Single particle characterization of black carbon aerosol in the Northeast Tibetan Plateau, China

Abstract. Refractory black carbon (rBC) mass, size distribution, and mixing state were measured with a ground-based Single Particle Soot Photometer (SP2) at Qinghai Lake (QHL), a rural area in the Northeastern Tibetan Plateau of China, during October 2011. The average measured rBC mass concentration of 0.36 μg STP-m⁻³ is significantly higher than the concentrations measured in background and remote regions around the globe. The diurnal variation of rBC concentration showed nocturnal peak and afternoon low concentrations and showed a loose anticorrelation to the variation of mixed layer depths, indicating nighttime trapping of emissions and daytime ventilation. The high rBC values and their diurnal behavior strongly suggest that the QHL area was heavily influenced by local rBC sources. The mass size distribution of rBC showed a primary mode peak at 175-nm diameter and a small secondary mode peak at 495 nm volume-equivalent diameter assuming 2 g cm⁻³ void free density. About 40% of the observed rBC particles within the detectable size range were mixed with large amounts of non-refractory materials present as a thick coating. A comparison of the Aethalometer and SP2 measurements suggests that there are non-BC species strongly affecting the Aethalometer measurement and, therefore, the Aethalometer measurements are not reliable for rBC determinations in the Tibetan Plateau region without artifact corrections. The apparent black-carbon specific, mass-absorption cross section derived from the Aethalometer and SP2 data was 37.5 m² g⁻¹ at a wavelength of 880 nm. A strong correlation was found between rBC and CO with a slope of 1.5 ± 0.1 ng STP-m⁻³ ppbv⁻¹, similar to values of mixed rural emissions.