emittere‧mi‧tter /ɪˈmɪtə $ -ər/ noun [countable] 
a person, organization, or country that produces a greenhouse gas such as carbon dioxide or a substance that pollutes the environment: 
Which country is the biggest emitter of greenhouse gases?
a person, organization, or country that produces a greenhouse gas such as carbon dioxide or a substance that pollutes the environment: Which country is the biggest emitter of greenhouse gases?