If you have set the participation type in your registration values (ex. In person registration has been set to In-person), then you can filter in several queries to only email by that participation type.
Click on Dashboard
Click on Attendee module
Go to Reports or Email/Download
Select the Complete attendees query
Click Go
Select Email
Select the participation type you want to email - this will email only those who have a main registration value that has been denoted as that participation type