# Secondary Organic Aerosol from OH Oxidation of Acyclic Terpenes Is More Viscous and Less Volatile than That of Their Cyclic Analogs

## TL;DR

This study shows that aerosols from acyclic terpenes are more viscous and less volatile than those from cyclic terpenes, affecting climate predictions.

## Contribution

The study reveals new insights into the chemical and physical properties of SOA from acyclic terpenes under plant stress conditions.

## Key findings

- Acyclic terpene SOA has higher viscosity and lower volatility than cyclic terpene SOA.
- Acyclic terpene SOA retains more volume after evaporation compared to cyclic terpene SOA.
- Acyclic terpene SOA contains more extremely low-volatility compounds than cyclic terpene SOA.

## Abstract

Biogenic volatile organic compounds (BVOCs), a dominant
source
of secondary organic aerosol (SOA) globally, exhibit emission rates
and compositions that are plant species-specific and vary with environmental
stressors. A common outcome of plant stress is increased emissions
of acyclic terpenes. The paucity of information about acyclic terpene
SOA chemistry contributes to uncertainties in predictions of SOA global
loadings and impacts on Earth’s radiative budget, particularly
in a changing climate where acyclic terpene emissions could become
more prominent. This study compared properties of SOA derived from
hydroxyl radical (OH) oxidation of acyclic and cyclic monoterpenes
(β-ocimene, α-pinene) and sesquiterpenes (β-farnesene,
β-caryophyllene). Single-particle mass spectrometry was used
for assessing shape, density, and evaporation kinetics of size-selected
SOA particles, and nanospray desorption electrospray ionization high-resolution
mass spectrometry (nano-DESI-HRMS) was used to measure the molecular
composition of SOA. Acyclic terpene SOA exhibited higher viscosity
and lower volatility compared to cyclic terpene SOA, and had a greater
volume fraction remaining (VFR) after ∼24 h of evaporationapproximately
1.3–1.6 times higher VFR than that of cyclic terpene SOA. Additionally,
HRMS analysis revealed greater chemical diversity and higher fractions
of extremely low-volatility compounds (56–62% ELVOC/LVOC) in
acyclic terpene SOA compared to cyclic counterparts (25–37%
ELVOC/LVOC). Our findings highlight the potential importance of accounting
for acyclic terpene aerosol chemistry under conditions of plant stress
to improve predictions of SOA loadings and impacts.