Improving global simulations of biomass-burning SOA through IVOC-focused sensitivity studies

Biomass burning is a major contributor to the global burden of secondary organic aerosol (SOA), with significant impacts on air quality, public health, and the Earth’s radiation balance. Observations from laboratory experiments and field campaigns increasingly show that intermediate-volatility organic compounds (IVOCs) emitted from wood combustion dominate SOA formation, potentially accounting for more than half of biomass-burning SOA (1,2). However, IVOC emissions and their atmospheric evolution remain highly uncertain and are often substantially underestimated in global chemistry–climate models, limiting our ability to accurately simulate organic aerosol distributions and trends. In this study, we quantify the sensitivity of biomass-burning SOA formation to key assumptions related to IVOC emissions, oxidation chemistry, and volatility distributions. We implement recent literature-based developments into ORACLE, the organic aerosol chemistry submodule of the EMAC global chemistry–climate model. ORACLE represents primary and secondary organic aerosol using a volatility basis set (VBS) framework, accounting for gas–particle partitioning, chemical aging through multigenerational oxidation, and changes in volatility and molecular mass during atmospheric processing (3). This framework enables a process-based evaluation of how uncertainties in emissions and chemistry propagate to global SOA burdens.

We perform a comprehensive suite of global sensitivity simulations for the period 2012–2016, corresponding to the most recent fully published GFED biomass-burning emission inventory and providing broad observational coverage for model evaluation. The sensitivity experiments address three major sources of uncertainty. First, we investigate alternative IVOC emission scaling approaches, including scaling IVOC emissions relative to emitted organic carbon (OC), as commonly assumed in global models (4), and scaling relative to volatile organic compound (VOC) emissions (5,2). These methods reflect differing assumptions about the relationship between IVOCs and primary combustion emissions and lead to substantially different global IVOC source strengths. Second, we assess uncertainties in SOA formation efficiency and chemical processing. This includes exploring reported ranges in aerosol mass yields under different NO_x regimes (2), as recent experiments indicate that NO_x strongly modulates SOA formation from biomass-burning IVOCs. In addition, we examine the sensitivity of modelled SOA to uncertainties in the OH reaction rate of IVOCs, which controls their atmospheric lifetime and spatial distribution. Third, we evaluate the influence of alternative volatility distribution of the IVOC oxidation products across VBS bins. Previous studies propose contrasting assumptions regarding whether the dominant SOA yield is associated with lower- or higher-volatility oxidation products (4,5), which leads to implications for SOA formation, transport, and lifetime.

By systematically disentangling the influence of IVOC emissions, chemical processing, and volatility assumptions, this work aims to identify parameterizations that are both physically representative of diverse biomass-burning conditions and computationally feasible for global applications. The results provide new constraints on biomass-burning SOA formation and support ongoing efforts to improve organic aerosol representation in global chemistry–climate models, thereby reducing long-standing discrepancies between simulated and observed SOA burdens.

 

References

(1) Bruns et al., 2016; doi: 10.1038/srep27881

(2) Li et al., 2024; doi: 10.1093/nsr/nwae014

(3) Tsimpidi et al., 2016; doi: 10.5194/acp-16-8939-2016

(4) Ciarelli et al., 2017; doi: 10.5194/gmd-10-2303-2017

(5) Tilmes et al., 2019; doi: 10.1029/2019MS001827