A new aerosol wet removal scheme for the Lagrangian particle model FLEXPART v10
A new, more physically based wet removal scheme for aerosols has been implemented in the Lagrangian particle dispersion model FLEXPART. It uses three-dimensional cloud water fields from the European Centre for Medium-Range Weather Forecasts (ECMWF) to determine cloud extent and distinguishes between...
| Main Authors: | , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2017-04-01
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| Series: | Geoscientific Model Development |
| Online Access: | http://www.geosci-model-dev.net/10/1447/2017/gmd-10-1447-2017.pdf |
| Summary: | A new, more physically based wet removal scheme for aerosols has been
implemented in the Lagrangian particle dispersion model FLEXPART. It uses
three-dimensional cloud water fields from the European Centre for
Medium-Range Weather Forecasts (ECMWF) to determine cloud extent and
distinguishes between in-cloud and below-cloud scavenging. The new in-cloud
nucleation scavenging depends on cloud water phase (liquid, ice or
mixed-phase), based on the aerosol's prescribed efficiency to serve as ice
crystal nuclei and liquid water nuclei, respectively. The impaction
scavenging scheme now parameterizes below-cloud removal as a function of
aerosol particle size and precipitation type (snow or rain) and intensity.<br><br>Sensitivity tests with the new scavenging scheme and comparisons with
observational data were conducted for three distinct types of primary
aerosols, which pose different challenges for modeling wet scavenging due to
their differences in solubility, volatility and size distribution:
(1) <sup>137</sup>Cs released during the Fukushima nuclear accident attached mainly
to highly soluble sulphate aerosol particles, (2) black carbon (BC) aerosol
particles, and (3) mineral dust. Calculated <i>e</i>-folding lifetimes of
accumulation mode aerosols for these three aerosol types were 11.7, 16.0, and
31.6 days respectively, when well mixed in the atmosphere. These are longer
lifetimes than those obtained by the previous removal schem, and, for mineral
dust in particular, primarily result from very slow in-cloud removal, which
globally is the primary removal mechanism for these accumulation mode
particles.<br><br>Calculated <i>e</i>-folding lifetimes in FLEXPART also have a strong size
dependence, with the longest lifetimes found for the accumulation-mode
aerosols. For example, for dust particles emitted at the surface the
lifetimes were 13.8 days for particles with 1 µm diameter and a
few hours for 10 µm particles. A strong size dependence in
below-cloud scavenging, combined with increased dry removal, is the primary
reason for the shorter lifetimes of the larger particles. The most frequent
removal is in-cloud scavenging (85 % of all scavenging events) but it
occurs primarily in the free troposphere, while below-cloud removal is more
frequent below 1000 m (52 % of all events) and can be important
for the initial fate of species emitted at the surface, such as those
examined here.<br><br>For assumed realistic in-cloud removal efficiencies, both BC and sulphate
have a slight overestimation of observed atmospheric concentrations (a factor
of 1.6 and 1.2 respectively). However, this overestimation is largest close
to the sources and thus appears more related to overestimated emissions
rather than underestimated removal. The new aerosol wet removal scheme of
FLEXPART incorporates more realistic information about clouds and aerosol
properties and it compares better with both observed lifetimes and
concentration than the old scheme. |
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| ISSN: | 1991-959X 1991-9603 |