Secondary Brown Carbon Formed by a Microreactor of a Levitated Aqueous Fe (III) Droplet with Fumaric Acid

This study investigates the influence of fumaric acid on the optical and microphysical properties of aqueous FeCl₃ microdroplets and how aging affects them. This process replicates a pathway for brown carbon (BrC) formation in the atmosphere. The experiment combines a Paul electrodynamic trap (PET), which captures a single particle, and a dual-wavelength cavity ring-down spectroscopy (CRDS) system. Initially, measurements were conducted under controlled humidity cycling, obtaining the particle phase function at a 532 nm wavelength. Retrievals reveal an irreversible increase in particle radius and complex refractive index (<i>m</i> _λ = <i>n</i> _λ + <i>ik</i>·_λ) after a dehydration-hydration cycle. The second part involves measuring a single particle trapped from the FeCl₃ + fumaric acid solution after 24 h in darkness. Instrumental flexibility enabled complementary measurements of the particle phase function at 473, 532, and 660 nm wavelengths and the extinction cross-section (σ_ext,λ) at 405 and 532 nm wavelengths. The most significant result was the retrieval of multiwavelength <i>m</i> _λ, revealing a strong spectral dependence of <i>k</i> _λ, which decreased from 0.014 at 405 nm to 0.000 at 660 mn. Radiative effects were evaluated and compared with other oxidation pathways of fresh biomass tar proxies, highlighting the need for precise BrC characterization in climate models, particularly in the UV range.