GACP Projects
FORM A: GACP ACCOMPLISHMENT REPORT
Name: Giovanni Pitari, Vincenzo Rizi and Eva Mancini.
Institution: Dipartimento di Fisica, Universita' L'Aquila, Italy
TITLE: A time-dependent study of aerosol-ozone interactions with a low-resoluton climate-chemistry coupled model.
ABSTRACT: A low-resolution climate-chemistry coupled model is used to study the complex chemical-radiative-dynamical interactions of aerosols and ozone. A time-dependent numerical experiment will be designed covering the past 20 years of satellite aerosols measurements and the next 50 years, with the purpose of investigating the effects on the ozone recovery rate of potentially increasing sulfate aerosols in the troposphere and lower-stratosphere. Future changes of sulfate aerosols will be calculated as a result of expected trends in surface emissions of sulfur precursors, aircraft emissions of sulfur dioxide and carbon soot particles, temperature and circulation changes due to growing emissions of greenhouse gases (carbon dioxide in particular). The time-dependent aerosol radiative forcing will be calculated taking also into account the related ozone changes, looking in particular at the effects on climate and on the lower stratospheric circulation during the years of large volcanic perturbations of the stratospheric sulfate aerosol layer (i.e. after El Chichon and Pinatubo eruptions). A comparison of model calculated total ozone with TOMS data will help understand the role of aerosol induced circulation changes after these large volcanic perturbations.
GOALS: Better understanding of the potential role of future aerosol trends on the ozone recovery rate. Study of chemical and radiative feedback processes of sulfate aerosols on atmospheric tracers.
OBJECTIVES:
- To assess the effects of sulfate aerosol perturbations on stratospheric ozone in the last 20 years, using aerosol extinction and surface area density data from satellite measurements and also calculated values from a microphysical code coupled with multi-dimensional transport models.
- To compare the model calculated ozone trend in the last 20 years with TOMS, SBUV and SAGE measurements with and without inclusion of the aerosol radiative forcing, with the purpose of having a better understanding of the relative role of chemically and dynamically driven ozone changes in the presence of large stratospheric aerosol perturbations.
- To calculate the aerosol radiative forcing on climate during the years of large perturbations of the stratospheric sulfate aerosol layer (i.e. after El Chichon and Pinatubo eruptions), taking also into account the feedback of ozone changes related to the aerosol perturbations.
- To study the role of potentially increasing tropospheric and lower stratospheric sulfate aerosols on the rate of ozone recovery following the compliance of the Montreal Protocol on limitations of CFCs, HCFCs and halons production in the next century. This study will include both chemical and radiative feedbacks of aerosol particles.
- To calculate past and future time-dependent radiative forcing of tropospheric sulfate aerosols as a function of growing surface and aircraft emissions of sulfur precursors and as a function of changing temperature and atmospheric chemical composition due to increasing greenhouse gases (carbon dioxide in particular).
APPROACH: Time-dependent runs of a climate-chemistry coupled model are made to accomplish the research objectives.
- Time-dependent run for 1980-2000, without aerosol radiative forcing (sulfate aerosol surface area density is taken from satellite observations).
- As above, but including the sulfate aerosol radiative forcing. A comparison of the results from simulations (a) and (b) allows to estimate the aerosol forcing effects on ozone during the volcanically active periods, taking also into account the role of changing ozone concentrations in the radiation budget. The total ozone trend calculated from runs (a) and (b) is compared to that deduced from TOMS observations. This comparison helps understand if circulation changes during volcanically active years may partially explain the large differences found between model calculated and observed ozone trends.
- As in (b), except that now the time-dependent sulfate aerosol size distribution is explicitly calculated using a microphysics code coupled to the model. Numerical simulations include now the calculated sulfate aerosol extinction data in the photolysis and heating calculations. A comparison of the results from simulations (b) and (c) will tell us how reliable is the model prediction in terms of sulfate aerosol distribution for the future simulations.
- Time-dependent run for 2000-2050, with aerosol distributions (sulfate, carbonaceous, sea-salt and dust) fixed at the calculated values for the year 2000.
- As in (d), but with aerosol distributions recalculated including future predictions for sulfur and aerosol surface fluxes. Future trends of aircraft emissions are also included. A comparison of the results from simulations (d) and (e) will give an estimate of the future climatic role of aerosols, in the hypothesis that they are changing mainly as an effect of changing surface and in-situ fluxes, water vapor, oxidants, temperature, circulation, precipitation. This comparison will also give an indication of the potential role of aerosols in the future recovery of atmospheric ozone.
THIRD YEAR ACTIVITY
TASKS COMPLETED: The low-resolution climate-chemistry model available at ULAQ was run between 1990 and 1995 in order to study the effects of Pinatubo aerosols on the lower stratospheric circulation and on the trends of longlived atmospheric tracers (ozone, methane and N2O in particular). Results with and without sulfate aerosol radiative forcing on climate were compared. (Subset of the above points a-b).
RESULTS: Our study finds that the dynamical perturbation is twofold: (a) the stratospheric mean meridional circulation is affected by local aerosol radiative heating (mostly located in the tropical lower stratosphere); (b) the planetary wave propagation in the mid-high latitude lower stratosphere is altered as a consequence of decreasing atmospheric stability due to the climatic perturbation. Dynamical results of the climate model are compared with available observations and a similarity with the dynamical regime of the easterly phase of the equatorial quasi-biennial oscillation is highlighted. Major findings of our study are: (a) radiatively forced changes of the stratospheric circulation during the first two years after the eruption may to a large extent explain the observed trend decline of longlived greenhouse gases (CH4 and N2O, in particular); (b) the dynamical perturbation helps explain why simple photochemical studies of the ozone trends during 1991-1993 generally fail in reproducing the satellite observed feature consisting in a 2% additional global ozone depletion during 1993 with respect to 1992. In both cases we conclude that an increase of the mid-high latitude downward flux at the tropopause is the key factor for explaining the behavior of these atmospheric tracers during 1991/92.
SUMMARY OF THE THREE-YEAR PROJECT ACTIVITY
The future evolution of stratospheric aerosols and ozone has been studied with time-dependent and steady-state runs of a climate-chemistry coupled model. The same model was also used to investigate the radiative forcing of stratospheric volcanic aerosols from a major explosive eruption (i.e. Pinatubo). The ability of the University of L'Aquila (ULAQ) model in predicting the tropospheric distribution of different aerosol types has been tested in the IPCC assessment calculations. The calculated extinction profiles at different latitudes have been found to be reasonable with respect to SAGE-II observations both in the stratosphere and in the upper troposphere (i.e. above 6 km altitude).
The most important results of our study in terms of future ozone sensitivity to stratospheric aerosol changes are summarize here.
(a) Future changes in global and regional emissions of SO2 may perturb the amount of SSA in such a way the future ozone recovery is decreased: -0.3% in global ozone during 2030.
(b) PSC are also going to be perturbed in the next decades as a consequence of climate changes produced by GHG increases; the calculated PSC enhancement forces an additional slow down of the ozone recovery rate: -0.8% during 2030.
(c) Stratospheric circulation changes in future climate are essentially represented by a reinforcement of the lower stratospheric residual circulation with enhanced upwelling in the tropics and stronger subsidence at mid-latitudes. The latitudinal gradient of the 50-200 hPa diabatic heating is enhanced in GHG richer atmosphere; the tropical UT/LS tends to warm up with respect to mid-latitudes through CO2-H2O-O3 IR cooling and tropospheric convective heating forced by a warmer surface. Tropospheric temperature changes tend to increase the atmospheric stability, so that less wave activity is found in stratosphere: polar vortices are more stable, thus enhancing O3 depletion by PSC activated Cl/Br. The net effect on ozone depends non-linerly on the combination of a more efficient transport toward mid-high latitudes and a more efficient lower stratospheric chemical removal in the polar regions produced by more stable vortices and by a simultaneous increase of PSCs at polar latitudes. Our calculations predict a positive change of global ozone produced by future climate changes (+0.2%).
(d) Net changes of the O3 profile result from the simultaneous action of the above three mechanisms.
PUBLICATIONS RELEVANT TO GACP
Pitari, G., and E. Mancini: Stratospheric sulfate aerosols in volcanically quiet conditions, in 'Aviation, aerosols, contrails and cirrus clouds (A2C3)', European Commission 'air pollution research report 74', U. Schumann and G.T. Amanatidis Eds., pp. 27-32, 2001.
Pitari, G., E. Mancini, A. Bregman, H.L. Rogers, J.K. Sundet, V. Grewe, and O. Dessens: Sulphate particles from subsonic aviation: Impact on upper tropospheric and lower stratospheric ozone, Phys. Chem. of Earth C, 26/8, 563-569, 2001.
Penner, J., D. Hegg, M. Andreae, D. Leaitch, G. Pitari, H. Annegarn, D. Murphy, J. Nganga, L. Barrie, and H. Feichter: IPCC, Climate Change 2001; Chapter 5: Aerosols and Indirect Cloud Effects, IPCC Third assessment report, J. Houghton et al. Eds., Cambridge University Press, 2001.
Pitari, G.: Stratospheric aerosols, in 'Encyclopedia of Global Environmental Change', Vol. 1: The Earth system: physical and chemical dimensions of global environmental change, M.C. McCracken and J.S. Perry Eds., Wiley, in press, 2001.
Pitari, G.: Polar stratospheric cloud aerosols, in 'Encyclopedia of Global Environmental Change', Vol. 1: The Earth system: physical and chemical dimensions of global environmental change, M.C. McCracken and J.S. Perry Eds., Wiley, in press, 2001.
Pitari, G., E. Mancini, V. Rizi, and D.T. Shindell: Impact of future climate and emission changes on stratospheric aerosols and ozone, J. Atmos. Sci., in press, 2001.
Penner, J.E., S.Y. Zhang. C.C. Chuang, M. Chin, J. Feichter, Y. Feng, P. Ginoux, M. Herzog, A. Higurashi, D. Koch, C. Land, U. Lohmann, M. Mishchenko, T. Nakajima, G. Pitari, B. Soden, I. Tegen, L. Stowe: A comparison of model- and satellite-derived optical depth and reflectivity, J. Atmos. Sci., in press, 2001.
Pitari, G., E. Mancini: Short-term climatic impact of the 1991 volcanic eruption of Mt. Pinatubo and effects on atmospheric tracers, Natural Hazards and Earth System Sciences, submitted, 2001.
