== Overview ==
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[[NoteBox(note,This page is part of the **Aerosol model** (SALSA) documentation. \\ It contains a description of SALSA. \\ For an overview of all SALSA-related pages\, see the **[wiki:doc/tec/aerosol SALSA main page]**.)]]

The aerosol module SALSA ([wiki:doc/app/salsaref/ Kokkola et al., 2008]) embedded in PALM ([wiki:doc/app/salsaref/ Kurppa et al., 2019]) can be used to simulate the aerosol particle concentrations, size distributions and chemical compositions. In SALSA, the aerosol size distribution is represented as a discrete set of size bins (by default 10 bins). 
The number n,,i,, (m^-3^) and mass concentration m,,c,i,, (kg m^-3^) of each size bin i and chemical component c are the model prognostic variables. Currently, the following chemical components can be included: sulphuric acid (H,,2,,SO,,4,,), organic carbon (OC), black carbon (BC), nitric acid (HNO,,3,,), ammonium (NH,,3,,), sea salt (SS), dust (DU) and water (H,,2,,O). Furthermore, the gaseous concentrations of H,,2,,SO,,4,,, HNO,,3,,, NH,,3,, and semi- and non-volatile organic vapours (OCSV and OCNV) are also default prognostic variables. 
The aerosol dynamic processes included are
 * coagulation,
 * condensation of H,,2,,SO,,4,,, organics (both OCSV and OCNV) and water vapour, 
 * dissolutional growth by HNO,,3,, and NH,,3,,, 
 * nucleation and dry deposition on horizontal and vertical surfaces and resolved-scale vegetation. 

SALSA can be coupled with the [/wiki/doc/app/chemistry_parameters#chemistrymodule "chemistry module"]. In that case, the five gaseous compounds (H,,2,,SO,,4,,, HNO,,3,,, NH,,3,,, OCNV and OCSV) will be imported to SALSA from the chemistry module and must thus be included in the chemical mechanism applied (see an [https://palm.muk.uni-hannover.de/trac/browser/palm/trunk/UTIL/chemistry/gasphase_preproc/mechanisms/def_salsa%2Bsimple "example"] for a chemical mechanism including the gases needed in SALSA).

SALSA is enabled by adding the NAMELIST {{{salsa_parameters}}} with appropriate parameters to the INPUT parameter file ({{{<run_identifier>_p3d}}}).

== Boundary conditions for the SALSA variables ==

By default, SALSA inherits its boundary conditions from PALM. Additionally, SALSA can be run with horizontal boundary conditions that differ from the remaining PALM code. For example, it is possible to:

* Use non-cyclic boundary conditions for the SALSA variables, while cyclic conditions are used for all other variables. If there are aerosol sources/sinks in the simulation domain, using cyclic conditions for the SALSA variables would lead to accumulation and a constant increase/decrease of concentrations in the domain.
* Disable self-nesting for SALSA variables and run SALSA e.g., only within a child domain (see [wiki:doc/app/salsa_parameters#nesting_salsa nesting_salsa]).
* Disable offline nesting for SALSA variables (see [wiki:doc/app/salsa_parameters#nesting_offline_salsa nesting_offline_salsa]).

All the scenarios above require setting the respective boundary condition parameters [wiki:doc/app/salsa_parameters#bc_aer_n bc_aer_n], [wiki:doc/app/salsa_parameters#bc_aer_s bc_aer_s], [wiki:doc/app/salsa_parameters#bc_aer_l bc_aer_l],[wiki:doc/app/salsa_parameters#bc_aer_r bc_aer_r], [wiki:doc/app/salsa_parameters#bc_aer_b bc_aer_b] and [wiki:doc/app/salsa_parameters#bc_aer_t bc_aer_t].