Last year, regional monitors briefly hit 400 ppm in the Arctic. But those monitoring stations aren’t seen as a world mark like the one at Mauna Loa, Hawaii.
Generally carbon levels peak in May then fall slightly, so the yearly average is usually a few parts per million lower than May levels.
NOAA monitoring at Mauna Loa: http://www.esrl.noaa.gov/gmd/ccgg/trends/weekly.html
Seth Borenstein can be followed at http://twitter.com/borenbears