Hydrocarbon and particulate matter evolution in the exhaust manifold of a spark-ignited engine under cold-start operation

•Oxidation reactions persist into the exhaust manifold under cold-start operation.•THC, CO, and H2 are consumed while NOx is generated in exhaust manifold reactions.•Engine cylinder out particulate matter is mostly in organic carbon form.•Organic carbon particulate is consumed in exhaust manifold reactions.•Cold-start CFD models must include exhaust manifold to capture exhaust gas species.