Experimental Study of the Heterogeneous Interaction of SO₃ and H₂O:  Formation of Condensed Phase Molecular Sulfuric Acid Hydrates

Abstract

The interaction of SO(3) and H(2)O at low temperatures upon an inert surface has been studied with infrared spectroscopy and compared to the predictions of recent computational studies. At low temperatures and low water partial pressures, amorphous deposits of molecular H(2)SO(4) complexed with variable amounts of H(2)O in a ratio of between 1:1 and 2:1 are formed. Upon annealing, this material ejects water and converts first to a 1:1 H(2)SO(4).H(2)O complex and subsequently to anhydrous H(2)SO(4). Adding water to the amorphous molecular hydrate results in the formation of a new species, which on the basis of its thermal behavior and by comparison to theoretical predictions can be attributed to a molecular polymer with a repeat unit of (H(2)SO(4).(H(2)O)(2))(n)(). Implications of these observations for the initial stages of the formation of sulfate aerosol in the atmosphere and their surface reactivity are discussed.

Attenuated Total Reflection Fourier Transform Infrared Spectroscopy to Investigate Water Uptake and Phase Transitions in Atmospherically Relevant Particles

“…The absorption bands observed in the spectrum of (NH 4 ) 2 SO 4 are assigned in Table II. 16,51,52 Absorption bands associated with water being taken up by the salt increase as the ammonium sulfate is exposed to increasing relative humidity conditions (2,3,15,20,31,35,43,48,57,61,66,69,72,73,75,77,79,81,84, and 87% RH). As seen in Fig.…”

“…The reaction of SO 3 and H 2 O has also been studied with this technique. 15 In these studies, gas-phase SO 3 was allowed to react with water adsorbed on the ATR-FT-IR crystal so that a better understanding of sulfate aerosol formation in the atmosphere could be obtained. ATR-FT-IR spectroscopy has also recently been used to study air-liquid interfaces in conjunction with Raman spectroscopy and sum frequency generation to examine the difference between the water structure at the air-water-air interface compared to that of the bulk for a number of different atmospherically relevant aqueous salt solutions.…”

Dipole moment of the H2SO4‐H2O complex

“…In light of the importance of sulfuric acid–water clusters and the potential role of charge–dipole interactions in nucleation processes, we present in this work a determination of the dipole moment of the gas phase complex H 2 SO 4 ‐H 2 O. The adduct has been observed in cryogenic matrices [ Given et al , 1998; Couling et al , 2003a] and in aerosols nucleated from SO 3 and H 2 O [ Couling et al , 2003b]. A recent microwave study has determined its structure and provided the first high resolution spectroscopic work [ Fiacco et al , 2002].…”

The role of surface molecular hydrates in the heterogeneous interaction of NH3 with sulfuric acid monohydrate

“…1050 cm À1 . This has previously been identified 12 as a polymeric hydrate species with probable composition [H 2 SO 4 (H 2 O) 2 ] n (based in part on theoretical work by Ianni and Bandy 21,22 ). Bulk films of this material can be grown below 195 K, above which it converts irreversibly to bulk SAM.…”

“…9,10 Recent work to characterise the spectral features of SAM and other hydrates of sulfuric acid have also led to the production of a detailed database of ion and molecule fingerprints through which changes in surface and bulk structure in ultrathin films can be reliably identified. [11][12][13] Exposure of an acid to a base inevitably leads to the formation of a salt: here, the rate of such a reaction for a gas interacting with a condensed phase is monitored in order to identify surface and bulk changes. Several reactive components are identified on the basis of previously assigned spectral fingerprints.…”