Organosulfates and organic nitrates are widely abundant secondary organic aerosols (SOAs). Alpha pinene, a common monoterpene emits from plants is a precursor of these SOA formations. While the gas-phase photochemical reactions of alpha-pinene are widely understood, our knowledge of its’ nighttime chemistry on mineral dust surfaces remains scarce. In this study, transmission Fourier Transform infrared spectroscopy (t-FTIR) and High-Resolution Mass Spectroscopy (HRMS) were used to study the reactions of alpha-pinene with SO2 and NO2 gases on hematite surfaces. Alpha-pinene adsorbed reversibly on hematite, a major reactive component in mineral dust. Pre-adsorbed SO2 on hematite reacts with gas-phase alpha-pinene to produce organosulfates C10H17O3S (m/z = 217) and C10H17O4S (m/z = 233) in the absence of photoirradiation and water vapor. Further, these organosulfate formations involve the adsorbed sulfites and surface hydroxyl groups on hematite and are irreversibly adsorbed to the surface. Reactions of pre-adsorbed NO2 on hematite with alpha-pinene in similar conditions suggest the formation of oxidized pinene products such as alcohols and carboxylic acids (3429 cm-1, 3615 cm-1, 1081 cm-1) and nitro compounds (1500 – 1580 cm-1). In contrast to this, when gas phase NO2 was allowed to react with pre-adsorbed alpha-pinene on hematite, suggests the formation of carbonyl compounds. This observation suggests the role of the hematite surface in different product formation pathways. These alpha-pinene products with NO2 are irreversibly adsorbed to the surface. Moreover, our studies indicate these alpha-pinene product formations occur only in the presence of the surface, giving insights on surface-driven SOA formation in industrialized areas.

Reactions of alpha-pinene with SO2 and NO2 on hematite surface

ACS Spring 2022

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