A new aerosol wet removal scheme for the Lagrangian particle model FLEXPART
Henrik Grythe1,2,3, Nina I. Kristiansen2, Christine D. Groot Zwaaftink2, Sabine Eckhardt2, Johan Ström1, Peter Tunved1, Radovan Krejci1,4, and Andreas Stohl21Department of Applied Environmental Science (ITM), Atmospheric Science Unit, Stockholm University, S 106 91 Stockholm, Sweden 2Norwegian Institute for Air Research (NILU), P.O.Box 100, 2027 KJELLER 3Finnish Meteorological Institute (FMI), Air Quality Research, Erik Palmenin aukio 1, P.O.Box 503, FI-00101 Helsinki 4Division of Atmospheric Sciences, Department of Physics, University of Helsinki, P.O.Box 64 (Gustaf Hällströmin katu 2a) FI-00014 University of Helsinki, Finland
Received: 12 Oct 2016 – Accepted for review: 18 Oct 2016 – Discussion started: 24 Oct 2016
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 in-cloud nucleation scavenging differentiates between cloud water phases (liquid, ice or mixed-phase) to allow for aerosol and cloud type specific removal. The impaction scavenging scheme parameterizes below-cloud removal as a function of aerosol 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 modelling 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, (2) black carbon (BC) aerosol, 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. The long lifetimes of mineral dust in particular are primarily a result of slow in-cloud removal which is the primary removal mechanism.
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 deposition and gravitational settling, 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.
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, Geosci. Model Dev. Discuss., doi:10.5194/gmd-2016-267, in review, 2016.