Abstract
Abstract. Secondary organic aerosol (SOA) formation from photooxidation of α-pinene has been investigated in a photochemical reaction chamber under varied inorganic seed particle acidity levels at moderate relative humidity. The effect of particle acidity on SOA yield and chemical composition was examined under high- and low-NOx conditions. The SOA yield (4.2–7.6 %) increased nearly linearly with the increase in particle acidity under high-NOx conditions. In contrast, the SOA yield (28.6–36.3 %) was substantially higher under low-NOx conditions, but its dependency on particle acidity was insignificant. A relatively strong increase in SOA yield (up to 220 %) was observed in the first hour of α-pinene photooxidation under high-NOx conditions, suggesting that SOA formation was more effective for early α-pinene oxidation products in the presence of fresh acidic particles. The SOA yield decreased gradually with the increase in organic mass in the initial stage (approximately 0–1 h) under high-NOx conditions, which is likely due to the inaccessibility to the acidity over time with the coating of α-pinene SOA, assuming a slow particle-phase diffusion of organic molecules into the inorganic seeds. The formation of later-generation SOA was enhanced by particle acidity even under low-NOx conditions when introducing acidic seed particles after α-pinene photooxidation, suggesting a different acidity effect exists for α-pinene SOA derived from later oxidation stages. This effect could be important in the atmosphere under conditions where α-pinene oxidation products in the gas-phase originating in forested areas (with low NOx and SOx) are transported to regions abundant in acidic aerosols such as power plant plumes or urban regions. The fraction of oxygen-containing organic fragments (CxHyO1+ 33–35 % and CxHyO2+ 16–17 %) in the total organics and the O ∕ C ratio (0.52–0.56) of α-pinene SOA were lower under high-NOx conditions than those under low-NOx conditions (39–40, 17–19, and 0.61–0.64 %), suggesting that α-pinene SOA was less oxygenated in the studied high-NOx conditions. The fraction of nitrogen-containing organic fragments (CxHyNz+ and CxHyOzNp+) in the total organics was enhanced with the increases in particle acidity under high-NOx conditions, indicating that organic nitrates may be formed heterogeneously through a mechanism catalyzed by particle acidity or that acidic conditions facilitate the partitioning of gas-phase organic nitrates into particle phase. The results of this study suggest that inorganic acidity has a significant role to play in determining various organic aerosol chemical properties such as mass yields, oxidation state, and organic nitrate content. The acidity effect being further dependent on the timescale of SOA formation is also an important parameter in the modeling of SOA.
Highlights
Secondary organic aerosols (SOA) formed by oxidation of biogenic and anthropogenic volatile organic compounds (VOC) comprise a substantial portion of submicron aerosol particles in the atmosphere (Kanakidou et al, 2005; Zhang et al, 2007a)
The enhanced formation of SOA and organic sulfates has been reported from the acid-catalyzed reactive uptake of VOC oxidation products in ambient aerosols that are acidic enough to promote this multiphase chemistry (Hawkins et al, 2010; Lin et al, 2012a; Rengarajan et al, 2011; Zhang et al, 2012; Zhou et al, 2012), which is contrary to other field studies showing no apparent evidence of acid-catalyzed SOA formation (Peltier et al, 2007; Takahama et al, 2006; Tanner et al, 2009; Zhang et al, 2007b)
The dependence of SOA formation on aerosol acidity generally has not been incorporated in many atmospheric chemistry models far due to the large uncertainties associated with the quantification of acidity effects, with the exception of the acidity effect for SOA via isoprene epoxydiol uptake (Marais et al, 2016; Pye et al, 2013)
Summary
Secondary organic aerosols (SOA) formed by oxidation of biogenic and anthropogenic volatile organic compounds (VOC) comprise a substantial portion of submicron aerosol particles in the atmosphere (Kanakidou et al, 2005; Zhang et al, 2007a). Kristensen et al (2014) found that the increase of aerosol acidity has a negligible effect on SOA formation from ozonolysis of α-pinene under low-NOx conditions These inconsistent results reported previously are most likely attributed to the varied experimental parameters such as particle acidity, initial hydrocarbon concentration, oxidant type and level, NOx level, temperature, and relative humidity (RH). As a result of the above issues, it is highly important for laboratory studies to investigate the acidity effect on SOA formation under more realistic conditions approaching those of the ambient atmosphere. This would facilitate an accurate parameterization of the acidity effect for incorporation into air quality models. The potential significance of the observed acidity effect in the ambient atmosphere is summarized
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