Dispararon al aire.

Dispararon is the preterite form of disparar in the 3rd person plural.

So it can mean:

• they fired / they shot
• sometimes you all fired / shot, if the subject is ustedes

In a sentence by itself, the most natural reading is usually they.

Spanish often drops subject pronouns because the verb ending already shows who the subject is.

Here, dispararon already tells us the subject is 3rd person plural, so Spanish does not need ellos or ellas unless the speaker wants emphasis or contrast.

So:

• Ellos dispararon al aire = correct, but more explicit
• Dispararon al aire = also correct, and very natural

Because a + el contracts to al in Spanish.

So:

• a + el aire → al aire

This contraction is mandatory in normal Spanish whenever a is followed by el.

A very similar contraction is:

• de + el → del

Because a here expresses direction: they fired toward / into the air.

So disparar al aire is the normal expression for firing into the air or shooting upward.

By contrast:

• en el aire usually means in the air
• it describes location, not direction

So:

• Dispararon al aire = they fired into the air
• Algo está en el aire = something is in the air

Nothing is missing. Disparar can be used without a direct object.

That means it can simply mean to fire / to shoot.

Examples:

• Disparó = he/she fired
• Dispararon al aire = they fired into the air

It can also take an object in other sentences, for example:

• Disparó una pistola = he/she fired a pistol
• Disparó una bala = he/she fired a bullet

So in your sentence, the focus is on the action itself and the direction, not on what weapon or projectile was used.

That is exactly right: nouns ending in -e can be masculine or feminine, so you usually have to learn their gender individually.

In this case, it is masculine:

• el aire

That is why the sentence has al = a + el.

Yes, grammatically it can.

The form dispararon is used for both:

• ellos/ellas = they
• ustedes = you all

However, in Spain, informal plural you is usually vosotros, whose preterite form would be:

• disparasteis

So in Spain, without context, dispararon is more likely to be understood as they fired.
If the speaker is addressing people formally, ustedes dispararon is still possible.

Because by the normal stress rules of Spanish, dispararon does not need one.

It ends in -n, so the stress naturally falls on the second-to-last syllable:

• dis-pa-RA-ron

Since that natural stress is already correct, no written accent is needed.

If the stress were somewhere unusual, then an accent mark would be required.

Yes, it is a very common and natural expression.

It is often used for situations like:

• warning shots
• celebratory gunfire
• shots fired upward rather than at a person or object

So a learner should treat disparar al aire as a useful chunk meaning to fire into the air.