You have to subscribe to the state of continuous presence - to imply that an object has been invisibly mentioned and whose pre-existence is inextricably required in the situation:
She heard the bell ring. She opened a window. She walked down the stairs. She opened the door.
Would you say,
She heard a bell ring. She opened a window. She walked down a flight of stairs. She opened a door.
Of course not, because it is presumed and implied the door bell is present, the flight of stairs is right there outside her bedroom, and there is one and only one a front door in most middle-class houses. But which window did she open?
Unless you are the Q of E living in many palaces with many doors and flights of stairs. I think during the time of Cromwell (am I right?), when taxes were levied on windows, and most houses had only one window, you would say, "She opened the window."
The air properties of an experiment. "The" is a determinate article. There is one and only one set of air properties from the beginning of the experiment. You would not say,
Define a set of air properties for the experiment.
That would mean conjuring any set of air properties which you would need to coerce into the experiment.
Let us write the beginning of an "interesting" novel. This is the beginning. Besides the usual preface and introduction, these are the first sentences.
I alighted from the car. I saw the ocean. The morning was beautiful. The birds were chirping. Spring has finally sprung itself from its slumber.
Has "the" car, "the" ocean, "the" morning or "the" birds been previously mentioned?
This pattern of English uses the definite article "the" to indicate to the reader to focus on the existing (implied pre-mentioned) singleton entities.
(singleton = one and only one).
Define "the" existing air properties of "the" experiment, not any irrelevant air properties from any neighbourhood.
Let's say you have been invited to a friend's house which you have never visited. On arrival, you tweeted, "I am @ G's house. The barn is gorgeous."
Would you tweet, "A barn is gorgeous"? Why not? You had never seen or even mentioned or had someone mention to you about any barn. You had not even known that the house had a barn.