# Hydroxyl Dicarboxylic Acids at a Mountainous Site in Hong Kong: Formation Mechanisms and Implications for Particle Growth

## TL;DR

This study investigates the formation of hydroxyl dicarboxylic acids in Hong Kong and their role in particle growth and climate impact.

## Contribution

The study uses time-resolved measurements and machine learning to reveal new insights into the formation mechanisms of hydroxyl dicarboxylic acids.

## Key findings

- Malic acid concentrations were higher in noncontinental air compared to continental air.
- Aqueous photochemical processes contribute to malic acid formation.
- OHDCA-like SOA may facilitate particle growth during specific times of the day.

## Abstract

Secondary organic aerosol (SOA) has been shown to significantly
impact climate, air quality, and human health. Hydroxyl dicarboxylic
acids (OHDCA) are generally of secondary origin and ubiquitous in
the atmosphere, with high concentrations in South China. This study
explored the formation of representative OHDCA species based on time-resolved
measurements and explainable machine learning. Malic acid, the most
commonly studied OHDCA, had higher concentrations in the noncontinental
air (63.7 ± 33.3 ng m–3) than in the continental
air (7.5 ± 1.4 ng m–3). Machine learning quantitatively
revealed the high relative importance of aromatics and monoterpenes
SOA, as well as aqueous processes, in the noncontinental air, due
to either shared precursors or similar formation pathways. Isoprene
SOA, particle surface area, and ozone corrected for titration loss
(O
x
) also elevated the concentrations
of malic acid in the continental air. Aqueous photochemical formation
of malic acid was confirmed given the synergy between LWC, temperature,
and O
x
. Moreover, the OHDCA-like SOA might
have facilitated a relatively rare particle growth from early afternoon
to midnight in the case with the highest malic acid concentrations.
This study enhances our understanding of the formation of OHDCA and
its climate impacts.