[Mister Micawber]

Using the indefinite article is easy if you are referring to no specific or aforementioned friendly atmosphere:

'I enjoy a tavern with a friendly atmosphere when I go drinking.'
'All museums have an atmosphere of reverence.'


'Maintain' is better if you are speaking of a working environment or other relatively objective situation. 'Preserve' suggests something more precious, perhaps: 'historical artifacts should be preserved'; 'a friendship should be preserved'.