The Area 51 FAQ page answers the question:
Should my idea be part of an existing site, or its own site?
In general, if a site makes sense as part of a bigger site, it's better to have one big site than a bunch of little niche sites. Site X should be subsumed by site Y if:
Almost all X questions are on-topic for site Y
If Y already exists, it already has a tag for X, and nobody is complaining
You're not creating such a big group that you don't have enough experts to answer all possible questions
There's a high probability that users of site Y would enjoy seeing the occasional question about X
At the rate that people are voting for questions... it will take years to get this proposed site into beta. In my opinion, the subject content of "air quality modeling" is too narrow to attract the number of votes it takes to make it successful. It would be better for the air quality modeling community to take advantage of the current Earth Science stack exchange site, especially since there are already many questions about air quality modeling on the site.
That being said, there may be a happy middle-ground. If the site name was changed to simply "Air Quality", it would broaden the focus (thereby attracting more users/votes) and also provide the means to Question/Answer related questions like "What does the new Regional Haze Rule mean for my state?" or "What are the different types of FRMs and FEMs for PM2.5?".
Or, if we wanted to really attract a high-volume of traffic without losing the theme of modeling, the name could be changed to "Air Quality & Climate Modeling", which could attract a surge of both scientist and non-scientist users/voters. This type of site might even get the momentum to get out of Beta someday.
Related to this question, I would like to point out that the lead-in statement for the Air Quality Modeling proposed site is narrow in focus. It says:
Proposed Q&A site for scientists, environmental regulators, and air quality modeling experts from around the world
In my opinion, a public stack exchange site should serve the purpose of anyone interested in the topic, not just professionals and university students. The site should be a public site that welcomes people from all disciplines, not just people that work in our field. If the purpose of the site is to cater to mostly just active air quality model users, a public stack exchange may not be the place for it.
Overall, I think if people could start asking/answering questions about CMAS-related topics now, it would be of great benefit to the user community. Furthermore, I don't see any benefit from having a narrow-focused site, especially if the community has to wait years for it to be enacted.