Volcanoes release complex plumes of gas and particles. Volcanic gases, like SO₂, can additionally condense, once released, to form particles, sulphate aerosol (SA). Observing simultaneously SO₂+SA is important: their proportion provides information on the internal state of volcanoes, and can be used to predict plumes' atmospheric evolution and their environmental and climatic impacts. We developed a new method to observe simultaneously, for the first time, SO₂+SA using infrared remote sensing.