"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."
In Brief: 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 around the planet all shows the same upward trend. The longest record of direct measurements of carbon dioxide (CO2) in Earth’s...
climate.nasa.gov