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. Feedback mechanisms are both chemical and radiative.
OBJECTIVES:
a. 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.
b. 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.
c. 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.
d. 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.
e. 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.
a. Time-dependent run for 1980-2000, without aerosol radiative forcing
(sulfate aerosol surface area density is taken from satellite observations).
b. 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.
c. 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.
d. Time-dependent run for 2000-2050, with aerosol distributions (sulfate,
carbonaceous, sea-salt and dust) fixed at the calculated values for the
year 2000.
e. 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.
TASKS COMPLETED: The activity during the second year of GACP has focused
on points c-d-e outlined above. A validation of the model predicted aerosol
extinction at 1.0 um wavelength has been made using SAGE-II data for
volcanically quiet years since 1984. Future changes of sulfur precursor
emissions at the surface have been taken into account using surface fluxes
provided for the IPCC-TAR assessment. The feedback of these changing emissions
on stratospheric sulfate aerosols has been studied with the model, along with
the effect of climate changes on stratospheric temperature and PSC formation.
FUTURE PLANS: Model calculations outlined above as (a) and (b) will be
completed.
RESULTS: 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 prodiced 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.
FORM C: FUTURE PLANS
Name: Giovanni Pitari, Vincenzo Rizi and Eva Mancini.
Institution: Dipartimento di Fisica, Universita' L'Aquila, Italy
The low-resolution GCM available at ULAQ will be run between 1980 and 2000
including the sulfate aerosol radiative forcing during the volcanically active
years (i.e. El Chichon and Pinatubo). This will help understand the role of
aerosol driven perturbations of the lower stratospheric circulation on the
observed total ozone trends. Time-dependent runs (a) and (b) of the ULAQ
climate-chemistry model will be completed.
a. Time-dependent run for 1980-2000, without aerosol radiative forcing
(sulfate aerosol surface area density is taken from satellite observations).
b. 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.
