structural equation modeling indicated that soil and microbial C:N and N:P ratios had the greatest influences on soil bacterial and fungal diversity, and elemental C:N:P ratios had a greater effect on soil fungal than bacterial diversity. Our findings emphasize the importance of elemental C:N:P ratios on soil microbial diversity, which is critical for formulating policies for sustainable biodiversity conservation in terrestrial ecosystems. (c) 2021 Published by Elsevier B.V
Records of fire and its controls on coastal plain of Laizhou Bay, China since 5000 years
No full textDisentangling fire and its controls over thousands of years ago is a major challenge in the study of paleofires. In this paper, we synthesize sedimentary charcoal and black carbon from the coastal plain of Laizhou Bay, China to improve our ability to reconstruct fire history in the past 5000 years, and to assess the controls of fire using principal component analysis and multiple regression approach. The results show that there were three distinct fire episodes at the intervals of 5300-5100 yr BP., 1190-1060 yr BP., and 300-0 yr BP., based on the Changyi (CY) profile. A large-scale fire episode was synchronous with a rapid climate variability that occurred 5300 yr BP., which had frequent drought and cold epoch. High fluxes of black carbon and charcoal were closely related to mass migration and the use of fire for land reclamation from 1190 to 1060 yr BP. Subsequently, anthropogenic biomass burning for cereal cultivation has reached a higher level over the past 300 years. However, between 4600 and 4000 yr BP. the shifts in the frequency of high-magnitude floods and climate aridity consequently resulted in the decline of fire-episode frequency and the abandonment of the late Neolithic settlements in the study region.
Therefore, climate is still a prerequisite factor for fire occurrence on the southern coast of Laizhou Bay in the past 5000 years. A rapid climate variability was not only responsible for various fire patterns, but also could affect human settlements and the mode of production during middle-late Holocene. In the past 1000 years, human land use has been a more important control of fire. However, temperature has become one of the evidently identified controls of fire across the study area in recent decades. To synthesize the methods of Principal Component Analysis and Multiple Regression Analysis with multi-proxy evidence is necessary for us to better understand fire and its controls on coastal plain of Laizhou Bay at the region
Impacts of Transboundary Transport on Coastal Air Quality of South China
No full textTransboundary transport plays an important role in air pollution formation in China. The coastal area of south China (CA-SCHN) frequently experiences air pollution in spring and autumn, but the contribution of transboundary transport to the air quality is still not clear. Meteorological field analyses reveal that large-scale synoptic patterns over east China in spring and autumn provide favorable situations facilitating southward transport of air pollutants originated from the North China Plain (NCP) and Yangtze River Delta (YRD). A springtime case study using the WRF-Chem model shows that trans-boundary transport of air pollutants from the NCP and YRD contributes to 27% and 46% of ozone (O-3) and fine particulate matter (PM2.5) concentrations in the CA-SCHN, respectively. In the autumn case, the contribution is around 11% for O-3 and 24% for PM2.5. In the spring, air pollutants in the NCP and YRD are transported over seas by large-scale synoptic systems to the South China Sea and re-circulated by the sea breeze to the CA-SCHN during daytime. In the autumn, the transport is driven by northerly winds over the land induced by large-scale synoptic systems, and is also modulated by the local mountain-valley breeze circulation. The results provide support for design and implementation of air pollutants control strategies in the CA-SCHN
<p>Worsening summertime ozone pollution in the Guanzhong Basin, China from 2014 to 2018: Impacts of synoptic conditions and anthropogenic emissions</p>
No full textThe ozone (O-3) pollution in the Guanzhong Basin (GZB), China has progressively deteriorated from 2014 to 2018, with the summertime near-surface maximum daily 8 h average (MDA8) O-3 concentration ([O-3]) increasing from about 104 to 141 mu g m(-3), although the emission mitigation strategies have been carried out since 2013. However, it remains elusive about whether anthropogenic emissions or meteorological conditions contribute to the worsening of O-3 pollution in the GZB. A subjective classification method is first used to classify the synoptic patterns influencing the GZB into favorable and unfavorable conditions. The unfavorable synoptic conditions generally correspond to less precipitation, more solar radiation and higher near-surface temperature, which facilitate formation of the O-3 pollution, and it is opposite for the favorable conditions. The increasing trend of MDA8 [O-3] is correlated well with the increase in occurrence of unfavorable synoptic conditions and near-surface temperature from 2014 to 2018. Sensitivity experiments using the WRF-Chem model reveals that the emission mitigation strategies from 2014 to 2018 enhance MDA8 [O-3] by 19.2% on average in the GZB, which counts about 54% of the observed MDA8 O-3 increasing trend. Particularly, the particulate matter decrease due to the emission mitigation contributes about 11.5% of the observed trend, caused by the reduced HO2 heterogeneous uptake and enhanced photolysis. The rest 46% of the observed MDA8 O-3 increasing trend can be mainly contributed by the variation of occurrence days of unfavorable synoptic situations