Journal cover Journal topic
Geoscientific Model Development An interactive open-access journal of the European Geosciences Union
Geosci. Model Dev., 10, 1447-1466, 2017
http://www.geosci-model-dev.net/10/1447/2017/
doi:10.5194/gmd-10-1447-2017
© Author(s) 2017. This work is distributed
under the Creative Commons Attribution 3.0 License.
Development and technical paper
07 Apr 2017
A new aerosol wet removal scheme for the Lagrangian particle model FLEXPART v10
Henrik Grythe1,2,3, Nina I. Kristiansen2, Christine D. Groot Zwaaftink2, Sabine Eckhardt2, Johan Ström1, Peter Tunved1, Radovan Krejci1,4, and Andreas Stohl2 1Department of Applied Environmental Science (ITM), Atmospheric Science Unit, Stockholm University, 106 91 Stockholm, Sweden
2Norwegian Institute for Air Research (NILU), P.O. Box 100, 2027 Kjeller, Norway
3Finnish Meteorological Institute (FMI), Air Quality Research, Erik Palmenin aukio 1, P.O. Box 503, 00101 Helsinki, Finland
4Division of Atmospheric Sciences, Department of Physics, University of Helsinki, P.O. Box 64 (Gustaf Hällströmin katu 2a), 00014 Helsinki, Finland
Abstract. 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.

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) 137Cs 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 e-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.

Calculated e-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.

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.


Citation: Grythe, H., Kristiansen, N. I., Groot Zwaaftink, C. D., Eckhardt, S., Ström, J., Tunved, P., Krejci, R., and Stohl, A.: A new aerosol wet removal scheme for the Lagrangian particle model FLEXPART v10, Geosci. Model Dev., 10, 1447-1466, doi:10.5194/gmd-10-1447-2017, 2017.
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Short summary
A new and more physically based treatment of how removal by precipitation is calculated by FLEXPART is introduced to take into account more aspects of aerosol diversity. Also new is the definition of clouds and cloud properties. Results from simulations show good agreement with observed atmospheric concentrations for distinctly different aerosols. Atmospheric lifetimes were found to vary from a few hours (large aerosol particles) up to a month (small non-soluble particles)
A new and more physically based treatment of how removal by precipitation is calculated by...
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