Sulphur dioxide emission rate measurements are an important tool for volcanic monitoring and eruption risk assessment. The SO₂ camera technique remotely measures volcanic emissions by analysing the ultraviolet absorption of SO₂ in a narrow spectral window between 300 and 320 nm using solar radiation scattered in the atmosphere. The SO₂ absorption is selectively detected by mounting band-pass interference filters in front of a two-dimensional, UV-sensitive CCD detector. One important step for correct SO₂ emission rate measurements that can be compared with other measurement techniques is a correct calibration. This requires conversion from the measured optical density to the desired SO₂ column density (CD). The conversion factor is most commonly determined by inserting quartz cells (cuvettes) with known amounts of SO₂ into the light path. Another calibration method uses an additional narrow field-of-view Differential Optical Absorption Spectroscopy system (NFOV-DOAS), which measures the column density simultaneously in a small area of the camera's field-of-view. This procedure combines the very good spatial and temporal resolution of the SO₂ camera technique with the more accurate column densities obtainable from DOAS measurements. This work investigates the uncertainty of results gained through the two commonly used, but quite different, calibration methods (DOAS and calibration cells). Measurements with three different instruments, an SO₂ camera, a NFOV-DOAS system and an Imaging DOAS (I-DOAS), are presented. We compare the calibration-cell approach with the calibration from the NFOV-DOAS system. The respective results are compared with measurements from an I-DOAS to verify the calibration curve over the spatial extent of the image. The results show that calibration cells, while working fine in some cases, can lead to an overestimation of the SO₂ CD by up to 60% compared with CDs from the DOAS measurements. Besides these errors of calibration, radiative transfer effects (e.g. light dilution, multiple scattering) can significantly influence the results of both instrument types. The measurements presented in this work were taken at Popocatépetl, Mexico, between 1 March 2011 and 4 March 2011. Average SO₂ emission rates between 4.00 and 14.34 kg s⁻¹ were observed.

Volcanic plumes are common and far-reaching manifestations of volcanic activity during and between eruptions. Observations of the rate of emission and composition of volcanic plumes are essential to recognize and, in some cases, predict the state of volcanic activity. Measurements of the size and location of the plumes are important to assess the impact of the emission from sporadic or localized events to persistent or widespread processes of climatic and environmental importance. These observations provide information on volatile budgets on Earth, chemical evolution of magmas, and atmospheric circulation and dynamics. Space-based observations during the last decades have given us a global view of Earth's volcanic emission, particularly of sulfur dioxide (SO₂). Although none of the satellite missions were intended to be used for measurement of volcanic gas emission, specially adapted algorithms have produced time-averaged global emission budgets. These have confirmed that tropospheric plumes, produced from persistent degassing of weak sources, dominate the total emission of volcanic SO₂. Although space-based observations have provided this global insight into some aspects of Earth's volcanism, it still has important limitations. The magnitude and short-term variability of lower-atmosphere emissions, historically less accessible from space, remain largely uncertain. Operational monitoring of volcanic plumes, at scales relevant for adequate surveillance, has been facilitated through the use of ground-based scanning differential optical absorption spectrometer (ScanDOAS) instruments since the beginning of this century, largely due to the coordinated effort of the Network for Observation of Volcanic and Atmospheric Change (NOVAC). In this study, we present a compilation of results of homogenized post-analysis of measurements of SO₂ flux and plume parameters obtained during the period March 2005 to January 2017 of 32 volcanoes in NOVAC. This inventory opens a window into the short-term emission patterns of a diverse set of volcanoes in terms of magma composition, geographical location, magnitude of emission, and style of eruptive activity. We find that passive volcanic degassing is by no means a stationary process in time and that large sub-daily variability is observed in the flux of volcanic gases, which has implications for emission budgets produced using short-term, sporadic observations. The use of a standard evaluation method allows for intercomparison between different volcanoes and between ground- and space-based measurements of the same volcanoes. The emission of several weakly degassing volcanoes, undetected by satellites, is presented for the first time. We also compare our results with those reported in the literature, providing ranges of variability in emission not accessible in the past. The open-access data repository introduced in this article will enable further exploitation of this unique dataset, with a focus on volcanological research, risk assessment, satellite-sensor validation, and improved quantification of the prevalent tropospheric component of global volcanic emission. Datasets for each volcano are made available at https://novac.chalmers.se (last access: 1 October 2020) under the CC-BY 4 license or through the DOI (digital object identifier) links provided in Table 1.

Fechamientos K-Ar y resultados paleomagnéticos para la región de Chapala nos permiten estimar la edad de las rocas del Grupo Tizapan (Mioceno Tardío-Plioceno Temprano), Grupo Chapala (Mioceno Tardío-Plioceno Temprano), Grupo Travasaño (Plioceno Temprano), Grupo Grande, Grupo Palo Verde y Grupo Zacoalco (Plioceno Tardío-Pleistoceno Temprano), Grupo Santa Cruz y Grupo Acatlán (Pleistoceno Temprano). En el Campo Volcánico de Michoacán-Guanajuato (CVMG) los estudios magnetoestratigráficos permiten documentar los patrones temporales y espaciales del vulcanismo. Las características del vulcanismo en el CVMG sugieren una migración de la actividad volcánica de aproximadamente 90 km hacia la trinchera durante el período comprendido entre los Crones Brunhes y Matuyama (0.78 Ma). En la región de Chapala se observa un patrón diferente, consistente en una migración espacial transicional y de menor magnitud ocurrida entre el Plioceno y, el Pleistoceno. Esta migración implica un cambio en la evolución magmática que se manifiesta en una disminución en el tamaño de los edificios volcánicos.