Abstract
Abstract. A recent development in the representation of aerosols in climate models is the realization that some components of organic aerosol (OA), emitted from biomass and biofuel burning, can have a significant contribution to shortwave radiation absorption in the atmosphere. The absorbing fraction of OA is referred to as brown carbon (BrC). This study introduces one of the first implementations of BrC into the Community Atmosphere Model version 5 (CAM5), using a parameterization for BrC absorptivity described in Saleh et al. (2014). Nine-year experiments are run (2003–2011) with prescribed emissions and sea surface temperatures to analyze the effect of BrC in the atmosphere. Model validation is conducted via model comparison to single-scatter albedo and aerosol optical depth from the Aerosol Robotic Network (AERONET). This comparison reveals a model underestimation of single scattering albedo (SSA) in biomass burning regions for both default and BrC model runs, while a comparison between AERONET and the model absorption Ångström exponent shows a marked improvement with BrC implementation. Global annual average radiative effects are calculated due to aerosol–radiation interaction (REari; 0.13±0.01 W m−2) and aerosol–cloud interaction (REaci; 0.01±0.04 W m−2). REari is similar to other studies' estimations of BrC direct radiative effect, while REaci indicates a global reduction in low clouds due to the BrC semi-direct effect. The mechanisms for these physical changes are investigated and found to correspond with changes in global circulation patterns. Comparisons of BrC implementation approaches find that this implementation predicts a lower BrC REari in the Arctic regions than previous studies with CAM5. Implementation of BrC bleaching effect shows a significant reduction in REari (0.06±0.008 W m−2). Also, variations in OA density can lead to differences in REari and REaci, indicating the importance of specifying this property when estimating the BrC radiative effects and when comparing similar studies.
Highlights
One of the key areas of uncertainty in climate models is their representation of aerosols (Anderson et al, 2003; Myhre et al, 2013)
To understand how the BRC model brown carbon (BrC) plays a role in aerosol–cloud interaction, we looked at vertical profiles of land (western Gulf of Mexico (GM), South America (SA), northeastern China (NEC)) and oceanic regions (Weddell Sea (WS), western Antarctic coast (AC), northeast Pacific (NEP)) with significant positive and negative BrC REaci (Fig. 9)
This study has shown that a BrC parameterization in Community Atmosphere Model version 5 (CAM5).4 can bring about significant global radiative effects in Community Earth System Model (CESM)
Summary
One of the key areas of uncertainty in climate models is their representation of aerosols (Anderson et al, 2003; Myhre et al, 2013). Another treatment assumes a core-shell organization of aged aerosols whereby the primary aerosol core is coated by a volume mean internal mixture of higher volatility, stronger scattering secondary aerosols (Jacobson, 2001; Feng et al, 2013; Saleh et al, 2015) This can act to increase absorption given an absorbing center (e.g., BC) by refracting intercepted light into the primary core (Bond et al, 2006), known as the lensing effect. In Feng et al (2013), global mean absorption aerosol optical depth (AAOD) increases with two different BrC absorption assumptions: moderately (Chen and Bond, 2010) and strongly (Kirchstetter et al, 2004) absorbing, with 66 % of BB and BF organic carbon emissions assumed to be BrC.
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