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

## TL;DR

This study explores how fumaric acid affects the properties of iron chloride droplets, simulating the formation of brown carbon in the atmosphere and its impact on light absorption.

## Contribution

The study introduces a novel microreactor method using a levitated aqueous Fe(III) droplet to simulate and analyze brown carbon formation pathways.

## Key findings

- Dehydration–hydration cycles caused irreversible increases in particle radius and refractive index.
- Multiwavelength measurements revealed a strong spectral dependence of the extinction coefficient, decreasing from 0.014 at 405 nm to 0.000 at 660 nm.
- The results highlight the importance of accurate brown carbon characterization for climate models, especially in the UV range.

## Abstract

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 (m
λ = n
λ + ik·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 (σext,λ) at 405 and 532 nm wavelengths. The most significant
result was the retrieval of multiwavelength m
λ, revealing a strong spectral dependence of k
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.