Constraining the direct radiative effect of Black and Brown Carbon on climate: an innovative experimental study on their spectral optical properties – B2C

The B2C project proposes an original mechanistic study of the spectral optical properties of BC and BrC in the CESAM large simulation chamber. CESAM is a 4.2 m3 pressure? temperature? and humidity?controlled chamber where particles can be generated, maintained in suspension, processed, and deposited under controlled conditions. The chamber is equipped with a source of realistic solar irradiance and is equipped with the most up?to?date analytical tools to characterize the chemical and the physical state of gas and particulate phases and the aerosol spectral optical properties.
The experimental strategy of the B2C project will consists at injecting into the CESAM chamber freshly generated BC aerosols or pre?prepared solutions of BrC, or generating in situ BrC from chemical reactions, then subjecting them to aging processes, and measuring their spectral optical properties at the same time as their physico?chemical properties, and at a time resolution allowing to study their coupled variations.

To achieve the objectives of the B2C project, strategic choices are made:
1. In order to study the optical properties of BC and to be able to compare/combine the results with most of the available literature, primary absorbing BC aerosols will be generated by a commercial soot generator taken at the standard: the miniCAST (by JING), at present the most used soot generator in chamber laboratory experiments.
2. In order to represent the change in BC optical properties due to atmospheric aging, the heterogeneous reaction with atmospheric constituents inducing the formation of inorganic / organic coatings on particles will be simulated in order to represent aged primary absorbing BC aerosols. The inorganic coating on the BC will be obtained by reaction with sulfur dioxide or sulfur and nitric acids. Organic coatings will be achieved by interacting BC with SOA produced in the chamber by the most abundant natural/anthropogenic VOC precursors.
3. Chemically?generated secondary absorbing BrC aerosols will be generated in the chamber by two different approaches: 1/ by atomizing pre?prepared proxy solutions of BrC obtained by the mixing of different compounds representative of different sources, as wood burning; 2/ by chemical reactions in CESAM including both non?aqueous and aqueous phase reactions.
4. Particle generation and its aging will be studied by simulating different ambient conditions, i.e. with different concentrations (low/high) of the precursor gases, high and low RH, with and without light.
The deliverable of the B2C project will be the spectrally?resolved complex refractive index, single scattering albedo and mass absorption, scattering and extinction cross sections for BC and BrC in dry and wet conditions, for different aging states, and their relation to particle chemistry & morphology

The work carried out so far has mainly consisted of the experimental activity dedicated to the study of primary carbonaceous aerosols and their ageing in the CESAM chamber.
A PhD thesis on the project started in January 2021. Several weeks of experiments in the simulation chamber have been carried out to achieve different objectives. First, to evaluate and optimise the experimental protocol for injection, ageing, and analysis of particulate compounds generated from a miniCAST type generator (Jing 6204C, computer controlled, generation of primary aerosols by incomplete combustion of propane gas). Secondly, to evaluate the different operating points of the miniCAST generator in order to determine for each of them the fraction of BC generated and the physico-chemical state of the particles (morphology, dimensional distribution, bulk and surface chemical composition). Thirdly, to study the interactions between these aerosols, in particular BC, with inorganic (ozone, sulphates) and organic (AOS) compounds, and the evolution of the optical properties for external and internal mixtures of BC with aerosols from these compounds.
A protocol for the injection of aerosols from the miniCAST generator was set up and five different operating points producing different EC/OC fractions (EC elemental carbon, OC organic carbon) were characterized. For one selected operating point very rich in elemental carbon, different ageing conditions were studied in the CESAM chamber. These included: ageing without any forcing and under dry conditions for times of 24-30 hours at different aerosol concentrations; ageing under dry/wet conditions, with/without irradiation, in the presence of ozone and SO2, and in the presence of AOS generated by the ozonolisation of alpha pinene.
The analysis of this first set of observations is in progress. The results are very encouraging and suggest that it is possible to spectrally resolve the variation of optical properties of combustion aerosols as a function of different generation and ageing conditions. The analysis of the optical and physico-chemical measurements supports the idea that differences in composition, shape and size result in a variation of the optical properties of the aerosols, in a repeatable manner for the same type of experiment and in relation to the ageing processes undergone.

The analysis of the experiments carried out is in progress and will be valorised by publications to be written in the coming months and focused on the measurement of absorption, as well as on the characterization of the physico-chemical and optical properties of the investigated aerosols. All the data will be integrated in order to establish the relationship between the optical properties, especially the refractive index, and the chemical and morphological state of the particles. New sets of experiments at CESAM are planned in 2022 and 2023 to extend the set of experimental observations to various chemical systems.

The scientific production from the first 18 months of the projects is associated to poster and oral presentations in national and international workshops and conferences, as listed below:.
1. Di Biagio, C., J. Yon, A. Marinoni, M. Cazaunau, E. Pangui, P. Ausset, S. Chevaillier, P. Decorse, P. Formenti, P. Laj, M. Maillé, C. Perruchot, S. Triquet, and J.?F. Doussin, Laboratory Investigation of the Spectral Optical Properties of Black Carbon Aerosols: Impact of Composition, Morphology and Aging, European Aerosol Conference 2020 – EAC 2020, 31 August – 4 September 2020, online.
2. Di Biagio, C., J. Heuser, A. Bergé, M. Cazaunau, S. Chevaillier, P. Formenti, A. Gratien, M. Maillé, G. Noyalet, E. Pangui, B. Picquet-Varrault, M. Zanatta, P. Ausset, T. Bourrianne, R. Ceolato, P. Decorse, C. Denjean, A. Faccinetto, P. Laj, A. Marinoni, D. Massabo, I. Ortega, C. Perruchot, D. Petitprez, P. Prati, L. Renzi, J. Yon, and J.F. Doussin, Contraindre l'effet radiatif direct du black carbon et du brown carbon sur le climat: une étude expérimentale innovante sur leurs propriétés optiques spectrales, Workshop ACTRIS-FR, 18-21 Octobre 2021, Oléron, France.
3. Heuser, J., C. Di Biagio, A. Bergé, M. Cazaunau, S. Chevaillier, P. Formenti, A. Gratien, M. Maillé, G. Noyalet, E. Pangui, B. Picquet-Varrault, M. Zanatta, P. Ausset, T. Bourrianne, R. Ceolato, C. Denjean, P. Decorse, A. Faccinetto, P. Laj, A. Marinoni, D. Massabo, I. Ortega, C. Perruchot, D. Petitprez, P. Prati, L. Renzi, J. Yon, and J.F. Doussin, Exploring the spectral optical properties of soot aerosols and the impact of ageing: a mechanistic study in the large CESAM simulation chamber, GDR Suies annual meeting, Lyon, France, 3-5 November 2021.

The objective of the B2C project is to spectrally resolve the optical properties of carbonaceous aerosols (black and brown carbon: BC, BrC) and their variability in link with the particle formation process and atmospheric aging in order to provide the required parameterizations to climate models and remote sensing retrievals so to constrain their global and regional radiative effect. The B2C project proposes an original mechanistic study in the controlled and well-characterized atmosphere of the large CESAM simulation chamber and takes advantages of its recent instrumental developments including the new UV-Vis spectrometer that allows measuring aerosol extinction spectra at high spectral resolution and at different relative humidity levels, as required by climate models and remote sensing. B2C builds up on the established expertise of the PI and the LISA team involved in the project to design original laboratory experiments to study the optical properties of 1/ Primary absorbing BC aerosols, taken as representative of freshly emitted combustion BC particles, 2/ Aged primary absorbing BC aerosols, so BC particles after they have undergone heterogeneous reaction with atmospheric constituents inducing the formation of inorganic/organic coatings on them, and 3/ Chemically-generated secondary absorbing BrC aerosols, so BrC particles formed by in situ atmospherically-relevant chemical processing of Secondary Organic Aerosols, including both non-aqueous and aqueous-phase processes. The strategy of the B2C project will consist at generating aerosols in CESAM and make them to age in realistic and atmospheric-relevant conditions, and at characterizing simultaneously their spectral optical properties and physico-chemical state based on state-of-the-art instrumentation, established laboratory protocols, and advanced modelling approaches. B2C will quantify the impact of chemical composition and morphology on the essential optical properties of these particles: the mass extinction, absorption, and diffusion efficiencies (MEC, MAC, MSC), the complex refractive index and the single scattering albedo (SSA). The B2C project will provide original and complementary data to previous studies so permitting the characterization of optical properties over the entire UV-Vis-IR spectrum. These data are required in models to constrain the direct radiative effect of BC and BrC on climate.
The deliverable of the B2C project will be the spectrally-resolved CRI, MAC, MSC, MEC and SSA for BC and BrC aerosols in dry and wet conditions, for different aging states, and their relation to particle chemistry and morphology.