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. Sulfuric acid (H2SO4) is commonly accepted as a key precursor for atmospheric new particle formation (NPF). However, directmeasurements of [H2SO4] remain challenging, thereby preventing the determination of this important quantity, and, consequently, a completeunderstanding of its contribution to the NPF process. Several proxies have been developed to bridge the gaps, but their ability to predict[H2SO4] under very specific conditions, such as those encountered in volcanic plumes (including, in particular, high sulfur dioxide mixingratios), has not been evaluated so far. In this context, the main objective of the present study was to develop new proxies for daytime[H2SO4] under volcanic plume conditions and compare their performance to that of the proxies available in the literature. Specifically, thedata collected at Maïdo during the OCTAVE (Oxygenated organic Compounds in the Tropical Atmosphere: variability and atmosphere–biosphere Exchanges) 2018 campaign, in the volcanic eruption plume of the Piton de la Fournaise, were first used to deriveseven proxies based on knowledge of the sulfur dioxide (SO2) mixing ratio, global radiation, condensation sink (CS) and relative humidity(RH). A specific combination of some or all of these variables was tested in each of the seven proxies. In three of them (F1–F3), all consideredvariables were given equal weight in the prediction of [H2SO4], whereas adjusted powers were allowed (and determined during the fittingprocedure) for the different variables in the other four proxies (A1–A4). Overall, proxies A1–A4 were found to perform better than proxies F1–F3,with, in particular, improved predictive ability for [H2SO4] &gt; 2 × 108 cm−3. The CS was observed to play animportant role in regulating [H2SO4], whereas the inclusion of RH did not improve the predictions. A last expressionaccounting for an additional sink term related to cluster formation, S1, was also tested and showed a very good predictive ability over the wholerange of measured [H2SO4]. In a second step, the newly developed proxies were further evaluated using airborne measurements performed inthe passive degassing plume of Etna during the STRAP (Synergie Transdisciplinaire pour Répondre aux Aléas liés aux Panaches volcaniques) 2016 campaign. Increased correlations between observed and predicted [H2SO4] wereobtained when the dependence of predicted [H2SO4] on the CS was the lowest and when the dependence on [SO2] was concurrentlythe highest. The best predictions were finally retrieved by the simple formulation of F2 (in which [SO2] and radiation alone were assumedto explain the variations in [H2SO4] with equal contributions), with a pre-factor adapted to the STRAP data. All in all, our resultsillustrate the fairly good capacity of the proxies available in the literature to describe [H2SO4] under volcanic plume conditions, but they concurrentlyhighlight the benefit of the newly developed proxies for the prediction of the highest concentrations([H2SO4] &gt; 2–3 × 108 cm−3). Moreover, the contrasting behaviours of the new proxies in the two investigateddatasets indicate that in volcanic plumes, like in other environments, the relevance of a proxy can be affected by changes in environmentalconditions and that location-specific coefficients do logically improve the predictions.

Investigation of several proxies to estimate sulfuric acid concentration under volcanic plume conditions