Impact of geo-engineering on the ion composition of the stratosphere

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1 Click Here for Full Article GEOPHYSICAL RESEARCH LETTERS, VOL. 35, L10812, doi: /2008gl033186, 2008 Impact of geo-engineering on the ion composition of the stratosphere Gufran Beig 1 Received 4 January 2008; revised 11 March 2008; accepted 16 April 2008; published 28 May [1] A remedy called geo-engineering solution has been recently proposed by some scientists to handle the global warming problem through injection of sulfates high aloft into the stratosphere. However, this idea may have some other side impacts. We have investigated the perturbation caused by geo-engineering solution on the stratospheric charged species using a coupled neutral-ion photochemical model. Model calculations indicate additional production of sulfuric acid immediately after the injection which further leads to increased abundance of heavy negative ion family by several orders of magnitude over the ambient. After 2 months, most of the H 2 SO 4 vapor condensed to H 2 SO 4 aerosols and the density of charged aerosol increases several folds and the effect spread further in the tropics. The perturbation in ionic species spread globally after about 1 year but became weaker in magnitude. The ion perturbation has implications on the electrical properties of the atmospheric medium. Citation: Beig, G. (2008), Impact of geo-engineering on the ion composition of the stratosphere, Geophys. Res. Lett., 35, L10812, doi: /2008gl Introduction [2] It has been proposed by some scientists that a possible partial solution to global warming may be to emulate the effects of a volcanic eruption by injecting material into the stratosphere as a form of geo-engineering [Crutzen, 2006; Wigley, 2006]. It is suggested that balloons bearing heavy guns be used to carry sulfates high aloft and fire them into the stratosphere. While carbon dioxide keeps heat from escaping Earth, substances such as sulfur dioxide, a common air pollutant, reflect solar radiation, helping cool the planet. The sulfur dioxide, a pollutant on Earth, would form sulfate aerosol particles to shade the planet. However, global warming is not caused by increased sunshine, rather it arises from the increased greenhouse effect owing to the buildup of greenhouse gases such as carbon dioxide from burning of fossil fuels and other human activities by trapping outgoing radiations and thus warming the planet. The geo-engineering seems increasingly likely to be the only route to staving off a cataclysm in the short term before new, clean energy sources are developed sufficiently [Wigley, 2006]. [3] Robock [2000] has reviewed the detailed work related to the impact of volcano (which can be considered as emulating geo-engineering solution) on temperature and composition of the atmosphere. Several other studies have reported the effects of the Mount Pinatubo volcanic eruption and resulting stratospheric aerosols on the subsequent 1 Indian Institute of Tropical Meteorology, Pune, India. Copyright 2008 by the American Geophysical Union /08/2008GL033186$05.00 climate [Harries and Futyan, 2006] and composition [Beig, 1999] of the atmosphere. Several studies have compared model simulated impacts of volcano on stratospheric constituents with observed perturbations [Tie et al., 1994]. If these perturbations were indeed associated with the stratospheric veil of aerosol that resulted from the eruption, then it has direct implications for the suggestions of geo-engineering solutions to global warming [Crutzen, 2006]. [4] In recent times, some authors have argued that the central concern is the adverse impact of geo-engineering solution. Recently, Trenberth and Dai [2007] have reported some major adverse effects, including drought, which could arise from geo-engineering solutions. However, no work has been done to study the impact of such geo-engineering solution to ion composition of the stratosphere. Ions may play a significant role under perturbed case which may influence neutrals and even lead to ozone destruction through well known catalytic cycles [Beig, 2000]. In the region extending from 0 to 50 km, ions are of considerable interest due to their role in controlling the electrical properties of the atmospheric medium. The main aim of this paper is to study the evolution of perturbations caused by the geo-engineering solution on the charged species of the stratosphere using a coupled neutral-ion photochemical model. For this purpose, different model runs have been made D Model and Ion Chemical Schemes [5] A two-dimensional model of radiation dynamics and chemistry coupled with ion chemical model is used in the present study which is described elsewhere in detail [Beig et al., 1993a, 1993b]. It extends from surface to 85 km and from pole to pole with a spatial resolution of 1 km in altitude and 5 in latitude. There are 64 neutral chemical species belonging to different families and 32 ionic species belonging to both positive any negative ions which are considered in the model. In a normal background case, the source of stratospheric sulfur is OCS, which gets into this altitude mainly through eddy diffusion process from the troposphere. It is then converted to SO 2 and finally produces gas phase sulfuric acid through a number of photochemical reactions. To calculate the aerosol distribution, this model was coupled with a microphysical model [Tie et al., 1994]. [6] The basic ion chemical schemes, which were coupled with this 2D-model for positive and negative ions are described earlier by us in detail [Beig et al., 1993a, 1993b]. The positive ion composition in the stratosphere consists of mainly two families namely proton hydrate ions (PH) of the type H + (H 2 O) n (dominant in the upper stratosphere) and non-proton hydrate ions (NPH) of the type H + (CH 3 CN) p (H 2 O) n (dominates in lower part of L of5

2 multiple cluster ions. The stated model has been validated earlier [Beig and Mitra, 1997; Beig, 2000]. Figure 1. Latitudinal distribution of the percentage change in aerosol number density (/cc) for runs A, B and C as compared to background. stratosphere) under normal case [Beig et al., 1993a]. In the presence of aerosol particles, the process of aerosol attachment with NPH ion takes place and complex heavy aerosol cluster ions are formed as per the following reaction [Beig, 2000]: H þ ðch 3 CNÞ p ðh 2 OÞ m þ Aerosols! H þ ðch 3 CNÞ p ðaerosolþðh 2 OÞ n [7] This reaction path would particularly be effective during periods following SO 2 injection. The product of the above reaction is called as aerosol-water-cluster ion and represented as A + (NPH). These aerosol ions are ultra fine particles (with a mass of typically amu) with an ion attached. The production of ions is considered by cosmic rays and the loss is by attachment and ion-ion recombination [Beig et al., 1993a, 1993b]. The chemical scheme for negative ions used in this work is from Beig et al. [1993b]. Two major negative ion families present in the stratosphere are- NO 3 (HNO 3 ) n (called NO 3 -core ions) and HSO 4 (HNO 3 ) n + HSO 4 (H 2 SO 4 ) m (called HSO 4 -core ions). The HSO 4 -core ions (which are driven by H 2 SO 4 vapor) dominate mainly for km whereas NO 3 -core ion family dominates in rest of the stratosphere [Beig et al., 1993b]. The H 2 SO 4 give rise to the formation of HSO 4 -core ion family by reacting with NO 3 -core ions and they can also get attached to HSO 4 -core ions to form 3. Model Simulations and Scenario Building [8] The geo-engineering experiment in the model is carried out by injecting 15 Mt of SO 2 into the stratosphere for the latitude belt of 10 S to10 N and an altitude of km. It is assumed in the model that the gas phase SO 2 is distributed uniformly in the above grid field and mixes instantaneously. In the present work, detailed chemistry of H 2 SO 4 vapor is considered which is described in detail elsewhere [Beig et al., 1993b]. The injection also causes the change in temperature, which affects the temperature dependent reaction rates. [9] There are number of processes that occur following the SO 2 injection. Initially SO 2 converted to gas phase sulfuric acid vapor through a number of chemical reactions involving OH. The concentration of OH is independently calculated in the model, which is critical in controlling the conversion. However the lifetime of H 2 SO 4 vapor is short and after nucleation it gets converted to H 2 SO 4 water droplets. Recently, Lee et al. [2003] have reported the particle formation by ion nucleation in this region. The production of these sulfur aerosols follows as a result of low vapor pressure of H 2 SO 4 at stratospheric temperature and humidity [Beig et al., 1993b]. The aerosols can get attached to both positively and negatively charged species so as to influence their abundance in the stratosphere. [10] Four model integrations were performed one for unperturbed background case and the remaining three for perturbed experiments (A, B and C). We have focused to three time frames and examined the changes in atmospheric composition with reference to the background case (A) immediately after a week of injection (B) two months after the injection and (C) one year after the injection. The surface mixing ratio of OCS is kept constant at 0.5 ppb. The sulfur chemistry in the normal background run is mainly arises from the surface OCS transportation to stratosphere and its photolysis. 4. Results and Discussion [11] Figure 1 shows the latitudinal distribution of the percentage change in aerosol number density for all the 3 model runs. The background aerosol number density for the lower stratosphere is roughly 10 30/cc for unperturbed case. The percentage variation in aerosol number density as compared to background case is found to be 2100% (an increase from 28/cc to 630/cc) for run A, but confined to the narrow grid with a maximum between km and 10 S to 10 N. For run B, the aerosol perturbation is found to be most significant and intense. Once the supersaturation of H 2 SO 4 vapor exceeds the critical value, as a result of low vapor pressure of H 2 SO 4 at stratospheric temperature and humidity, a large number of H 2 SO 4 /H 2 O droplets are formed. The complete process of the conversion to sulfate aerosols may take two to three months following the injection. The maximum variation is increased to about 4000% (an increase by /cc) around 22 km near the equator. The perturbation has now spread to 30 S 35 N and km. At this period of time, most of the 2of5

3 Figure 2. Latitudinal distribution of the percentage change in the number density (/cc) of A + (NPH) ions for runs A, B and C as compared to background. H 2 SO 4 vapor is converted to particle form. After one year of injection (C), the magnitude of variation in aerosol density is relatively less but attains a global coverage. The maximum variation is close to equator which is /cc (800%). [12] It is rather difficult to compare the present results with those of volcanic eruptions in quantitative terms. One major difference is that the volcanic eruption injects fly ash and other fine aerosol particles in the lower part of the atmosphere which is not the case with the present experiment. The amount of SO 2 injection in the stratosphere may also vary depending upon the particular eruption. Volcano plume also provide the local electrification. Nevertheless, we can find some similarities and the results are expected to agree in broad qualitative terms. We attempted to compare the present model results of zonal mean aerosol area density distributions with those observed by the Infrared Limb-sounding instruments CLAES (Cryogenic Limb Array Etalon Spectrometer) and ISAMS (Improved Stratospheric and Mesospheric Sounder) on board Upper Atmosphere Research Satellite (UARS) [Lambert et al., 1997] after the Mount Pinatubo volcanic eruption of June It is clear from the model results that the aerosol area density is 38 mm 2 /cm 3 after 2 3 months of injection and is confined to the tropics for the altitude region of 24 to 29 km. It is roughly in agreement with UARS-measurements which show a variation from 30 to 40 mm 2 /cm 3 for the same time period and close to equatorial region. The simulated results of surface area density after one year of eruption show that the coverage is found to be global but weak in magnitude which is broadly in agreement with observed data of volcanic eruption. [13] The impact of perturbation in aerosol number density and changes in temperature is reflected in positive ion composition of the stratosphere. Figure 2 shows the percentage change in the concentration of A + (NPH), for all the three runs as compared to normal case. The abundance of these heavy ions increases dramatically through 20 to 30 km in the initial few months following the injection. The increase in A + (NPH) is quite substantial and is found to be of the order of 1000% (an increase from 51/cc to /cc) within the latitudinal belt 10 S to10 N for run A. The maximum increase is found for run B which is now spread wider in latitude. The maximum increase in A + (NPH) for this run is found to be 1800% (an increase by /cc) at about 22 km through the latitudes 5 S to5 N. This is the period when the conversion of H 2 SO 4 - vapor to aerosol is found to be most significant and intense. Above 30 km, the concentration of aerosols is controlled by equilibrium saturation pressure of H 2 SO 4 - vapor over aerosol and highly temperature dependent. Increase in temperature above 30 km does not allow sulfate particles to accumulate. Hence, change in A + (NPH) comes back to ambient level above about 30 km for run A and B. In case of run C, when perturbation attained a global coverage but became weak, the perturbation in temperature settled down to ambient level. Model simulated result for run C indicates that the magnitude of variation in A + (NPH) has increased at all the latitudes which maximizes (600% i.e., variation of /cc) at the tropics (15 S to10 N) at 20 km. Beyond the tropical latitudes, the magnitude of increase reduces. [14] Figure 3a show the percentage change in the H 2 SO 4 vapor density as compared to normal case for the first two runs (A and B) considered here at 0 o N. The variation for run C is negligible as compared to other 2 runs and hence not plotted in Figure 3a. The sulfuric acid vapor, resulting from the conversion of injected SO 2, increases rapidly in the first few days after the injection. The typical number density of H 2 SO 4 vapor for the unperturbed case is of the order of 10 5 /cc in the lower stratosphere. The percentage change in H 2 SO 4 increases by 1000 times (which is an increase from /cc to /cc), for the altitude range of km. The magnitude of change decreases rapidly above 27 km which becomes 100 times ( /cc) around 30 km and 200% at 34 km. It is clear from the meridional distribution (not shown) that the high concentration of H 2 SO 4 which is centered around 24 km is confined to 10 S 15 N with little variation beyond this range. However, above mentioned high concentration of sulfuric acid vapor cannot remain in this form for a longer period as H 2 SO 4 vapor quickly gets converted to H 2 SO 4 / H 2 O water droplets. It can be noticed from Figure 3a that the concentration of H 2 SO 4 for run B drops down rapidly as compared to run A. The percentage change in H 2 SO 4 for run B increases by 100 times (a variation by /cc) for the altitude range of km. The magnitude of change decreases rapidly above 27 km and below 20 km. However it is still more than the normal case and the effect of geo-engineering now spreads beyond tropical region. This is due to the fact that although the 3of5

4 Figure 3. Vertical distribution of the percentage change for runs A and B at 0 N: (a) in sulfuric acid vapor number density and (b) in HSO 4 -core negative ion family. conversion of H 2 SO 4 vapor to aerosol particles catch momentum, the increase in temperature at lower stratospheric heights, increases the equilibrium saturation pressure of H 2 SO 4 over H 2 SO 4 /H 2 O aerosol, as a result of which H 2 SO 4 vapor concentration increases and sulfate aerosols formation seize down up to certain extent. [15] Because of the variation in H 2 SO 4 vapor, the concentration of negative ions gets highly influenced. Figure 3b show the percentage change in the fractional abundance of relatively heavier HSO 4 -core ions as compared to normal case for the first two runs (A and B) at 0 o N. For run A, the percentage change in HSO4-core ions is as high as 2000 % around 20 km (an increase from 15/cc to /cc) as most of the H 2 SO 4 in the initial few days will remain in vapor form and help in enhancing the concentration of this family of ions. It decreases significantly above and below this height. These perturbations are confined to the grid box of 10 S 10 N. The percentage change in HSO 4 core ions for run B in Figure 3b indicates that the maximum perturbation is peaking around 20 km (1500%). However, the magnitude of change decreases rapidly with height. It becomes 700% ( /cc increase) around 24 km and about 100% around 30 km. The perturbation is now spread in entire tropics. However, the concentrations of HSO 4 core ions tend to come back to background level with time and a declining trend is noticed for run C which now attains a global coverage. [16] Data for the stratospheric ion composition and that of H 2 SO 4 vapor are available only at one location in France (44 N) even for unperturbed case. Only a few results are reported on these parameters during post-volcanic eruption period [Arnold and Bührke, 1983; Qiu and Arnold, 1984]. Arnold and Bührke [1983] reported a distinct increase in H 2 SO 4 vapor density which is attributed to SO 2 injection by the El Chichon Mexican Volcano of April Qui and Arnold [1984] have later confirmed the above finding and reported that H 2 SO 4 has increased during the early evolutionary stage of volcano and finally decreases in the later stage but concentration of H 2 SO 4 aerosol should increase significantly. These results are in agreement with present results as shown in Figures 3a and 3b. [17] As per the above discussion, it is clear that during the geo-engineering scenario, the dominance of heavy ions [A + (NPH) and HSO 4 core ions] increases and the mass of the ions increases several fold as compared to normal background case. This will decrease the mobility and hence the conductivity of the medium which will in turn reduce the air-earth current (I) in the fair weather regions of the Global Electric Circuit (GEC). The GEC represents the current contour formed by bottom ionosphere and terrestrial surface conducting layers, with thunderstorm generators as the basic electrical sources, and the areas of a free atmosphere as zones of returnable currents [Rycroft et al., 2000]. If the columnar resistance of the troposphere and the potential of the ionosphere are assumed constant then any reduction in I can be inferred from the results of Cobb and Wells [1970] as associated with the reduction in conductivity in the stratosphere. The changing conductivity of the stratospheric ion layer affects the global electric circuit, and has a direct influence on radio communication [Harrison, 2004]. The disturbances in radio communication will affect the point to point communication used extensively during military operation. Changes in the GEC has often been linked with the climatic change, global warming in particular [Williams, 1992]. Global warming has been shown to result in an increase in global lightning frequency. Even an increase in conductivity due to enhancement in the radioactivity of the troposphere has been suggested to increase the frequency of lightning flashes [Israelsson et al., 1987]. Even small atmospheric electrical modulations on the aerosol size distribution and ions (which occurs during geo-engineering scenario) can affect cloud properties, temperature change, strengthening and weakening of winter cyclones [Tinsley et al., 2007] and modify the radiative balance of the atmosphere, through changes communicated globally by the atmospheric electrical circuit [Harrison, 2004]. 5. Conclusions [18] Geo-engineering perturbs both positive and negative ion composition. The perturbation in negative ion composition was maximum, immediately after the eruption due to the enhanced concentration of sulfuric acid vapor by several 4of5

5 orders in magnitude. The positive ion composition indicates maximum perturbation about 2 months after the injection when aerosol in the stratosphere is found to be at peak. Charged aerosol particles become significant only under geo-engineering scenario. Above perturbations were confined to a narrow grid box in the initial stage of injection but it attained a global coverage within one year. The perturbation in ion composition continued to be unabated even after one year of eruption. Domination of heavy charged species in the stratosphere following the injection would reduce the mobility and conductivity of the atmospheric medium which may play a significant role in influencing the Global Electric Circuit. Creating a risk of influencing the electrical properties of the atmospheric medium to cut down on global warming does not seem like an appropriate fix. However, in recent months, several scientists are considering doing just that. [19] Acknowledgment. I thank A. K. Kamra (IITM, Pune) for very fruitful discussion. References Arnold, F., and T. Bührke (1983), New H 2 SO 4 and HSO 3 vapour measurements in the stratosphere Evidence for a volcanic influence, Nature, 301, Beig, G. (1999), Perturbation in atmospheric charged species after the eruption of Mount Pinatubo, Geophys. Res. Lett., 27, Beig, G. (2000), The relative importance of solar activity versus anthropogenic influences on ion composition, temperature and associated neutrals of the middle atmosphere, J. Geophys. Res., 105, 19,841 19,856. Beig, G., and A. P. Mitra (1997), Atmospheric and ionospheric response to trace gas perturbations through the ice age to the next century in the middle atmosphere, part II Ionization, J. Atmos. Sol. Terr. Phys., 59, Beig, G., S. Walters, and G. Brasseur (1993a), A two-dimensional model of ion composition in the stratosphere: 1. Positive ions, J. Geophys. Res., 98, 12,767 12,773. Beig, G., S. Walters, and G. Brasseur (1993), A two dimensional model of ion composition in the stratosphere: 2. Negative ions, J. Geophys. Res., 98, 12,775 12,781. Cobb, W. E., and H. J. Wells (1970), The electrical conductivity of oceanic air and its correlation to global atmospheric pollution, J. Atmos. Sci., 27, Crutzen, P. J. (2006), Albedo enhancement by stratospheric sulfur injections: A contribution to resolve a policy dilemma?, Clim. Change, 77, Harries, J. E., and J. M. Futyan (2006), On the stability of the Earth s radiative energy balance: Response to the Mt. Pinatubo eruption, Geophys. Res. Lett., 33, L23814, doi: /2006gl Harrison, R. G. (2004), The global atmospheric electrical circuit and climate, Surv. Geophys., 25, , doi: /s Israelsson, S., T. Schutte, E. Pisler, and S. Lundquist (1987), Increased occurrence of lightning flashes in Sweden during 1986, J. Geophys. Res., 92, 10,996 10,998. Lambert, A., R. G. Grainger, C. D. Rodgers, F. W. Taylor, J. L. Mergenthaler, J. B. Kumer, and S. T. Massie (1997), Global evolution of the Mt. Pinatubo volcanic aerosols observed by the infrared limb-sounding instruments CLAES and ISAMS on the Upper Atmosphere Research Satellite, J. Geophys. Res., 102, Lee, S.-H., et al. (2003), Particle formation by ion nucleation in the upper troposphere and lower stratosphere, Science, 301, Qiu, S., and F. Arnold (1984), Stratospheric in-situ measurements of H 2 SO 4 and HSO 3 vapors during a volcanically active period, Planet. Space Sci., 32, Robock, A. (2000), Volcanic eruptions and climate, Rev. Geophys., 38, Rycroft, M., S. Israelson, and C. Price (2000), The global atmospheric electric circuit, solar activity and climate change, J. Atmos. Sol. Terr. Phys., 62, Tie, X. X., et al. (1994), Two-dimensional simulation of Pinatubo aerosol and its effect on stratospheric ozone, J. Geophys. Res., 99, 20,545 20,562. Tinsley, B. A., et al. (2007), The role of the global electric circuit in solar and internal forcing of clouds and climate, Adv. Space Res., 40, Trenberth, K. E., and A. Dai (2007), Effects of Mount Pinatubo volcanic eruption on the hydrological cycle as an analog of geoengineering, Geophys. Res. Lett., 34, L15702, doi: /2007gl Wigley, T. M. L. (2006), A combined mitigation/geoengineering approach to climate stabilization, Science, 314, Williams, E. R. (1992), The Schumann resonance: A global thermometer, Science, 256, G. Beig, Indian Institute of Tropical Meteorology, Pune , India. (beig@tropmet.res.in) 5of5