Abstract: An apparatus comprises: (i) a lean burn internal combustion engine; (ii) an exhaust system for treating a flowing exhaust gas from the engine, which system comprising: (a) a first substrate monolith comprising a NOx adsorber catalyst (NAC); and (b) a catalysed soot filter (CSF) comprising a filter substrate; and (iii) means for enriching the exhaust gas to provide an enriched exhaust gas composition intermittently during normal lean running operation for removing sulfate adsorbed on the NAC, wherein the exhaust system comprises: (c) a compound located downstream of at least some of the NAC, which compound is effective to remove and/or convert at least some hydrogen sulfide in enriched exhaust gas derived from removing sulfate adsorbed on the NAC.