Robert, congratulations. You're the 1,000,000th person to notice volcanoes near the Mauna Loa Observatory. It is not affecting the observationsNormally during Climate discussions, one encounters the Mauna Loa claim as to amount of CO2. Well hell, it sits on the same island as does 2 active volcanos.
Mauna Loa Observatory sits in an isolated location so it can capture clean air flowing over the Pacific Ocean. There is little direct influence from pollution due to the vastness of the ocean, but there are some local, natural processes that can change the amount of CO2 over short amounts of time (such as a few hours). For instance, nearby plants and trees can temporarily draw CO2 out of the air through photosynthesis.
Gases from Mauna Loa’s crater, which is 4 miles (6 kilometers) away from the observatory, can temporarily raise the amount of CO2 in the air. Prevailing winds generally prevent volcanic gases from reaching the observatory, but when the wind is light, it can happen. However, the influx of volcanic CO2 is immediately noticeable because the readings jump upward sharply, and by amounts far larger than normal seasonal fluctuations. These brief spikes are not included in the final data because they’re caused by short-term, local variations, and they don’t reflect the long-term average that the observatory strives to measure.
Another way to answer the question is to look at CO2 data from other parts of the world. If Mauna Loa data were contaminated by the volcano, then those measurements would look different from air samples collected in Alaska or at the South Pole. Air samples are regularly collected from observatories, tall towers, aircraft, and weather balloons at 86 locations worldwide. These data from around the planet all follow the same trend.

How Do We Know Mauna Loa Carbon Dioxide Measurements Don't Include Volcanic Gases? - NASA Science
The amount of CO2 in the atmosphere is measured at Mauna Loa Observatory, Hawaii, and all around the world. NASA also measures CO2 from space. Data from

It is important but I don't know how you would decide which is the MOST important gas. How did you decide CO2 got the title?Carbon Dioxide is Earths most important gas.