Dynamic instability in tropospheric photochemistry: An excitability threshold

Dynamic equations describing photoxidation of tropospheric chemical polluta nts are nonlinear, containing complex feedback loops. Such nonlinearity is known to give rise to various dynamical instabilities including multiple st eady states, oscillation, and even chaos. A related type of instability, ex citability, is demonstrated here using a two-variable (reduced from six) mo del of CH4 photoxidation in which perturbation of a stable but excitable st eady state beyond a threshold is dramatically amplified before the steady s tate is reapproached. Such switching/amplification responses may have impor tant implications for atmospheric/climatic modeling. A phase-plane analysis describes the origin of this excitability and suggests that it may be a re latively common phenomenon in environmental models.