# Spectroscopic Analysis of the Complex Refractive Indices for Imine Brown Carbon Aerosol Particles

## TL;DR

The study measures optical properties of imine brown carbon aerosols, finding light absorption only under basic conditions.

## Contribution

New precise measurements of refractive indices and density for imine BrC aerosols using advanced spectroscopy techniques.

## Key findings

- Light absorption in imine BrC aerosols occurs only at pH ≈ 9.5 with k values 0.0016–0.0018.
- Aerosols from acidic and neutral solutions showed no detectable light absorption (k < 10–4).
- Aerosolization and drying under basic conditions enhanced chromophore formation.

## Abstract

Brown carbon (BrC) contributes substantially to light
absorption
by atmospheric aerosols, which represents a key uncertainty in estimates
of aerosol radiative forcing. Among its diverse constituents, imine
BrC formed from aqueous reactions between glyoxal and nitrogen-containing
species is of widespread interest. We report sensitive measurements
of particle size-resolved optical cross sections, associated retrievals
of complex refractive indices (m = n + ik), and effective density measurements for imine
BrC aerosols generated from aqueous solutions containing glyoxal and
nitrogen species across a wide pH range. An Aerodynamic Aerosol Classifier
was coupled with cavity ring-down and photoacoustic spectroscopy to
measure extinction and absorption cross sections at the short-visible
wavelength of 405 nm, enabling retrievals of n and k with high precision and accuracy while eliminating multiply
charged particle artifacts that impair more common mobility-based
approaches. Detectable light absorption was observed under basic conditions
(pH ≈ 9.5) only, yielding values of k in the
range 0.0016–0.0018 and n of ∼1.50,
demonstrating that imine BrC may contribute modest absorption at short-visible
wavelengths. Meanwhile, aerosols generated from aqueous solutions
under acidic and near-neutral conditions were nonabsorbing (k < 10–4). Comparison with bulk UV/vis
absorption spectra indicates that aerosolization and drying enhanced
chromophore formation under basic conditions, likely through the supersaturation
of dissolved reacting solutes in drying aqueous aerosol droplets.
Measured values for effective particle density were consistent with
the formation of partially oligomerized glyoxal hydrates.