It's not a particle, your example is a な-adjective.
Adjectives generally fall into two major groups, い-adjectives and な-adjectives.
I'm going to try to explain, but quickly since I've been drinking and am quite tired...
The main difference between these types of adjectives is in how they conjugate and connect to other words. い-adjectives sit right up against the words they modify (ex: 怖い犬 [こわいいぬ]), while な-adjectives need な to come in-between (ex: きれいな帽子 [きれいなぼうし]). Though when な-adjectives aren't being used to directly modify something, they often drop their な (ex: あの女の人がきれいです [あのおんなのひとがきれいです]).
That explanation is super simple and leaves out a lot and probably has a lot of holes, but I hope it might tide you over until someone with a clearer head, and most likely, a better grasp of the intricacies of Japanese can give you a better explanation.