Before the end of 2023, cars entering the Low Emission Zones of cities with more than 50,000 inhabitants will be required to display a sticker. Notable cities in this category are Valencia, Alicante, Gandia, Benidorm and Barcelona.
As a driver you have to be aware in these cities the Low Emission Zones will be signposted with a sign like that below, and that you are only allowed to enter with the appropriate sticker displayed.
In theory the stickers can be obtained from any Correos office, but in practice they often seem to have used up their stock. The stickers can also be ordered online from
https://www.pegatinas-dgt.com/en
Cost is 6.5 Euros.
You enter your car registration number and follow on from there.
You need a photo of your driving licence and vehicle permiso card (green one)
You add these images to upload, give your delivery address and payment details.
Ref
https://n332.es/how-will-low-emission-zones-be-managed-in-spain/