Dynamic equations describing photoxidation of tropospheric chemical pollutants are nonlinear, containing complex feedback loops. Such nonlinearity is known to give rise to various dynamical instabilities including multiple steady states, oscillation, and even chaos. A related type of instability, excitability, is demonstrated here using a two-variable (reduced from six) model of CH4 photoxidation in which perturbation of a stable but excitable steady state beyond a threshold is dramatically amplified before the steady state is reapproached. Such switching/amplification responses may have important implications for atmospheric/climatic modeling. A phase-plane analysis describes the origin of this excitability and suggests that it may be a relatively common phenomenon in environmental models. ¿ 2001 American Geophysical Union