“ NE ” ( not estimated ) for existing emissions by sources and removals by sinks of greenhouse gases which have not been estimated . Where “ NE ” is used in an inventory for emissions or removals of CO2 , N2O , CH4 , HFCs , PFCs , or SF6 , the Party should indicate , using the completeness table of the common reporting format , why emissions could not be estimated ; “ NA ” ( not applicable ) for activities in a given source/sink category that do not result in emissions or removals of a specific gas .