# Connecting Brown Carbon Composition and Physicochemical Properties of Aqueous Urban PM2.5 to their Photosensitized Production of Singlet Oxygen and Organic Triplet Excited States

## TL;DR

This study connects the composition of brown carbon in urban PM2.5 to its ability to produce reactive oxygen species through photosensitization.

## Contribution

A new framework is introduced to predict and apportion photosensitizers in brown carbon using optical measurements and statistical modeling.

## Key findings

- Highly oxygenated organic aerosols mainly contribute to singlet oxygen production.
- Less oxygenated organic aerosols mainly contribute to organic triplet excited states production.
- Statistical models successfully predict reactive species concentrations and quantum yields from optical data.

## Abstract

Oxidants, including
singlet oxygen (1O2*)
and organic triplet excited states (3C*) formed from the
photoexcitation of brown carbon (BrC), drive many chemical processes
in atmospheric waters. However, due to the chemical complexity of
atmospheric BrC, many questions remain about the specific BrC chromophores
and physicochemical properties that primarily control 1O2* and 3C* production. In this study, we present
a framework for apportioning photosensitizers and predicting the production
of 1O2* and 3C* based on measurable
physicochemical properties of BrC. This is achieved by combining photochemical
experiments with absorbance and fluorescence measurements and statistical
modeling of a year-long data set of PM2.5 extracts from
Hong Kong SAR, China. Parallel Factor and Non-negative Matrix Factorization
analyses of the fluorescence data revealed that highly oxygenated
organic aerosols were the main contributors to 1O2* production, whereas less oxygenated organic aerosols were the main
contributors to 3C* production. Next, we developed Orthogonal
Partial Least Squares-Multiple Linear Regression models that successfully
predicted 1O2* and 3C* steady-state
concentrations ([1O2*]ss and [3C*]ss) and quantum yields (
ΦO2*1
and Φ
3C*) from
standard optical measurements. These models revealed that while [1O2*]ss and [3C*]ss depended on parameters that reflected the quantities of BrC chromophores,
ΦO2*1
and Φ
3C* were
influenced by the specific types (i.e., quality) of BrC chromophores
present. Overall, this combined approach provides a powerful tool
for identifying key BrC chromophore components and specific physicochemical
properties that drive 1O2* and 3C*
production.