Particulate matter
In this Guide we explain how particulate matter (PM) is computed in GEOS-Chem 14.7.0 and later versions. (For PM definitions in versions prior to 14.7.0, please see the PM2.5 and PM10 in older GEOS-Chem versions section.)
PM2.5 definition
PM2.5 refers to particulate matter having a diameter of \(2.5 \ \mu m\) or less. We use the following PM2.5 definition in GEOS-Chem, as approved by the Aerosols Working Group.
Growth factor |
Multiplies these species |
Value at 35% RH |
Value at 50% RH |
|---|---|---|---|
SIA_GROWTH |
SO4, NIT, NH4, HMS |
1.10 |
1.35 |
ORG_GROWTH |
OCPI, SOA |
1.05 |
1.07 |
SSA_GROWTH |
SALA |
1.86 |
1.86 |
The OA changes at both RH, and the SIA change at 50% RH are straightforward changes to yield consistency between with the current Kappa-Kohler hygroscopicity parameterization in GEOS-Chem based on Latimer and Martin [2019].
The SIA recommendation at 35% RH is less certain since it depends on the efflorescence RH of the SIA in the aerosol mixture under the variable conditions of the instruments, collection media, and laboratories involved. Given knowledge gaps about the aerosol phase at low RH, the proposed growth factor of 1.1 assumes that half of the particles are aqueous (growth factor of 1.19 for Kappa-Kohler) and the other half are crystalline (growth factor of unity).
These growth factors are calculated using the change in radius between different RH. Essentially, the change in radius between the dry (i.e. 0% RH) and wet (35% or 50% RH) aerosol is treated as a shell of water for the purposes of calculating the additional mass associated with the wet particle. Under this condition, it can be shown that:
where:
\(R_{w}\) is the wet aerosol radius
\(R_{d}\) is the dry aerosol radius
\(\rho_{H_2O}\) is the density of water
\(\rho_{d}\) is the dry aerosol density
Emissions from the Anthropogenic Fugitive, Combustion and Industrial Dust (AFCID) (cf Philip et al. [2017]) are automatically added to DSTbin1, DSTbin2, DSTbin3, and DSTbin4 in most GEOS-Chem simulations. AFCID is activated by default but can be disabled by the user if so desired.
The DSTbin4 size bin includes mineral dust aerosols with diameter both smaller and larger than \(2.5 \ \mu m\). Dandan Zhang has determined that 54.6% of DSTbin4 should be included in PM2.5 (cf. Zhang et al. [2025]).
In summary, PM2.5 in GEOS-Chem is computed as:
PM2.5 = ( NH4 + NIT + SO4 + HMS ) * SIA_GROWTH
+ BCPI
+ BCPO
+ ( OCPO + ( OCPI * ORG_GROWTH ) ) * ( OM/OC ratio )
+ DSTbin1
+ DSTbin2
+ DSTbin3
+ ( DSTbin4 * 0.546 )
+ ( SALA * SSA_GROWTH )
+ ( SOA * ORG_GROWTH )
Note
Some modifications to this basic definition are necessary, depending on the SOA species that are used in a given GEOS-Chem simulation. See PM2.5 and PM10 diagnostics below for details.
For 35% RH this evaluates to:
PM2.5 = ( NH4 + NIT + SO4 + HMS ) * 1.10
+ BCPI
+ BCPO
+ ( OCPO + ( OCPI * 1.05 ) ) * ( OM/OC ratio )
+ DSTbin1
+ DSTbin2
+ DSTbin3
+ ( DSTbin4 * 0.546 )
+ ( SALA * 1.86 )
+ ( SOA * 1.05 )
For 50% RH, this evaluates to:
PM2.5 = ( NH4 + NIT + SO4 + HMS ) * 1.35
+ BCPI
+ BCPO
+ ( OCPO + ( OCPI * 1.07 ) ) * ( OM/OC ratio )
+ DSTbin1
+ DSTbin2
+ DSTbin3
+ ( DSTbin4 * 0.546 )
+ ( SALA * 1.86 )
+ ( SOA * 1.07 )
By default, the OM/OC ratio is set to a constant value of 1.4. For users who seek more information on the seasonal and spatial variation of OM/OC in the lower troposphere, we provide the option to use the seasonal gridded dataset developed by Philip et al. [2014]. This dataset has some uncertainty, but offers more information than a global-mean OM/OC ratio in regions where primary organic aerosols have a large fossil fuel source.
Lastly, PM2.5 is converted to STP for diagnostic archival in GEOS-Chem according to the ideal gas law:
PM10 definition
PM10 refers to particulate matter with a diameter of \(10 \ \mu m\) or less. We compute PM10 in GEOS-Chem as follows:
PM10 = PM2.5
+ ( DSTbin4 * 0.454 )
+ DSTbin5
+ DSTbin6
+ ( DSTbin7 * 0.156 )
+ ( SALC * SSA_GROWTH )
For 35% RH, this evaluates to:
PM10 = PM2.5
+ ( DSTbin4 * 0.454 )
+ DSTbin5
+ DSTbin6
+ ( DSTbin7 * 0.156 )
+ ( SALC * 1.86 )
For 50% RH, this evaluates to:
PM10 = PM2.5
+ ( DSTbin4 * 0.454 )
+ DSTbin5
+ DSTbin6
+ ( DSTbin7 * 0.156 )
+ ( SALC * 1.86 )
DSTbin4 cont
The constant scale factors for DSTbin4 (0.454) and DSTbin7 (0.156) are taken from Zhang et al. [2025]. They represent the fraction of aerosols in DSTbin4 with \(D \gt 2.5 \ \mu m\), and the fraction of aerosols in DSTbin7 with \(D \le 10.0 \ \mu m\).
Lastly, PM10 is converted to STP for diagnostic archival in GEOS-Chem according to the ideal gas law:
PM2.5 and PM10 diagnostics
The PM2.5 and PM10 diagnostics belong to the AerosolMass collection in the GEOS-Chem History diagnostics). They are computed according to the code below, which may be found in aerosol_mod.F90.
Avoid double-counting of ISOAAQ species
It was determined that the PM2.5 diagnostic was erroneously including the ISOAAQ species in the accounting of PM2.5 when the Simple SOA option was used. After discussion with the Aerosols Working Group, the PM2.5 and PM10 diagnostic computations were modified accordingly:
SOA option |
Add this to PM2.5 and AOD diagnostics |
|---|---|
When Complex SOA is selected |
TSOA + ASOA + ISOAAQ |
Otherwise |
SOAS (simple SOA species) |
The GEOS-Chem benchmark simulations carry both Simple SOA and Complex SOA species, but only the Simple SOA species (SOAS) is included in diagnostic output.
PM2.5 and PM10 in older GEOS-Chem versions
GEOS-Chem versions prior to 14.7.0 used 4 bulk dust species (DST1, DST2, DST3, DST4) instead of 7. These versions used the following definition for PM2.5:
PM2.5 = ( NH4 + NIT + SO4 + HMS ) * SIA_GROWTH
+ BCPI
+ BCPO
+ ( OCPO + ( OCPI * ORG_GROWTH ) ) * ( OM/OC ratio )
+ DST1
+ ( DST2 * 0.30 )
+ ( SALA * SSA_GROWTH )
+ ( SOA * ORG_GROWTH )
and for PM10:
PM10 = PM2.5
+ ( DST2 * 0.7 )
+ DST3
+ ( DST4 * 0.9 )
+ ( SALC * SSA_GROWTH )
The growth factors SIA_GROWTH, SSA_GROWTH, and
ORG_GROWTH are defined above.