Short answers, pulled from the story.
The principal volcanic gases are water vapor, carbon dioxide, sulfur dioxide or hydrogen sulfide depending on temperature, nitrogen, argon, helium, neon, methane, carbon monoxide, and hydrogen. Water vapor is consistently the most abundant, normally comprising more than 60 percent of total emissions, with carbon dioxide typically making up 10 to 40 percent.
The eruption of Mount Pinatubo in the Philippines on the 15th of June 1991 released approximately 18 plus or minus 4 teragrams of SO2. It was a Volcanic Explosivity Index 6 event, a scale of eruption that occurs only once every 50 to 100 years.
Volcanoes release far more gas between eruptions through passive degassing than during active eruptions. Fischer et al. (2019) estimated that from 2005 to 2015, non-eruptive passive degassing produced 23.2 teragrams of SO2 per year compared to 2.6 teragrams per year during eruptions. Eruptive CO2 accounts for less than 10 percent of total annual volcanic CO2 output.
Scientists measure volcanic gases using remote spectroscopy methods such as Differential Optical Absorption Spectroscopy (DOAS), Fourier Transform Infrared Spectroscopy (FTIR), and the Multi-Component Gas Analyzer System (Multi-GAS). Direct sampling uses evacuated flasks known as Giggenbach bottles, a method refined by German chemist Werner F. Giggenbach (1937-1997). Sulfur dioxide is the preferred monitoring target because it absorbs strongly in ultraviolet wavelengths and has low atmospheric background concentrations.
Passive degassing is a continuous process in which a volcano slowly releases gases even when not actively erupting, and it can persist for years. It accounts for the large majority of total volcanic gas output. Even small emissions from passive degassing can alter soil and water chemistry and affect local ecosystems.
Le Quere et al. estimated that human burning of fossil fuels and cement production released the equivalent of 34.1 gigatons of CO2 per year from 2006 through 2015. Volcanic CO2 estimates vary; Fischer et al. (2019) put total volcanic output at about 53 teragrams per year, while Werner et al. (2019) estimated 220-300 teragrams per year including diffuse soil degassing. Human activities currently account for the majority of atmospheric CO2 increases.