Understanding temperature variability especially elevation dependent warming (EDW) in high-elevation mountain regions is critical for assessing the impacts of climate change on water resources including glacier melt, degradation of soils, and active layer thickness. EDW means that temperature is warming faster with the increase of altitude. In this study, we used observed temperature data during 1979-2017 from 23 meteorological stations in the Qilian Mountains (QLM) to analyze temperature trend with Mann-Kendall (MK) test and Sen's slope approach. Results showed that the warming trends for the annual temperature followed the order of T_min > T_mean > T_max and with a shift both occurred in 1997. Spring and summer temperature have a higher increasing trend than that in autumn and winter. T_mean shifts occurred in 1996 for spring and summer, in 1997 for autumn and winter. T_max shifts occurred in 1997 for spring and 1996 for summer. T_min shifts occurred in 1997 for spring, summer and winter as well as in 1999 for autumn. Annual mean diurnal temperature range (DTR) shows a significant decreasing trend (-0.18 degrees C/10a) from 1979 to 2017. Summer mean DTR shows a significant decreasing trend (-0.26 degrees C/10a) from 1979 to 2017 with a shift occurred in 2010. After removing longitude and latitude factors, we can learn that the warming enhancement rate of average annual temperature is 0.0673 degrees C/km/10a, indicating that the temperature warming trend is accelerating with the continuous increase of altitude. The increase rate of elevation temperature is 0.0371 degrees C/km/10a in spring, 0.0457 degrees C/km/10a in summer, 0.0707 degrees C/km/10a in autumn, and 0.0606 degrees C/km/10a in winter, which indicates that there is a clear EDW in the QLM. The main causes of warming in the Qilian Mountains are human activities, cloudiness, ice-snow feedback and El Nino phenomenon.

The optical properties and radiative forcing of atmospheric aerosol (ARF) and black carbon (BC) aerosol (BCRF) in ultraviolet (UV), visible (VIS), near-infrared (NIR), and shortwave (SW) spectra were investigated under haze conditions based on the observations of the Aethalometer and sun-sky radiometer and simulations from libRadtran. The results show that the BC concentrations increased greatly from 2.73 mu g/m(3) under clear-air conditions to 7.95 mu g/m(3) under severe haze conditions, while BC aerosol optical depth (AOD) increased from 0.025 to 0.092. A high correlation (R-2 = 0.62) was found between BC AOD and absorbing aerosol optical depth (AAOD) derived from the sun-sky radiometer. The BCRF in SW (BCRFSW) varied from -10.20 W/m(2) under clear-air conditions to -25.40 W/m(2) under severe hazy conditions. However, its fraction in ARF (ARF(SW)) decreased from 19% to 17% simultaneously, which is mainly related to the decrease of the ratio of BC AOD to AOD. The fraction of ARF in VIS in ARF(SW) decreased from 56.3% under clear-air conditions to 50.5% under severe haze conditions, while the fraction of BCRF in VIS in BCRFSW was much larger, and increased from 72.9% to 73.8%. The BCRF efficiency (BCRFE) was much larger than ARF efficiency (ARFE), and both of them decreased with the development of haze. The ARFE in SW decreased from -173.84 W/m(2) under clear-air conditions to -112.75 W/m(2) under severe haze conditions while BCRFESW varied from -482.50 W/m(2) to -321.88 W/m(2). The decrease of ARFE and BCRFE is related to the increase of aerosol loading and asymmetry factor (ASY). The ASY increased and the forward scattering was enhanced with the development of haze due to the hygroscopic growth of aerosol particles, which reduced the extinction efficiency of aerosols including BC on solar radiation and the cooling effect on the surface. (C) 2020 Elsevier Ltd. All rights reserved.