Patricia A. Nadeau Christine R. Sealing Michael J. Cappos Christoph Kern Brianna Lopez Hannah Calleja 20260305 The data are both tabular digital data (SO2 emission rates) and individual *.std ASCII files (spectra). Hilo, HI U.S. Geological Survey https://doi.org/10.5066/P1N2DWRY https://www.sciencebase.gov/catalog/item/698fc90db66b01ea6aa36728 Sulfur dioxide (SO2) emission rates are a fundamental measurement for the study and monitoring of active volcanoes. Such emissions inform scientists about the level of volcanic activity or unrest, and they also present a hazard to surrounding populations. Here we present a continuation of a long-term dataset collected by the Hawaiian Volcano Observatory for over four decades using UV spectrometers in below-plume traverses [Casadevall and others, 1987; Elias and others, 1998; Elias and Sutton, 2002; Elias and Sutton, 2007; Elias and Sutton, 2012; Elias and others, 2018a; Elias and others, 2018b; Elias and others, 2020; Kern and others, 2020; Nadeau and others, 2023]. Traverse-based emission rate measurements included herein were compiled between 2023 and 2025 and span six eruptions at Kīlauea, as well as the quiescent periods between eruptions. No SO2 emission rate measurements were made at other Hawaiian volcanoes (e.g., Mauna Loa) during the timeframe of this data release; only Kīlauea was emitting SO2 in quantities measurable by UV spectrometer. The dataset begins with measurements obtained during the January 2023 summit eruption of Kīlauea. SO2 emissions remained elevated as the eruption persisted into March; following the end of the eruption, emissions dropped to background (~100 tonnes/day, or t/d) until the onset of the June 2023 summit eruption. Again, increased SO2 emissions persisted through to the end of the roughly two-week eruption. SO2 emission rates dropped to background once more, and remained low until the September 2023 summit eruption, which lasted only one week. No additional eruptive activity occurred in 2023. In June 2024, however, the first eruption in 50 years on Kīlauea’s Southwest Rift Zone (SWRZ) took place over the course of approximately nine hours. Unlike most prior eruptions, including those in 2023 included here, SO2 emissions remained elevated for a few days following the end of lava effusion. The next eruption took place over the course of roughly one week in September, on Kīlauea’s Middle East Rift Zone (MERZ), in and near Nāpau Crater. For the duration of the eruption, SO2 emissions varied with the intensity of eruptive activity, but were very low by the end of the eruption and no longer detectable just a few days later. The final portion of this data release covers the December 2024 summit eruption, which, after an initial few days of fissure-based activity, established itself as a prolonged, episodic lava fountain eruption, with pauses on the order of days to weeks between episodes. As such, associated emission rates are variable: all emission rates are above background, but fountaining events emitted tens of thousands of tonnes per day of SO2 while inter-episode emissions were typically over an order of magnitude less. Also adding to the variability of the measured emission rates was the presence of gas pistoning cycles [e.g., Nadeau and others, 2015; Patrick and others, 2016] during many of the pauses from mid-March 2025 onward. In this data release, we include raw spectra collected by ultraviolet (UV) spectrometers on traverses beneath SO2 plumes, derived SO2 emission rates and associated metadata for each individual traverse, and means of SO2 emission rates for each separate traverse campaign/set of traverses. Data are separated by the three eruption sites/degassing locations: Kīlauea summit (Halemaʻumaʻu), Kīlauea MERZ (Nāpau Crater), and Kīlauea SWRZ. References Casadevall, TJ., Stokes, J.B., Greenland, L.P., Malinconico, L.L., Casadevall, J.R., and Furukawa, B.T. (1987). SO2 and CO2 emission rates at Kilauea Volcano, 1979-1984, chap. 29 in Decker, R.W., Wright, T.L., and Stauffer, P.H., eds., Volcanism in Hawaii: U.S. Geological Survey Professional Paper 1350, v. 1, p. 771-780. Elias, T., and Sutton, A. J. (2002). Sulfur Dioxide Emission Rates of Kīlauea Volcano, Hawai‘i, an Update: 1998–2001. U.S. Geological Survey Open-File Report 02–460, 29. Available at: https://pubs.usgs.gov/of/2002/of02-460/ Elias, T., and Sutton, A. J. (2007). Sulfur Dioxide Emission Rates from Kīlauea Volcano, Hawai‘i, an Update: 2002–2006. U.S. Geological Survey Open-File Report 2007–1114, Version 1.0, 37. Available at: https://pubs.usgs.gov/of/2007/1114/ Elias, T., and Sutton, A. J. (2012). Sulfur Dioxide Emission Rates from Kīlauea Volcano, Hawai‘i, 2007–2010. U.S. Geological Survey Open-File Report 2012–1107, 25. Available at: https://pubs.usgs.gov/of/2012/1107/ Elias, T., Sutton, A. J., Stokes, J.B., Casadevall, T.J. (1998). Sulfur Dioxide Emission Rates of Kīlauea Volcano, Hawai‘i, 1979-1997. U.S. Geological Survey Open-File Report 98-462 Version 1.0. Available at: https://pubs.usgs.gov/of/1998/of98-462/ Elias, T., Kern, C., Horton, K., Garbeil, H., and Sutton, A.J. (2018a). SO2 emission rates from Kilauea Volcano, Hawaii (2014-2017): U.S. Geological Survey data release, https://doi.org/10.5066/F7794402 Elias, T., Kern, C., Horton, K. A., Sutton, A. J., & Garbeil, H. (2018b). Measuring SO2 Emission Rates at Kīlauea Volcano, Hawaii, Using an Array of Upward-Looking UV Spectrometers, 2014–2017. Frontiers in Earth Science, 6(214). https://doi.org/10.3389/feart.2018.00214 Elias, T., Kern, C., Sutton, A.J., and Horton, K. (2020). Sulfur dioxide emission rates from Kīlauea Volcano, Hawaii, 2008-2013: U.S. Geological Survey data release, https://doi.org/10.5066/P9K0EZII Kern, C., Lerner, A. H., Elias, T., Nadeau, P. A., Holland, L., Kelly, P. J., Werner, C. A., Clor, L. E., & Cappos, M. (2020). Quantifying gas emissions associated with the 2018 rift eruption of Kīlauea Volcano using ground-based DOAS measurements. Bulletin of Volcanology, 82(7), 55. https://doi.org/10.1007/s00445-020-01390-8 Nadeau, P.A., Werner, C.A., Waite, G.P., Carn, S.A., Brewer, I.D., Elias, T., Sutton, A.J., and Kern, C. (2015). Using SO2 camera imagery and seismicity to examine degassing and gas accumulation at Kīlauea Volcano, May 2010. Journal of Volcanology and Geothermal Research, 300, 70-80. https://doi.org/10.1016/j.jvolgeores.2014.12.005 Nadeau, P.A., Kern, C., Cappos, M.J., Elias, T., Warren, S.M., Lerner, A.H., Sealing, C.R., Slagle, C.L., Moisseeva, N., Holland, L.D., Clor, L., and Werner, C.A. (2023). Sulfur dioxide emission rates from Hawaiian volcanoes, 2018-2022: U.S. Geological Survey data release, https://doi.org/10.5066/P9SNW2B7. Patrick, M.R., Orr, T., Sutton, A.J., Lev, E., Thelen, W., and Fee, D. (2016). Shallowly driven fluctuations in lava lake outgassing (gas pistoning), Kīlauea Volcano. Earth and Planetary Science Letters, 433, 326-338. https://doi.org/10.1016/j.epsl.2015.10.052 The data were collected to track emission rates of sulfur dioxide at volcanoes in Hawaii as a means of monitoring and better understanding volcanic activity. 20230101 20251231 ground condition Complete None planned -155.3584 -155.1167 19.4465 19.2807 ISO 19115 Topic Category geoscientificInformation None Volcano Sulfur dioxide SO2 emission rate DOAS Spectroscopy USGS Thesaurus volcanology USGS Metadata Identifier USGS:698fc90db66b01ea6aa36728 None Hawaii Kīlauea None. Please see 'Distribution Info' for details. None. Users are advised to read the dataset's metadata thoroughly to understand appropriate use and data limitations. Patricia A Nadeau U.S. Geological Survey, ALASKA REGION Research Geologist mailing and physical

1266 Kamehameha Ave., Suite A8

Hilo HI 96720 US 808-967-8806 pnadeau@usgs.gov Spectral data has a signal to noise ratio of better than 250:1. Dark noise is less than 50 RMS counts per co-added spectrum. Stray light is specified to less than 0.1% at 435 nm. Corrected linearity of response is better than 99%. SO2 emission rates data have been filtered for outliers that are not representative of likely true emission rates from the volcanoes, as with instrumental issues or poor atmospheric conditions. Data are considered complete for the information presented, as described in the abstract. Users are advised to read the rest of the metadata record carefully for additional details. No formal positional accuracy tests were conducted No formal positional accuracy tests were conducted Step 1: Spectral acquisition These are the raw data acquired by the DOAS spectrometers. Aside from the on-board linearity correction which occurs prior to transfer to disk, no additional processing was applied to the raw spectra provided in this dataset. 20260201 Step 2: Plume speed estimation Derivation of SO2 emission rates from spectrometer traverses relies on an accurate estimation of plume propagation speed, with calculated emission rates being linearly dependent on the assumed plume speed. During trade wind conditions, wind speed (as a proxy for plume speed) was determined as in Nadeau and others [2023] using HVO’s nearby telemetered anemometers. During kona wind conditions, depending on the specific wind direction and plume behavior, the same anemometer network was sometimes used. In the case of kona winds that advected the plume either too high or too far away from our ground anemometer network, we relied on modeled data from the European Centre for Medium-Range Weather Forecasts (ECMWF) weather forecast model (https://www.ecmwf.int/) or from colleagues at the Vog Measurement and Prediction (VMAP) project [Businger and others, 2015; Moisseeva and others; 2023]. References Businger, S., Huff, R., Pattantyus, A., Horton, K., Sutton, A. J., Elias, T., & Cherubini, T. (2015, 01 Oct. 2015). Observing and Forecasting Vog Dispersion from Kīlauea Volcano, Hawaii. Bulletin of the American Meteorological Society, 96(10), 1667-1686. https://doi.org/https://doi.org/10.1175/BAMS-D-14-00150.1 Moisseeva, N., Businger, S., & Elias, T. (2023). VogCast: a framework for modeling volcanic air pollution and its application to the 2022 eruption of Mauna Loa volcano, Hawai`i. Journal of Geophysical Research: Atmospheres, 128, e2023JD039281. http://doi.org/10.1029/2023JD039281 Nadeau, P.A., Kern, C., Cappos, M.J., Elias, T., Warren, S.M., Lerner, A.H., Sealing, C.R., Slagle, C.L., Moisseeva, N., Holland, L.D., Clor, L., and Werner, C.A. (2023). Sulfur dioxide emission rates from Hawaiian volcanoes, 2018-2022: U.S. Geological Survey data release, https://doi.org/10.5066/P9SNW2B7. 20260201 Step 3: Emission rate calculation Methodology for this data release is identical to that described in a previous data release of SO2 emission rates for 2018-2022 [Nadeau and others, 2023] with the exception of wavelength fit windows. All data were processed using a variable wavelength fit window between 305 and 340 nm with the exception of one date in 2023 that used a fixed window (within the same range as the variable window). Unlike lower East Rift Zone SO2 emission rate measurements included in Nadeau and others [2023] and Kern and others [2020], no additional specialized processing was required for measurements included in this dataset. References Kern, C., Lerner, A. H., Elias, T., Nadeau, P. A., Holland, L., Kelly, P. J., Werner, C. A., Clor, L. E., & Cappos, M. (2020). Quantifying gas emissions associated with the 2018 rift eruption of Kīlauea Volcano using ground-based DOAS measurements. Bulletin of Volcanology, 82(7), 55. https://doi.org/10.1007/s00445-020-01390-8 Nadeau, P.A., Kern, C., Cappos, M.J., Elias, T., Warren, S.M., Lerner, A.H., Sealing, C.R., Slagle, C.L., Moisseeva, N., Holland, L.D., Clor, L., and Werner, C.A. (2023). Sulfur dioxide emission rates from Hawaiian volcanoes, 2018-2022: U.S. Geological Survey data release, https://doi.org/10.5066/P9SNW2B7. 20260201 Point Point 0.000001 0.000001 Decimal degrees KIL_site_SO2_2023-2025_indiv.csv Comma Separated Value (CSV) file containing data for individual traverses, where site is one of MERZ, summit, or SWRZ. Producer Defined Date/Time [UTC] Data acquisition date and time in UTC Producer Defined 1/1/2023 12/31/2025 M/D/Y HH:MM in UTC time SO2 Emission Rate [t/d] Emission rate of sulfur dioxide (SO2) in metric tonnes per day Producer Defined ND SO2 not detected Producer defined 26 214277 tonnes per day (t/d) SO2 Emission Rate Error [t/d] Calculated error on SO2 emission rate Producer Defined NAN no data Producer defined 5 51050 tonnes per day (t/d) Wind Speed [m/s] Wind speed used in calculation of SO2 emission rate Producer Defined 3.14 13.17 meters per second (m/s) Wind Speed Error [m/s] Estimated error on wind speed used in SO2 emission rate calculation Producer Defined 1 3 meters per second (m/s) Wind Direction [deg] Wind direction used in SO2 emission rate calculation Producer Defined 0 359 degrees Wind Direction Error [deg] Estimated error on wind direction used in SO2 emission rate calculation Producer Defined 0 81 degrees Wind Data Source Source from which the wind data used in the SO2 emission rate calculation was obtained Producer Defined MG21_KIL Anemometer data from the Kīlauea multi-GAS monitoring station, operated by the Hawaiian Volcano Observatory Producer defined HRSDH x 1.2 Anemometer data from the Sand Hill high-resolution SO2 monitoring station (multiplied by 1.2), operated by the Hawaiian Volcano Observatory Producer defined GFS Global Forecast System weather model from the National Weather Service Producer defined VMAP Output from the VMAP (Vog Measurement and Prediction) project models Producer defined HRCPK/HRSDH average x 1.2 Average of anemometer data from Cone Peak and Sand Hill high-resolution SO2 monitoring stations, operated by the Hawaiian Volcano Observatory Producer defined HRCPK/HRSDH/HRPKE average Average of anemometer data from Cone Peak, Sand Hill, and Pu‘ukoa‘e high-resolution SO2 monitoring stations, operated by the Hawaiian Volcano Observatory Producer defined HRCPK Anemometer data from the Cone Peak high-resolution SO2 monitoring station, operated by the Hawaiian Volcano Observatory Producer defined HRPKE Anemometer data from the Pu‘ukoa‘e high-resolution SO2 monitoring station, operated by the Hawaiian Volcano Observatory Producer defined HRHLK Anemometer data from the Halona Kahakai high-resolution SO2 monitoring station, operated by the Hawaiian Volcano Observatory Producer defined DOAS Fit Range [nm] Wavelength range used for deriving SO2 column path amounts, in nanometers Producer Defined 305-340; variable fit 305-340 nanometers, with a smaller subset of the range possible as determined by the variable fit model in the data processing software Producer defined 308-340 308 to 340 nanometers Producer defined Traverse Method Means by which traverses beneath the plume were conducted Producer Defined car DOAS was attached to a car Producer defined helicopter DOAS was attached to a helicopter Producer defined Vent location for processing (latitude) Latitude (WGS84) of the vent location for the emissions measured Producer Defined 19.366461 19.4062 degrees (latitude) Vent location for processing (longitude) Longitude (WGS84) of the vent location for the emissions measured Producer Defined -155.146008 -155.311725 degrees (longitude) KIL_site_SO2_2023-2025_means.csv Comma Separated Value (CSV) file containing data for means of sets of traverses, where site is one of MERZ, summit, or SWRZ. Producer Defined Date/Time [UTC] Mean (average) data acquisition date and time in UTC Producer Defined 1/1/2023 12/31/2025 M/D/Y HH:MM in UTC time Mean SO2 Emission Rate [t/d] Mean (average) emission rate of sulfur dioxide (SO2) in metric tonnes per day Producer Defined ND SO2 not detected Producer defined 38 193223 tonnes per day (t/d) Mean SO2 Emission Rate Error [t/d] Mean (average) calculated error on SO2 emission rate Producer Defined NAN no data Producer defined 9 47537 tonnes per day (t/d SO2 Emission Rate Standard Deviation [t/d] Standard deviation of all SO2 emission rates in a set of traverses Producer Defined NAN no data Producer defined 11 20501 tonnes per day (t/d) Number of Traverses [n] Number of traverses in a set of traverses Producer Defined 1 14 Raw spectra Spectra are saved in three compressed *.zip files. SWRZ.zip contains spectra acquired beneath gas plumes emitted from active fissures in the Southwest Rift Zone. MERZ.zip contains spectra collected downwind of the East Rift Zone. Summit.zip contains spectra collected while traversing under gas plumes emitted from the volcano's summit. Each zip file contains a number of directories with the naming convention YYYY.MM.DD, where YYYY is the 4-digit year, MM is the 2-digit month, and DD is the 2-digit day of the date of data collection in Hawaiʻi Standard Time (UTC-10). Each of these directories contains one or multiple subdirectories corresponding to the one or multiple data acquisition segments performed on a given day. These subdirectories are named runXX, where XX is the 2-digit sequential number of the data acquisition segment. Inside each of these subdirectories, individually acquired radiance spectra are saved in the *.std ASCII format. These files are numbered chronologically as they were recorded. Besides the sequentially numbered measurement spectra, each subdirectory contains one or multiple reference spectra. Files beginning with the identifier ‘dark’ correspond to spectra recorded at a fixed exposure time with no light entering the spectrometer. Also collected with no light, the ‘darkcur’ identifier corresponds to spectra recorded to characterize the instrument’s dark current signal and spectra labeled ‘offset’ were recorded to characterize the spectrometer’s electronic offset. Finally, spectra labeled ‘sky’ were measured at the start of each data acquisition segment, typically (but not always) without any volcanic gas overhead. Each spectrum is saved in a *.std ASCII file. These files begin with a 3-line header specifying the file format and the number of spectral channels. Then, 2048 individual upward-looking radiances are reported. The spectral data are followed by metadata associated with each spectrum, including WGS84 Longitude (decimal degrees), WGS84 Latitude (decimal degrees), Altitude (m), ExposureTime of the spectral acquisition (ms), and NumScans which reports the number of individual spectra added together to form this reported measurement spectrum. The rest of the reported metadata is for internal use when opening the spectra in the freely available DOASIS software (https://novac-community.org/software) The spectra were measured by the authors with differential optical absorption spectroscopy instruments. Spectral radiance Relative intensity of incident radiation measured at numerous discrete wavelengths, in spectrometer 'counts' Producer Defined 0 4095 per co-added spectrum unitless measurement of relative intensity, often referred to as spectrometer 'counts' GS ScienceBase U.S. Geological Survey mailing address

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Denver CO 80225 United States 1-888-275-8747 sciencebase@usgs.gov Unless otherwise stated, all data, metadata and related materials are considered to satisfy the quality standards relative to the purpose for which the data were collected. Although these data and associated metadata have been reviewed for accuracy and completeness and approved for release by the U.S. Geological Survey (USGS), no warranty expressed or implied is made regarding the display or utility of the data for other purposes, nor on all computer systems, nor shall the act of distribution constitute any such warranty. Digital Data https://doi.org/10.5066/P1N2DWRY None 20260305 Patricia A Nadeau U.S. Geological Survey, ALASKA REGION Research Geologist mailing and physical

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Hilo HI 96720 US 808-967-8806 pnadeau@usgs.gov FGDC Content Standard for Digital Geospatial Metadata FGDC-STD-001-1998