Atmospheric chemistry of a 33–34 hour old volcanic cloud from Hekla Volcano (Iceland): Insights from direct sampling and the application of chemical box modeling

Abstract

[1] On 28 February 2000, a volcanic cloud from Hekla volcano, Iceland, was serendipitously sampled by a DC-8 research aircraft during the SAGE III Ozone Loss and Validation Experiment (SOLVE I). It was encountered at night at 10.4 km above sea level (in the lower stratosphere) and 33-34 hours after emission. The cloud is readily identified by abundant SO 2 ( 1 ppmv), HCl ( 70 ppbv), HF ( 60 ppbv), and particles (which may have included fine silicate ash). We compare observed and modeled cloud compositions to understand its chemical evolution. Abundances of sulfur and halogen species indicate some oxidation of sulfur gases but limited scavenging and removal of halides. Chemical modeling suggests that cloud concentrations of water vapor and nitric acid promoted polar stratospheric cloud (PSC) formation at 201-203 K, yielding ice, nitric acid trihydrate (NAT), sulfuric acid tetrahydrate (SAT), and liquid ternary solution H 2 SO 4 /H 2 O/HNO 3 (STS) particles. We show that these volcanically induced PSCs, especially the ice and NAT particles, activated volcanogenic halogens in the cloud producing >2 ppbv ClO x . This would have destroyed ozone during an earlier period of daylight, consistent with the very low levels of ozone observed. This combination of volcanogenic PSCs and chlorine destroyed ozone at much faster rates than other PSCs that Arctic winter. Elevated levels of HNO 3 and NO y in the cloud can be explained by atmospheric nitrogen fixation in the eruption column due to high temperatures and/or volcanic lightning. However, observed elevated levels of HO x remain unexplained given that the cloud was sampled at night.

Halogen emissions from a small volcanic eruption: Modeling the peak concentrations, dispersion, and volcanically induced ozone loss in the stratosphere

“…These were found to increase HNO 3 :3H 2 O (NAT) and ice polar stratospheric cloud (PSC) threshold temperatures substantially over the first 35 hours [Rose et al, 2006]. To test if this result remains true when plume dilution is included, we extend previous studies of the evolution of the Hekla plume chemistry, which ran on short timescales and neglected plume dilution, by using a 3D chemical transport model to investigate the impacts of both mixing and chemical evolution on plume composition for two weeks following the eruption.…”

“…Ash falls from Icelandic eruptions are notably F-rich [Oskarsson, 1980]. Limited data suggest that hydrogen halides comprise 0.5-1 mole% of Icelandic volcanic gases [Rose et al, 2006;Thordarson et al, 1996;Moune et al, 2006].…”

“…The main gases from Hekla were introduced in SLIMCAT at one grid point (64°N, 341°E) below the volcanic plume cloud top height (lowest 2 levels, decreasing to lowest level after 20:00 UT 26 February 2006) for the duration of the eruption: H 2 O (6 ppm), SO 2 (1.2 ppm), H 2 SO 4 (70.4 ppb), HF (60 ppb), HCl (45 ppb), CO (20 ppb), ClONO 2 (8 ppb), HNO 3 (4 ppb) and HBr (800 ppt), as in work by Rose et al [2006]. A passive SO 2 tracer, assumed not to undergo chemical reaction, was added to SLIMCAT so that its concentration could be used to diagnose plume dilution.…”

“…The stratospheric volcanic plume from this eruption was intercepted on several occasions by an instrumented NASA DC-8 aircraft [Hunton et al, 2005;Rose et al, 2006], revealing high plume HCl (>80 ppb) and HF(>60 ppb) concentrations. Modeling studies diagnosed volcanic enhancement of ClO x concentrations to between 2 and 14 ppb and complete O 3 destruction within the plume within the first 35 hours in agreement with O 3 measurements [Rose et al, 2006]. The increases in halogen gas concentrations seen in the Hekla plume represent a significant perturbation to stratospheric chemistry and halogen gases at these levels have seldom been modeled in the stratosphere.…”

“…Average polar vortex ClO x is also enhanced by a factor of 3 initially, by the combination of increased inorganic Cl availability and increased heterogeneous activation. DC-8 based measurements of ClO x in the plume on 5 and 9 March peaked at 50 ppt [Rose et al, 2006], SLIMCAT vortex average ClO x ranges between 40 to 80 ppt during this time.…”