Abstract
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 (O3)-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 time-of-flight mass spectrometry (UPLC/ESI-HR-Q-TOFMS), with a focus on the formation of carboxylic acids and high-molecular 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 (NOx) 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 NOx, 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 O3-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.
Highlights
The formation of biogenic secondary organic aerosol (BSOA) through atmospheric processing of naturally emitted biogenic volatile organic compounds (BVOCs) comprises a major fraction of secondary organic aerosol (SOA) in the Published by Copernicus Publications on behalf of the European Geosciences Union.K
We investigate the effect of oxidant species (OH vs. O3), relative humidity (RH) and seed aerosol acidity in order to obtain a better understanding of the conditions leading to the formation of the dimers and how these parameters may affect the formation and chemical composition of SOA
We propose that the α-pinene-derived dimers may arise via gas-phase reactions involving stabilized Criegee intermediates (CI) from the ozonolysis of α-pinene rather than esterification in the aerosol phase, which is kinetically inconsistent with the experimentally observed timescale for dimer formation
Summary
The formation of biogenic secondary organic aerosol (BSOA) through atmospheric processing of naturally emitted biogenic volatile organic compounds (BVOCs) comprises a major fraction of secondary organic aerosol (SOA) in the Published by Copernicus Publications on behalf of the European Geosciences Union. Species having compositions corresponding to structures tentatively identified as higher molecular weight esters originating from α- and β-pinene have been observed in both laboratory-generated and ambient SOA (Müller et al, 2008, 2009; Camredon et al, 2010; Yasmeen et al, 2010; Gao et al, 2010; Kristensen et al, 2013) These compounds have been referred to as esters based on the assumed formation by liquid phase esterification or their structure, but in this work we use the term dimers. Using on-line atmospheric pressure chemical ionization mass spectrometry (APCI-MS) with high time resolution, Müller et al (2008) observed that the intensity of dimers increased more rapidly than the intensity of their monomer precursors in freshly nucleated aerosol in the α-pinene / O3 system This result led to the suggestion that gas-phase dimer formation could be important for homogeneous nucleation, the exact reaction mechanism has remained elusive. We propose that the α-pinene-derived dimers may arise via gas-phase reactions involving stabilized Criegee intermediates (CI) from the ozonolysis of α-pinene rather than esterification in the aerosol phase, which is kinetically inconsistent with the experimentally observed timescale for dimer formation
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