HO2 reactive uptake on organic aerosols: a molecular level study

Significant uncertainties are still associated to chemical reaction mechanisms used in atmospheric models, in particular for RO_x radicals (OH, HO₂, RO₂). Recent measurements of radicals in forested areas characterized by low NO_x (NO₂, NO) concentrations indicate that models can significantly overestimate peroxy radical concentrations.^1,2 These results question the ability of models to correctly simulate the oxidative capacity of the troposphere since peroxy radicals are a main source of the hydroxyl radical (OH), one of the most important oxidative species in the atmosphere.³  One possible explanation is the occurrence of heterogeneous processes (uptake of radicals) on the surface of aerosols that are either misrepresented or not included in models. While recent studies have reported uptake coefficients of HO₂ on different types of aerosols, the process is not completely understood yet.

Molecular dynamics combined with ab-initio calculations have been used to study HO₂ reactive uptake on organic aerosols. The sticking process of HO₂ and its reactivity have been modelled on a nanometer size aerosol particle.⁴ Those theoretical calculations provide insight into the uptake process at the molecular scale and are planned to be compared to experimental measurements carried out with an aerosol flow tube.

This work is supported by the CaPPA project (Chemical and Physical Properties of the Atmosphere), funded by the French National Research Agency (ANR) through the PIA (Programme d’investissement d’avenir) and by the regional council “Hauts-de-France”. The authors also thank CPER Climibio and FEDER for their financial support. Calculations were performed using HPC resources from GENCI-TGCC (Grant 2020- A0070801859).

References

• [1] T. Griffith et al., Atmos. Chem. Phys. 13, 5403 (2013)
• [2] Mao et al., Atmos. Chem. Phys. 12, 8009 (2012)
• [3] Stone et al., Chem. Soc. Rev. 41, 6348 (2012)
• [4] Roose et al., ACS Earth Space Chem. 3, 380 (2019)