Abstract
Abstract. 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 NOx 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.
Highlights
Biogenic secondary organic aerosol (BSOA) is formed through atmospheric oxidation and processing of naturally emitted volatile organic compounds (VOC), and comprises a major fraction of secondary organic aerosols (SOA) in ambient air (Hallquist et al, 2009)
Dimers have often been identified according to their molecular weight determined by mass spectrometry, but based on previously proposed structures (Table 1) we suggest a more clear nomenclature where the names reflect the components of the dimer esters according to present understanding of their molecular structures
In this work we report concentrations and diurnal variations of first- and second-generation oxidation products of α- and β-pinene in aerosol samples collected above a ponderosa pine plantation in California, USA, during the Biosphere Effects on Aerosols and Photochemistry Experiment (BEARPEX) 2007 and 2009
Summary
Biogenic secondary organic aerosol (BSOA) is formed through atmospheric oxidation and processing of naturally emitted volatile organic compounds (VOC), and comprises a major fraction of secondary organic aerosols (SOA) in ambient air (Hallquist et al, 2009). The first-generation oxidation products of monoterpenes constitute a complex group of primarily carbonyl compounds, carboxylic acids, and organic nitrates (e.g., Yu et al, 1999; Glasius et al, 2000; Larsen et al, 2001; Hallquist et al, 2009; Camredon et al, 2010; Perraud et al, 2010; Fry et al, 2009), which can undergo further atmospheric processing such as oxidation (e.g., Szmigielski et al, 2007; Muller et al, 2012), formation of organosulfates (e.g., Surratt et al, 2008; Iinuma et al, 2007) and oligomerization (e.g., Tolocka et al, 2004; Kalberer et al, 2004). Formation of oligomers, including dimers, from α-pinene ozonolysis products is an important SOA transformation process (e.g., Tolocka et al, 2004; Gao et al, 2004; Muller et al 2008; Putman et al, 2012) that has been estimated to contribute as much as 50 % to nonvolatile SOA mass (Hall IV and Johnston, 2011)
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