Chapter 14 Chemistry of HO_x radicals in the upper troposphere

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 24 h 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 dominanted by the O(¹D)+H₂O reaction when [H₂O]>100 ppmv, 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 factors 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 ppbv d^-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.