The slow-down effect in the nighttime mesospheric chemistry of hydrogen radicals

Abstract

We investigate the time behaviour of the total odd hydrogen radical concentration [HOx] = [H] + [OH] + [HO2] in the nighttime mesosphere. We use for the analysis data calculated by our global 3D chemical transport model (CTM). The calculations show that after sunset (χ > 100°) in the dark atmosphere, the total concentration of HOx decreases and is marked by rising characteristic time. The decline of the concentration of HOx can be approximately described by an inverse power function of the form HOX=A/t-CB with an exponent B smaller than unity. In double logarithmic coordinates, the time behaviour of the total hydrogen radical concentration looks like a decreasing straight line but with the tendency to flatten weakly with time. We deduce the parameters A, B, and C from the 3D calculations and derive a differential equation that describes the time behaviour of [HOx]. The characteristic time of the HOx family increases linearly with time, but the gradient of increase depends on the exponent B. Therefore, it is impossible to characterize the time behaviour of the nighttime mesosphere HOx-system by a constant or averaged characteristic time. Such response of increasing characteristic times is a frequently observed behaviour for nonlinear systems characterized by internal feedback.