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<p>For over 6 months, the 2014–2015 effusive eruption at Holuhraun, Iceland, injected considerable amounts of sulfur dioxide (<span class="inline-formula">SO₂</span>) into the lower troposphere with a daily rate of up to one-third of the global emission rate, causing extensive air pollution across Europe. The large injection of <span class="inline-formula">SO₂</span>, which oxidises to form sulfate aerosol (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M3" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">SO</mi><mn mathvariant="normal">4</mn><mrow><mn mathvariant="normal">2</mn><mo>-</mo></mrow></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="29pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="6060a0eb6022af681aa55d19b3180df9"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-24-1939-2024-ie00001.svg" width="29pt" height="17pt" src="acp-24-1939-2024-ie00001.png"/></svg:svg></span></span>), provides a natural experiment offering an ideal opportunity to scrutinise state-of-the-art general circulation models' (GCMs) representation of aerosol–cloud interactions (ACIs). Here we present Part 1 of a two-part model inter-comparison using the Holuhraun eruption as a framework to analyse ACIs. We use <span class="inline-formula">SO₂</span> retrievals from the Infrared Atmospheric Sounding Interferometer (IASI) instrument and ground-based measurements of <span class="inline-formula">SO₂</span> and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M6" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">SO</mi><mn mathvariant="normal">4</mn><mrow><mn mathvariant="normal">2</mn><mo>-</mo></mrow></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="29pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="a8455a3a3390243c17ea2f3ca419ac4e"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-24-1939-2024-ie00002.svg" width="29pt" height="17pt" src="acp-24-1939-2024-ie00002.png"/></svg:svg></span></span> mass concentrations across Europe, in conjunction with a trajectory analysis using the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model, to assess the spatial and chemical evolution of the volcanic plume as simulated by five GCMs and a chemical transport model (CTM). IASI retrievals of plume altitude and <span class="inline-formula">SO₂</span> column load reveal that the volcanic perturbation is largely contained within the lower troposphere. Compared to the satellite observations, the models capture the spatial evolution and vertical variability of the plume reasonably well, although the models often overestimate the plume altitude. HYSPLIT trajectories are used to attribute to Holuhraun emissions 111 instances of elevated sulfurous surface mass concentrations recorded at European Monitoring and Evaluation Programme (EMEP) stations during September and October 2014. Comparisons with the simulated concentrations show that the modelled ratio of <span class="inline-formula">SO₂</span> to <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M9" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">SO</mi><mn mathvariant="normal">4</mn><mrow><mn mathvariant="normal">2</mn><mo>-</mo></mrow></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="29pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="06a1e144313624090049b6627390d3e8"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-24-1939-2024-ie00003.svg" width="29pt" height="17pt" src="acp-24-1939-2024-ie00003.png"/></svg:svg></span></span> during these pollution episodes is often underestimated and overestimated for the young and mature plume, respectively. Models with finer vertical resolutions near the surface are found to better capture these elevated sulfurous ground-level concentrations. Using an exponential function to describe the decay of observed surface mass concentration ratios of <span class="inline-formula">SO₂</span> to <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M11" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">SO</mi><mn mathvariant="normal">4</mn><mrow><mn mathvariant="normal">2</mn><mo>-</mo></mrow></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="29pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="815783a157bc15e547bdd7a24388d96b"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-24-1939-2024-ie00004.svg" width="29pt" height="17pt" src="acp-24-1939-2024-ie00004.png"/></svg:svg></span></span> with plume age, the in-plume oxidation rate constant is estimated as 0.032 <span class="inline-formula">±</span> 0.002 <span class="inline-formula">h⁻¹</span> (1.30 <span class="inline-formula">±</span> 0.08 <span class="inline-formula">d</span> <span class="inline-formula"><i>e</i></span>-folding time), with a near-vent ratio of 25 <span class="inline-formula">±</span> 5 (<span class="inline-formula">µg m⁻³</span> of <span class="inline-formula">SO₂</span> <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M20" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="78c74280a32911099c6aadbec3864e34"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-24-1939-2024-ie00005.svg" width="8pt" height="14pt" src="acp-24-1939-2024-ie00005.png"/></svg:svg></span></span> <span class="inline-formula">µg m⁻³</span> of <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M22" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">SO</mi><mn mathvariant="normal">4</mn><mrow><mn mathvariant="normal">2</mn><mo>-</mo></mrow></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="29pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="47e678c4680581e78f7a83f3b1df9ebc"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-24-1939-2024-ie00006.svg" width="29pt" height="17pt" src="acp-24-1939-2024-ie00006.png"/></svg:svg></span></span>). The majority of the corresponding derived modelled oxidation rate constants are lower than the observed estimate. This suggests that the representation of the oxidation pathway/s in the<span id="page1940"/> simulated plumes is too slow. Overall, despite their coarse spatial resolutions, the six models show reasonable skill in capturing the spatial and chemical evolution of the Holuhraun plume. This capable representation of the underlying aerosol perturbation is essential to enable the investigation of the eruption's impact on ACIs in the second part of this study.</p>