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