Global modeling of brown carbon: impact of temperature- and humidity-dependent bleaching

Abstract. Brown carbon (BrC), a light-absorbing component of organic aerosols, undergoes bleaching in the atmosphere, a process where its light-absorption capacity diminishes over time. A recent study suggests that the lifetime of fresh BrC against bleaching (τ_BrC) is influenced by ambient temperature and relative humidity. In this study, we incorporate the improved τ_BrC parameterization into the atmospheric chemical transport model (GEOS-Chem) to assess its impact on atmospheric chemistry and radiative effects. Our results show that τ_BrC varies strongly with altitude, ranging from 1–10 hours in the planetary boundary layer (PBL) to over 100 hours in the upper troposphere, where bleaching becomes negligible. Dry regions (e.g., Northern Africa, South Asia) exhibit longer surface τ_BrC, while humid regions (e.g., tropics) show shorter τ_BrC. The updated τ_BrC parameterization triples global fresh BrC burdens compared to the baseline parameterization with uniform τ_BrC, increasing its effective lifetime from 0.45 to 1.45 days and amplifying its direct radiative effect (DRE) by 48 % (from +0.059 to +0.088 W m^-2). Lofted wildfire emissions experience prolonged τ_BrC due to reduced bleaching in the free troposphere, underscoring the importance of fire injection height. Additionally, BrC absorption suppresses photochemical activity, reducing JNO₂ by up to 7.4 %, surface ozone by 0–2.5 % and tropospheric OH by 0–6.9 %. These effects intensify during major wildfire events, such as Siberian fires in 2019 that caused JNO₂ and ozone to drop by 36.3 % and 17.5 %, respectively, highlighting BrC's role in perturbing atmospheric oxidation capacity.