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Understanding The Effect Of Anthropogenic Aerosol Weekly
Cycles Upon The Climate Using A Global Model Of Aerosol
Processes (GLOMAP)
School of
Earth & Environment
Environment
David A. Ridley, Dominick Spracklen, Kirsty Pringle, Ken Carslaw, Martyn Chipperfield
School of Earth and Environment, University of Leeds, UK
Introduction
The Global Model of Aerosol Processes (GLOMAP) is a detailed microphysical model simulating the transport and evolution of
aerosol species on a 3D global scale. Its purpose is to study the importance of different aerosol processes in the atmosphere so
that parameterisation within General Circulation Models (GCMs) can be improved.
Sensitivity studies of aerosol concentrations have been undertaken and comparisons of model results with in-situ observations
from different campaigns are found to be positive.
We intend to simulate aerosol weekly cycles within GLOMAP to determine the effects upon aerosol burden and cloud formation,
and hence elucidate a connection between observed changes in the radiation budget of the Earth and anthropogenic aerosol
emissions.
GLOMAP
Model Design
The Global Model of Aerosol Processes (GLOMAP) is an
extension of the chemical transport model TOMCAT and
includes processes such as aerosol nucleation, growth,
coagulation and deposition.
TOMCAT
•3D Offline CTM
•Forced by ECMWF Winds
•Convective transport
•Convective and resolved rain
Sources
Coagulation
•Semi-implicit fast numerical
solution
Removal
Dry Deposition
•Dry deposition of aerosol
Clouds
•Convective and frontal rain
•In-cloud nucleation scavenging
•Below cloud scavenging
Hygroscopic Growth
•Equilibrium size given by
solution of Kohler equation
Fig. 1 Schematic of GLOMAP
As well as total aerosol number, size distributions can be
obtained from the model allowing calculation of condensation
nuclei
(CN)
and
cloud
condensation
nuclei
(CCN)
concentrations.
SO2
Fig. 4 DTR difference between Saturday-Monday average
and Wednesday-Friday average over the USA2
By simulating weekly cycles in emissions we can determine
if this has any noticeable effect upon aerosol size distribution
and consequent cloud formation.
Weekly variations in cloud cover are a possible cause for the
so-called weekend effect.
Emissions
Weekly Cycles
Weekly cycles are useful in characterising anthropogenic
effects. If significant climatic changes are detected on this
scale then we can begin to understand how emissions will
affect the climate in the long term.
CCN @ 0.3%
December 1995 (Monthly Average)
Fig 2. Global maps of surface layer SO2 concentration and
CCN concentration at 0.3% supersaturation1
GLOMAP
The diurnal temperature range (DTR) is an important
climatic indicator.
A recent study has shown variations
between weekday and weekend DTR comparable in magnitude
to long term temperature trends2.
This must be an anthropogenic effect, however the
mechanism is still unclear.
Aerosol size spectrum (~ 1nm –
24µm)
Nucleation and
Condensation
•Binary H2SO4/H2O nucleation
•Condensational growth
Sulfur Chemistry
•8 sulfur species
•8 sulfur reactions
•Aqueous phase chemistry
•Oxidants from full chemistry run
‘Weekend Effect’
GLOMAP
Microphysics
Emissions
•Anthrop + volcanic SO2
emissions
•DMS emissions from wind stress
and DMS sea surface
concentration
•Sea salt aerosol generation
function
DTR
Comparison With
Observations
Comparisons of CN concentration measurements from
aircraft
observations
(during
the
Intercontinental
Transportation of Ozone Precursors [ITOP] campaign) and
GLOMAP show favourable results.
Fig 5. Daily SO2 levels from Manchester, UK averaged over 4 years.
Values are displayed as a fraction of the mean
When urban SO2 measurements are averaged over several
years a clear weekly trend can be seen. This is used to
weight the SO2 emissions in GLOMAP.
Research
Future Plans
• Weekly cycles in other pollutants, such as NOx from
transport emissions, may have stronger weekly cycles and
hence contribute significantly to enhanced CCN formation on
weekdays.
• Including a primary aerosol component in the emissions
could act to simulate the nucleation that occurs in the dense
emission plumes.
• An offline cloud microphysics model can be used to examine
possible cloud responses to modelled changes aerosol burden.
Fig. 3 Comparison of P3 aircraft CN concentration measurements (blue) with concurrent
GLOMAP CN concentration (orange). Aircraft pressure is also shown (black)
The ultimate aim is to quantify how this aerosol indirect
effect changes the radiation budget and hence whether it can
account for the observed DTR weekend effect.
1 Spracklen DV, Pringle KJ, A global off-line model of size-resolved aerosol microphysics: Model development and prediction of aerosol properties, submitted Atmos. Phys. Chem. 2004
2 Forster PMD, Solomon S, Observations of a "weekend effect" in diurnal temperature range, PNAT 11225-11230 SEP 30 2003
Contact email: [email protected]