PAST, PRESENT, AND FUTURE MODELED OZONE TRENDS WITH COMPARISONS TO OBSERVED TRENDS

The NASA Goddard Space Flight Center (GSFC) two-dimensional (2-D) mode l of stratospheric transport and photochemistry has been used to predi ct ozone changes that have occurred in the past 20 years from anthropo genic chlorine and bromine emissions, solar cycle ultraviolet flux var iations, the changing sulfate aerosol abundance due to several volcani c eruptions including the major eruptions of El Chichon and Mount Pina tubo, solar proton events (SPEs); and galactic cosmic rays (GCRs). The same linear regression technique has been used to derive profile and total ozone trends from both measurements and the GSFC model. Derived 2-D model ozone profile trends are similar in shape to the Solar Backs cattered Ultraviolet (SBUV) and SBUV/2 trends with highest percentage decreases in the upper stratosphere at the highest latitudes. The gene ral magnitude of the derived 2-D model upper stratospheric negative oz one trend is larger than the trends derived from the observations, esp ecially in the northern hemisphere. The derived 2-D model negative tre nd in the lower stratosphere at middle northern latitudes is less than the measured trend. The derived 2-D model total ozone trends are smal l in the tropics and larger at middle and high latitudes, a pattern th at is very similar to the Total Ozone Mapping Spectrometer (TOMS) deri ved trends. The differences between the derived 2-D model and TOMS tre nds are generally within 1-2% in the northern hemisphere and the tropi cs. The derived 2-D model trends are generally more in southern middle and high latitudes by 2-4%. Our 2-D model predictions are also compar ed with the temporal variations in total ozone averaged between 65 deg rees S and 65 degrees N over the TOMS observing period (1979-1993). In clusion of anthropogenic chlorine and bromine increases, solar cycle u ltraviolet flux variations, and the changing sulfate aerosol area abun dance into our model captures much of the observed TOMS global total o zone changes. The model simulations predict a decrease in ozone of abo ut 4% from 1979 to 1995 due to the chlorine and bromine increases. The changing sulfate aerosol abundances were computed to significantly af fect ozone and result in a maximum decrease of about 2.8% in 1992 in t he annually averaged almost global total ozone (AAGTO) computed betwee n 65 degrees S and 65 degrees N. Solar ultraviolet flux variations are calculated to provide a moderate perturbation to the AAGTO over the s olar cycle by a maximum of +/-0.6% (about 1.2% from solar maximum to m inimum). Effects from SPEs are relatively small, with a predicted maxi mum AAGTO decrease of 0.22% in 1990 after the extremely large events o f October 1989. GCRs are computed to cause relatively minuscule variat ions of a maximum of +/-0.02% in AAGTO over a solar cycle.