Direct Measurements of Covalently Bonded Sulfuric Anhydrides from Gas-Phase Reactions of SO 3 with Acids under Ambient Conditions.

Sulfur trioxide (SO ₃ ) is an important oxide of sulfur and a key intermediate in the formation of sulfuric acid (H ₂ SO ₄ , SA) in the Earth's atmosphere. This conversion to SA occurs rapidly due to the reaction of SO ₃ with a water dimer. However, gas-phase SO ₃ has been measured directly at concentrations that are comparable to that of SA under polluted mega-city conditions, indicating gaps in our current understanding of the sources and fates of SO ₃ . Its reaction with atmospheric acids could be one such fate that can have significant implications for atmospheric chemistry. In the present investigation, laboratory experiments were conducted in a flow reactor to generate a range of previously uncharacterized condensable sulfur-containing reaction products by reacting SO ₃ with a set of atmospherically relevant inorganic and organic acids at room temperature and atmospheric pressure. Specifically, key inorganic acids known to be responsible for most ambient new particle formation events, iodic acid (HIO ₃ , IA) and SA, are observed to react promptly with SO ₃ to form iodic sulfuric anhydride (IO ₃ SO ₃ H, ISA) and disulfuric acid (H ₂ S ₂ O ₇ , DSA). Carboxylic sulfuric anhydrides (CSAs) were observed to form by the reaction of SO ₃ with C ₂ and C ₃ monocarboxylic (acetic and propanoic acid) and dicarboxylic (oxalic and malonic acid)-carboxylic acids. The formed products were detected by a nitrate-ion-based chemical ionization atmospheric pressure interface time-of-flight mass spectrometer (NO ₃ ^- -CI-APi-TOF; NO ₃ ^- -CIMS). Quantum chemical methods were used to compute the relevant SO ₃ reaction rate coefficients, probe the reaction mechanisms, and model the ionization chemistry inherent in the detection of the products by NO ₃ ^- -CIMS. Additionally, we use NO ₃ ^- -CIMS ambient data to report that significant concentrations of SO ₃ and its acid anhydride reaction products are present under polluted, marine and polar, and volcanic plume conditions. Considering that these regions are rich in the acid precursors studied here, the reported reactions need to be accounted for in the modeling of atmospheric new particle formation.