Steady State Instability and Oscillation in Simplified Models of Tropospheric Chemistry

Abstract

Three components have been identified as being common to oscillation in five related but increasingly complex models of tropospheric chemistry. The first of these components is an NOx-empty/refill cycle in which [NOx] grows or declines, depending upon the relative source rates of NOx and HOx, the latter being generally proportional to the rate of photolysis of [O3]. The second component is a complex O3-production/loss cycle dependent upon [NOx], [HOx], [CO], and [O3]. The third component is nonlinear coupling (both direct and indirect) of the first two, which allows each of the two cycles to affect the other. This coupling also introduces a positive feedback that autocatalytically accelerates O3 production at high [CO] and [O3] when [NOx] and [NO]/[NO2] are simultaneously low, thus destabilizing the steady state. A schematic model is provided that illustrates the interaction of these three components and indicates that the positive feedback indeed is necessary for oscillation to occur. The major features governing the behavior of this dynamic instability and related oscillation in simpler models also are dominant in a larger oscillatory model of tropospheric CH4 photooxidation. Thus dynamical instability and oscillation appear to be common features of tropospheric chemical mechanisms, regardless of the particular reaction set chosen and over significant ranges of parameter values, and appear to result from complex nonlinear coupling of NOx-empty/refill and O3-production/loss cycles.