Read these plz :
it says : pascal's symbol is ' Pa '
http://www.wikipedia.org/wiki/Pascal_(unit)
http://www.wikipedia.org/wiki/Pressure
http://www.wikipedia.org/wiki/Atmosphere_(unit)
dont forget that in st's book , p.397
Pa , isnt written like that ( Pa ) ... It is written with special physical form which makes the letter (a) under letter ( P)