Particulate matter in GEOS-Chem

On this page we provide information about how to compute particulate matter concentrations from GEOS-Chem output.

Definitions of PM2.5 and PM10 for GEOS-Chem

PM2.5 definition

Below is the definition of PM2.5 used in GEOS-Chem and approved by the Aerosols Working Group.

This table lists hygroscopic growth factors for PM2.5 constituent species:

Scale factor	Multiplies these species	Value at 35% RH	Value at 50% RH
SIA_GROWTH	SO4, NIT, NH4	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:

GrowthFactor = 1 + [{(radiusAtRH_wet / radiusAtRH_dry)^3 - 1} x (Density_Water / Density_DrySpecies)]

Emissions from the Anthropogenic Fugitive, Combustion and Industrial Dust (AFCID) (cf Philip et al (2017) are automatically added to the DST1 bin in most GEOS-Chem simulations. AFCID is activated by default but can be disabled by the user if so desired.

The DST2 bin includes aerosols with diameter both smaller and larger than 2.5 um. Fangqun Yu has recently determined that 30% of DST2 should be included in PM2.5. (The prior value of 38%, which had been established by Duncan Fairlie, Aaron van Donkelaar, Colette Heald, Jeff Pierce and Noelle Selin, was used until GEOS-Chem 13.4.0.)

In summary, PM2.5 at 35% RH should be computed as:

PM25 = ( NH4 + NIT  + SO4 ) * 1.10
     + BCPI 
     + BCPO 
     + ( OCPO + ( OCPI * 1.05 ) ) * (OM/OC ratio)  # OM/OC ratio = 2.1 by default
     + DST1 
     + DST2 * 0.30                                 # F. Yu suggests 30% of DST2 (Nov 2011); prior value was 38% of DST2
     + SALA * 1.86
     + SOA  * 1.05

By default, the OM/OC ratio is set to a constant value of 2.1. 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.

NOTE: Some modifications to this basic definition are necessary, depending on the SOA species that are used in a given GEOS-Chem simulation. See the PM2.5 and PM10 diagnostics for GEOS-Chem section below for details.

PM10 definition

In GEOS-Chem 13.4.0 and later versions, PM10 at 35% RH is computed according to the following formula:

PM10 = PM2.5 
     + ( DST2 * 0.7  )
     + DST3
     + ( DST4 * 0.9  )
     + ( SALC * 1.86 )   # NOTE: The value of 1.86 is the SSA_GROWTH factor at 35% RH

The constant scale factors for DST2 (70%) and DST4 (90%) were determined by Fanqun Yu from APM aerosol microphysics simulations. For more information, please follow this link..

NOTE: Some modifications to this basic definition are necessary, depending on the SOA species that are used in a given GEOS-Chem simulation. See the PM2.5 and PM10 diagnostics for GEOS-Chem section below for details.

PM2.5 and PM10 diagnostics for GEOS-Chem

The PM2.5 and PM10 diagnostics belong to the the AerosolMass collection in the GEOS-Chem History diagnotics). They are computed according to the code below, which may be found in GeosCore/aerosol_mod.F90.

       !==============================================================
       ! P A R T I C U L A T E   M A T T E R
       !
       ! See this GEOS-Chem wiki page for the most up-to-date
       ! definitions of PM2.5 and PM10 used in GEOS-Chem:
       !
       ! http://wiki.geos.chem.org/Particulate_Matter_in_GEOS-Chem
       !==============================================================

       ! Particulate matter < 2.5um [kg/m3]
       PM25(I,J,L) = NH4(I,J,L)        * SIA_GROWTH + &
                     NIT(I,J,L)        * SIA_GROWTH + &
                     SO4(I,J,L)        * SIA_GROWTH + &
                     BCPI(I,J,L)                    + &
                     BCPO(I,J,L)                    + &
                     OCPO(I,J,L)                    + &
                     OCPI(I,J,L)       * ORG_GROWTH + &
                     SALA(I,J,L)       * SSA_GROWTH + &
                     SOILDUST(I,J,L,1)              + &   ! + 100% of DST1
                     SOILDUST(I,J,L,2)              + &   !
                     SOILDUST(I,J,L,3)              + &   ! 
                     SOILDUST(I,J,L,4)              + &   ! 
                     SOILDUST(I,J,L,5) * 0.3_fp           ! + 30%  of DST2

       ! Particulate matter < 10um [kg/m3]
       PM10(I,J,L) = PM25(I,J,L) +                    &   ! PM2.5
                     SOILDUST(I,J,L,5) * 0.7_fp     + &   ! + 70%  of DST2
                     SOILDUST(I,J,L,6)              + &   ! + 100% of DST3
                     SOILDUST(I,J,L,7) * 0.9_fp     + &   ! + 90%  of DST4
                     SALC(I,J,L)       * SSA_GROWTH

       ! Include either simple SOA (default) or Complex SOA in
       ! PM2.5 calculation.  In simulations where both Simple SOA and
       ! Complex SOA species are carried (i.e. "benchmark"), then
       ! only the Simple SOA will be added to PM2.5, in order to avoid
       ! double-counting. (bmy, 5/11/18)
       IF ( Is_SimpleSOA ) THEN
          PM25(I,J,L) = PM25(I,J,L) + ( SOAS(I,J,L) * ORG_GROWTH )
          PM10(I,J,L) = PM10(I,J,L) + ( SOAS(I,J,L) * ORG_GROWTH )

       ELSE IF ( Is_ComplexSOA ) THEN 
          PM25(I,J,L) = PM25(I,J,L)                 + &
                        TSOA(I,J,L)   * ORG_GROWTH  + &
                        ASOA(I,J,L)   * ORG_GROWTH  + &
                        ISOAAQ(I,J,L) * ORG_GROWTH        ! Includes SOAGX

          PM10(I,J,L) = PM10(I,J,L)                 + &  
                        TSOA(I,J,L)   * ORG_GROWTH  + &
                        ASOA(I,J,L)   * ORG_GROWTH  + &
                        ISOAAQ(I,J,L) * ORG_GROWTH        ! Includes SOAGX

          ! Need to add OPOA to PM2.5 for complexSOA_SVPOA simulations
          ! -- Maggie Marvin (15 Jul 2020)
          IF ( Is_OPOA ) THEN
             PM25(I,J,L) = PM25(I,J,L) + ( OPOA(I,J,L) * ORG_GROWTH )
             PM10(I,J,L) = PM10(I,J,L) + ( OPOA(I,J,L) * ORG_GROWTH )
          ENDIF
       ENDIF

       ! Apply STP correction factor based on ideal gas law
       PM25(I,J,L) = PM25(I,J,L) * ( 1013.25_fp / PMID(I,J,L) ) * &
                                   ( T(I,J,L)  / 298.0_fp     )

       PM10(I,J,L) = PM10(I,J,L) * ( 1013.25_fp / PMID(I,J,L) ) * &
                                   ( T(I,J,L)   / 298.0_fp    )

Also note that there are some calculations that were not included in the basic definitions of PM2.5 and PM10 in the preceding sections. These are:

Avoid double-counting of ISOAAQ species

Jenny Fisher rightly pointed out 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, we modified the PM2.5 and PM10 diagnostic computations accordingly:

To avoid double-counting of SOA, we do the following:

• When the Complex SOA option is selected, we add TSOA + ASOA + ISOAAQ to the PM2.5 and AOD diagnostics instead the simple SOA species SOAS.
• Otherwise, we add SOAS to the PM2.5 and AOD diagnostics instead of TSOA + ASOA + ISOAAQ.

NOTE: 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.

Save out PM2.5 diagnostic at STP conditions

Aaron van Donkelaar wrote:

As was originally implemented in GEOS-Chem v11-01, the PM2.5 diagnostic outputs were at ambient conditions. While this is not technically an error, most PM2.5 monitors measure at STP conditions which will cause disagreement during comparison with observations and inconsistency during application of any health response curves (generally determined from the STP observations).
As a result, I’d recommend applying an STP correction factor based on ideal gas law after PM2.5 is calculated:

        PM2.5 = PM2.5 * ( 1013.25 / P ) * ( T / 298 )
        PM10  = PM10  * ( 1013.25 / P ) * ( T / 298 )

--Bob Yantosca (talk) 19:41, 3 November 2021 (UTC)