は is sometimes used as particle(or postposition?) e.g. わたしはにほんじんです。(watashi wa nihon jin desu/ I am Japanese.) On the other hand, わ is not particle(or postposition) は and わ are the same pronounciation "wa" in case that は is worked as postposition. But the other situation, は is pronounced "ha". sorry that I don't know the reason we Japanese pronounce は(wa and ha)