Particulate matter in GEOS-Chem

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

PM2.5

Definition

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)

PM2.5 in the 1-yr benchmark output

Here is the definition of PM2.5 that we use to create the plots from 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 )
           + ( ( BCPI + BCPO )       * ppb_ugm3 * 12 )
           + ( ( OCPI + OCPO ) * 2.1 * ppb_ugm3 * 12 )
           + ( ( NO3         )       * ppb_ugm3 * 62 )
           + ( ( DST1 + DST2 )       * ppb_ugm3 * 29 )

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

--Bob Y. 10:20, 9 April 2014 (EDT)