Reply on RC2

Abstract

Tibetan Plateau (TP, hereafter), known as “Third Pole”, is surrounded by the highly polluted regions, such as Indo-Gangetic Plain, Taklimakan and Gobi Deserts and Sichuan Basin. However, the previous in-situ aerosol measurements mainly focused on the southern and northern slopes, while less observations and studies were conducted at the eastern slope of the TP (ESTP). The scientific knowledge on optical properties of aerosols is extremely limited over the ESTP, and in-situ observations at varying altitudes from the heavily polluted regions to the relatively clean Plateau were important for better understanding the light absorption and radiative forcing over the TP. Sichuan Basin (SCB), a highly polluted region due to more rapid economic development, is located in the east side of TP. Therefore, we conducted the first aerosol field experiment at six sites (Chengdu, Sanbacun, Wenchuan, Lixian, Maerkang, Hongyuan) along eastern slope of the TP extending elevation from 500 m to 3500 m. Light-absorbing aerosols are considered to be a key climate driver, and their role may be underestimated in the high-altitude regions. The light absorption of brown carbon (BrC) accounting for that of total carbon increases from 20 % to 50 %, and the mass absorption efficiency of BrC over TP is 2–3 times higher than that inside SCB, especially in winter, which is mainly related to high ratio of secondary to primary organic carbon due to stronger secondary formation and less primary emissions at high altitudes. Contrary to BrC aerosols, EC (elemental carbon) mass absorption efficiency declines with altitude in winter, induced by source difference between the TP and SCB. The more urban sources (motor vehicles, industries, etc.) inside the SCB fail to be transported to the TP due to stable air inside the basin in winter, which also is favorable for aerosol aging 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 OC / EC ratio over the TP than SCB, and thus the enhanced radiative forcing of BrC relative to EC from polluted SCB to pristine TP is because the concentration of OC decreases more slowly with altitude than does EC. South Asia, a highly particulate matter (PM) pollution region, is an important origin of aerosol particles at the region from western Sichuan Basin to eastern Tibetan Plateau, which is significantly dependent on seasons.