Using GECKO-A to derive mechanistic understanding of secondary organic aerosol formation from the ubiquitous but understudied camphene
Authors: Afreh Isaac Kwadjo , Aumont Bernard , Camredon Marie , Barsanti Kelley Claire 
Published: July 2021

Abstract. Camphene, a dominant monoterpene emitted from both biogenic and pyrogenic
sources, has been significantly understudied, particularly in regard to
secondary organic aerosol (SOA) formation. When camphene represents a
significant fraction of emissions, the lack of model parameterizations for
camphene can result in inadequate representation of gas-phase chemistry and
underprediction of SOA formation. In this work, the first mechanistic study of SOA formation from camphene was performed using the Generator for Explicit Chemistry and Kinetics of Organics in the Atmosphere (GECKO-A). GECKO-A was used to generate gas-phase chemical mechanisms for camphene and two well-studied monoterpenes, α-pinene and limonene, as well as to predict SOA
mass formation and composition based on gas/particle partitioning theory. The
model simulations represented observed trends in published gas-phase reaction
pathways and SOA yields well under chamber-relevant photooxidation and dark
ozonolysis conditions. For photooxidation conditions, 70 % of the
simulated α-pinene oxidation products remained in the gas phase
compared to 50 % for limonene, supporting model predictions and
observations of limonene having higher SOA yields than α-pinene under
equivalent conditions. The top 10 simulated particle-phase products in the
α-pinene and limonene simulations represented 37 %–50 % of
the SOA mass formed and 6 %–27 % of the hydrocarbon mass reacted. To
facilitate comparison of camphene with α-pinene and limonene, model
simulations were run under idealized atmospheric conditions, wherein the
gas-phase oxidant levels were controlled, and peroxy radicals reacted equally
with HO2 and NO. Metrics for comparison included gas-phase
reactivity profiles, time-evolution of SOA mass and yields, and
physicochemical property distributions of gas- and particle-phase
products. The controlled-reactivity simulations demonstrated that (1)
in the early stages of oxidation, camphene is predicted to form very low-volatility products, lower than α-pinene and limonene, which condense
at low mass loadings; and (2) the final simulated SOA yield for camphene
(46 %) was relatively high, in between α-pinene (25 %) and
limonene (74 %). A 50 % α-pinene + 50 % limonene mixture was then used as a surrogate to represent SOA formation from camphene; while simulated SOA mass and yield were well represented, the volatility distribution of the particle-phase products was not. To demonstrate the potential importance of including a parameterized representation of SOA formation by camphene in air quality models, SOA mass and yield were predicted for three wildland fire fuels based on measured monoterpene distributions and published SOA parameterizations for α-pinene and limonene. Using the 50/50 surrogate mixture to represent camphene increased predicted SOA mass by 43 %–50 % for black spruce and by 56 %–108 % for Douglas fir. This first detailed modeling study of the gas-phase oxidation of camphene and subsequent SOA formation highlights opportunities for future measurement–model comparisons and lays a foundation for developing chemical mechanisms and SOA parameterizations for camphene that are suitable for air quality modeling.