Brown carbon (BrC), a light-absorbing aerosol, plays a pivotal role in influencing global climate. However, assessing BrC radiative effects remains challenging because the required observational data are hardly accessible. Here we develop a new BrC radiative effect estimation method combining conventional observations and numerical models. Our findings reveal that BrC absorbs up to a third of the sunlight at 370 nm that black carbon does, highlighting its importance in aerosol radiative effects.