Black carbon aerosol in winter northeastern Qinghai–Tibetan Plateau, China: the source, mixing state and optical property

Abstract. Black carbon (BC) aerosol at high altitudes of the Qinghai–Tibetan Plateau has potential effects on the regional climate and hydrological cycle. An intensive measurement campaign was conducted at Qinghai Lake (~ 3200 m above sea level) at the edge of the northeastern Qinghai–Tibetan Plateau during winter using a ground-based single particle soot photometer (SP2) and a photoacoustic extinctiometer (PAX). The average concentration of refractory BC (rBC) and number fraction of coated rBC were found to be 160 ± 190 ng m⁻³ and 59 % for the entire campaign, respectively. Significant enhancements of rBC loadings and number fraction of coated rBC were observed during a pollution episode, with an average value of 390 ng m⁻³ and 65 %, respectively. The mass size distribution of rBC particles showed log-normal distribution, with a peak diameter of ~ 187 nm regardless of the pollution level. Five-day backward trajectory analysis suggests that the air masses from north India contributed to the increased rBC loadings during the campaign. The potential source contribution function (PSCF) model combined with the fire counts map further proves that biomass burning from north India is an important potential source influencing the northeastern Qinghai–Tibetan Plateau during the pollution episode. The rBC mass absorption cross section (MAC_rBC) at λ = 532 nm was slightly larger in clean days (14.9 m² g⁻¹) than during the pollution episode (9.3 m² g⁻¹), likely due to the effects of brown carbon and the uncertainty of the MAC_rBC calculation. The MAC_rBC was positively correlated with number fraction of coated rBC during the pollution episode with an increasing rate of 0.18 (m² g⁻¹) %⁻¹. The number fraction of coated rBC particles showed positive correlation with light absorption, suggesting that the increase of coated rBC particles will enhance the light absorption. Compared to rBC mass concentration, rBC mixing sate is more important in determining absorption during the pollution episode, estimated from the same percentage-wise increment of either rBC mass concentration or the number fraction of coated rBC. The estimated BC direct radiative forcing was +0.93 W m⁻² for the pollution episode, which is 2 times larger than that in clean days. Our study provides insight into the potential climatic impacts of rBC aerosol transported to the Qinghai–Tibetan Plateau from south Asian regions, and is also useful for future modeling studies.