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> 顔の大きさの半分 **はある**
> であろう大きなマスク。([source](https://www3.nhk.or.jp/news/html/20180801/k10011558961000.html?utm_int=news-
> life_contents_list-items_014))

I've been told that this sentence is describing the size of the mask being
half the size of the face, Can't we simply say 「顔の大きさの半分であろう大きなマスク」 instead?
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grammar particle-は
What is the purpose of はある in this sentence?
While you probably already know this, [であろう] is a literary/formal way of
saying だろう/でしょう. It should not be separated into で and あろう any more than でしょう
should be separated into で and しょう.

Let's rephrase the quote in question:

> 顔の大きさの半分はある **でしょう** 、大きなマスク。

Maybe it is easier to see now how the でしょう relates to the clause
[顔の大きさの半分はある], or 'The mask was as large as half the size of a face.'. The end
result (including でしょう) is a sentence more equivalent to '____ probably as
large as half the size of a face.'.

The はある expression would, in English, be stated as 'there exists/existed', or
'it is/was'.

If you search for 「はあるでしょう」or 「はあるだろう」, there are many results. On
[Kotonoha](http://www.kotonoha.gr.jp/shonagon/search_form) alone, there are
over 400 combined examples. If you change the particle は to も, there are over
1000 combined results.

Just to pick one out of the results:

> 「そんな経験が 誰にも 一度はある だろう。」 'That type of experience, for everyone there
> probably exists that one time.', or more colloquially, 'Everyone has
> probably had that type of experience.'

Now, to remove the であろう expression entirely. Again, Kotonoha has over 1300
results for 「はある。」. Here is one:

> 一度見る価値 はある。'It is worth seeing once.'

From the above sentence and how it relates to your main question, I hope that
you can see that はある helps to complete (round out) a clause.
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This type of は means "at least". That is, the mask covers roughly 55-70% of
the area of the face.

  * [Does は mean 'at least' in this sentence?](https://japanese.stackexchange.com/q/29438/5010)
  * [Can は be used to mean "at least"](https://japanese.stackexchange.com/q/33243/5010)

The sentence can be rephrased as 顔の大きさの半分はありそうな大きなマスク, 顔の大きさの半分以上あるであろう大きなマスク,
etc. But 顔の大きさの半分であろう大きなマスク does not have the same meaning.
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answer