# Difference between revisions of "Particulate matter in GEOS-Chem"

## PM2.5

### Definition

Randall Martin wrote:

The PM2.5 calculation should account for aerosol water of PM2.5 in a way that is consistent with the PM2.5 measurements. The water content of PM2.5 is operationally defined. In the United States an RH of 35% is often used and would be appropriate for the benchmark. In Europe an RH of 50% is often used.
Below are the factors that account for aerosol water in a consistent manner as used in jv_spec.dat from GC v10-01-01, as calculated by Aaron van Donkelaar and Sajeev Philip.
At 35% RH:
1.33 for SO4, NIT, and NH4
1.16 for OCPI and SOA
1.86 for SALA
At 50% RH the values are
1.51 for SO4, NIT, and NH4
1.24 for OCPI and SOA
2.42 for SALA
These growth factors are calculated using the change in radius between different RH within jv_spec.dat. 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:
WetMass2DryMassRatio = 1 + [{(radiusAtRH_wet / radiusAtRH_dry)^3 - 1} x (Density_Water / Density_DrySpecies)]
The DST2 bin includes aerosols with diameter both smaller and larger than 2.5 um. Lengthy discussion with Duncan Fairlie, Aaron van Donkelaar, Colette Heald, Jeff Pierce and Noelle Selin led to the conclusion that 38% of the DST2 bin should be included in the calculation of PM2.5.
In summary, the PM2.5 benchmark and wiki documentation should be changed to
```  PM25     = 1.33 (NH4 + NIT  + SO4) + BCPI + BCPO + 2.1 (OCPO + 1.16 OCPI) + 1.16 SOA
+ DST1 + 0.38 DST2 + 1.86 SALA
```
where the tracer units in ug/m3 at STP, and the value of 2.1 is the global mean OM/OC as recommended by the Aerosol WG.
SOA should be defined to include all SOA tracers used in the benchmark. This will evolve as the Aerosol WG recommends new SOA options. At present, the best way to represent SOA is as
• TSOA0, TSOA1, TSOA2, TSOA3 = Lumped semivolatile aerosol products of monoterpene + sesquiterpene ox.
• ISOA1, ISOA2, ISOA3, = Lumped semivolatile aerosol products of isoprene oxidation
• ASOAN, ASOA1, ASOA2, ASOA3 = Lumped nonvolatile aerosol products of light aromatics and IVOCs
The molecular weights are 150 g/mol for all SOA tracers.
For users that seek more information on the seasonal and spatial variation of OM/OC in the lower troposphere, we make available the seasonal gridded dataset developed by Philip et al. (2014). This dataset may slightly underestimate OM/OC in biomass burning regions, but offers more information than a global-mean OM/OC ratio in regions where primary organic aerosols have a large fossil fuel source.

--Melissa Sulprizio (talk) 13:19, 30 June 2016 (UTC)

### PM2.5 in the 1-yr benchmark plots

#### GEOS-Chem v11-01 and later versions

In GEOS-Chem v11-01 the definition of PM2.5 was changed following the recommendation by Randall Martin and the Aerosol WG as described above. The definition now used to create the plots for GEOS-Chem 1-year benchmark simulations is:

```  ; Convert ppbv to ug/m3
STP_P     = 1013.25
STP_T     = 298.
ppb_ugm3  = 1e6 / 8.314 * 100. * STP_P/STP_T *1e-9

MWaer     = [18, 12, 12, 62, 96, 29, 31.4] ; NH4, EC, OC, NIT, SO4, DUST, SALA

NH4_ugm3  = NH4( indlon, indlat, 0)*ppb_ugm3*MWaer(0)
NIT_ugm3  = NIT( indlon, indlat, 0)*ppb_ugm3*MWaer(3)
SO4_ugm3  = SO4( indlon, indlat, 0)*ppb_ugm3*MWaer(4)
BCPI_ugm3 = BCi( indlon, indlat, 0)*ppb_ugm3*MWaer(1)
OCPI_ugm3 = OCi( indlon, indlat, 0)*ppb_ugm3*MWaer(2)
BCPO_ugm3 = BCo( indlon, indlat, 0)*ppb_ugm3*MWaer(1)
OCPO_ugm3 = OCo( indlon, indlat, 0)*ppb_ugm3*MWaer(2)
DST1_ugm3 = Dst1(indlon, indlat, 0)*ppb_ugm3*MWaer(5)
DST2_ugm3 = Dst2(indlon, indlat, 0)*ppb_ugm3*MWaer(5)
SALA_ugm3 = Dst2(indlon, indlat, 0)*ppb_ugm3*MWaer(6)

; Compute PM2.5
PM25 = 1.33 * ( NH4_ugm3 + NIT_ugm3 + SO4_ugm3 ) + &
( BCPI_ugm3 + BCPO_ugm3 ) + 2.1*( OCPI_ugm3 + OCPO_ugm3 ) + &
1.12*SOA_ugm3 + DST1_ugm3 + (0.38 * DST2_ugm3) + (1.86 * SALA_ugm3)
```

where the tracer units are ug/m3 at STP, and the value of 2.1 is the global mean OM/OC as recommended by the Aerosol WG.

--Melissa Sulprizio (talk) 13:19, 30 June 2016 (UTC)

#### GEOS-Chem v10-01 and earlier versions

Here is the definition of PM2.5 that we use to create the plots for GEOS-Chem 1-year benchmark simulations:

```  ; Convert ppbv to ug/m3
STP_P    = 1013.25
STP_T    = 298.
ppb_ugm3 = 1e6 / 8.314 * 100. * STP_P/STP_T *1e-9

; Compute PM2.5
PM25     = ( ( NH4         )       * ppb_ugm3 * 18 )
+ ( ( NIT         )       * ppb_ugm3 * 62 )
+ ( ( SO4         )       * ppb_ugm3 * 96 )
+ ( ( BCPI + BCPO )       * ppb_ugm3 * 12 )
+ ( ( OCPI + OCPO ) * 2.1 * ppb_ugm3 * 12 )
+ ( ( DST1 + DST2 )       * ppb_ugm3 * 29 )
```

Where NH4, NIT, SO4, BCPI, BCPO, OCPI, OCPO, NO3, DST1, and DST2 have units of ppbv.

We use the value 2.1 for OM:OC, but note from Jeff Pierce's comment in the section above that this is an open question.

Colette Heald wrote:

For completeness PM2.5 should include accumulation mode sea salt (SSa), but that's not included in the PM2.5 calculations used in the benchmarks, I think just for historical reasons since the code was written for continental US sites. It may be worth adding this comment to the wiki for folks who are using this as a guideline for calculating PM2.5.

--Bob Y. 17:11, 13 February 2015 (EST)

### PM2.5 diagnostic

In GEOS-Chem v11-01g, PM2.5 concentrations were added to the ND42 diagnostic for SOA concentrations. The PM2.5 calculation in diag42_mod.F follows the recommendation by Randall Martin and the Aerosol WG as described above.

--Melissa Sulprizio (talk) 16:48, 1 September 2016 (UTC)

### Past discussions

Xinyi Dong wrote:

I am trying to get PM2.5 from GEOS-Chem and have a question regarding how to sum the aerosol species to get PM2.5. In one paper Tai et al., 2012, the author said "total PM2.5 in GEOS-Chem is taken to be the sum of sulfate, nitrate, ammonium, OC and EC". While in another paper Liu et al., 2004, JGR, the author also include fine dust and sea salt (although he was examing AOT, not exactly PM2.5, but the fine dust aerosols have diameter less than 2.5um). I could not find an official equation for PM2.5 from GEOS-Chem website, so what aerosol species I need to sum in order to get PM2.5 ?
Here is the titles of the two papers:
1. Tai et al, 2012: Meteorological modes of variability for fine particulate matter (PM2.5) air quality in the United States: implications for PM2.5 sensitivity to climate change PDF
2. Liu et al., 2004: Mapping annual mean ground-level PM2.5 concentration using multiangle imaging spectroradiometer aerosol optical thickness over the contiguous United States.

Jeff Pierce replied:

Sulfate + Nitrate + Ammonium + OC + EC + SOA + FineSeaSalt + FineDust is probably the best bet for PM2.5 (don't forget to convert OC to OM). However, there is no perfect way in GEOS-Chem since size distributions vary and there will be some fraction of all species that are larger than 2.5 microns.

Xinyi Dong wrote:

Now I see why [the two papers] use different equations; my understanding is one paper is compared GEOS-Chem with surface observations having few impacts from sea salt and dust, while another compared with satellite AOT, so sea salt and dust were included.
I did a quick search for the conversion from OC to OM, one paper say the OM/OC ratio is 2.1, another say 1.4, so is there an official value suggested for v9-01-02?

Jeff Pierce replied:

OM:OC is an open question! 2.1 is generally more representative of aged organics, 1.4 would be fresh. I think many people use 1.8 if they choose a single value.

--Bob Y. 16:39, 7 February 2013 (EST)