Abstract
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.
Highlights
IntroductionSources of atmospheric monoterpene (C10H16) emissions are diverse and include biogenic sources (Geron et al, 2000; Guenther et al, 1995; Hayward et al, 2001; Kesselmeier and Staudt, 1999; Kim et al, 2010; Ludley et al, 2009; Maleknia et al, 2007; Rinne et al, 2000; Steinbrecher et al, 1999; Tani et al, 2003; White et al, 2008), as well as pyrogenic sources (Akagi et al, 2011, 2013; Gilman et al, 2015; Hatch et al, 2015; Simpson et al, 2011)
For limonene photooxidation, experimental data show a linear trend in the secondary organic aerosol (SOA) yield as a function of SOA mass, and the SOA yield does not plateau at higher SOA mass loadings
At SOA mass < 100 μg m−3, the modeled SOA yield is within range of the observations, towards the lowest values; between SOA mass > 100 and < 200 μg m−3, the modeled SOA yield is lower than the observations
Summary
Sources of atmospheric monoterpene (C10H16) emissions are diverse and include biogenic sources (Geron et al, 2000; Guenther et al, 1995; Hayward et al, 2001; Kesselmeier and Staudt, 1999; Kim et al, 2010; Ludley et al, 2009; Maleknia et al, 2007; Rinne et al, 2000; Steinbrecher et al, 1999; Tani et al, 2003; White et al, 2008), as well as pyrogenic sources (Akagi et al, 2011, 2013; Gilman et al, 2015; Hatch et al, 2015; Simpson et al, 2011). Studies across biogenic source types (e.g., terrestrial vegetation, soil, and marine) typically include up to 14 individual monoterpenes, with α-pinene, β-pinene, camphene, 3-carene, limonene, myrcene, p-ocimene, and sabinene being the most widely reported and having the highest emissions (Ambrose et al, 2010; Bäck et al, 2012; Fehsenfeld et al, 1992; Geron et al, 2000; Hayward et al, 2001; Rinne et al, 2000; White et al, 2008; Yassaa et al, 2008). The identities and quantities of monoterpenes from pyrogenic sources (e.g., biomass burning) vary as a function of plant species and fuel component (Hatch et al, 2019). 30 monoterpene isomers have been observed from biomass burning sources, with α-pinene, β-pinene, camphene, 3-carene, limonene, and myrcene being commonly detected (Akagi et al, 2013; Gilman et al, 2015; Hatch et al, 2015)
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