Response to Referee #1

Abstract

Black carbon (BC) aerosol has important effects on the climate and hydrology of the Tibetan Plateau (TP). An intensive measurement campaign was conducted at Lulang (∼ 3300 m a.s.l.-above sea level), southeastern TP, from September to October 2015, to investigate the sources and physicochemical characteristics of refractory BC (rBC) aerosol. The average rBC mass concentration was 0.31 ± 0.55 µg m −3 , which is higher than most prior results for BC on the TP. A clear diurnal cycle in rBC showed high values in the morning and low values in the afternoon. A bivariate polar plot showed that rBC loadings varied with wind speed and direction, which also reflected the dominant transport direction. The estimated net surface rBC transport intensity was +0.05 ± 0.29 µg s −1 m −2 , indicating stronger transport from outside the TP compared with its interior. Cluster analysis and a concentration-weighted trajectory model connected emissions from north India to the high rBC loadings, but the effects of internal TP sources should not be overlooked. The average mass median diameter (MMD) of rBC was 160 ± 23 nm, with smaller MMDs on rainy days (145 nm) compared with non-rainy days (164 nm). The average number fraction of thickly coated rBC (F rBC) was 39 ± 8 %, and it increased with the O 3 mixing ratios from 10:00 to 14:00 LT, indicating that photochemical oxidation played a role in forming rBC coatings. The average rBC absorption enhancement (E abs) was estimated to be 1.9, suggesting that light absorption by coated rBC particles was greater than for uncoated ones. The E abs was strongly positively correlated with the F rBC , indicating an amplification of light absorption for internally mixed rBC. For rBC cores < 170 nm, E abs was negatively correlated with MMD, but it was nearly constant for rBC cores > 170 nm. Our study provides insight into the sources and evolution of rBC aerosol on the TP, and the results should be useful for improving models of the radiative effects of carbonaceous aerosols in this area. 1 Introduction The Tibetan Plateau (TP) is the world's largest high-elevation region. It holds the largest ice mass on the planet outside the polar regions and is sometimes called the Earth's "Third Pole" (Yao et al., 2008). The snow and associated glacial meltwater on the TP provides fresh water for drinking and irrigation for more than 1 billion people downstream (Immerzeel et al., 2010). The TP exerts significant thermal and dynamic impacts on hydrological processes in South and East Asia. For example, changes in the area covered by glaciers and snowpack on the TP affect the heat fluxes and water exchange between the atmosphere and the earth's surface, and that, in turn, affects the atmospheric circulation