Direct Measurements of Covalently Bonded Sulfuric Anhydrides from Gas-Phase Reactions of SO3 with Acids under Ambient Conditions

Sulfur trioxide (SO₃) is an important oxide of sulfur and a key intermediate in the formation of sulfuric acid (H₂SO₄, SA) in the Earth's atmosphere. This conversion to SA occurs rapidly due to the reaction of SO₃ with a water dimer. However, gas-phase SO₃ 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 SO₃. 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 SO₃ 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 (HIO₃, IA) and SA, are observed to react promptly with SO₃ to form iodic sulfuric anhydride (IO₃SO₃H, ISA) and disulfuric acid (H₂S₂O₇, DSA). Carboxylic sulfuric anhydrides (CSAs) were observed to form by the reaction of SO₃ with C₂ and C₃ 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 (NO₃^--CI-APi-TOF; NO₃^--CIMS). Quantum chemical methods were used to compute the relevant SO₃ reaction rate coefficients, probe the reaction mechanisms, and model the ionization chemistry inherent in the detection of the products by NO₃^--CIMS. Additionally, we use NO₃^--CIMS ambient data to report that significant concentrations of SO₃ 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.