Reply on RC2

Abstract

The fraction of organic aerosols, predominantly emitted from biomass burning and biofuel use, that strongly absorbs ultraviolet and short visible light is referred to as brown carbon (BrC). The lifecycle and the optical properties of BrC are still highly uncertain, thus contributing to the uncertainty of the total aerosol radiative effect. This study presents the implementation of BrC aerosols into the atmospheric component of the CNRM climate model and particularly in its aerosol scheme TACTIC, using a BrC parameterization based on the optical properties of Saleh et al. (2014). Several simulations have been carried out with this global climate model, over the period 2000–2014, to analyze the BrC radiative and climatic effects. Model evaluation has been achieved by comparison of single-scattering albedo (SSA), aerosol optical depth (AOD) and absorption aerosol optical depth (AAOD) with AERONET stations at the local scale and with different satellite products at the global scale. This work has mainly shown an improvement, thanks to the BrC implementation and its bleaching parameterization, in total SSA and AAOD, at 440 nm, whether at several AERONET stations or at the regional scale, notably over regions of Africa (AFR) and South America (AME) where large quantities of biomass burning aerosols are emitted. The annual global BrC effective radiative forcing (all-sky conditions) has been calculated in terms of aerosol-radiation interactions (ERFari, 0.029 ± 0.006 W m-2) and in terms of aerosol–cloud interactions (ERFaci, -0.024 ± 0.066 W m-2). Over the AFR and AME regions, the study shows respectively a positive ERFari of 0.292 ± 0.034 and of 0.085 ± 0.032 W m-2 on annual average, close to the BrC radiative effect calculated in other studies. This work also shows that the inclusion of BrC causes a statistically significant low-level cloud fraction increase over the South-East Atlantic Ocean during the burning season caused in part by a vertical velocity decrease at 700 hPa (semi-direct aerosol effect). Lastly, this study also highlights that the low-level cloud fraction changes, associated with more absorbing biomass burning aerosols when BrC aerosols are included, contribute to an increase, at 700 hPa, of both solar heating rate and air temperature over this region.