There are CO2 concentration measurements made in places other than Mauna Loa.
Here, for instance, is a plot of the annual growth in CO2 from monitoring stations in Mauna Loa (MLO), American Samoa (SMO), and at the South Pole (SPO).
If one looks at the seasonally adjusted absolute concentration time series, there is a lag between the northern hemisphere concentration (i.e. MLO) and the southern hemisphere of about a year, but they otherwise track well.
More recently, there are satellite observations that can track regional concentrations and their diffusion through the atmosphere.
Of all the metrics associated with anthropogenic climate change theory, that CO2 is rising and it's due to human emissions is one of the least uncertain.