Pressure and Temperature Dependence of the Gas-Phase Reaction of SO₃ with H₂O and the Heterogeneous Reaction of SO₃ with H₂O/H₂SO₄ Surfaces

Abstract

The gas-phase reaction of SO3 with H2O and the heterogeneous reaction of SO3 with H2O−H2SO4 surfaces have been studied in a fast flow reactor coupled to a chemical ionization mass spectrometer (CIMS) for species detection. The gas-phase reaction was studied under turbulent flow conditions over the pressure range from 100 to 760 Torr N2 and the temperature range from 283 to 370 K. The loss rate of SO3 was measured under pseudo-first-order conditions; it exhibits a second-order dependence on water vapor concentration and has a strong negative temperature dependence. The first-order rate coefficient for the SO3 loss by gas-phase reaction shows no significant pressure dependence and can be expressed as k I(s-1) = 3.90 × 10-41 exp(6830.6/T)[H2O]2 where [H2O] is in units of molecule cm-3 and T is in Kelvin. The overall uncertainty of our experimentally determined rate coefficients is estimated to be ±20%. At sufficiently low SO3 concentrations (<1012 molecule cm-3) the rate coefficient is independent of the initial SO3 level, as expected for a gas-phase reaction mechanism involving one SO3 and two H2O molecules. However, at higher concentrations and lower temperatures, increased rate coefficients were observed, indicating a fast heterogeneous reaction after the onset of binary homogeneous nucleation of acid hydrate clusters leading to particle formation, which was verified by light-scattering experiments. The heterogeneous loss of SO3 to the reactor walls has also been investigated under low pressure (1.1−12.5 Torr) laminar flow conditions. The loss rate is highly dependent on the humidity of the surface. In the presence of excess water the reactive sticking coefficient approaches unity and the wall loss rate is gas diffusion limited; under dry conditions it approaches zero, as expected. The atmospheric implications of the homogeneous and heterogeneous SO3−water reaction are discussed.

Evaluated kinetic and photochemical data for atmospheric chemistry: Volume I - gas phase reactions of O&lt;sub&gt;x&lt;/sub&gt;, HO&lt;sub&gt;x&lt;/sub&gt;, NO&lt;sub&gt;x&lt;/sub&gt; and SO&lt;sub&gt;x&lt;/sub&gt; species

“…Wang et al (1989) obtained an upper limit to the rate coefficient which is more than two orders of magnitude lower than the value of Castleman et al (1975) by treatment of the flow tube walls to reduce wall effects, and the studies of Reiner et al (1993;1994), using the laminar flow tube method, obtained the lowest values for the rate constant. The flow studies of Kolb et al (1994), and Jayne et al (1997) Reliability log k = ±0.2 at 298 K.…”

SO&lt;sub&gt;2&lt;/sub&gt; oxidation products other than H&lt;sub&gt;2&lt;/sub&gt;SO&lt;sub&gt;4&lt;/sub&gt; as a trigger of new particle formation. Part 1: Laboratory investigations

“…The latest recommendation for atmospheric chemistry (Atkinson et al, 2004) (Payne et al, measured in the presence of 20 torr H 2 O, the other studies have been performed under dry conditions), the impact of this pathway producing OH radicals and SO 3 was investigated. SO 3 produces H 2 SO 4 in a fast consecutive hydrolysis step (Jayne et al, 1997).…”

“…Assuming for the reaction of SO 3 with NO and NO 2 the rate coefficient measured for the NO 2 reaction (Penzhorn and Canosa, 1983), the SO 3 life-times regarding the NO and NO 2 reaction are in the order of 80 days. That means that also the hydrolysis of SO 3 via pathway (7) (with a life-time of ∼20 microseconds assuming a H 2 O concentration of 3×10 17 molecule cm −3 (Jayne et al, 1997)) cannot be affected by NO and NO 2 . Consequently, NO and NO 2 for the concentrations considered here, cannot affect H 2 SO 4 formation via pathways (5)-(7).…”

Water Dimers in the Atmosphere II: Results from the VRT(ASP-W)III Potential Surface

“…Proposed contexts include the excess atmospheric absorption of solar radiation, [1][2][3] which has been ascribed to the water dimer, "the water continuum" absorption in the far-infrared, 4 and catalysis of the formation of atmospheric H 2 SO 4 , 5,6 which has has been confirmed by experimental results in the laboratory. 7,8 In addition, the abundance of water dimers in the atmosphere may be important for modeling water condensation 9 and for the formation of radical complexes such as HO 2 ‚H 2 O, which have been predicted to exist in relatively high concentrations in the atmosphere. 10,11 As shown in the seminal work by Vaida et al, 12 the search for such a water dimer spectroscopic signature in the atmosphere requires a highly accurate determination of its concentrations.…”