Particle-scale Diversity in Organic Composition of Fresh and Aged Wildfire Emissions

JOHANNES PASSIG, Ellen Iva Rosewig, Aleksandrs Kalamasnikovs, Haseeb Hakkim, Marco Schmidt, Thomas Gröger, Mika Ihalainen, Anni Hartikainen, Markus Somero, Pasi Yli-Pirilä, Olli Sippula, Kerneels Jaars, Pieter Gideon van Zyl, Kajar Köster, Stefan Siebert, Saara Peltokorpi, Liqing Hao, Angela Buchholz, Annele Virtanen, Ville Vakkari, Andreas Walte, Ralf Zimmermann, Mass Spectrometry Centre;Rostock University/Helmholtz Munich

     Abstract Number: 216
     Working Group: Aerosol Processes and Properties in Changing Environments in the Anthropocene

Abstract
Wildfire smoke significantly impacts air quality, human health, climate, and Earth system processes (van der Werf et al., 2017). The atmospheric aging of these aerosols involves complex chemical and microphysical transformations that alter their optical properties, radiative forcing, and cloud-forming potential. For example, organic coatings can enhance light absorption via lensing effects and increase hygroscopicity (Cheng et al., 2024). A central but poorly understood factor in this context is the aerosol mixing state—the distribution of optically and nucleation-active substances across the particle ensemble.

In this study, we present single-particle mass spectrometry data from a boreal forest smoke experiment, resolving the distribution of both hydrophilic molecules and hydrophobic polycyclic aromatic hydrocarbons (PAHs). We show that glyoxal and methylglyoxal are directly emitted during combustion, contributing to the initial hygroscopicity of the particles. Upon photochemical aging, we observed the rapid formation of oxalate, coinciding with a moderate increase in hygroscopicity. At the same time, high initial levels of the softwood combustion marker retene and other PAHs rapidly declined. The similar timescales of oxalate formation and PAH degradation suggest a prolonged co-existence during aging.

Importantly, our single-particle analysis shows that these hydrophilic and hydrophobic substances are not segregated into different particle types but are broadly co-distributed across the particle ensemble. This co-existence implies that both optical lensing and droplet activation effects are widespread in aged wildfire smoke and must be considered in atmospheric models and climate impact assessments.

This work was supported by the Helmholtz Association (International Laboratory aeroHEALTH – Interlabs-0005), by the Academy of Finland (grant no 337550, 341597, 343359) and by the European Commission under the Horizon 2020 Programme, H2020-INFRAIA-2020-1, Grant: 101008004.

[1] Cheng, Z. et al. (2024), Nat. Commun. 15, 10326.
[2] Van der Werf, G. et al. (2017), Earth Syst. Sci. Data, 9.