Gassurface Reactions

A number of important chemical reactions in the atmosphere involve a gas molecule striking the surface of an airborne particle. For gas molecules A in three-dimensional random motion, the number of molecules of A striking a unit area per unit time is

where nA is the gas-phase concentration of A and vA is the mean speed of the A molecules:

\nmAJ

Then the number of collisions per second with a single spherical particle of radius Rp is (|"ava)(4h/?p). Usually one is interested in reaction occurring with an ensemble of particles, all of different sizes. If the particle population has a total surface area per unit volume of air of Ai,(cm2cm-3), then the total number of collisions of A molecules with particles is (jnAvA)Ap.

When a gas molecule strikes the surface of a particle, usually not every collision leads to reaction. One can define a reaction efficiency or uptake coefficient y as the probability of reaction, y is usually determined experimentally as the ratio of the number of collisions that result in reaction to the theoretical total number of collisions, and it depends in general on temperature and particle type. Therefore, the rate of the heterogeneous reaction is expressed as pseudo-first-order

where | yvAAp is the first-order rate coefficient.

An important heterogeneous reaction in the atmosphere is that of gaseous N205 with water molecules on the surface of atmospheric particles:

The ionic state of H20 on the particle allows this reaction to proceed. For this reaction y depends on the particie type. This reaction will play an important role in both the stratosphere and troposphere, and we defer a calculation of its rate until these chapters.

Sources of Kinetic Data for Atmospheric Chemistry Two major sources of evaluated kinetic and photochemical data for atmospheric chemistry are

Sander et al. (2003) NASA Panel for Data Evaluation

Publication from the Jet Propulsion Laboratory, Pasadena. CA htt p ://j pldatae val.jpl.nasa.gov/

Atkinson et al. (2004)

IUPAC Subcommittee on Gas Kinetic Data Evaluation for Atmospheric Chemistry http://www.iupac-kinetic.ch.cam. ac.uk/summary/ IUPACsu mm_web_latest.pdf

Termoiecuiar Reactions that Lead to Thermally Unstable Products Several termolecular atmospheric reactions lead to products that can thermally decompose back to the reactants at atmospheric temperatures. Two important reactions of this type are formation of N205 and CH3C(0)02N02:

N02 + N03 + M s=i N2Os + M CH?C(0)02 + N02 + M m CH3C(0)02N02 -I- M

N205 plays an important role in both stratospheric and tropospheric chemistry. The CH3C{0)02N02 molecule, namely, peroxyacetyl nitrate and abbreviated PAN, is influential in tropospheric chemistry. We will study the chemistry of these two specics in Chapters 5 and 6.

Because both forward and reverse reactions involve the participation of air molecules (M). the reaction rate coefficients for the forward and reverse reactions are represented by the general tcrmolecular form (3.25). The reverse (decomposition) reactions in these types of reactions are typically highly temperature-dependent. Reaction rate coefficients for these two reactions are given in both the IUPAC (Atkinson et al. 2004) and JPL (Sander et al. 2003) kinetic data evaluations. In the IUPAC evaluation both forward and reverse rate coefficients are given. The JPL evaluation presents the forward rate coefficients and the equilibrium constants for the reactions. At equilibrium the rates of the forward (/) and reverse (r) reactions are equal, so the equilibrium constant, Kf<r, is directly related to the forward, kf, and reverse, kr, rate coefficients. For example, for N205 formation.

where kf has second-order units (cm3 molecule-1 s~') and kr has first-order units (s-1). Given kf and Kf r, one can therefore obtain kr.

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