Abstract
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.
Highlights
Aerosols are of concern for urban air quality, and an important part of the climate system
This paper describes and tests a new scheme for aerosol wet removal implemented into the Lagrangian particle dispersion model FLEXPART
To explore how frequent in-cloud and below-cloud scavenging events are and where they occur, we used a 3-month (December 2006 to February 2007) global European Centre for MediumRange Weather Forecasts (ECMWF) data set (1◦ × 1◦ with 92 vertical layers) and classified each grid cell as being either in a cloud-free column or, if clouds exist in the column, in, below or above the cloud
Summary
Aerosols are of concern for urban air quality, and an important part of the climate system. The efficiency of aerosol particles in serving as cloud condensation nuclei (CCN) depends on their size and chemical properties as well as on the ambient conditions. If the cloud water precipitates to the surface, CCN or IN contained in the droplets or ice crystals are removed from the atmosphere. Since each drop of precipitation can account for millions of cloud droplets, nucleation scavenging is the most important mechanism for wet removal (Rogers and Yau, 1989). This paper describes and tests a new scheme for aerosol wet removal implemented into the Lagrangian particle dispersion model FLEXPART. It is based on the mechanisms of nucleation removal within the cloud and impaction removal below the cloud.
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