They circulate at any time around the city, so they are very comfortable when traveling at night or if you are carrying weight, although, of course, their cost is much higher than in public transport.
The flag drop is €2.30 and they charge a supplement both to go to or come from the Airport, Fira de Barcelona, Moll Adossat Maritime Station and transport stations,
If you want to take a taxi from the airport:
T1: in the arrivals area (floor 0) and in the Barcelona-Madrid corridor.
T2: in front of terminals T2A, T2B and T2C.
Extra fees: Pick up or drop off at the airport or cruise terminal: €4.30. Pick up or drop off at the Fira Barcelona exhibition center: €2.50. Pick up or drop off at transport stations (Estació de Sants, França station or Nord bus station: €2.50.