Heterogeneous physicochemistry of the polar ozone hole

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Abstract
The heterogeneous physical and chemical processes that occur in the presence of and involve polar stratospheric clouds (PSCs) are investigated. The theory developed here is guided by, and compared for consistency with, the extensive observations from the Airborne Antarctic Ozone Experiment. We first describe the characteristics of PSCs that affect chemical processes, such as particle composition, cloud surface area and mass, and aerosol mechanical time constants. The vapor pressures of trace compounds measured over ice in laboratory settings are discussed and shown to be consistent with in situ observations and simple thermodynamics. The mechanism for the formation of nitric acid haze (type I PSC) is elucidated. To estimate key chemical time constants, we derive expressions for the rates of mass transfer to PSC particles and reaction rates on surfaces; here, laboratory measurements of “sticking coefficients” are related to the fundamental parameters of surface physics and chemistry. By comparing relative rates of physical and chemical processes, together with data and simulations on the seasonal evolution of polar ice clouds (in a companion paper (Toon et al., 1989)), we reach several important conclusions. The HCl + ClONO2, ClONO2 + H2O, N2O5 + HCl, and N2O5 + H2O reactions can occur on early forming type I PSC (haze) particles, converting inert chlorine to active chlorine (Cl2, HOCl, and ClNO2) and active nitrogen to HNO3 relatively quickly. Denitrification occurs somewhat later in the winter season with the formation of type II PSC (ice cirrus) clouds, which can absorb HNO3 in solid solution and remove the HNO3 by sedimentation; the degree of denitrification is sensitive to the cooling rate and the time constant for condensation of nitric acid haze. Dechlorination does not occur as efficiently as denitrification because the HCl reservoir is effectively depleted by conversion into active chlorine before the onset of type II cloud formation and denitrification. If HF is soluble in ice, defluorination is expected at the same time as denitrification, although the extent of defluorination would be effectively limited by a more pronounced vertical separation between HF and PSCs. The reactions listed above provide pathways to deplete the inert chlorine reservoir (HCl, ClONO2) on a time scale of days to weeks in the presence of type I clouds (although photochemical recycling of chlorine can occur on this time scale as well). Type II PSCs and, to a lesser extent, type III PSCs (lee wave clouds) contribute to chemical processing late in the winter that sustains the chemical imbalance of the polar stratosphere. We also discuss nonlinearities in the combined heterogeneous/homogeneous chemical system and show, using a simple model, that the decadal evolution of the Antarctic ozone hole may be understood, in connection with the accumulation of fluorocarbons in the atmosphere, through nonlinearities in the heterogeneous chemistry, with possible contributing effects of variations in stratospheric temperatures and water vapor concentrations, which appear to have caused an increase in PSC frequency, extent, and duration in recent years.