Specifically, black carbon is a small, dark particle emitted, when fuel burns incompletely.

Why is black carbon not included in IMO's GHG strategy?

Firstly, during negotiations some oil producing countries support that black carbon is not a 'gas', thus it shouldn't be included in a GHG strategy.

Secondly, there is an argument that IMO is already working to decide if it should regulate black carbon emissions from ships.

A progress has been made from  marine engine manufacturers, as well as the governments of Canada, Denmark, Finland, Germany, Japan, the Republic of Korea, and the United States.

In addition, ships are a source of black carbon emissions and they're the only ones sailing in the Arctic and deposit black carbon exactly where they shouldn't; On snow and ice.

The shipping sector still expects IMO's decision on regulating black carbon.

As the Association states

We expect IMO delegates to start discussing potential black carbon control policies in 2020. It’s not clear what type of policy IMO member states and organisations will propose. If we look to the past, IMO has regulated fuel quality for all ships, set efficiency requirements for new ships, and limited emissions from new ships. Let’s take a look at some policy options.

A solution would be IMO setting a black carbon standard for international ships when operating in particularly sensitive areas such as the Arctic or, given that emissions of black carbon outside the Arctic can affect the Arctic environment, the standard could apply to all ships.

Therefore, ships could comply by using low-black carbon fuels or aftertreatment technologies such as diesel particulate filters.

In light of an immediate way of cutting black carbon emissions is to alter from residual fuels to distillate ones, with the latter reducing black carbon by 33%, on average.

Concluding, efficiency regulations for new ships under IMO’s Energy Efficiency Design Index (EEDI) are expected to continue to become more stringent and, by consequence, new ships will burn less fuel and emit less black carbon than their predecessors.