# Exploring the spectral optical properties of soot aerosols and the impact of ageing: a mechanistic study in the large CESAM simulation chamber

Johannes Heuser, Claudia Di Biagio, Antonin Bergé, Mathieu Cazaunau, Servanne Chevaillier, Paola Formenti, Aline Gratien, Michel Maille, Gael Noyalet, Edouard Pangui, et al.

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Johannes Heuser, Claudia Di Biagio, Antonin Bergé, Mathieu Cazaunau, Servanne Chevaillier, et al. Exploring the spectral optical properties of soot aerosols and the impact of ageing: a mechanistic study in the large CESAM simulation chamber. 1st ACTRIS Science Conference, May 2022, Visio, France. hal-03763736

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# Exploring the spectral optical properties of soot aerosols and the impact of ageing: a mechanistic study in the large CESAM simulation chamber

J. HEUSER¹, C. DI BIAGIO², A. BERGE², M. CAZAUNAU², S. CHEVAILLIER², P. FORMENTI², A. GRATIEN², M. MAILLE¹, G. NOYALET², E. PANGUI², B. PICQUET-VARRAULT¹, M. ZANATTA¹,¹¹, P. DECORSE⁴, A. FACCINETTO⁵, P. LAJ⁶, A. MARINONI⁷, D. MASSABO⁸, C. PERRUCHOT⁴, D. PETITPREZ⁵, P. PRATI⁸, L. RENZI⁷, J. YON¹⁰, and J.F. DOUSSIN¹

¹ Univ Paris Est Creteil and Université de Paris, CNRS, LISA, F-94010 Créteil, France; ² Université de Paris and Univ Paris Est Creteil, CNRS, LISA, F-75013 Paris, France; ³ CNRM/Toulouse; ⁴ ITODYS/UP Paris; ⁵ PC2A/Lille; ⁶IGE/Grenoble; ⁷ CNR/Bologna; ⁸Univ of Genoa/Italy; ⁹ ONERA/Palaiseau; ¹⁰ CORIA/Rouen; ¹¹ KIT/Allemagne

Keywords: BC, Optical Properties, Atmospheric Simulation Chamber, Aerosol Ageing.

## INTRODUCTION

Atmospheric aerosol particles have both direct and indirect influences on the climate (IPCC,2013). The majority of aerosol particles are generally considered to produce negative direct radiative forcing meaning they are considered to force cooling of the Earth-atmosphere system. Certain types of particles are instead found to exert a positive radiative effect, in particular, the strong light-absorbing BC-containing particles (BC, Black Carbon) (e.g. Bond et al.,2013). Currently, the radiative properties of BC-containing particles governing their interaction with atmospheric radiation, still have significant uncertainties and remain difficult to represent in climate models and remote sensing retrieval algorithms. This is due to the poor knowledge on how these spectral optical properties vary at emission and modify during atmospheric ageing in link with the variability and modification of the particle physicochemical properties and mixing state. Particularly, little knowledge exists on the spectral complex refractive index (CRI) of BC-containing aerosols, the basic intensive parameter describing the capacity of particles to absorb and scatter radiation.

## Methods

A set of original experiments are performed in the atmospheric simulation chamber CESAM (https://cesam.cnrs.fr/) at LISA, Créteil, in order to advance our understanding of the variability of radiative properties of BC-containing particles in relation to the processes of formation and atmospheric ageing. CESAM allows the simulation of different ageing processes as it provided lifetimes of more than 24h for submicron aerosol particles and it enables the measurement of the physicochemical properties (i.e.on size distribution, composition, effective density and morphology) at the same time as the spectral optical properties by combining in-situ measurement, ex-situ measurement by commercial instrumentation (e.g. cavity attenuated phase shift spectroscopy, nephelometer, aethalometer, scanning mobility particle size, aerosol chemical speciation monitor) and off-line analysis of aerosol samples collected out of the chamber. The latter for example were used to analyse aerosol composition and to retrieve morphological information via transmission electron microscopy which will be used in future to initiate shape-advanced optical calculations to retrieve the CRI of the aerosol particles.

Three CESAM measurement campaigns on BC-containing aerosols were performed over the course of 2021. Soot aerosols generated from the incomplete combustion of propane from a commercial diffusion flame soot generator (miniCAST model 6204 TYPE C by JING) were chosen as proxy for the BC-containing aerosols. Experiments at CESAM were set up in order to explore (i) the change of optical properties for fresh soot particles generated with different fuel/oxygen rates and therefore varying sizes and OC/EC contents and (ii) the change of the soot optical properties during aerosol ageing.
Both physical ageing, to evaluate the influence of particle morphological restructuration in dry conditions, and chemical/photochemical ageing in humid conditions, to evaluate the changes due to illumination under different atmospheric conditions as well as the potential formation of non-absorbing inorganic and organic coating on the particles, were simulated.
<figure><img src="image_2.png" /><figcaption>Figure 1: Times series of a photochemical ageing process in an ozone rich environment and the change in optical properties measured with cavity attenuated phase shift spectrometers at 450 nm and 630 nm (the protocol of water injection includes disconnection of the instruments) [Preliminary Results]</figcaption></figure>

At the core of the work are for now 26 experiments that allowed the measurement of the soot properties for both above-mentioned points thus including experiments focused on the (i) soot generation and (ii) soot ageing while also including multiple repetitions to ensure repeatability and control experiments. The processes simulated in the chamber during those experiments range from more or less purely physical ageing in a dry-dark particle-free air-like mixture (80/20 N₂/O₂-mixture) up to more complex systems focusing on the above-mentioned chemical/photochemical ageing. These simulated atmospheric conditions for the ageing did include until this point experiments with and without radiation, relative humidity and additional chemical compounds like ozone, sulphur dioxide and α-pinene, a natural Volatile Organic Compound (VOC) in a variety of combinations aiming to ultimately have systems that generate significant amounts of the above-mentioned coatings. Besides identifying the influence of the different ageing systems, the experiments were set up to allow the identification of the influence of each component of the ageing system on the measured properties of the BC-containing aerosol.

## Perspectives

The first three campaigns resulted in a large database of a broad spectrum of variations. The first analysis of the measured properties of fresh soot particles generated with different fuel/oxygen rates indicate that the generated BC-containing particles cover a range of different absorbing properties and especially possess diverse wavelength-dependent absorption behaviours while also covering a large range of different OC/EC-ratios and other physicochemical properties. This should allow to indicate and to a large part constrain the variability of the optical properties from freshly generated propane soots.

Figure 1 and Figure 2 show both a selection of preliminary results for the measured optical and physical properties during one of the photochemical ageing experiments of EC-rich soot. The ageing of the soot was performed in a humid (blue line [35% RH]), ozone containing (brown line) and illuminated (yellow line)
environment. Changes in both optical and physical properties during the ageing process were recorded. While the particle concentration displayed in Figure 1 was not influenced by the processes, the optical parameters acquired with four CAPS devices especially visible in the SSA showed a significant change due to the ageing. Further, the effective density measured with a DMA-CPMA-CPC setup displayed in Figure 2 also showed significant differences for the particles measured after the photochemical ageing (brown data).

<figure><img src="image_3.png" /><figcaption>Figure 2: Effective density for the particle diameter measured with a CMPA-DMA-CPC for the experiment shown in Figure 1, EXP_5_Fresh is measured shortly after soot injection, EXP_5_H2O after the water vapour injection (to 35% RH), and, EXP_5_O3 shortly after turning on the light [Preliminary Results]</figcaption></figure>

At this point, the analysis of the physiochemical processes and changes in the spectral optical properties during the ageing is still ongoing but key descriptive parameters of the optical properties like MAC, MEC and MSC could already be retrieved for differently aged BC-containing aerosols using a variety of different methods to calculate both the optical parameters as well as the mass. Further and more complex parameters retrieved out of the initial measurements are to follow as well as further ageing experiments to enlarge the range of observed ageing states of BC-containing aerosol. In this presentation, we are giving an overview of the 2021 campaigns, their initial results and the perspectives for future data analysis and chamber experiments.

## ACKNOWLEDGEMENTS

This study has been supported by the French National Research Agency (ANR) through the B2C project (contract ANR-19-CE01-0024-01), by the French National program LEFE-CHAT (Les Enveloppes Fluides et l'Environnement – Chimie Atmosphérique), by the European Union's Horizon 2020 research and innovation programme through the EUROCHAMP-2020 Infrastructure Activity under grant agreement No 730997. by Paris Region in the framework of call for projects from DIM Qi2 and it has been done in the framework of the GDR CNRS 3622 (GDR SUIE). CNRS-INSU is gratefully acknowledged for supporting the CESAM chamber as a national instrument as well as the AERIS data center for distributing and curing the data produced by the CESAM chamber.

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