Dimers in α-pinene secondary organic aerosol: effect of hydroxyl radical, ozone, relative humidity and aerosol acidity

摘要

The formation of secondary organic aerosol (SOA) from both ozonolysis andhydroxyl radical (OH)-initiated oxidation of α-pinene underconditions of high nitric oxide (NO) concentrations with varying relativehumidity (RH) and aerosol acidity was investigated in the University ofNorth Carolina dual outdoor smog chamber facility. SOA formation fromozonolysis of α-pinene was enhanced relative to that fromOH-initiated oxidation in the presence of initially high-NO conditions.However, no effect of RH on SOA mass was evident. Ozone (O)-initiatedoxidation of α-pinene in the presence of ammonium sulfate (AS) seedcoated with organic aerosol from OH-initiated oxidation of α-pineneshowed reduced nucleation compared to ozonolysis in the presence of pure ASseed aerosol.The chemical composition of α-pinene SOA was investigated byultra-performance liquid chromatography/electrospray ionizationhigh-resolution quadrupole time-of-flight mass spectrometry(UPLC/ESI-HR-Q-TOFMS), with a focus on the formation of carboxylic acids andhigh-molecular weight dimers. A total of eight carboxylic acids and fourdimers were identified, constituting between 8 and 12% of the totalα-pinene SOA mass. OH-initiated oxidation of α-pinene in thepresence of nitrogen oxides (NO) resulted in the formation of highlyoxidized carboxylic acids, such as 3-methyl-1,2,3-butanetricarboxylic acid(MBTCA) and diaterpenylic acid acetate (DTAA). The formation of dimers wasobserved only in SOA produced from the ozonolysis of α-pinene in theabsence of NO, with increased concentrations by a factor of two athigher RH (50–90%) relative to lower RH (30–50%). The increasedformation of dimers correlates with an observed increase in new particleformation at higher RH due to nucleation. Increased aerosol acidity wasfound to have a negligible effect on the formation of the dimers. SOA massyield 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 dimersthrough gas-phase reactions of the stabilized Criegee Intermediate (sCI)formed from the ozonolysis of α-pinene. The high molecular weightand polar nature of dimers formed in the gas phase may explain increasedparticle number concentration as a result of homogenous nucleation. Sincethree of these dimers (i.e. pinyl-diaterpenyl dimer (MW 358),pinyl-diaterebyl dimer (MW 344) and pinonyl-pinyl dimer (MW 368)) have beenobserved in both laboratory-generated and ambient fine organic aerosolsamples, we conclude that the dimers observed in this study can be used astracers for the O-initiated oxidation of α-pinene, and aretherefore indicative of enhanced anthropogenic activities, and that the highmolecular weight and low volatility dimers result in homogenous nucleationunder laboratory conditions, increasing the particle number concentration.