In short, it is difficult to control {HEAD ~ both the filter temperature} and the concentration of {TAIL ~ nitrogen dioxide} in the filter to optimum values simultaneously 
of
In short , it is difficult to control { HEAD ~ both the filter temperature } and the concentration of { TAIL ~ nitrogen dioxide } in the filter to optimum values simultaneously
OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH