The effect of particle acidity on secondary organic aerosol formation from iα/i-pinene photooxidation under atmospherically relevant conditions

摘要

Secondary organic aerosol (SOA) formation from photooxidation of iα/i-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-NOsubix/i/sub conditions. The SOA yield (4.2–7.6?%) increased nearly linearly with the increase in particle acidity under high-NOsubix/i/sub conditions. In contrast, the SOA yield (28.6–36.3?%) was substantially higher under low-NOsubix/i/sub 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 iα/i-pinene photooxidation under high-NOsubix/i/sub conditions, suggesting that SOA formation was more effective for early iα/i-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-NOsubix/i/sub conditions, which is likely due to the inaccessibility to the acidity over time with the coating of iα/i-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-NOsubix/i/sub conditions when introducing acidic seed particles after iα/i-pinene photooxidation, suggesting a different acidity effect exists for iα/i-pinene SOA derived from later oxidation stages. This effect could be important in the atmosphere under conditions where iα/i-pinene oxidation products in the gas-phase originating in forested areas (with low NOsubix/i/sub and SOsubix/i/sub) are transported to regions abundant in acidic aerosols such as power plant plumes or urban regions. The fraction of oxygen-containing organic fragments (Csubix/i/subHsubiy/i/subOsub1/subsup+/sup 33–35?% and Csubix/i/subHsubiy/i/subOsub2/subsup+/sup 16–17?%) in the total organics and the O?∕?C ratio (0.52–0.56) of iα/i-pinene SOA were lower under high-NOsubix/i/sub conditions than those under low-NOsubix/i/sub conditions (39–40, 17–19, and 0.61–0.64?%), suggesting that iα/i-pinene SOA was less oxygenated in the studied high-NOsubix/i/sub conditions. The fraction of nitrogen-containing organic fragments (Csubix/i/subHsubiy/i/subNsubiz/i/subsup+/sup and Csubix/i/subHsubiy/i/subOsubiz/i/subNsubip/i/subsup+/sup) in the total organics was enhanced with the increases in particle acidity under high-NOsubix/i/sub 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