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 due to chemical transformation. A recent study suggests that the lifetime of freshly emitted, unbleached BrC (referred to as fresh BrC) against bleaching (τBrC) is influenced by ambient temperature and relative humidity. In this study, we incorporate the improved τBrC parameterization into an atmospheric chemical transport model (GEOS-Chem) to assess its atmospheric chemical and radiative effects. Our results show that τBrC varies strongly with altitude, ranging from 1–10 h in the planetary boundary layer (PBL) to over 100 h in the upper troposphere, where bleaching becomes negligible. Dry regions (e.g., northern Africa and South Asia) exhibit longer surface τBrC, while humid regions (e.g., the tropics) show shorter τBrC. The updated τBrC parameterization triples the global burden of fresh BrC compared to the baseline parameterization with uniform τBrC, increasing its effective lifetime from 0.45 to 1.45 d and amplifying the direct radiative effect (DRE) of BrC by 48 % (from +0.059 to +0.088 W m−2). Lofted wildfire emissions experience prolonged τBrC due to reduced bleaching in the free troposphere, underscoring the importance of fire injection height. Additionally, BrC absorption suppresses photochemical activity, reducing the photolysis rate of NO2 (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 the 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.