Q: 2009 Macbook Air capable of running OS X El Capitan

Yes, it's quite capable of running El Capitan. In fact you may find the computer performs better than with Lion. You don't mention how much RAM is installed, but Lion and later really need at least 4 GBs of RAM for somewhat decent performance. With 2 GBs, they will all be slow if you try running more than two applications concurrently and load them up with third-party utilities.

I would stick to Lion or add more RAM before you upgrade OS X.

You should at least consider using El Capitan.

Upgrade to OS X El Capitan - Apple Support

Regardless of whether you upgrade to El Capitan or not, the more important lesson for the future is to not simply stay on the operating system version that comes with the machine, but rather make the smaller, incremental steps of upgrading whenever operating system upgrades are released.  The whole process of evolving to a current system would have been so much simpler if you didn't overlook:

• Mountain Lion (OS X 10.8)
• Mavericks (OS X 10.9)
• Yosemite (OS X 10.10)

... and all the smaller incremental updates that have occurred since 2009.