Chemistry of HO x radicals in the upper troposphere

Abstract

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 +2H 2O 2+2CH 3OOH+HNO 2+HNO 4) 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+NO 2). 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)+H 2O reaction when [H 2O]>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 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 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 –O 3 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 .