# Dynamic Evolution of Mass and Physical Properties of Atmospheric Organic Aerosol Under Solar Irradiance

## TL;DR

This study explores how sunlight changes the mass and properties of organic aerosol particles in the atmosphere, impacting climate models.

## Contribution

The study introduces a kinetic model that explains how photolytic aging alters aerosol mass and hygroscopicity.

## Key findings

- Photolytic aging reduces 40–66% of low-volatility OA mass under solar irradiance.
- Up to ±50% changes in OA hygroscopicity are observed during photolytic aging.
- A kinetic model explains the observed changes in mass, volatility, and hygroscopicity.

## Abstract

Organic aerosol (OA) particles constitute a substantial
fraction
of submicron particulate mass in the atmosphere and play a critical
role in climate system. OA undergoes dynamic aging processes in the
atmosphere, with photolytic aging induced by ultraviolet solar irradiance
being an important yet poorly characterized mechanism. Knowledge gaps
persist regarding the role of volatility transformations during photolytic
aging on the OA mass decay kinetics and the evolution of climate-relevant
properties, such as hygroscopicity, hindering the model evaluation
of OA spatiotemporal distributions and atmospheric budgets. In this
study, we conduct isothermal photolytic aging experiments on both
laboratory-generated secondary organic aerosols and ambient-collected
particles from urban Atlanta, utilizing a high-sensitivity Quartz
Crystal Microbalance. Our results reveal that photolytic aging reduces
40–66% of the low-volatility OA mass with lifetimes ranging
from 8 to 200 h under solar irradiance, and 44–92% of the photolytic
mass loss is through slow evaporation of semi- or intermediate-volatile
products, kinetically limited by their volatility. We observe up to
±50% changes in OA hygroscopicity with the transformation of
fresh OA to photorecalcitrant low-volatility products, associated
with changes in oxygen-to-carbon ratio and molecular weight. A kinetic
model incorporating photolytic volatility transformation provides
a cohesive explanation for the observed photolysis-induced changes
in mass, volatility, and hygroscopicity. Our results can help constrain
model representation of the dynamic evolutions of mass and climate-relevant
properties during photolytic aging processes of the ambient OA, improving
our understanding of OA atmospheric behavior and climate impact.