where A is the pre-exponential or frequency factor (which may involve a small dependence on temperature) and E a is the activation energy. While more precise definitions are available, the activation energy is usually interpreted as the minimum energy (kinetic plus potential, relative to the lowest state of reactants) that reactants must have to form products and the pre-exponential factor is a measure of the rate at which collisions occur. If a reaction obeys the Arrhenius equation, then a plot of ln k versus 1 T should produce a straight line, whose slope and intercept at the origin are -Ea R and A, respectively. However, many reactions of wide current interest do not obey the Arrhenius equation and/or have negative activation energies (rate constants that decrease when the temperature is increased). At the same time, the experimental characterization of several reactions of atmospheric interest is made difficult by the involvement of highly reactive species, such as free radicals or ions.