MIT Center for Global Change Science
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May 1993
Submitted to the Dept. of Earth, Atmospheric and Planetary Sciences on September 11, 1992 in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Meteorology
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Electrified convection provides a unique setting for atmospheric chemistry. While the chemistry of lightning is sometimes thought of as predominantly the chemistry of the hot (≈ 30,000K) lightning channel, the large radial electric fields (> breakdown strength of air) surrounding the lightning channel results in high ion and electron production rates in the "corona sheath". Additionally, the high-temperature lightning channel releases large amounts of short-wavelength, ionizing, UV radiation (λmax ≈ 100nm) that is absorbed in the surrounding region. The relative volumes of the hot lightning channel and the surrounding corona sheath are also noteworthy. If a typical lightning channel has a radius of a few centimeters and the surrounding corona sheath is a few tens of meters then the ratio of the volume of the coronal sheath to that of the lightning channel is 106:1.
The fate of the highly reactive, charged products formed outside of the hot channel determine, in part, the net chemical effect of electrified convection. These products can dominate over the hot-channel chemistry and alter the local concentrations of all the major chemical families considered in standard photochemical studies. Additionally, it is of interest to know whether electron-capturing gases with very long tropospheric lifetimes, such as SF6 may be removed by in-cloud ion chemistry at a rate sufficient to materially alter their total atmospheric lifetime.
To examine the chemical effects of the reactions induced by the ions, electrons, and photons produced in and around a lightning channel, a two-dimensional, axisymmetric dynamical/chemical model of electrified convection has been developed. This model represents the first effort to model the physical/chemical phenomena associated with the production of corona, the subsequent chemical reactions, and these reactions' integrated effects on the chemistry of electrified convection. This model considers ∼ 800 thermal and photochemical reactions among 165 neutrals, ions, water clusters, and electrons; effects of pressure, temperature, and electric fields upon the reaction coefficients are explicitly considered. Because the aim of this thesis is to focus on ion production and their subsequent gas-phase reactions, aqueous-phase chemistry has not been considered (although heterogeneous loss is represented) and the dynamics and microphysics are specified in a relatively simple manner.
The in-cloud lifetimes and storm-averaged production rates for each species considered in the (Electrified) model are computed and compared with their corresponding values computed by the model with all electrical processes turned off (the Base Case model run). Additionally, the relative chemical-source strengths of the hot channel vis-a-vis the surrounding regions are compared for selected chemical species.
The ion and UV-photon generation mechanisms dominate the overall production of many neutrals such as atomic O, N, and H, and are responsible for elevated mixing rations of the sparingly soluble or short-lived chemical families that derive from these species (i.e. NOx, Ox). For example, the average mixing ratio of NOx is increased from 20 ppt in the Base Case model run to 100 ppb in the Electrified model run. Similarly, the maximum OH mixing ratio of the Electrified model run (5 x 10-10) is 5 orders of magnitude higher than in the Base Case model run, and the domain-average mixing ratio is one order of magnitude larger.
The model-domain-averaged effect of lightning on the highly soluble chemical families such as HOx, is relatively small because of high in-cloud scavenging rates that mask locally-high rates of production. For example, the model-domain-average concentration of HOx changes by less than 2% between the Base Case and Electrified model runs, yet the maximum HOx mixing ratios in the main ionization regions of the Electrified model are four orders of magnitude larger than in the Base Case model run.
In general, the ion and UV-induce reactions contribute equally to the production of both O(3P) and O(1D) and consequently are equally important in the overall chemistry of O3 and OH and other derivative chemical species. Ion processes dominate the production of both the neutral N (and consequently its derivative species, e.g. NO) as well as charged species such as O2+, O4+, N2+, N4+, and the ions that ultimately derive from them, primarily water clusters H3O+ · (H2On).
Extrapolating the results of this model to the global average number of thunderstorms (≈ 1000 at any given time) results in an annual, global production of 20 Tg of O3, 0.64 Tg of Nitrogen as NOx, and 0.34 Tg of Nitrogen as N2O. These values can be compared to the currently estimated global stratospheric source of O3 of 680 Tg/yr and to the currently estimated tropospheric source strengths of 6.8 Tg of Nitrogen as NOx, and 9.7 Tg of Nitrogen as N2O. The predicted mixing ratios of NO in the outflow of this model thunderstorm agrees with corresponding observations.
The Electrified model represents a net-loss process for some chemical species, primarily due to enhanced in-cloud OH levels. On a global basis this model accounts for the annual destruction of 1.8 Tg of CO, 0.45 Tg of CH4, and 2 x 10-4 of Tg of OCS. Once again, these are small fractions of the currently estimated annual source strengths of these species (1600 Tg CO, 525 Tg CH4, and 0.4 Tg OCS).
Although the global budgets of the major chemical families and compounds are not greatly affected by electrified convection, the meso-0scale regime in which the thunderstorms are embedded certainly are affected to a greater or lesser extent depending on the assumptions regarding the heterogeneous-loss rates of sparingly soluble species in the outflow.
In addition to the meso-scale chemical effects of the thunderstorm outflow, the electric-field-driven capture of ions and the heterogeneous loss of neutrals to cloud particles greatly alters the normal aqueous-phase chemistry of clouds, although this model does not accurately quantify the extent of this perturbation. Nevertheless, high levels of (millimolar concentrations) of water-soluble oxidants such as H2O2 can reasonably be expected to occur in the vicinity (< 100s of meters) of corona and lightning events.
The quantities of almost all chemical species formed in the cooling hot channel are of little importance when compared with the corresponding quantities formed in the surrounding regions. The only exception is NOx; while the local mixing ratios of NO and NO2 can approach 0.01 in the cooled lightning channel, the relatively small volume of the hot lightning channel (≈ 10 m3) compared to the much larger volume of air surrounding the channel dominated by ion chemistry (≈ 5 x 107 m3) results in ≈ 50% of the in-cloud production of NOx coming from the hot channel. The ion-induced processes occurring in the regions surrounding the lightning chennel dominate the production or loss of all other species and chemical families (e.g. N2O, Ox, HOy).
For the cases of CF4, SF6, and CCl4 their mean in-cloud chemical lifetimes are reduced by a factor of ≈ 2 from their base-case lifetimes of 6.1 x 107, 6.1 x 105, 2.2 x 104 years respectively. Given the small fraction of the Earth's troposphere that is in electrified convection (≈ 6 x 10-4), this in-cloud loss cannot compete with other known loss mechanisms, such as stratospheric or mesospheric photodissociation and electron impact that result in these gases having global lifetimes estimated to be of order centuries for CF4 and SF6, and decades for CCl4.
This model is now being refined to include (1) a better representation of heterogeneous loss to ice phase and (2) feedback between space charge (ions) and the electric field and it is being expanded to include the full range of aqueous-phase chemistry. Additionally (1) a better understanding of the electric-field-strength dependent reactions of e- and O2 with water would be valuable in quantifying the production of the HOy family and (2) modeling the production of corona from water would improve the simulation of aqueous-phase ion chemistry. With these modifications in hand it will then become possible to make a complete first-principles model of the chemistry of lightning and its impact upon cloud chemistry.
