Engineering topics
Publications and source records attributed to Salawitch, Ross J..
The current status of stratospheric research on stratospheric ozone is reviewed. Special attention is given to ozone chemistries in the upper and the lower stratosphere at the Antarctic, Arctic, and tropical latitudes, as well as at midlatitudes. The consequences these changes have on global climate are discussed in the framework of a one-dimensional radiative-convective model.
Growth of nitric acid trihydrate (NAT) particles on background stratospheric aerosols is examined for an isolated air parcel cooled at a uniform rate. During the process of nucleation, the saturation ratio of HNO3 vapor reaches a maximum value between 2 and 15, corresponding to supercooling by 1-4 K. If cooling rates exceed 0.5-1 K/day, small particles of NAT are produced. A major fraction of the available condensation nuclei is activated and removal of HNO3 by gravitational settling is slow. If cooling rates are less than 0.5-1 K/day, the number of aerosols that nucleate is reduced, leading to differential growth of large NAT particles. Observations of 5 micron radius particles in clouds at temperatures above the water frost point may reflect condensation of NAT on ice particles that fall through a column of air as it is cooled. Rapid condensation of HNO3 on ice particles is promoted by the high supersaturation attained during nucleation and maintained during subsequent cooling. This process provides a mechanism for irreversible removal of HNO3.
It is argued that the reaction COF2 + H2O - 2HF + CO2 should proceed on surfaces of polar stratospheric clouds, based on laboratory evidence for this reaction in condensed phase and on analysis of column observations of HF during the Airborne Arctic Stratospheric Expedition. If the hypothesis is confirmed, observations of COF2 and HF could provide unambiguous indication of heterogeneous processing of polar air, and could help elucidate the influence of heterogeneous chemistry on concentrations of HCl, ClNO3, and chlorine oxide radicals.
Measurements of ClO (Brune et al., 1990) acquired during the Airborne Arctic Stratospheric Expedition are used to infer concentrations of reactive chlorine (ClO + 2 x Cl2O2). Observed fields of potential temperature and potential vorticity are used to extrapolate in situ data to larger regions of the vortex. Calculated values of the loss rate of O3, based on estimates of reactive chlorine and measurements of BrO (Toohey et al., 1990), suggest that the loss of O3 was abut 12 pct for levels of the atmosphere with potential temperatures between 440 and 470 K over the 39 day duration of the ER-2 flights into the polar vortex. Calculated loss rates agree with observed rates of removal of O3, although significant uncertainties exist for each.
The current knowledge of the chemistry of the stratosphere is reviewed, with particular consideration given to the measurements from the Atmospheric Trace Molecule Spectroscopy (ATMOS) experiment and from the Airborne Antarctic Ozone Experiment. Analysis of the ATMOS data at 30 deg N suggests that the current understanding of the contemporary-stratosphere chemistry at mid-latitudes is relatively complete, except for possible problems with the diurnal variations of N2O5 at low altitudes, and with ClNO3 at higher altitudes. Except for some difficulties with these two compounds, the data from ATMOS agree well with the gas phase models for nitrogen and chlorine species at 30 deg N in spring. It is emphasized that, in addition to the HOCl mechanism proposed by Solomon et al. (1986), the ClO-BrO scheme proposed by McElroy et al. (1986), and the ClO dimer mechanism introduced by Molina and Molina (1987), other processes exist that are responsible for ozone removal.
The current understanding of stratospheric chemistry is reviewed with particular attention to the influence of human activity. Models are in good agreement with measurements for a variety of species in the mid-latitude stratosphere, with the possible exception of ozone (O3) at high altitude. Rates calculated for loss of O3 exceed rates for production by about 40 percent at 40 kilometers, indicating a possible but as yet unidentified source of high-altitude O3. The rapid loss of O3 beginning in the mid-1970s at low altitudes over Antarctica in the spring is due primarily to catalytic cycles involving halogen radicals. Reactions on surfaces of polar stratospheric clouds play an important role in regulating the abundance of these radicals. Similar effects could occur in northern polar regions and in cold regions of the tropics. It is argued that the Antarctic phenomenon is likely to persist: prompt drastic reduction in the emission of industrial halocarbons is required if the damage to stratospheric O3 is to be reversed.
Precipitation of nitrate in polar stratospheric clouds (PSCs) can provide a significant sink for Antarctic stratospheric odd nitrogen. It is argued that the depth of the Ozone Hole is sensitive to the occurrence of temperatures below about 196 K. An increase in the prevalence of temperatures below 196 K would enhance ozone loss by increasing the spatial extent and persistence of PSCs, and by decreasing the level of HNO3 that remains following PSC evaporation. Concentrations of halogen gases in the 1960s and earlier were insufficient to support major ozone loss, even if thermal conditions were favorable.
Previous observations of OClO over the Antarctic obtained in 1986 are used to constrain the stratospheric abundance of bromine. The observed diurnal variation is consistent with the production of OClO via the reaction of ClO with BrO if the branching ratio to the BrCl channel is between 3.5 and 14 percent. Otherwise, an additional source in twilight is indicated. The present results suggest a stratospheric bromine concentration of 16 + or - 4 pptv, a value which is consistent with independent measurements.
Interferometric measurements of HCl, ClNO3, HNO3, NO2, and NO obtained over the Antarctic in 1986 are used to model the chemistry of the atmosphere in the region of the Ozone Hole. The low abundance noted in stratospheric HCl is attributed to incorporation of HCl in polar stratospheric clouds and subsequent reaction of HCl with ClNO3. The results point to a net loss of HNO3 from the stratosphere and to the suppression of the abundance of odd nitrogen at high altitudes in the vortex. O3 loss is suggested to be due to the catalytic influence of halogen radicals.
Chemical explanations for the spring decline of Antarctic O3 involve reactions of ClO or interactions between BrO and ClO. Reaction schemes involving Br radicals have the highest efficiency for removal of O3. The chemical mechanisms require low levels of NO(x). Recent cooling of the stratosphere may have triggered formation of HNO3 monohydrate, removing NO(x) and setting the stage for enhanced loss of ozone over Antarctica.