Atmospheric Chemistry in Volcanic Plumes
Total Page：16
File Type：pdf, Size：1020Kb
Atmospheric chemistry in volcanic plumes Roland von Glasow1 School of Environmental Sciences, University of East Anglia, Norwich, NR4 7TJ, United Kingdom Edited by Barbara J. Finlayson-Pitts, University of California, Irvine, Irvine, CA, and approved February 22, 2010 (received for review November 15, 2009) Recent field observations have shown that the atmospheric plumes The model setup is based on observations for Mount Etna, Sicily, of quiescently degassing volcanoes are chemically very active, during nonexplosive stages (3, 7, 8, 9). The crater altitude of the pointing to the role of chemical cycles involving halogen species modeled volcano is 3,300 m; the modeled plume is nonconden- and heterogeneous reactions on aerosol particles that have pre- sing. It is important to mention that most volcanic emissions are viously been unexplored for this type of volcanic plumes. Key into the free troposphere with significantly increased lifetimes features of these measurements can be reproduced by numerical compared to emissions in the boundary layer. A plume that is models such as the one employed in this study. The model shows emitted into the boundary layer from a low-altitude volcano sustained high levels of reactive bromine in the plume, leading to might show significantly different behavior, partly due to the extensive ozone destruction, that, depending on plume dispersal, potential direct contact of the plume with the surface and can be maintained for several days. The very high concentrations of vegetation. sulfur dioxide in the volcanic plume reduces the lifetime of the OH Initial Plume Composition radical drastically, so that it is virtually absent in the volcanic plume. This would imply an increased lifetime of methane in volcanic Where volcanic volatiles and air meet there is a very rapid transi- plumes, unless reactive chlorine chemistry in the plume is strong tion from reducing to oxidizing conditions resulting in a very enough to offset the lack of OH chemistry. A further effect of bro- rapid change of the composition of the gas-particulate mixture. mine chemistry in addition to ozone destruction shown by the For an assessment of the atmospheric effects of volcanic emis- model studies presented here, is the oxidation of mercury. This re- sions, it is crucial to be able to quantify the initial plume compo- lates to mercury that has been coemitted with bromine from the sition. Experimentally it is rather difficult to measure the exact volcano but also to background atmospheric mercury. The rapid composition of volcanic volatiles, and in the literature the oxidation of mercury implies a drastically reduced atmospheric life- majority of the data about volcanic volatiles that are not already time of mercury so that the contribution of volcanic mercury to the influenced by atmospheric processing stem from measurements atmospheric background might be less than previously thought. at fumaroles. Gerlach (10) suggested that the assumption of ther- However, the implications, especially health and environmental ef- modynamic equilibrium might be applicable for high-tempera- fects due to deposition, might be substantial and warrant further ture mixtures of volcanic volatiles and ambient air and studies, especially field measurements to test this hypothesis. proposed, based on results of a thermodynamic equilibrium mod- el [HSC chemistry (11)] that “high-temperature reaction of mag- halogen chemistry ∣ mercury chemistry ∣ oxidation capacity matic gases with air and/or in-plume heterogeneous chemical processes involving aerosols during plume transport” might be raditionally, emissions of volcanoes were regarded to be required to explain observed high mixing ratios of halogen oxides mainly of importance for the atmospheric sulfur cycle, acid in volcanic plumes. Bobrowski et al. (3) and Roberts et al. (12) T based their modeling studies on this idea. In the following, I refer deposition, and stratospheric effects from explosive eruptions. “ ” This view changed recently, mainly because of the observation to an effective source region where high-temperature volcanic of very high concentrations of bromine oxide, BrO, in the tropo- volatiles are assumed to be mixed with a certain volume fraction spheric, nonexplosive plume of Soufrière Hills, Montserrat (1). of air and where the assumption of thermodynamic equilibrium Since then, an increasing number of observations of reactive holds for this mix of hot gases. The addition of oxygen dramati- halogens have been made in noneruptive plumes of various vol- cally changes the composition in the effective source region. canoes (2–4). Many more measurements of the early plumes of The model used here is an improved version of the model used various craters including the chemical composition of aerosol (5) in ref. 3; please see Materials and Methods for more details. The in recent years showed many more facets of the exciting chemical main measurements that are available for a model evaluation with regard to reactive halogen chemistry are, in addition to processes in noneruptive volcanic plumes. For more details and measurements of precursor gases at the crater rim, the remote further references, see, for example, von Glasow et al. (6). It is sensing of bromine oxide, BrO, and sulfur dioxide, SO2, at varying now clear that there is potential for a substantial influence of distances from the crater as performed by, e.g., Oppenheimer nonexplosive volcanic plumes on atmospheric chemistry, which et al. (2) and Bobrowski et al. (3). The variability in SO2 fluxes is why it is necessary to come to a quantitative assessment of these from volcanoes is very large, so that ratios of gases are often used. effects. This paper aims at contributing to this assessment. The main relevance of reactive chemistry in volcanic plumes, In this case, the ratio of the vertical columns (VC) of BrO and and especially halogen chemistry, is changes to the budgets of SO2 could be derived from the field measurements. Fig. 1 shows ozone and other oxidants, the contribution of volcanic bromine the modeled VC of BrO and SO2 and their ratio as well as to the observed free tropospheric background of reactive bro- measurements from Mount Etna; the range of the absolute values and the ratio of the measurements by Bobrowski et al. (3) can be mine, and changes to the atmospheric mercury burden. Ozone reproduced in the model. This is true only for the “base run” is a key driver of atmospheric oxidation and an important green- house gas in the troposphere. Mercury, in its methylated form, is highly toxic. Author contributions: R.v.G. designed research; R.v.G. performed research; R.v.G. analyzed This paper uses a state-of-the-art numerical model to investi- data; and R.v.G. wrote the paper. gate the implications of reactive chemistry in the plumes of quies- The authors declare no conflict of interest. cently (i.e., nonexplosive) degassing volcanoes, with a focus on This article is a PNAS Direct Submission. mercury chemistry. In this paper “volcanic plume” refers to the 1To whom correspondence should be addressed. E-mail: [email protected]. mixture of volcanic volatiles and particulates with ambient air; This article contains supporting information online at www.pnas.org/cgi/content/full/ only the atmospheric evolution of the plume is considered. 0913164107/DCSupplemental. 6594–6599 ∣ PNAS ∣ April 13, 2010 ∣ vol. 107 ∣ no. 15 www.pnas.org/cgi/doi/10.1073/pnas.0913164107 Downloaded by guest on September 29, 2021 scenario (see Materials and Methods); a model run that does not additional bromine has to be “activated” from other bromine re- employ the idea of a modified initial plume composition in the servoirs in order to explain the continued increase in the BrO to SPECIAL FEATURE effective source region (“pure volcanic volatiles”) shows an in- SO2 ratio. The only relevant gas phase loss of HBr is reaction with crease in BrO∶SO2 ratios that lags behind the measurements. OH; however, the model results show that under all conditions In a model run without aerosol chemistry, neither the BrO VC studied, the core of the plume is OH-free, due to its very rapid nor the BrO∶SO2 ratio can be reproduced, highlighting the loss by reaction with SO2. In this case, HBr either has to react importance of these processes. It has to be stressed that the varia- with OH at the plume edge, where ambient air is in contact with bility in the measurements is very large, partly caused by the the plume or it has to be taken up on aerosol particles, where it variability in emissions and partly by atmospheric variability. reacts with HOBr (stemming from the gas phase) and acidity to The latter includes wind speed, which determines the time that Br2, which is very insoluble and is released to the gas phase where the plume was exposed to atmospheric processing before getting it photolyzes, producing two Br radicals (see, e.g., ref. 14 for a to a certain measurement site, and the presence/absence of general overview of atmospheric halogen chemistry): clouds or a condensed plume, which has a strong influence on heterogeneous chemistry inside the plume. Almost all published BrO þ HO2 → HOBr þ O2; [1] measurements of BrO in volcanic plumes lack such ancillary meteorological observations. HBr → HBraq; [2] Bromine Chemistry in the Plume Of the gas phase bromine compounds that are expected to be − þ present in volcanic plumes only HBr (e.g., ref. 7) and BrO have HOBraq þ Br þ H → Br2;aq þ H2O; [3] been measured so far. Typically it is assumed that most bromine is released in the form of HBr; however, the concept of an effective → ; [4] source region implies that significant amounts of either Br radi- Br2;aq Br2 cals and/or BrCl and Br2 would be present in the early plume (see, e.g., ref. 10 and Table S2).