The blower is pressurizing the A-coil region of the air handler, so the air coming out of the condensate drain line is expected but certainly not desirable. After all, it is cool, conditioned air that you are blowing directly into your attic! As you suspect, you need to add a p-trap to the condensate discharge line. The draining condensate sets up a water column in the p-trap that resists the air pressure but still lets water drain through the trap. I would also run two drains lines out to daylight, one the p-trapped line from the air handler and the other from the the emergency pan. IMO, the emergency pan is there for emergencies and should only be used when the primary drain plugs. If you only have one single drain line, eventually the emergency pan will fail, rust out, etc., and then you will have an emergency!

https://www.amazon.com/Rectorseal-83113-113-Trap-4-Inch/dp/B00BMV6QU4/ref=asc_df_B00BMV6QU4/?tag=hyprod-20&linkCode=df0&hvadid=194019181411&hvpos=&hvnetw=g&hvrand=7634480883287024367&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9016116&hvtargid=pla-309643599825&psc=1

Thanks for your time!

However, this isn't the main condensate line, that already has a trap and is not leaking air. This one is overflow to the pan in case the main line plugs. If there were a trap to prevent airflow, I imagine it would dry out soon enough.