Air quality The BlueHDi exhaust line is fitted with three pollution control subassemblies: p an oxidation catalyst; p an additive particulate filter that enables the removal of particulates in number, regardless of their size and regardless of driving conditions; p an after-treatment system called selective catalytic reduction (SCR) positioned upstream of the additive particulate filter, which eliminates up to 90% of nitrogen oxide (NOx) emitted by the engine.