Sensitivity of organic aerosol optical properties to improved representation of brown carbon in the EMAC model

Abstract

Brown Carbon (BrC) is a wide class of aerosol species whose optical properties span from near dark Black Carbon (BC) to transparent/reflective and Organic Carbon (OC), are potent in their impact on Earth's climate radiative forcing (RF) and air quality. To date, the uncertainties about their contributions are larger compared to that of BC/OC, trace gases and other factors  (IPCC, 2021, Chapter 6). This is to a considerable extent due to current Earth System Models (ESMs) lacking sufficient representation of BrC, whilst no consistent unified classification and framework for BrC implementation in ESMs has been developed yet. Here, we review such implementation options and offer an advanced implementation of the BrC in the atmospheric chemistry general circulation model EMAC (Pozzer et al., 2021). EMAC includes all relevant processes (detailed aerosol physicochemistry, optical radiation calculation, online emission, etc.) for the comprehensive simulation of organic aerosol (OA). Our BrC intermediate-complexity implementation includes primary (e.g. biomass- or fossil-fuel burning) and secondary (e.g. oxidation of phenolic precursors) formation processes and includes “fresh” and “aged” mixture states defining final optical properties. Because most of available ambient measurements do not allow such differentiation, BrC categories are assigned refractive properties obtained chiefly in controlled lab experiments. The optical properties of OC and BC were adjusted to account for BrC presence and updated to recent recommendations. Ultimately, the new parameterization aims at more accurate reproduction of primary (POA) and secondary (SOA) organic aerosol optical properties and/under their atmospheric aging. Our preliminary simulations with EMAC indicate that POA and SOA optical properties are sensitive to representation of the newly implemented BrC-contributed and updated OC/BC parts, compared at selected AERONET observational stations. Further sensitivities are associated with the primary/secondary BrC emission proportion varying with the source sector. The overall refractive index (RI) of BrC results in an intensified absorption of the C-inclusive aerosol than the simulated with the former OC/BC-only speciation. On a global scale, changes to the top-of-atmosphere global RF may reach non-negligible extra 0.45 W/m2 (upper limit of POA absorption efficiency), whereas up to 50% larger negative RF changes are obtained at the surface. Due to BC and selected BrC species intense absorption in the UV range, we also quantify the effects of using the new parameterisation on ozone photolytic formation and loss. In summary, our findings suggest that an improved representation of BrC indicates a prior underestimation of its contribution to th e OA light-absorbing efficiency, consequently affecting the simulated RF in EMAC. A wavelength-resolved analysis of refractive indices against observational data is planned for subsequent in-depth analyses. This work was funded by the European Commission Horizon Europe project FOCI, Non-CO2 Forcers and Their Climate, Weather, Air Quality and Health Impacts (No. 101056783, see https://www.project-foci.eu).