This question has been discussed extensively in the Development Q&A’s with Jorg and Seb and Martial. Getting atmospheric particulates correct is on the roadmap, I think for next year sometime at this point. I think they talked about this in December last year or something. You may find it listed in the development roadmap.
Watch some of the Development Q&A’s there’s a ton of information in them about what Asobo is thinking and when they plan to work on different aspects of the sim. Speculating what and when Asobo will introduce things is fun, but, kind of pointless when they already have said what they are working on and sometimes about when they plan on implementing things. Right now, their main focus through the end of the year is on the flight model and implementing helicopters and computational fluid dynamics and its affect on the surfaces of aircraft. Weather is also important and development around it is ongoing.
Why are atmospheric effects difficult? Because getting it right is super, super hard. I realize it may look easy if you play a lot of games, but, as you can see by all the posts here, people expect it to be right. So the biggest problem is, how to recreate something that typically takes massive supercomputers to calculate and simplifying the problem to be handled by a small machine.