They are described in the HYSPLIT user's guide:
https://www.ready.noaa.gov/hysplitusersguide/S441.htm =>
"Other conversion rates or a greater number of pollutants can be defined by editing the CHEMRATE.TXT file in the local directory. This file is created automatically each time the Save button is pressed when the ICHEM=2 option is selected. The file is only required when conversions other than the default case. The file consists of one or more records, each record defining a pollutant conversion. The data are free-format and consist of four fields, the integer "from" and "to" pollutant index numbers, and the real hourly conversion "rate", and molecular weight adjustment "factor". For instance, if the file were to be defined for the default case, the one data record would have the following values: (1 2 0.10 1.0). The molecular weight adjustment factor can be used to account for other reactions not considered in the simple conversion module. For instance, if one were to define pollutants #1 and #2 as SO2 and SO4, respectively, then the molecular weight adjustment factor should be 1.5 as SO2 transforms to SO4 (the conversion picks up two additional oxygen molecules)."