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The study of air pollution in a valley is a classic research subject. Compared with flat terrain, the formation and development of haze pollution are more complicated and unique within a deep basin. How a basin or valley plays a role in the horizontal and vertical distribution of air pollutants is poorly understood in highly industrialized deep basins in China due to scarce field observations. We conducted a collaborative experiment of three-dimensional (3D) boundary layer meteorology and pollution at the western Sichuan Basin (SCB) close to the Tibetan Plateau (TP). Generally, the concentrations of PM1 (particulate matter smaller than 1.0 μm), NO, and NO2 largely decline with elevation, while O3 shows a slight increasing trend inside the SCB. Three different types of pollutant profiles and the formation mechanisms are described below. The high PM1 near the surface layer corresponds to the vertical clockwise circulation, i.e., a wind shift with increasing altitude in a clockwise direction. The air pollutants at the central and eastern SCB can be transported to the eastern foothills of the TP by southeasterly winds and then are trapped within the western SCB by a strong surface temperature inversion. The pollutants over the eastern TP also can be dispersed to above the SCB by westerly winds. More aerosol particles are concentrated at about 2.0 km MSL by jointly ascending and descending motion below and above the layer over the valley. The relative uniform PM1 in the vertical direction correlates to the counterclockwise circulation. The trapped pollutants in the western SCB can be transported to the eastern region by westerly winds and then are dispersed to the upper air by unstable stratification. [ABSTRACT FROM AUTHOR]

Scientific knowledge about light absorption by aerosols is extremely limited at the eastern slope of the Tibetan Plateau (ESTP). We conducted the first aerosol field experiment at six sites (Chengdu, Sanbacun, Wenchuan, Lixian, Maerkang, Hongyuan) along the ESTP, ranging in elevation from 500 to 3500 m. The fraction of light absorption by brown carbon (BrC) to total carbon increases from 20 % to 50 % with altitude, and the mass absorption efficiency (MAE) of BrC over the Tibetan Plateau (TP) is 2–3 times higher than that inside the Sichuan Basin (SCB), especially in winter. In contrast, the MAE of elemental carbon (EC) in winter decreases with altitude. The contrasting variation of EC and BrC MAE with altitude is mainly attributed to source differences between the TP and SCB. Emissions from the more urban sources (motor vehicles, industries, etc.) inside the SCB fail to be transported to the TP due to the stable air in winter inside the basin, which is also favourable for aerosol ageing to enhance absorption efficiency. The radiative forcing of BrC relative to EC varies from 0.10 to 0.42 as altitude increases with the higher organic carbon (OC) to EC ratio over the TP than SCB. Thus, the reason for the enhanced relative BrC to EC radiative forcing from polluted SCB to pristine TP is that the BrC concentration decreases more slowly than the EC concentration with altitude. This study contributes to the understanding of the difference in light absorption by EC and BrC with altitude, from polluted lower-altitude basins to the pristine TP, and provides a data set for regional climate model validation. [ABSTRACT FROM AUTHOR]

ABSTRACT: Actual evapotranspiration (ETa) is an important component of the hydrologic cycle. In this study, ETa on the Tibetan Plateau (TP) is calculated using the advection‐aridity model (AA model) with data from 86 meteorological stations during the period from 1961 to 2010. Results show that the mean annual ETa over the TP was 543 mm and range from 147 to 687 mm, with higher values in the southern part and lower values in the northern part of the TP. During 1961–2010, annual and seasonal mean ETa show statistically increasing trend at most all stations. Annual ETa in area‐averaged over the TP changed +1.01 mm year−1. Among the four seasons, the changes were most pronounced in March, April, May (MAM) and June, July, August (JJA). ETa variation is significantly and positively correlated with a drought index, defined as the difference between precipitation (P) and potential evapotranspiration (ETp), i.e. P − ETp, showing that the ETa increases over the TP tend to be affected by an increasing soil water supply associated with global warming, such as the retreat of permafrost, increase in P, and decline of ETp. [ABSTRACT FROM AUTHOR]

The annual trend in thunderstorm activity during a 42-year period from 1971 to 2012 over the transition region of the Tibetan Plateau and the Loess Plateau in China was documented. The delimiting factor was the number of days with thunderstorms collected from 64 stations over the study area. Reanalysis data from JRA-55, NCEP and ERA-Interim were used to understand the underlying reasons for the trend in the number of thunderstorm days. The results show that thunderstorms in this area were most active in the summer, followed by spring and autumn, while no thunderstorms were reported during the winter. Two centers with high number of thunderstorm days were revealed, one of which is located in the Gannan area close to the Tibetan Plateau and the other is in the Yongdeng and Wushaoling area east of the Qilian Mountains. The Loess Plateau in comparison has a lower frequency of thunderstorm days. There was no trend in the mean annual thunderstorm days from 1971 to the early 1990s, but a significant decreasing trend occurred afterward (until 2012), with the most important trend occurring during the summer when thunderstorms were the most numerous. The decrease in thunderstorm days was associated with a nonsignificant decrease in the convective available potential energy. Circulation patterns and moisture fluxes were analyzed to understand the observed difference between the two periods. Years with relatively high thunderstorm days (1971-1990) were characterized by relatively high water vapor flux to the study region, as well as having the western ridge of the western Pacific subtropical high located further east than during the years 1991-2012 with fewer thunderstorms. There were also relatively low 500 hPa geopotential heights in the north of China and the Mongolia in the earlier period. It is suggested that the relative changes in the position of the subtropical high and the 500 hPa trough are related to changes in the intensity of the Asian summer monsoon. [ABSTRACT FROM AUTHOR]

A comparative analysis of the geochemical characteristics of sediments from the Oligocene Zhuhai Formation (32-23.8 Ma), the Miocene Zhujiang Formation (23.8-16.5 Ma), and the Hanjiang Formation (16.5-10.5 Ma) and a comprehensive analysis of the geochemical characteristics of rocks surrounding the paleo-Pearl River drainage contribute to understanding the influences of the Tibetan Plateau uplift on provenance evolution of the paleo-Pearl River. The results show that the geochemical characteristics of sediments from the Oligocene Zhuhai Formation are very different from the geochemical characteristics of sediments from the Miocene Zhujiang and Hanjiang Formations. The ∑ rare earth elements (REE) of mudstone is relatively high in the Zhuhai Formation, 204.07-293.88 ppm (average 240.46 ppm), and low in the Zhujiang and Hanjiang Formations, 181.32-236.73 ppm (average 203.83 ppm) and 166.84-236.65 ppm (average 199.04 ppm), respectively. The chemical index of alteration (CIA) for these samples has a similar trend to the ∑ REE: the CIA of the Zhuhai Formation is relatively high and the CIA of the Zhujiang and Hanjiang Formations is relatively low. The uplift of the Tibetan Plateau is crucial to the westward expansion of the paleo-Pearl River drainage. [ABSTRACT FROM AUTHOR]