Recent advances in our understanding of atmospheric chemistry and climate made possible by on-line aerosol analysis instrumentation.

Sulfate particles play a key role in the air quality and the global climate, but the heterogeneous formation mechanism of sulfates on surfaces of atmospheric particles is not well established. Carbonates, which act as a reactive component in mineral dust due to their special chemical properties, may contribute significantly to the sulfate formation by heterogeneous processes. This paper presents a study on the oxidation of SO 2 by O 3 on CaCO 3 particles. Using Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS), the formation of sulfite and sulfate on the surface was identified, and the roles of O 3 and water in oxidation processes were determined. The results showed that in the presence of O 3 , SO 2 can be oxidized to sulfate on the surface of CaCO 3 particles. The reaction is first order in SO 2 and zero order in O 3 . The reactive uptake coefficient for SO 2 [(0.6–9.8)×10 14 molecule cm -3 ] oxidation by O 3 [(1.2–12)×10 14 molecule cm -3 ] was determined to be (1.4±0.3)×10 -7 using the BET area as the reactive area and (7.7±1.6)×10 -4 using the geometric area. A two-stage mechanism that involves adsorption of SO 2 followed by O 3 oxidation is proposed and the adsorption of SO 2 on the CaCO 3 surface is the rate-determining step. The proposed mechanism can well explain the experiment results. The atmospheric implications were explored based on a box model calculation. It was found that the heterogeneous reaction might be an important pathway for sulfate formation in the atmosphere.

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Kinetics and mechanism of heterogeneous oxidation of sulfur dioxide by ozone on surface of calcium carbonate

The issue of the acid ionization of HCl at the surface of stratospheric ice particles, which is relevant in connection with Antarctic stratospheric ozone depletion, is studied theoretically. This study extends that of Gertner and Hynes (Science, 1996, 271, 1563) by examination of 12 new sites and by quantization of the transferring proton's nuclear motion. Fourteen initial adsorption sites for the HCl are selected via arguments based on HCl incorporation mechanisms for a dynamic ice surface, for the basal-plane and prism face surfaces of ice. Acid ionization at 190 K in those sites is examined via combined electronic structure and classical molecular dynamics methodologies to estimate the reaction Gibbs energies and barriers. In each case examined, the acid ionization step of proton transfer from the HCl to a neighboring water molecule is estimated to be thermodynamically feasible and kinetically rapid.

Model molecular dynamics simulation of hydrochloric acid ionization at the surface of stratospheric ice

Heterogeneous reaction of SO2 on mineral dust seems to be an important sink for SO2. However, kinetic data about this reaction on authentic mineral dust are scarce and are mainly limited to low relative humidity (RH) conditions. In addition, little is known about the role of hydrogen peroxide (H2O2) in this reaction. Here, we investigated the uptake kinetics of SO2 on three authentic mineral dusts (i.e., Asian mineral dust (AMD), Tengger desert dust (TDD), and Arizona test dust (ATD)) in the absence and presence of H2O2 at different RHs using a filter-based flow reactor, and applied a parameter (effectiveness factor) to the estimation of the effective surface area of particles for the calculation of the corrected uptake coefficient (γc). We found that with increasing RH, the γc decreases on AMD particles, but increases on ATD and TDD particles. This discrepancy is probably due to the different mineralogy compositions and aging extents of these dust samples. Furthermore, the presence of H2O2 can promote th...

Kinetics of Heterogeneous Reaction of Sulfur Dioxide on Authentic Mineral Dust: Effects of Relative Humidity and Hydrogen Peroxide

The liquid-solid phase transitions of emulsified binary HNO{sub 3}/H{sub 2}O and ternary HNO{sub 3}/H{sub 2}SO{sub 4}/H{sub 2}O acid solutions have been investigated using differential scanning calorimetry in order to elucidate the mechanism of ice formation from stratospheric aerosols. The results indicate that binary solution droplets with less than 33 wt % HNO{sub 3} freeze to form ice at temperatures 37--65 K below the equilibrium ice melting temperatures. Droplets with higher concentrations (up to 65 wt %) do not freeze; instead, they form glasses at temperatures below 160 K. The results also show that the presence of H{sub 2}SO{sub 4} in amounts corresponding to less than 5 wt % depresses the freezing points of the 0--33 wt % HNO{sub 3} droplets by 5 to >30 K, in agreement with earlier observations. The implications of the new data for an understanding of the formation of polar stratospheric clouds have been investigated using a thermodynamic model. The results indicate that an ice saturation ratio of greater than 1.65 is required for ice particles to form from ternary aerosols at stratospheric temperatures, corresponding to a cooling of about 3 K below the ice frost point.

Phase transitions in emulsified HNO3/H2O and HNO3/H2SO4/H2O solutions

This paper reviews two physical chemistry topics related to atmospheric chemistry. First, we describe semiempirical models, based on thermochemistry and transition state theory, which are employed to estimate gas phase reaction rates over a wide range of temperatures and pressures. We also review briefly the experimental techniques utilized for measurements of elementary reaction rate constants, which provide the primary input to these models. We then address chemical reactions which take place in the atmosphere on the surface of solid, ice-like aerosol particles, discussing some current views on the mechanisms of these reactions and describing some laboratory techniques for the study of these heterogeneous processes.

Mario J. Molina and

Papers

Environmental Chemistry (Gas and Gas−Solid Interactions): The Role of Physical Chemistry