# OH-Initiated Photooxidation of Gas-Phase Atmospherically Relevant Monoterpene-Derived Organic Nitrates

## TL;DR

This study examines how organic nitrates from monoterpenes break down in the atmosphere, showing they contribute significantly to NOx recycling.

## Contribution

The study provides new rate constants and mechanisms for the photooxidation of monoterpene-derived organic nitrates.

## Key findings

- Photooxidation rate constants for MT-ONs range from 5.7 to 11.0 × 10–11 cm³ molecule–1 s–1.
- Up to 85% of 3°_ApHN contributes to NOx recycling, while 23% of 1°_BpHN does.
- CHO product prevalence depends on MT-ON structure and peroxy radical chemistry.

## Abstract

Monoterpene-derived organic nitrates (MT-ONs) can influence
NO
x
recycling, secondary organic aerosol,
and
ozone formation. While OH-initiated photooxidation is considered a
sink for MT-ONs, the rate constants and mechanisms remain poorly constrained.
We investigate the gas phase photooxidation of three synthetic ONs
derived from α-pinene, β-pinene, and d-limonene
(3°_ApHN, 1°_BpHN, and 2°_LmHN) through chamber experiments.
The photooxidation rate constants for MT-ONs range from (5.7 ±
0.5) to (11.0 ± 1.5) × 10–11 cm3 molecule–1 s–1, with corresponding
lifetimes of 1.7–3.2 h under ambient OH concentrations. If
we consider that products without a nitrooxy group (CHO) represent
MT-ONs undergoing photooxidation to form NO
x
or nitric acid, our measurements suggest that up to 85%, 49%,
and 23% of 3°_ApHN, 2°_LmHN, and 1°_BpHN, respectively,
contribute to NO
x
recycling. The prevalence
of CHO products is influenced by the molecular structure of MT-ONs
and different peroxy radical chemistry. These results are different
from prior studies that suggest that photooxidation leads to the complete
degradation of MT-ONs into NO
x
or nitric
acid. We propose photooxidation mechanisms, highlighting both OH addition
and H abstraction as important processes. This study provides fundamental
data to evaluate the contributions of MT-ON photooxidation to NO
x
recycling and offers new insights into the
broader implications of ON chemistry in the atmosphere.