MacBookAir 2014 MiniDP to DP 2560x1440
Yes. According to MacTracker.app:

External Resolution Up to 2560 by 1600 pixels.

Personally, I would connect the MBA to the Dell over HDMI, and get the highest quality MDP dongle. Apple used to sell its own, but now sells a Moshi Adaptor.

At my school, we use Belkin Adaptors sold  on Amazon (at this particular point in time) with a 66% discount price.