# Importance of Particle-Phase Reactions in the Growth of Newly Formed Particles

## TL;DR

This study shows that chemical reactions within newly formed particles, not just in the gas phase, play a key role in particle growth and aerosol formation.

## Contribution

The study reveals that particle-phase reactions, including dimer formation, significantly influence aerosol composition and volatility.

## Key findings

- Particle-phase reactions, such as accretion and decomposition, directly form biogenic OOM dimers.
- OOMs detected via UPLC-ESI Orbitrap MS/MS show isomer-specific fragmentation patterns.
- Volatility estimates based on elemental composition fail to capture isomer-specific volatility differences.

## Abstract

New particle formation (NPF) is a chemistry-driven process
that
results in the formation of secondary aerosols and is the main source
of global cloud condensation nuclei. Currently, the majority of NPF
parametrizations consider volatility-based gas-to-particle conversion
of oxygenated organic molecules (OOMs), formed only in the gas phase,
and assume thermodynamic equilibrium regardless of aerosol chemical
composition or environmental conditions. Here, we performed a comprehensive
chemical analysis of the OOMs produced from α-pinene ozonolysis
in a fast-flow reactor to elucidate the role of gas- and particle-phase
chemistry in the NPF processes. Gas- and particle-phase OOMs were
measured with an iodide high-resolution time-of-flight chemical ionization
mass spectrometer (HrTOF-CIMS) attached to the filter inlet for gas
and aerosol (FIGAERO). Additionally, particle-phase OOMs were detected
with off-line ultra-performance liquid chromatography-electrospray
ionization (UPLC-ESI) high-resolution Orbitrap MS/MS analysis. Mass
spectra of particle-phase OOMs detected with these two methods showed
surprisingly similar features, despite entirely different sampling,
ionization, and detection techniques. 100% of the OOMs detected with
the UPLC-ESI Orbitrap mass spectrometer contained 2–8 isomers
with different fragmentation ions. Volatility estimation methods based
on elemental composition alone cannot account for chemical functionalities
and molecular structures and thus do not differentiate isomers, despite
the potential for large volatility differences. Our analysis shows
that biogenic OOM dimers can also form directly within the particle
phase, via either accretion or decomposition reactions. These particle-phase
reactions can affect the chemical composition and volatilities of
the OOMs, and, in turn, can affect the phase state and diffusivity
of aerosols. Our observations strongly imply the importance of considering
gas- and particle-phase chemistry in the growth of freshly formed
particles.