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Edited by Mark H. Thiemens, University of California, La Jolla, CA, and approved June 21, 2011 (received for review March 29, 2011)
Abstract
Volcanic eruptions caused major weather and climatic changes on timescales ranging from hours to centuries in the past. Volcanic particles are injected in the atmosphere both as primary particles rapidly deposited due to their large sizes on time scales of minutes to a few weeks in the troposphere, and secondary particles mainly derived from the oxidation of sulfur dioxide. These particles are responsible for the atmospheric cooling observed at both regional and global scales following large volcanic eruptions. However, large condensational sinks due to preexisting particles within the plume, and unknown nucleation mechanisms under these circumstances make the assumption of new secondary particle formation still uncertain because the phenomenon has never been observed in a volcanic plume. In this work, we report the first observation of nucleation and new secondary particle formation events in a volcanic plume. These measurements were performed at the puy de Dôme atmospheric research station in central France during the Eyjafjallajokull volcano eruption in Spring 2010. We show that the nucleation is indeed linked to exceptionally high concentrations of sulfuric acid and present an unusual high particle formation rate. In addition we demonstrate that the binary H2SO4 - H2O nucleation scheme, as it is usually considered in modeling studies, underestimates by 7 to 8 orders of magnitude the observed particle formation rate and, therefore, should not be applied in tropospheric conditions. These results may help to revisit all past simulations of the impact of volcanic eruptions on climate.
Volcanic eruptions and their impact on climate have been extensively investigated in geological archives such as sediments (1), ice cores (2–5), and through modeling studies (6, 7) but processes taking place in the volcanic plume have rarely been directly observed. The sulfur dioxide emitted in large amounts during explosive volcanic eruptions (8) can be oxidized to sulfuric acid, thus many authors suspect that the formed sulfuric acid gives rise to the formation of new secondary particle from binary homogeneous nucleation mechanism (1, 9–12). Those new particles can then affect the atmospheric radiative balance both directly and indirectly because they can act as cloud condensation nuclei (CCN) (13). This leads to a cooling effect, which is on a time scale from months to years, depending on the location of the eruption and the height at which volcanic gases were emitted.
The Eyjafjallajokull volcano located in the south of Iceland [63°38′ N, 19°36′ W, summit 1,660 m above sea level (asl)] erupted on March 20, 2010. A major outbreak of the central crater under the covering ice cap followed on April 14, 2010 (Institute of Earth Sciences, 2010). A large volcanic plume composed of ashes and gases rose up to the tropopause level (approximately 10 km) for days and were observed by satellite and ground-based remote sensing instruments. The volcanic plume reached European altitude stations several times in the following days and until the end of the eruptive period on the May 21, 2010. This event was a unique opportunity to characterize the volcanic ash plume after it had been photochemically aged for several days, with a sophisticated instrumentation that can only be ground-based.
The puy de Dôme research station is located at 1465 m above sea level in central France (45°46′ N, 2°57′ E). The station is surrounded mainly by a protected area where fields and forests are predominant, the city of Clermont-Ferrand is located 16 km east of the station. Meteorological parameters, including wind speed and direction, temperature, pressure, relative humidity, and radiation (global, UV, and diffuse), atmospheric trace gases (O3, NOx, SO2, CO2) and particulate black carbon (BC) are monitored continuously throughout the year. In addition to those measurements, the station is equipped with a neutral cluster and air ion spectrometers (NAIS), a nanoparticle size distribution measurement device, which allow the detection of charged and neutral particles from 0.8 to 42 nm, a scanning mobility particle sizer (SMPS; 10–450 nm) and an optical particle counter (Grimm spectrometer, 0.3–20 μm). Atmospheric dynamics and stratification is monitored using a Rayleigh–Mie LIDAR (light detection and ranging) emitting at 355 nm, with parallel and perpendicular polarization channels. LIDAR measurements were conducted from the roof of the laboratory (45°45′ N, 3°6′ E, 410 m asl) located 11 km east of the puy de Dôme station.
The Volcanic Plume Detection
The volcanic plume was first detected over Europe at the high altitude station Zugspitze (2650 m, Germany) on the night from April 17 to April 18, 2010 and a second time in the afternoon on April 19, 2010. Both the SO2 and the particle number concentrations (Dp > 3 nm) where detected with concentrations exceeding the 99th-percentile value (years 2000–2007), and were highly correlated with each other (14). At the puy de Dôme station, a strong depolarization signal indicative of volcanic ash (15) was detected using LIDAR measurements (Fig. 1) on April 18 and April 19, 2010 and from May 18 to May 20, 2010. During April, the volcanic plume was only detected in the free troposphere (between 3,500 and 4,000 m asl) above the puy de Dôme site and neither variations of SO2 nor particles concentration compared to the mean diurnal variation level could be detected at the puy de Dôme station (1,465 m asl), indicating that the volcanic plume did not reach the lower troposphere and the planetary boundary layer. On the contrary, during the May episodes, the main volcanic ash plume was first detected around 3000 m asl and then mixed into the boundary layer, as witnessed by an increase of the depolarization ratio in the LIDAR signal and a peak of the SO2 concentration at the puy de Dôme station. Hence, those particular events observed in May 2010 can be analyzed in detail with specific ground-based instrumentation.
Fig. 1.
Depolarization ratio measured by LIDAR, the dotted line illustrates the height of the puy de Dôme research station.
Air masses origins and isobaric backtrajectories were simulated using the Hysplit model and were conducted for each hour of the day for the period between May 18 and May 20, 2010. Results confirm that the volcanic plume arrived during the night between May 18 and May 19, 2010 after 100 h of transportation. At the puy de Dôme station, the plume was first detected between 2,500 and 3,000 m asl on the May 18 at 23:45 when SO2 concentrations peaked until reaching a maximum value of 2.25 parts per billion by volume (ppbv) around 04:00 exceeding the 99th-percentile value measured at the station from 2005 to present days (Fig. 2). This boundary layer intrusion is also detectable from the LIDAR signal as an increase of the depolarization ratio in the atmospheric lowest layers (Fig. 1). One parameter determining if nucleation of new particles occurs in a given environment is the condensational sink (CS; see Materials and Methods). If the condensable vapor/CS ratio is too low prior to the potential onset of the nucleation event then condensable vapors condense onto preexisting particles rather than form new particles by nucleation. The evolution of the CS (Fig. 3, Lower) calculated from the preexisting particulate surface, following the method of Pirjola and coworkers (16), did not increase with the intrusion of the volcanic plume, hence indicating that large particles emitted by the volcano had already settled. This is confirmed by the low number concentration of supermicronic particles detected at the station (< 1 cm-3). As the sun is rising, photochemical reactions lead gas-phase SO2 to be oxidized to sulfuric acid. This chemical process was estimated using an indirect approach based on global radiation, SO2 amount, and CS, following the work of Petäjä et al. (17). The calculation indicates that the production of sulfuric acid reached 3.7 parts per trillion by volume (pptv, exceeding the 09–14:00 90th-percentile determined from long-term measurements (from 2005 to present days) using the same calculation procedure. The sulfuric acid levels that we calculate are in the same order of magnitude as the one directly measured at the Zugspitze in the Eyjafjalla plume in April 2010 (0.65 pptv) (14).
Fig. 2.
Evolution of the SO2 concentration at the puy de Dôme station (A, Upper) and calculated H2SO4 (B, Lower).
Fig. 3.
Evolutions of the total (charged and neutral) particle size distributions (A, Upper) and condensational sink (B, Lower) from May 18 to May 21, 2010.
New Particle Formation Events
Strongly correlated to the simulated H2SO4 concentrations, the particle concentration follow a clear diurnal variation, both on May 19 and May 20, 2010, with number concentrations multiplied by 2.5 at 14 h compared to night time concentrations and by 5 specifically in the size range from 0.8 to 42 nm. The size distributions of aerosol particles measured with the NAIS (Fig. 3, Upper) shows that these particles are formed from the nanometric scale by nucleation, and that they subsequently grow in the following hours. Events started respectively around 10:00 and 07:30 on May 19 and May 20, when the CS was respectively of 6.8 and 11.0 × 10-3 s-1. These CS values are slightly higher than the average CS observed on nucleation-event days and no-nucleation-event days calculated from a long-term study conducted at the site (18) (respectively 3.73 ± 0.11 and 5.17 ± 0.15 × 10-1 s-1), highlighting that an exceptionally high condensable vapor concentration was needed to onset the nucleation process.
The formation rate for 2-nm particles (J2) are classically calculated to evaluate the number of particles formed per time unit (see Materials and Methods). The nucleation events detected in the volcanic plume are characterized by J2 that are four times higher (J2 = 4.76 ± 2.63 s-1) than the average values computed from long-term measurements (2007–2011) at the station (
on 34 comparable events) and 10% higher than the J2 99th-percentile value (3.66 s-1 calculated on 34 comparable events). After the observations of the presence of H2SO4 and water in volcanic particles during the Pinatubo eruption by Deshler et al. (9), the majority of studies used the H2SO4 - H2O binary homogeneous nucleation (BHN) theory (6, 7, 12, 19) to estimate the particle formation rates. In the same manner, we can calculate from our data the H2SO4 - H2O binary homogeneous nucleation rate J2,BHN using Yu’s procedure (20), which is the closest to the BHN theory (21). The computed J2,BHN was found to be seven to eight orders of magnitude below the observed J2, suggesting that the volcanic nucleation events could not be adequately described with the H2SO4 - H2O binary homogeneous nucleation scheme as it was done in the previous modeling studies. Furthermore, we found that those nucleation events are characterized by an unusual low ion-induced nucleation rate (IIN = 1.2%) compared to the average value computed from long-term measurements [IIN = 12.49 ± 2.03%, calculated on 34 comparable events (18)], indicating that the observed nucleation should be described by a neutral nucleation and growth mechanism.
The average particle growth rate of newly formed particles calculated over the 1.3–20 nm size range, was 5.26 ± 0.76 nm·h-1, which is slightly lower than observed growth rates at the puy de Dôme station [6.20 ± 0.12 nm·h-1 (18)]. From this value, the minimal condensable vapor concentration needed to explain the particle growth velocity (22) was estimated to be 2.65 ± 0.71 × 10+07 molecules/cm3. The agreement between the calculated condensable vapor concentration and calculated sulfuric acid concentrations during the nucleation process (3.67 ± 0.78 × 10+07 molecules/cm3 in average) suggests that the nucleation events observed is likely linked to the H2SO4 produced from the atmospheric oxidation of the volcanic emitted SO2. As a consequence, this observation may also indicate that condensable vapors different than sulfuric acid are not needed to explain the observed new particle growth, contrarily to what it is usually observed during nucleation and growth events in the planetary boundary layer under remote conditions (23). When particles grow above a certain limit, between 50 and 100 nm diameter, they can act as CCN. The potential CCN concentration is classically estimated using the two ratios N50/Ntot and N100/Ntot where N50 and N100 are respectively the particle concentration with a diameter higher than 50 and 100 nm, Ntot is the total concentration. From SMPS data we calculated that freshly formed particle significantly contribute to increase the number of potential CCN. After the nucleation and growth event 38.25 ± 6.2% of super-10nm particles have reach the 50 nm diameter and 15.2 ± 3.0% the 100 nm diameter and therefore could act as CCN.
Conclusions
The observational data and analysis presented here demonstrate that nucleation and subsequently growth can derived from volcanic eruption gaseous released and that this new secondary particle formation event could occur within the lower troposphere at a large distance from the eruptive activity. The analysis of such events reveals that the binary H2SO4 - H2O homogeneous nucleation scheme implemented in modeling studies is not adapted to describe the processes observed in the low troposphere, even at high sulfuric acid concentrations. The underestimation of the formation rate of new secondary particles in volcanic plumes by seven to eight orders of magnitude when performed from calculations based on this nucleation scheme could lead to an underestimation of the CCN and the subsequent potential formation of low-level clouds. As a consequence, such results may help to revisit nucleation schemes implemented in all past simulations of the impact of volcanic eruptions on climate. Schemes involving a third species such as ammonium [ternary nucleation theory (24)] or the nano-Köhler theory of cluster activation (25) are examples of paths to explore to improve the representation of nucleation and particles growth to climate relevant sizes in global models.
In addition to volcanic explosive eruptions, large SO2 amounts are released in the low troposphere by a continuous volcanic degassing activity. Such geological activities represent an unused laboratory and offer a new field of experimentation for testing hypothesis and models to describe the tropospheric nucleation in volcanic plumes.
Materials and Methods
Growth rate, Nucleation Rate, and Condensational Sink Calculation.
Growth rate calculations were performed from the NAIS size distribution. The procedure involves fitting the size distribution with a log-normal structure and calculating the temporal change in the modal diameter (26). The condensational sink is classically estimated using sulfuric acid as a condensing molecule on the surface available from the total aerosol population (27). The formation rate of 2 nm particles was calculated from the aerosol size distribution (from 0.8 to 42 nm) obtained from the NAIS (27) 
[1]where N2-3 is the particle concentration in the size range from 2–3 nm, GR1.3-3 is the growth rate (GR) between 1.3 and 3 nm (assumed to be equal to the GR of 2 nm particle), and CoagS2 is the loss of particles by coagulation scavenging of 2 nm particles (27).
The aerosol condensation sink determines how rapidly molecules will condense onto preexisting particles and depends strongly on the shape of the size distribution and can be calculated using the following formula (27): 
[2]where Dvapor is the diffusion coefficient of the condensing vapor (here sulfuric acid), βMi is the transitional correction factor, ri and Ni are respectively the radius of the particle and its number concentration.
Gap-Filling the CS Data.
When SMPS data were not available, we estimated the CS from TEOM (Tapered Element Oscillating Microbalance, FDMS 8500C) and condensation particle counter (TSI CPC 3010). Those two instruments provide the mass and the number concentration of particles, from which we can infer a mean single particle volume. Using this average volume and size, we reconstruct an equivalent particle size distribution from which the CS is computed. This method was tested on TEOM/CPC data for which SMPS measurements were available and an agreement of more than 90% between the estimated and real CS was achieved.
Acknowledgments
We thank E. Freney and S. Valade for reading the manuscript. We also thank the Observatoire de Physique du Globe de Clermont-Ferrand for contributing to maintaining instrumentation at the puy de Dôme research station. This work has been partly funded by the European Commission program project EUSAAR, Contract n°026140 (EUSAAR).
Footnotes
- ↵1To whom correspondence should be addressed. E-mail: j.boulon{at}opgc.univ-bpclermont.fr.
Author contributions: K.S. and P.L. designed research; J.B. performed research; M.H. contributed new reagents/analytic tools; J.B. and M.H. analyzed data; and J.B. and K.S. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
References
Observations of nucleation of new particles in a volcanic plume
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