Mechanisms controlling dissolved CO2 over-saturation in the Three Gorges Reservoir area

The emission of CO2 to the atmosphere from inland waters is an important part of the global carbon cycle. In this study, we made spatial and temporal measurements of CO2 partial pressure (pCO2) along the Three Gorges Dam system. The pCO2 ranged from 619 to 2383 μatm and was supersaturated relative to atmospheric CO2. Further, pCO2 showed obvious spatial and temporal variations: pCO2 at the high-flow season was much lower than that at the low-flow season near the upstream part of the reservoir, whereas pCO2 in the reservoir water and after the dam showed an opposite seasonal trend. Organic matter mineralization produced more CO2 in the surface water of the reservoir area at the high-flow season and should be responsible for the δ13C-depleted dissolved inorganic carbon (δ13CDIC). In addition, organic carbon mineralization is sensitive to temperature variability, which is expected to be an important driver of the dissolved CO2 over-saturation in the reservoir. This study suggested that the construction of Three Gorges Reservoir increased the water transit time and accelerated the organic carbon mineralization in the Changjiang River. The results indicate that carbon cycling changes markedly in large impounded rivers

Juxtaposition of Western Pacific Subtropical High on Asian Summer Monsoon Shapes Subtropical East Asian Precipitation

Increasing lines of evidence question the homogenous response of Asian Summer Monsoon (ASM) precipitation patterns, requiring rethinking of the forcing mechanisms. Here we show a similar to 15,000-year quantitative precipitation history based on well-dated lake levels at Lake Chenghai, subtropical China. Lake levels and the inferred precipitation were high during the Bolling-Allerod, early and late Holocene, but low during the middle Holocene. The orbital scale precipitation trend is out of phase with boreal summer insolation, the later has been widely suggested as the driver of ASM precipitation. Lake Chenghai long-term lake levels are synchronous with trends in tropical Pacific sea surface temperatures, the related zonal sea surface temperature gradients, and interhemispheric temperature gradients. We propose that changes in either the interhemispheric or zonal Pacific temperature gradients modulate the intensity and location of the western Pacific subtropical high, which is juxtaposed on the ASM, leading to heterogeneous hydroclimatic conditions over subtropical East Asia

A New Structural Classification Scheme for Dissolved Organic Sulfur in Urban Snow from North China

The chemical composition of dissolved organic sulfur in snow is important in understanding the sources and scavenging processes of atmospheric organic matter. Snow samples collected simultaneously from four megacities in North China were analyzed using ultra-high-resolution Fourier transform ion cyclotron resonance mass spectrometry. The modified oxygen and redefined aromaticity index help in the interpretation of the possible structural information and evaluation of the aromaticity of sulfur-containing molecules. By extending these parameters, we provide a new structural classification for organic sulfur species in the atmosphere. With the new classification, the oxidized (O/S > 3) and less oxidized (O/S ≤ 3) sulfur-containing molecules can be easily distinguished. Typical known secondary organosulfates and sulfonates and anthropogenically derived anionic surfactants verified the validity of this new classification. The new classification was applied to the molecular characterization of dissolved organic sulfur in snow samples. More than one hundred (138–150) of the molecules with medium O/S ratios of 5–11 and a low to medium mass range of <500 Da are related with typical known secondary organosulfates and anthropogenically derived anionic surfactants. Our study provides new insights into the molecular compositions of organic sulfur species in ambient air, although their atmospheric behaviors between the snow–aerosol interfaces warrant further examination

Dramatic weakening of the East Asian summer monsoon in northern China during the transition from the Medieval Warm Period to the Little Ice Age

Changes in the intensity of the East Asian summer monsoon (EASM) are critical for regulating the regional hydrology, ecology, and human civilization, especially in the vicinity of the summer monsoon limit (SML). However, the detailed spatial variations of the SML in mainland China over the past 2000 years are uncertain due to the lack of high-resolution paleoclimate archives. As a result, the accurate location of the SML during the transition from the Medieval Warm Period (MWP) to the Little Ice Age (LIA), as well as its impacts on ecology and society, are poorly understood. Here, we report a potential location of the SML during the late Holocene by combining data from a lake sedimentary record and a compilation of paleoclimate records from arid northern China. We find that EASM intensity was strong during the MWP and that the SML in arid northern China was roughly located along the Yinshan Mountains, Yabulai Mountains, and north of Lake Qinghai. EASM intensity dramatically weakened during the MWP-LIA transition, and the SML retreated southeastward significantly, which may have primarily but nonlinearly been a response to the reduction in solar irradiance and its associated changes in atmospheric circulation (e.g., El Nino-Southern Oscillation and Siberian High) and could have had profound impacts on hydrology, ecology, and human civilization across northern monsoonal Asia