What's the meaning of "combustion scalar"and....
 

I was confused by the meaning of "combustion scalar" and "mass fraction" in coal combustion model. for example, the combustion scalar3 is the nox mass fraction,and now it equals 5E-4,then what's the meaning of the number 5E-4? Is it equals 500ppm??? or the mass fraction2 is the mass fraction of oxidant,and now it equals 0.4, then then the concentration of oxidant is 0.4*22%,is it right?