Abstract Brown carbon (BrC), a light‐absorbing organic aerosol, exhibits higher absorption coefficients at short wavelengths. Partial BrC can be formed via secondary process. In this study, we isolated the secondary BrC (σabs,sec BrC) between urban surface and mountain stations (which receive surface pollutants during convective mixing) to investigate how atmospheric lifetime variations influence its absorption properties. Although the surface provided more favorable conditions for precursor concentrations and physicochemical reactions, the surface station observed lower σabs,sec BrC during the most developed planetary boundary layer period. This was driven by low temperatures and relative humidity (RH) during uplift, which extended the atmospheric lifetime of high‐viscosity secondary BrC, thereby enhancing σabs,sec BrC observed at the mountain station. Variations in near‐surface emission intensity dominated peak σabs,sec BrC values observed at the mountain station during 14:00–15:00. Moreover, decreasing RH promoted higher viscosity, which enhanced σabs,sec BrC.