Nope, trillions. The mass of the atmosphere is about 5.15 × 10¹⁸ kg [1], 400 ppm of that are 2.06 x 10¹⁵ kg or 2.06 x 10¹² t which are about two trillion (short scale) or two billion (long scale) metric tons. I used the short scale [2]. And because the 400 pm are by volume and not by mass you end up with about three trillion metric tons if you take that into account, too. Of course only unless I messed things up.

[1] https://en.wikipedia.org/wiki/Atmosphere_of_Earth

[2] https://en.wikipedia.org/wiki/Long_and_short_scales