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  1. Abstract. While camphene is one of the dominant monoterpenesmeasured in biogenic and pyrogenic emission samples, oxidation of camphenehas not been well-studied in environmental chambers and very little is knownabout its potential to form secondary organic aerosol (SOA). The lack ofchamber-derived SOA data for camphene may lead to significant uncertaintiesin predictions of SOA from oxidation of monoterpenes using existingparameterizations when camphene is a significant contributor to totalmonoterpenes. Therefore, to advance the understanding of camphene oxidationand SOA formation and to improve representation of camphene in air qualitymodels, a series of experiments was performed in the University ofCalifornia Riverside environmental chamber to explore camphene SOA massyields and properties across a range of chemical conditions atatmospherically relevant OH concentrations. The experimental results werecompared with modeling simulations obtained using two chemically detailedbox models: Statewide Air Pollution Research Center (SAPRC) and Generatorfor Explicit Chemistry and Kinetics of Organics in the Atmosphere (GECKO-A).SOA parameterizations were derived from the chamber data using both thetwo-product and volatility basis set (VBS) approaches. Experiments performedwith added nitrogen oxides (NOx) resulted in higher SOA mass yields (upto 64 %) than experiments performed without added NOx (up to 28 %).In addition, camphene SOA mass yields increased with SOA mass (Mo) atlower mass loadings, but a threshold was reached at higher mass loadings inwhich the SOA mass yields no longer increased with Mo. SAPRC modelingof the chamber studies suggested that the higher SOA mass yields at higherinitial NOx levels were primarily due to higher production of peroxyradicals (RO2) and the generation of highly oxygenated organicmolecules (HOMs) formed through unimolecular RO2 reactions. SAPRCpredicted that in the presence of NOx, camphene RO2 reacts with NOand the resultant RO2 undergoes hydrogen (H)-shift isomerizationreactions; as has been documented previously, such reactions rapidly addoxygen and lead to products with very low volatility (i.e., HOMs). The endproducts formed in the presence of NOx have significantly lowervolatilities, and higher O : C ratios, than those formed by initial campheneRO2 reacting with hydroperoxyl radicals (HO2) or other RO2.Further analysis reveals the existence of an extreme NOx regime, whereinthe SOA mass yield can be suppressed again due to high NO / HO2 ratios.Moreover, particle densities were found to decrease from 1.47 to 1.30 g cm−3 as [HC]0 / [NOx]0 increased and O : C decreased. Theobserved differences in SOA mass yields were largely explained by thegas-phase RO2 chemistry and the competition between RO2+HO2, RO2+ NO, RO2+ RO2, and RO2 autoxidationreactions. 
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