Aerosol spectral optical properties in the Paris urban area and its peri-urban and forested surroundings during summer 2022 from ACROSS surface observations

Aerosol spectral optical properties in the Paris urban area and its peri-urban and forested surroundings during summer 2022 from ACROSS surface observations

<p>The complex refractive index (CRI; <span class="inline-formula"><i>n</i>−<i>i</i><i>k</i></span>) and the single scattering albedo (SSA) are key parameters driving the aerosol direct radiative effect. Their spatial, temporal, and spe...

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Main Authors:	L. Di Antonio, C. Di Biagio, P. Formenti, A. Gratien, V. Michoud, C. Cantrell, A. Bauville, A. Bergé, M. Cazaunau, S. Chevaillier, M. Cirtog, P. Coll, B. D'Anna, J. F. de Brito, D. O. De Haan, J. R. Dignum, S. Deshmukh, O. Favez, P.-M. Flaud, C. Gaimoz, L. N. Hawkins, J. Kammer, B. Language, F. Maisonneuve, G. Močnik, E. Perraudin, J.-E. Petit, P. Acharja, L. Poulain, P. Pouyes, E. D. Pronovost, V. Riffault, K. I. Roundtree, M. Shahin, G. Siour, E. Villenave, P. Zapf, G. Foret, J.-F. Doussin, M. Beekmann
Format:	Article
Language:	English
Published:	Copernicus Publications 2025-03-01
Series:	Atmospheric Chemistry and Physics
Online Access:	https://acp.copernicus.org/articles/25/3161/2025/acp-25-3161-2025.pdf
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Summary:	<p>The complex refractive index (CRI; <span class="inline-formula"><i>n</i>−<i>i</i><i>k</i></span>) and the single scattering albedo (SSA) are key parameters driving the aerosol direct radiative effect. Their spatial, temporal, and spectral variabilities in anthropogenic–biogenic mixed environments are poorly understood. In this study, we retrieve the spectral CRI and SSA (370–950 <span class="inline-formula">nm</span> wavelength range) from in situ surface optical measurements and the number size distribution of submicron aerosols at three sites in the greater Paris area, representative of the urban city, as well as its peri-urban<span id="page3162"/> and forested rural environments. Measurements were taken as part of the ACROSS (Atmospheric Chemistry of the Suburban Forest) campaign in June–July 2022 under diversified conditions: (1) two heatwaves leading to high aerosol levels, (2) an intermediate period with low aerosol concentrations, and (3) an episode of long-range-transported fire emissions. The retrieved CRI and SSA exhibit an urban-to-rural gradient, whose intensity is modulated by the weather conditions. A full campaign average CRI of <span class="inline-formula">1.41−0.037<i>i</i></span> (urban), <span class="inline-formula">1.52−0.038<i>i</i></span> (peri-urban), and <span class="inline-formula">1.50−0.025<i>i</i></span> (rural) is retrieved. The imaginary part of the CRI (<span class="inline-formula"><i>k</i></span>) increases and the SSA decreases at the peri-urban and forest sites when exposed to the influence of the Paris urban plume. Values of <span class="inline-formula"><i>k</i></span> <span class="inline-formula">></span> 0.1 and SSA <span class="inline-formula"><</span> 0.6 at 520 <span class="inline-formula">nm</span> are related to a black carbon mass fraction larger than 10 %. Organic aerosols are found to contribute to more than 50 % of the aerosol mass and up to 10 % (urban), 17 % (peri-urban), and 22 % (forest) of the aerosol absorption coefficient at 370 <span class="inline-formula">nm</span>. A <span class="inline-formula"><i>k</i></span> value of 0.022 (370 <span class="inline-formula">nm</span>) was measured at the urban site for the long-range-transported fire episode.</p>
ISSN:	1680-7316 1680-7324