Abstract. The formation of secondary organic aerosol (SOA) from both ozonolysis and hydroxyl radical (OH)-initiated oxidation of α-pinene under conditions of high nitric oxide (NO) concentrations with varying relative humidity (RH) and aerosol acidity was investigated in the University of North Carolina dual outdoor smog chamber facility. SOA formation from ozonolysis of α-pinene was enhanced relative to that from OH-initiated oxidation in the presence of initially high-NO conditions. However, no effect of RH on SOA mass was evident. Ozone (O 3 )-initiated oxidation of α-pinene in the presence of ammonium sulfate (AS) seed coated with organic aerosol from OH-initiated oxidation of α-pinene showed reduced nucleation compared to ozonolysis in the presence of pure AS seed aerosol.The chemical composition of α-pinene SOA was investigated by ultra-performance liquid chromatography/electrospray ionization high-resolution quadrupole timeof-flight mass spectrometry (UPLC/ESI-HR-Q-TOFMS), with a focus on the formation of carboxylic acids and highmolecular weight dimers. A total of eight carboxylic acids and four dimers were identified, constituting between 8 and 12 % of the total α-pinene SOA mass. OH-initiated oxidation of α-pinene in the presence of nitrogen oxides (NO x ) resulted in the formation of highly oxidized carboxylic acids, such as 3-methyl-1,2,3-butanetricarboxylic acid (MBTCA) and diaterpenylic acid acetate (DTAA). The formation of dimers was observed only in SOA produced from the ozonolysis of α-pinene in the absence of NO x , with increased concentrations by a factor of two at higher RH (50-90 %) relative to lower RH (30-50 %). The increased formation of dimers correlates with an observed increase in new particle formation at higher RH due to nucleation. Increased aerosol acidity was found to have a negligible effect on the formation of the dimers. SOA mass yield did not influence the chemical composition of SOA formed from α-pinene ozonolysis with respect to carboxylic acids and dimers.The results support the formation of the high-molecular weight dimers through gas-phase reactions of the stabilized Criegee Intermediate (sCI) formed from the ozonolysis of α-pinene. The high molecular weight and polar nature of dimers formed in the gas phase may explain increased particle number concentration as a result of homogenous nucleation. Since three of these dimers (i.e. pinyl-diaterpenyl dimer (MW 358), pinyl-diaterebyl dimer (MW 344) and pinonyl-pinyl dimer (MW 368)) have been observed in both laboratory-generated and ambient fine organic aerosol samples, we conclude that the dimers observed in this study can be used as tracers for the O 3 -initiated oxidation of α-pinene, and are therefore indicative of enhanced anthropogenic activities, and that the high molecular weight and low volatility dimers result in homogenous nucleation under laboratory conditions, increasing the particle number concentration.

This study investigates the contribution of high-molecular weight dimer esters to laboratory-generated α-pinene gas- and particle-phase secondary organic aerosol (SOA) and particulate matter (PM) collected at the Nordic boreal forest site of Hyytiälä, Finland. Laboratory flow reactor experiments (25 °C) show that dimer esters from ozonolysis of α-pinene contribute between 5 and 16% of the freshly formed α-pinene particle-phase SOA mass. An increased level of formation is observed at a higher relative humidity of ∼40%, and the presence of a hydroxyl radical (OH) scavenger is shown to affect the formation of dimer esters. Of the 28 dimer esters identified in laboratory α-pinene SOA, 15 are also observed in ambient PM samples, contributing between 0.5 and 1.6% of the total PM1. The observed esters show good correlation with known α-pinene SOA tracers in collected PM samples. This work reveals an, until now, unrecognized contribution of dimer esters from α-pinene oxidation to boreal forest PM.

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The formation of carboxylic acids and dimer esters from α-pinene oxidation was investigated in a smog chamber and in ambient aerosol samples collected during the Biosphere Effects on Aerosols and Photochemistry Experiment (BEARPEX). Chamber experiments of α-pinene ozonolysis in dry air and at low NO_x concentrations demonstrated formation of two dimer esters, pinyl-diaterpenyl (MW 358) and pinonyl-pinyl dimer ester (MW 368), under both low- and high-temperature conditions. Concentration levels of the pinyl-diaterpenyl dimer ester were lower than the assumed first-generation oxidation products cis-pinic and terpenylic acids, but similar to the second-generation oxidation products 3-methyl-1,2,3-butane tricarboxylic acid (MBTCA) and diaterpenylic acid acetate (DTAA). Dimer esters were observed within the first 30 min, indicating rapid production simultaneous to their structural precursors. However, the sampling time resolution precluded conclusive evidence regarding formation from gas- or particle-phase processes. CCN activities of the particles formed in the smog chamber displayed a modest variation during the course of experiments, with κ values in the range 0.06–0.09 (derived at a supersaturation of 0.19%).

The pinyl-diaterpenyl dimer ester was also observed in ambient aerosol samples collected above a ponderosa pine forest in the Sierra Nevada Mountains of California during two seasonally distinct field campaigns in September 2007 and July 2009. The pinonyl-pinyl ester was observed for the first time in ambient air during the 2009 campaign, and although present at much lower concentrations, it was correlated with the abundance of the pinyl-diaterpenyl ester, suggesting similarities in their formation. The maximum concentration of the pinyl-diaterpenyl ester was almost 10 times higher during the warmer 2009 campaign relative to 2007, while the concentration of cis-pinic acid was approximately the same during both periods, and lack of correlation with levels of cis-pinic and terpenylic acids for both campaigns indicate that the formation of the pinyl-diaterpenyl ester was not controlled by their ambient abundance. In 2009 the concentration of the pinyl-diaterpenyl ester was well correlated with the concentration of DTAA, a supposed precursor of diaterpenylic acid, suggesting that the formation of pinyl-diaterpenyl dimer was closely related to DTAA. Generally, the pinyl-diaterpenyl ester was found at higher concentrations under higher temperature conditions, both in the smog-chamber study and in ambient air aerosol samples, and exhibited much higher concentrations at night relative to daytime in line with previous results.

We conclude that analysis of pinyl dimer esters provides valuable information on pinene oxidation processes and should be included in studies of formation and photochemical aging of biogenic secondary organic aerosols, especially at high temperatures