Puzzled By The Low-Sulfur Focus

I mean, those cruise liners can really crank out the NOx! But if today's focus is sulfur, so be it:

Carnival Corp., the world’s largest cruise operator, agreed to spend $180 million to cut sulfur emissions from 32 ships, bowing to demands by the Environmental Protection Agency to clean up pollution it causes near ports.

Under the agreement with the EPA, announced today by Carnival and the agency, the company will to install scrubbers and diesel particulate filters on its ships to cut pollutants that can exacerbate smog or discharge microscopic particles dangerous to humans’ lungs.