<strong class="journal-contentHeaderColor">Abstract.</strong> Both size and mixing state of Black carbon (BC) are critical climate-relevant physical parameters. It remains a challenge for ambient measurements to characterize their variability across different atmospheric conditions particularly in outflow regions. To investigate how BC&rsquo;s physical properties are determined in source regions and altered during transport, we conducted 23 flight measurements of BC, CO, and CO₂ over the Yellow Sea from 2021 to 2022. The refractive BC mass concentration (M_rBC) varied by up to two orders of magnitude between near sea surface and around 5 km above sea level, and Planetary Boundary Layer height-dependency of M_rBC was stronger in winter than in spring. Smallest rBC&rsquo;s mass median diameter MMD (163.4 nm) observed in South Korea-sourced air indicated fresh urban emissions, whereas larger MMD, enhanced internal mixing, and higher rBC/CO slopes were exhibited in the air masses from North Korea and China, reflecting additional emissions from biomass and coal combustion. Both MMD and internal mixing tended to decrease with altitude, highlighting the wet scavenging effect during particle transport. When accumulated precipitation exceeded 1 mm, M_rBC decreased by more than 50 %, with moderate reductions in MMD and mixing state. As a result, overall BC transport efficiency declined to 1/e within 5.5 days. These findings emphasize the complex effects of source region, seasonality, and wet removal on varying rBC distributions in the outflow region. We believe that these findings offer valuable observational constraints for improving the physical realism of models.