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<p>The 6-month-long 2014–2015 Holuhraun eruption was the largest in Iceland for 200 years, emitting huge quantities of sulfur dioxide (<span class="inline-formula">SO₂</span>) into the troposphere, at times overwhelming European anthropogenic emissions. Weather, terrain and latitude made continuous ground-based or UV satellite sensor measurements challenging. Infrared Atmospheric Sounding Interferometer (IASI) data are used to derive the first time series of daily <span class="inline-formula">SO₂</span> mass present in the atmosphere and its vertical distribution over the entire eruption period. A new optimal estimation scheme is used to calculate daily <span class="inline-formula">SO₂</span> fluxes and average <span class="inline-formula"><i>e</i></span>-folding time every 12&thinsp;h. For the 6 months studied, the <span class="inline-formula">SO₂</span> flux was observed to be up to 200&thinsp;kt&thinsp;day<span class="inline-formula">⁻¹</span> and the minimum total <span class="inline-formula">SO₂</span> erupted mass was <span class="inline-formula">4.4±0.8</span>&thinsp;Tg. The average <span class="inline-formula">SO₂</span> <span class="inline-formula"><i>e</i></span>-folding time was <span class="inline-formula">2.4±0.6</span> days. Where comparisons are possible, these results broadly agree with ground-based near-source measurements, independent remote-sensing data and values obtained from model simulations from a previous paper. The results highlight the importance of using high-resolution time series data to accurately estimate volcanic <span class="inline-formula">SO₂</span> emissions. The <span class="inline-formula">SO₂</span> mass missed due to thermal contrast is estimated to be of the order of 3&thinsp;% of the total emission when compared to measurements by the Ozone Monitoring Instrument. A statistical correction for cloud based on the AVHRR cloud-CCI data set suggested that the <span class="inline-formula">SO₂</span> mass missed due to cloud cover could be significant, up to a factor of 2 for the plume within the first kilometre from the vent. Applying this correction results in a total erupted mass of <span class="inline-formula">6.7±0.4</span>&thinsp;Tg and little change in average <span class="inline-formula"><i>e</i></span>-folding time. The data set derived can be used for comparisons to other ground- and satellite-based measurements and to petrological estimates of the <span class="inline-formula">SO₂</span> flux. It could also be used to initialise climate model simulations, helping to better quantify the environmental and climatic impacts of future Icelandic fissure eruptions and simulations of past large-scale flood lava eruptions.</p>