Increased absorption by brown carbon: impact on estimation of black carbon radiative effect

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Description

Observational studies have suggested that a significant amount of the atmospheric aerosol absorption is attributable to brown carbon (BrC) in addition to black carbon (BC) and dust aerosols. A few global modeling studies of BrC recently estimate a direct radiative effect of +0.04 to +0.25 W m-2 due to the increased absorption, which is comparable to BC. However, while those studies may establish BrC as an important warming forcer, large uncertainty still remains in simulated aerosol absorption due to highly variable origins of BrC. Here we present a global simulation of BrC with the latest four-mode version of the Modal Aerosol Module (MAM4) in the Community Atmosphere Model version 5 (CAM5). The spectrally-resolved absorption by BrC is parameterized for biomass burning, biofuel and fossil fuel sources, respectively. Comparisons with the ground-based AERONET and ARM/ASR observations show improvement in representing the observed spatial variability in aerosol absorption spectrum. The CAM5 model estimates that BrC contributes about 13% of the fine-mode aerosol absorption at 550nm on a global average, and plays a more significant role at high latitudes and the Arctic region. The increased absorption by primary BrC inserts a global radiative effect of +0.04 W m-2, which is within and on the low end of previous estimates. On the other hand, the inclusion of BrC in CAM5 is also shown to lower the predicted all-BC direct radiative effect from +0.56 to +0.5 W m-2, compensating for the positive forcing by BrC. This mainly implies the nonlinear absorption by internally mixed carbonaceous aerosols. The simulated changes in meteorological and cloud fields due to inclusion of BrC will also be discussed in the estimation of BC radiative effect.