Adding a species not at an inlet but in the flow zone
 

So I have a cylindrical flowtube like in the picture below and I add the bulk species M and some species A. Now species A should be photolised in the yellow region to some species B. Of course this reaction does only take place within the yellow region so is it possible to define such reactions? Even more this reaction depends on the the time the fluid stays in the photolysis region. Thus regions in the center should become more concentrated compared to regions at the wall. Thx for any help.
http://fs5.directupload.net/images/161227/ucbxx2pu.png

Is that at all possible? or not?

Is it at least possible to define a concentration profile (of a species) at an inlet?!?