Yes, it's the species mass fraction, https://ansyshelp.ansys.com/account/Secured?returnurl=/Views/Secured/corp/v231/en/flu_udf/flu_udf_DataAccessMacros.html%23x1-4610004.2.3 Click on Help in the solver & paste the link into that browser.
The harder part is getting the cell that the particle is in to do that maths. That's covered in the DPM part, https://ansyshelp.ansys.com/account/Secured?returnurl=/Views/Secured/corp/v231/en/flu_udf/flu_udf_DPMDEFINE.html
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