An SP2and a CO instrument were deployed at Qinghai Lake in the NortheastTibetan Plateau from October 16 to 27, 2011 to characterize rBC mass size distribution and mixing state, and of the relationships of rBC loading to CO mass mixing ratio. The area was not pristine, with an average measured rBC mass concentration of 0.36 μg STP-m~(-3). rBC concentration peaked at night with low concentrations in the afternoon. This diurnal cycle was loosely anticorrelated with the mixed layer depth, consistent with trapping of local emissions at night, and increased ventilation during the day. Measurements in the late afternoon and early evening, when the mixed layer depths were the highest, were representative of the larger Tibetian Plateau region with an average rBC concentration of 0.23 μg STP-m~(-3). A lognormal primary mode with MMD of ～175-nm and a small secondary lognormal mode with MMD of 470-500 nm (volume-equivalent diameter assuming 2 g cm~(-3) void free density) were observed. Relative reduction in the secondary mode during a snow event supports recent work suggesting size dependent removal of BC by precipitation. rBC was strongly correlated with CO, with different correlation slopes depending on air mass and local wet deposition. The data, in addition to constraining rBC loadings and microphysical state in the sparsely sampled Tibetan plateau region, improves sampling of as yet poorly understood features of rBC aerosol in China and strengthens conclusions about size dependent removal of BC via precipitation.
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