Short answers, pulled from the story.
Charles David Keeling installed infrared gas analyzers on the slopes of Mauna Loa in 1958. This remote location sat far from continents and lacked vegetation to ensure clean data.
Measurements show a steady increase from 313 parts per million in March 1958. By November 2018, levels reached 406 parts per million.
Scientists chose the volcano specifically because it stood above the thermal inversion layer. Incoming ocean breezes carried air that represented the free atmosphere over the Northern Hemisphere.
Responsibility transferred to his son Ralph Keeling after the elder scientist passed away. On the fiftieth anniversary of the project start, Ralph wrote an article for Science magazine describing how the work evolved over time.
On the 9th of May 2013, daily mean concentration surpassed 400 parts per million. Estimates suggest carbon dioxide has not reached this level since the mid-Pliocene era between 2 and 4 million years ago.