An Improved Representation of Organic Aerosol Composition and Atmospheric Evolution in the EC-Earth3-AerChem model

Organic compounds can constitute roughly half of the sub-micron aerosol mass in the troposphere, necessitating an accurate representation of organic aerosol (OA) in Earth system models (ESMs) to better capture aerosol-climate feedbacks. The secondary fraction of OA (SOA), however, formed through the oxidation of various volatile organic compounds (VOCs) from both natural and anthropogenic sources, complicates the description of OA in ESMs. Most ESMs either assume a non-volatile SOA produced with a constant yield from known precursors or provide a simplistic depiction of its volatility derived from biogenic VOCs, treating the primary fraction of OA (POA) as non-reactive and non-volatile. This approach often fails to accurately reproduce observed OA atmospheric measurement. On the other hand, biological materials such as bacteria, fungal spores, and various fragments released by living organisms into the atmosphere have been widely identified as part of the super-micron OA mass, which most ESMs also inadequately represent.

In the context of the H.F.R.I. project REINFORCE, we focus on improving the representation of atmospheric composition in Earth System Models (ESMs). We present simulations using the volatility basis set (VBS) approach to represent SOA formation, along with incorporating the organic fraction of bioaerosols. These developments are implemented in the state-of-the-art ESM, EC-Earth version 3, which includes interactive aerosols and atmospheric chemistry (EC-Earth3-AerChem). A lite version of the well-documented aerosol module ORACLE, which allows for relatively limited computing resource consumption, has been coupled to the CTM component of EC-Earth3-AerChem to calculate the partitioning and chemical evolution of POA vapors and their changes in volatility. The formation of SOA from semivolatile organic compounds (SVOCs) and intermediate-volatility organic compounds (IVOCs) has been added to the existing SOA formation scheme from biogenic VOCs in the model. Moreover, the three main types of bioaerosols—bacteria, fungal spores, and pollen grains—have been implemented into the model based on interactive bioaerosol schemes that depend on ecosystem types, the leaf area index (LAI), and various meteorological parameters. Bioaerosols in EC-Earth3-AerChem can also be transferred to the soluble aerosol coarse mode due to atmospheric aging processes. Overall, our efforts aim to bridge the gap between model simulations and observations, thereby enhancing our understanding of OA climate impacts.