Charles David Keeling of the Scripps Institute of Oceanography began collecting CO2 measurements on Mauna Loa in 1958 and the graph illustrating the subsequent increase in CO2 to the present is often referred to as the Keeling Curve. The graph usually is based on monthly average CO2 concentrations and thus shows both the long term increase in CO2, but also the decline of CO2 in the Northern Hemisphere spring and summer due to plant growth and a rise in the fall due to plant decay.
I call the 400 parts per million milestone somewhat ill-defined because CO2 does fluctuate. We hit 400 this week, but the drawdown from plant growth means we will drop below it in the coming months. It will probably take about 3 years until CO2 is above 400 parts per million for the entire year.
The observatory at Mauna Loa is at high elevation in the central Pacific and one of the better places to measure the "background" CO2 concentrations. You can find higher CO2 concentrations in areas where emissions are high, especially when the atmosphere is poorly ventilated. Jim Ehleringer's group at the University of Utah maintains a number of CO2 sensors in the Salt Lake Valley. This morning, the CO2 concentration in Sugarhouse was as high as 460 parts per million.
These high values merely reflect the trapping of CO2 produced primarily by fossil fuel combustion near the ground. This CO2 eventually mixes into the free atmosphere and contributes to the ongoing increase reflected in the Keeling Curve.