Brown carbon (BrC) aerosols were explored in the northernmost megacity in China during a frigid winter and an agricultural-fire-impacted spring. BrC was more light absorbing at night for both seasons, with more pronounced diurnal variations in spring, and the dominant drivers were identified as regulations on heavy-duty diesel trucks and open burning, respectively. Agricultural fires resulted in unique absorption spectra of BrC, which were characterized by a distinct peak at ∼365 nm.