The trade-off in diesel engines is between more CO2 and more nitrogen oxides. Burning lean mixtures with lots of excess oxygen tends to reduce both particulates

The trade-off in diesel engines is between more CO2 and more nitrogen oxides. Burning lean mixtures with lots of excess oxygen tends to reduce both particulates (since they are more completely burned) and CO2 (since combustion is more efficient) at a cost of nitrogen-oxygen side reactions.