Evolution of particle properties¶

In FLEXPART, the modeling of particle movement involves a combination of interpolating properties from meteorological input data and computing properties that are intrinsic to each particle. This hybrid approach allows for a more comprehensive and realistic representation of particle behavior in the atmosphere.

In Particle transport, you can find how particles are advanced, using parameterisation schemes and meteorological data interpolated to the particle position. This interpolated meteorological data is necessary for computing the trajectories of particles, but such information can also be a valuable diagnostic tool on itself. For this reason, interpolated meteorological data along the trajectories of each particle, can be written to file setting the option IPOUT to a non-zero value in the COMMAND file.

In addition, FLEXPART goes beyond simple interpolation and incorporates intrinsic properties that are directly related to each individual particle. The main intrinsic property considered in the model is the mass of the particles. The mass of a particle can significantly impact its behavior in the atmosphere, affecting its movement and interactions with the environment (see Settling. During the simulation, FLEXPART takes into account various processes that can modify the mass of particles over time. For instance, particles can undergo deposition (Dry deposition and Wet deposition, where they are removed from the atmosphere by settling onto surfaces (e.g., ground or vegetation). Additionally, for particles resulting from nuclear disasters or radioactive emissions, radioactive decay is another important process that leads to a reduction in particle mass over time (see Radioactive decay and OH reaction).

By considering these intrinsic properties, particularly the evolving mass of the particles, FLEXPART can simulate the transport, dispersion, and fate of pollutants, aerosols, and other substances in a more realistic manner. It allows the model to account for the varying lifetimes of different particles based on their specific properties and the environmental conditions they encounter.

Dry deposition¶

In FLEXPART, the dry deposition module is used to calculate the dry deposition velocities of gases and particulate matter within the layer from 0 to 15 metres above ground (Eq. 48 in Stohl et al. 2005). Here, dry deposition is treated in a same way as in previous versions and can be found in drydepo_mod.f90. However, now the module considers different particle shapes as it is in the settling module (for details see Shape factor).

The limitation here is that the Stokes’ number, which is used to calculate the deposition layer resistance, is not resolved for non-spherical particles and remains to be investigated. Therefore, in FLEXPART 11 the Stokes’ number for particles of any shape is assumed to be similar to that for perfect spheres.

Wet deposition¶

Radioactive decay¶

FLEXPART can account for radioactive and/or chemical decay of particles by defining a half life \(T_{1/2}\) (parameter pdecay > 0 in the corresponding SPECIES file; off if pdecay < 0). The decay affects the particle mass while traveling through the atmosphere as well as the deposited mass following: \(m(t + \Delta t) = m(t) e^{−\Delta t/\beta}\) with \(\beta = \frac{T_{1/2}}{ln(2)}\) The treatment of radioactive and/or chemical decay remains unchanged compared to v10.4 and we refer to Pisso et al. 2019 for a comparison with observations.

OH reaction¶

Loss processes related to reactions with hydroxyl (OH) radicals are represented as a first-order, linear approximation in FLEXPART 11 — identical to v10.4 and therefore only shortly summarized here; for a detailed description see Pisso et al. 2019.

The OH radical is the most important oxidant in the troposphere and although reactions with atmospheric gases can be highly non-linear (e.g., CH\(_4\)), a first-order, linear loss approximation using prescribed OH fields is possible \textcolor{red}{(maybe add citation why this is fine to do?)}(for FLEXPART 11: monthly averaged 3\(^\circ\)\;x\;5\(^\circ\) OH fields with 17 vertical layers; following GEOS-Chem model by Bey et al. 2001 and read in from OH_variables.bin). Hourly OH variations are accounted for by modifying the monthly fields with the hourly photolysis rate of ozone \(j\) based on a simple could-free parameterization depending on the solar zenith angle:

\(OH = \frac{j}{j^*} OH^*\)

where \(j^*\) and \(OH^*\) are the monthly mean photolysis rate and OH concentration taken from OH_variables.bin, respectively.

The OH reaction can be turned on by providing positive reaction rates in the corresponding SPECIES file (parameters pohcconst, pohdconst, and pohnconst; turned off with negative values). The temperature-dependent OH reaction rate \(\kappa\) (s\(^{-1}\)) is then calculated as:

\(\kappa = C T^N e^{\frac{-D}{T}} OH\)

where C, N and D are the species-specific constants defined in the SPECIES file. T is the absolute temperature, and \(OH\) the OH concentration (Atkinson 1997). The OH-related mass loss is then calculated as:

\(m(t + \Delta t') = m(t) e^{−\kappa \Delta t'}\)

with \(\Delta t'\) being the reaction time step given by lsynctime.

In order to be able to use higher spatially and temporally resolved OH fields, the user has to modify the OH-related FLEXPART subroutines (readOHfield, gethourlyOH, ohreaction, located in the ohr_mod.f90 module) to be able to read in and process these other OH fields.