Aircraft observations from three recent missions (STRAT, SUCCESS, SONEX) are synthesized into a theoretical analysis of the factors controlling the concentrations of HO(x) radicals (HO(x)=OH+peroxy) and the larger reservoir family HO(y) (HO(y)=HO(x)+2H2O2+2CH3OOH+HNO2+HNO4) in the upper troposphere. Photochemical model calculations capture 66% of the variance of observed HO(x) concentrations. Two master variables are found to determine the variance of the 24h average HO(x) concentrations: the primary HO(x) production rate, P(HO(x)), and the concentration of nitrogen oxide radicals (NO(x)=NO+NO2). We use these two variables as a coordinate system to diagnose the photochemistry of the upper troposphere and map the different chemical regimes. Primary HO(x) production is dominated by the O(1D)+H2O reaction when [H2O]>100ppmv, and by photolysis of acetone (and possibly other convected HO(x) precursors) under drier conditions. For the principally northern midlatitude conditions sampled by the aircraft missions, the HO(x) yield from acetone photolysis ranges from 2 to 3. Methane oxidation amplifies the primary HO(x) source by a factor of 1.1-1.9. Chemical cycling within the HO(x) family has a chain length of 2.5-7, while cycling between the HO(x) family and its HO(y) reservoirs has a chain length of 1.6-2.2. The number of ozone molecules produced per HO(y) molecule consumed ranges from 4 to 12, such that ozone production rates vary between 0.3 and 5 ppbvd-1 in the upper troposphere. Three chemical regimes (NO(x)-limited, transition, NO(x)-saturated) are identified to describe the dependence of HO(x) concentrations and ozone production rates on the two master variables P(HO(x)) and [NO(x)]. Simplified analytical expressions are derived to express these dependences as power laws for each regime. By applying an eigenlifetime analysis to the HO(x)-NO(x)-O3 chemical system, we find that the decay of a perturbation to HO(y) in the upper troposphere (as from deep convection) is represented by four dominant modes with the longest time scale being factors of 2-3 times longer than the steady-state lifetime of HO(y). Copyright (C) 2000 Elsevier Science Ltd.
All Science Journal Classification (ASJC) codes
- Environmental Science(all)
- Atmospheric Science