Water-soluble organic carbon (WSOC) is a major carbonaceous component of fine particulate matter (PM2.5), with significant implications for air quality, climate change, and the global carbon cycle. However, limited observational data under regional background condition has hindered a comprehensive understanding of its seasonal variability, sources, and formation pathways, especially in North China. To address this gap, we carried out a year-long observation of WSOC at Beihuangcheng Island, a representative regional background site in North China. The results revealed an annual average WSOC concentration of 2.96 +/- 1.99 mu g m(-3), with marked seasonal variability-higher levels during the cold season and lower levels during the warm season. Strong positive correlations between WSOC and several chemical tracers in PM2.5 (e.g., K+, SO42-) as well as secondary organic carbon (SOC) indicate that WSOC is predominantly of secondary origin, mainly derived from anthropogenic emissions such as coal combustion and biomass burning. Source apportionment using the Positive Matrix Factorization (PMF) model identified secondary aerosol formation as the dominant contributor, accounting for 47.3 % of WSOC annually. This source is characterized by high loads of NO3-, SO42-, and a low organic carbon to elemental carbon (OC/EC) ratio, implicating fossil fuel-related emissions-especially from vehicles and coal combustion-in the secondary formation of WSOC. Biomass burning emerged as the second-largest contributor, responsible for 33.2 % of annual WSOC, with a notable rise to 40.0 % in winter. This seasonal increase highlights the substantial influence of residential heating and agricultural residue burning on WSOC levels during the cold season.