Nothing (in) particular

I think "nothing particular" looks more natural in both these contexts. In my opinion "in particular" is used to mention some thing as a better example of a condition, situation, event etc.

Hello. Let me ask you one question.

Which sentence do you think is more natural?

1. Are you busy now?
No, nothing particular. No, nothing in particular.

2. What do you do in your free timea?
Nothing particular. Nothing in particular.

Thanks in advance.

I've never heard "nothing particular."

As written, the first sentence is a "yes or no" question. It would be "more natural" using the word "doing":
What are you doing? Nothing in particular.

Then, sentence 2:
What do you do in your spare time?
Nothing in particular.

Of course, the younger generation would answer "hanging out."

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I'm no grammarian but phrases/sayings need gramatical ligitimacy. So I'd agree with ESC, ie:
Are you busy?
A direct yes/no question. So the answer in this case should be "No. Nothing in particular" because the full responses would be:
"No. [I'm doing] nothing in particular".
The other alternative:
"No. [I'm doing] nothing particular" puts the emphasis on the particular which is not the question we are answering.

On the other hand the free time question seeks information beyond a simple yes/no so:
"Nothing particular" works because the emphasis is on particular, answering the question.

Anyone who has read this far might also consider in response to are your busy?:
"No. [I'm doing] nothing interesting" - which might explain why I'm responding to this question rather than getting on with my paid employment!