# Changes in Secondary Organic Aerosol Composition and Volatility Going from a Low to a High HO2/RO2 Regime in α‑Pinene Photooxidation

## TL;DR

This study shows how changes in HO2 and RO2 radicals during α-pinene oxidation affect the composition and volatility of secondary organic aerosols.

## Contribution

The study experimentally compares SOA composition and volatility under low and high HO2/RO2 regimes, revealing significant differences in aerosol properties.

## Key findings

- High HO2/RO2 conditions reduced particle-phase monomer, fragment, and accretion product signals by 34%, 29%, and 78%, respectively.
- Gas-phase changes aligned with particle-phase changes within a factor of 2, with organic mass reduced by 39%.
- Bulk SOA volatility increased slightly, indicating suppression of low-volatility products and formation of high-volatility hydroperoxide monomers.

## Abstract

The mechanisms of secondary organic aerosol (SOA) formation
are
not yet fully understood. The relative abundance of hydroperoxyl radicals
(HO2) and peroxy radicals (RO2) affects SOA
properties, but chamber experiments often underemphasize the role
of HO2. To clarify their contribution, this study compares
the composition and volatility of SOA formed by the hydroxyl radical
(OH) oxidation of α-pinene under low and high HO2/RO2 regimes with a constant OH concentration. The particle-phase
was characterized with a Filter Inlet for Gases and AEROsols coupled
to an iodide Chemical Ionization Mass Spectrometer (FIGAERO CIMS),
and a CIMS with NO3
– ionization was used
for gas-phase measurements. High HO2/RO2 conditions
weakened the particle-phase monomer (C10), fragment (C4–9), and accretion product (C11–20) signals by 34%, 29%, and 78%, respectively, compared to low HO2/RO2 conditions. The only species with an increased
signal (180%) was C10H18O7. The gas-phase
changes align with those in the particle-phase within a factor of
2. Overall, the organic mass was reduced by 47% and 39% for particle
and gas-phases, respectively. Bulk SOA volatility (log C*) increased slightly from −0.22 μg m–3 to −0.1 μg m–3, reflecting the suppression
of low volatility accretion products but formation of high volatility
hydroperoxide monomers. This study highlights the importance of HO2 for SOA formation and model predictions.