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| Titel |
Use of aerosol optical properties for chemistry-transport model evaluation |
| VerfasserIn |
Jean-Christophe Péré, Marc Mallet, Véronique Pont, Bertrand Bessagnet |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250035888
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| Zusammenfassung |
| This study presents an aerosol optical scheme developed in the chemistry-transport model
CHIMERE dedicated to calculate optical properties of particles. Such developments are
very helpful as they complement the usual validation with PM (Particulate Matter)
ground-based measurements by using surface (AERONET/PHOTONS network) and
satellite (MODIS) remote sensing observations. To reach this goal, Aerosol Optical
Thickness (AOT), column-averaged Single Scattering Albedo (SSA) and asymmetry
parameter (g) are calculated at 440 nm, 675 nm, 870 nm and 1020 nm (AERONET
wavelengths) under three hypotheses on the particle mixing state (external, internally
homogeneous and core-shell). Furthermore and in addition to optical calculations, an
original development has been made to estimate column volume size distributions in
CHIMERE, directly comparable with AERONET retrievals. Comparisons between
simulations and observations are made over Western Europe for the year 2003 but also
for two specific cases focused on ammonium nitrate and on secondary organic
aerosols.
Results indicate that modeled SSA is fairly constant with increasing wavelength while it is
very sensitive to particle mixing. Indeed, the core-shell SSA exhibits close match (at the
annual mean) with AERONET values compared to external and internally homogeneous
mixings. Concerning the asymmetry parameter, discrepancies between observations and
simulations for the three mixings remain small at the four wavelengths with up to 10–15 %
differences (at the annual mean), which could indicate good consistencies between observed
and modeled particle type. Concerning the aerosol optical thickness, comparisons indicate
that the seasonal cycle of modeled AOT (few sensitive to particle mixing) agrees well with
AERONET observations (correlations in the range 0.50–0.74). However, modeling results
reveal also some biases according to seasons. In fall, winter and early spring, AOTtot and
AOTfine AERONET values are well reproduced by the model (with small negative
biases) compared to late spring and summer periods. Results obtained for a pollution
episode of ammonium nitrate (March 2003) reveal that the CHIMERE model rather
well estimates AERONET fine mode volume size distribution, leading to good
agreements between modeled and observed AOT. In parallel, the overestimation of
hydrophilic ammonium nitrate concentrations during this episode leads to important
discrepancies between AERONET and core-shell SSA, due to the thickness of the aerosol
shell.
In summertime, uncertainties in modeling secondary organic aerosol formation during warm
days cause an inconsistent simulated aerosol volume size distribution (for both the internal
and external mixings). This leads to a weak spectral dependence of modeled AOTfine in
contradiction with observations and to an important underestimation of the total and fine
AERONET AOT. Due to elevated temperatures over Western Europe during late spring 2003,
this discrepancy in modeled volume size distribution could also explain the underestimation
of AOT during this period.
Adsorption of SOA gaseous precursors onto background coarse particles over rural areas
could be a reason of the discrepancies observed on the volume size distribution. In parallel,
other processes not yet taken into account in CHIMERE, such as homogeneous nucleation of
gaseous VOC or their nucleation with sulfuric acid, could form finest SOA with higher
visible light extinction efficiency. In addition, underpredicted modeled concentrations of
gaseous VOC likely to nucleate over existing fine aerosols could also explain the
underestimation of AOT. |
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