R. C. Whitten

A One-Dimensional Model Describing Aerosol Formation and Evolution in the Stratosphere: I. Physical Processes and Mathematical Analogs

Abstract We have developed a time-dependent one-dimensional model of the stratospheric sulfate aerosol layer. In constructing the model, we have incorporated a wide range of basic physical and chemical processes in order to avoid predetermining or biasing the model predictions. The simulation, which extends from the surface to an altitude of 58 km, includes the troposphere as a source of gases and condensation nuclei and as a sink for aerosol droplets; however, tropospheric aerosol physics and chemistry are not fully analyzed in the present model. The size distribution of aerosol particles is resolved into 25 discrete size categories covering a range of particle radii from 0.01–2.56 µm with particle volume doubling between categories. In the model, sulfur gases reaching the stratosphere are oxidized by a series of photochemical reactions into sulfuric acid vapor. At certain heights this results in a supersaturated H2SO4–H2O gas mixture with the consequent deposition of aqueous sulfuric acid solution on th...

Noctilucent clouds: Simulation studies of their genesis, properties and global influences

Abstract The formation, evolution and properties of noctilucent clouds are studied using a timedependent one-dimensional model of ice particles at mesospheric altitudes. The model treats ice crystals, meteoric dust, water vapor and air ionization as fully interactive cloud elements. For ice particles, the microphysical processes of nucleation, condensation, coagulation and sedimentation are included; the crystal habits of ice are also accounted for. Meteoric dust is analyzed in the manner of Hunten et al. (1980). The simulated particle sizes range from 10 A to 2.6μm. The chemistry of water vapor and the charge balance of the mesosphere are also analyzed in detail. Based on model calculations, including numerous sensitivity tests, several conclusions are reached. Extremely cold mesopause temperatures ( 2 O. Ample cloud condensation nuclei are always present in the mesosphere; at very low temperatures, either meteoric dust or hydrated ions can act as cloud nuclei. To be effective, meteoric dust particles must be larger than 10–15 A in radius. When dust is present, water vapor supersaturations may be held to such low values that ion nucleation is not possible. Ion nucleation can occur, however, in the absence of dust or at extremely low temperatures ( −3 ) of large ice particles (>0.05 μm radius) and cloud optical depths (at 550 nm) ∼10 −4 , ion nucleation generally leads to a large number (∼10 3 cm −3 ) of smaller particles and optical depths ∼10 −5 ). However, because calculated nucleation rates in noctilucent clouds are highly uncertain, the predominant nucleus for the clouds (i.e., dust or ions) cannot be unambiguously established. Noctilucent clouds require several hours-up to a day-to materialize. Once formed, they may persist for several days, depending on local meteorological conditions. However, the clouds can disappear suddenly if the air warms by 10–20 K. The environmental conditions which exist at the high-latitude summer mesopause, together with the microphysics of small ice crystals, dictate that particle sizes will be ≲ 0.1 μm radius. The ice crystals are probably cubic in structure. It is demonstrated that particles of this size and shape can explain the manifestations of noctilucent clouds. Denser clouds are favored by higher water vapor concentrations, more rapid vertical diffusion and persistent upward convection (which can occur at the summer pole). Noctilucent clouds may also condense in the cold “troughs” of gravity wave trains. Such clouds are bright when the particles remain in the troughs for several hours or more; otherwise they are weak or subvisible. Model simulations are compared with a wide variety of noctilucent cloud data. It is shown that the present physical model is consistent with most of the measurements, as well as many previous theoretical results. Ambient noctilucent clouds are found to have a negligible influence on the climate of Earth. Anthropogenic perturbations of the clouds that are forecast for the next few decades are also shown to have insignificant climatological implications.

Predictions of the electrical conductivity and charging of the aerosols in Titan's atmosphere

Abstract The electrical conductivity and electrical charge on the aerosols in atmosphere of Titan are computed for altitudes between 0 and 400 km. Ionization of methane and nitrogen due to galactic cosmic rays (GCR) is important at night where these ions are converted to ion clusters such as CH + 5 CH 4 , C 7 H + 7 , C 4 H + 7 , and H 4 C 7 N + . The ubiquitous aerosols observed also play an important role in determining the charge distribution in the atmosphere. Because polycyclic aromatic hydrocarbons (PAHs) are expected in Titans atmosphere and have been observed in the laboratory and found to be electrophilic, we consider the formation of negative ions. During the night, the very smallest molecular complexes accept free electrons to form negative ions. This results in a large reduction of the electron abundance both in the region between 150 and 350 km over that predicted when such aerosols are not considered. During the day time, ionization by photoemission from aerosols irradiated by solar ultraviolet (UV) radiation overwhelms the GCR-produced ionization. The presence of hydrocarbon and nitrile minor constituents substantially reduces the UV flux in the wavelength band from the cutoff of CH 4 at 155 to 200 nm. These aerosols have such a low ionization potential that the bulk of the solar radiation at longer wavelengths is energetic enough to produce a photoionization rate sufficient to create an ionosphere even without galactic cosmic ray (GCR) bombardment. At altitudes below 60 km, the electron and positive ion abundances are influenced by the three-body recombination of ions and electrons. The addition of this reaction significantly reduces the predicted electron abundance over that previously predicted. Our calculations for the dayside show that the peaks of the charge distributions move to larger values as the altitude increases. This variation is the result of the increased UV flux present at the highest altitudes. Clearly, the situation is quite different than that for the night where the peak of the distribution for a particular size is nearly constant with altitude when negative ions are not present. The presence of very small aerosol particles (embryos) may cause the peak of the distribution to decrease from about 8 negative charges to as little as one negative charge or even zero charge. This dependence on altitude will require models of the aerosol formation to change their algorithms to better represent the effect of charged aerosols as a function of altitude. In particular, the charge state will be much higher than previously predicted and it will not be constant with altitude during the day time. Charging of aerosol particles, whether on the dayside or nightside, has a major influence on both the electron abundance and electrical conductivity. The predicted conductivities are within the measurement range of the HASI PWA instrument over most but not all, of the altitude range sampled.

An analysis of various nucleation mechanisms for sulfate particles in the stratosphere

Abstract A theoretical analysis of particle formation mechanisms under stratospheric conditions was carried out using a fully interactive one-dimensional model of aerosol formation and evolution. The formation mechanisms considered are homogeneous, ion and heterogeneous heteromolecular nucleation of H 2 SO 4 H 2 O systems, the clustering of sulfate radicals, and heterogeneous nucleation onto stable neutral ion—ion recombination complexes. We develop theoretical expressions for the nucleation rates, describe the manner in which the nucleation mechanisms are incorporated into the model, and present the results of model calculations. We find that although the different nucleation processes lead to greatly different rates of particle formation, the observed characteristics of the aerosol are hardly affected by the assumed particle formation mechanism. Consequently, it will be difficult to devise measurements to evaluate the relative importance of the various formation mechanisms. Our results show that the homogeneous and ion nucleation rates in the stratosphere are negligible. Heterogeneous nucleation onto stable ion—ion recombination products and the clustering of sulfate radicals are two processes which could lead to the generation of large numbers of particles in the stratosphere. Using presently available experimental techniques it is not possible to determine unambiguously which formation mechanism is responsible for the production of the stratospheric particles.

A One-Dimensional Model Describing Aerosol Formation and Evolution in the Stratosphere: II. Sensitivity Studies and Comparison with Observations

Abstract We have performed sensitivity tests on a one-dimensional physical-chemical model of the unperturbed stratospheric aerosols and have compared model calculations with observations. The sensitivity tests and comparisons with observations suggest that coagulation controls the particle number mixing ratio, although the number of condensation nuclei at the tropopause and the diffusion coefficient at high altitudes are also important. The sulfate mass and large particle number (r > 0.15 µm) mixing ratios are controlled by growth, sedimentation, evaporation at high altitudes and washout below the tropopause. The sulfur gas source strength and the aerosol residence time are much more important than the supply of condensation nuclei in establishing mass and large particle concentrations. The particle size is also controlled mainly by gas supply and residence time. OCS diffusion (not SO2diffusion) dominates the production of stratospheric H2SO4 particles during unperturbed times, although direct injection o...

An analysis of the physical, chemical, optical, and historical impacts of the 1908 Tunguska meteor fall

Abstract A detailed analysis of the physical nature and photochemical after effects of the explosive cometary meteor Tunguska is presented. The physical manifestations of the event (the acoustic and seismic waves, forest damage, and so on) are shown to be consistent with the entry of a 5-million-ton object into the Earths atmosphere at 40 km sec −1 . The meteor apparently had a very low effective density ( 3 ) due either to its intrinsic porous structure, to shattering in orbit far from the Earth, or to breakup upon initial impact with the Earths upper atmosphere. Aerodynamic calculations are used to demonstrate that the shock waves emanating from the falling meteor could have generated up to 30 million tons of nitric oxide (NO) in the stratosphere and mesosphere. The photochemical consequences of such an immense Tunguska-related NO injection are investigated with the aid of a fully interactive one-dimensional chemical-kinetics model of atmospheric trace constituents. The first year after the Tunguska fall (from mid-1908 to mid-1909) a 35–45% hemispherical ozone depletion is predicted with the model; declining but still substantial ozone depletions are calculated in subsequent years. Atmospheric transmission data collected by a research team of the Smithsonian Astrophysical Observatory (APO) at Mount Wilson, California, from 1908 to 1911 are analyzed for ozone absorption in the Chappuis bands. Statistical analysis of the APO data reveals an ozone variation of 30 ± 15% over this period, supporting the theoretical predictions. The optical anomalies which followed the Tunguska event are reviewed for evidence of NO x O x chemiluminescent emissions, NO 2 solar absorption, and meteoric dust turbidity. The chemical afterglows are shown to be intense enough to account for some of the unusual night-time light displays seen after the fall, but not widespread enough to explain the “light nights” and glowing skies reported throughout Eurasia. These phenomena appear to be related to the dust and water vapor deposited by the meteor at the cold summer mesopause, resulting in the formation of dense noctilucent clouds. Only circumstantial optical evidence for a large Tunguska NO 2 enhancement is found, which can not be used to calibrate independently the NO injection by the meteor. The suggestion of a dust veil created by the Tunguska explosion is revealed by the APO transmission data. We deduce that nearly 1 million tons of pulverized dust may have been deposited in the mesosphere and stratosphere by the Tunguska fall, which agrees with previous estimates of the meteor mass influx. Possible climate changes triggered by the Tunguska event are investigated. The most important climate anomaly identified in the post-Tunguska era is a 0.3°K cooling of the Northern Hemisphere which lasted for almost a decade. Several large volcanic eruptions occurred during this period which also played a role in the temperature change. However, radiation transport calculations are reported which suggest that Tunguska contributed to the cooling trend. The lessons of Tunguska for other important geophysical problems, such as ozone/weather coupling and the ancient extinction of the dinosaurs, are also explored. It is concluded that more rigorous investigations of the physics and chemistry of the Tunguska event are warranted.

Thermal Structure and Major Ion Composition of the Venus Ionosphere: First RPA Results from Venus Orbiter.

Thermal plasma quantities measured by, the retarding potential analyzer (RPA) are, together with companion Pioneer Venus measurements, the first in situ measurements of the Venus ionosphere. High ionospheric ion and electron temperatures imply significant solar wind heating of the ionosphere. Comparison of the measured altitude profiles of the dominant ions with an initial modlel indicates that the ionosphere is close to diffusive equilibrium. The ionopause height was observed to vary from 400 to 1000 kilometers in early orbits. The ionospheric particle pressure at the ionopause is apparently balanced at a solar zenith angle of about 70� by the magnetic field pressure with little contribution from energetic solar wind particles. The measured ratio of ionospheric scale height to ionopause radius is consistent with that inferred from previously measured bow shock positions.

Thermal Structure and Energy Influx to the Day-and Nightside Venus Ionosphere.

Pioneer Venus in situ measurements made with the retarding potential analyzer reveal strong variations in the nightside ionospheric plasma density from location to location in some orbits and from orbit to orbit. The ionopause is evident at night as a relatively abrupt decrease in the thermal plasma concentration from a few hundred to ten or fewer ions per cubic centimeter. The nightside ion and electron temperatures above an altitude of 250 kilometers, within the ionosphere and away from the terminator, are comparable in magnitude and have a value at the ionopause of approximately 8000 K. The electron temperature increases from a few tens of thousands of degrees Kelvin just outside the ionopause to several hundreds of thoussands of degrees Kelvin further into the shocked solar wind. The coldest ion temperatures measured at an altitude of about 145 kilometers are 140 to 150 K and are still evidently above the neutral temperature. Preliminary day-and nightside model ion and electron temperature height profiles are compared with measured profiles. To raise the model ion temperature to the measured ion temperature on both day-and nightsides, it was necessary to include an ion energy source of the order of 4 x 10–3 erg per square centimeter per second, presumably Joule heating. The heat flux through the electron gas from the solar wind into the neutral atmosphere averaged over day and night may be as large as 0.05 erg per square centimeter per second. Integrated over the planet surface, this heat flux represents one-tenth of the solar wind energy expended in drag on the sunward ionopause hemisphere.

The lower ionosphere of Titan

Abstract Ionization of the atmosphere of Titan by galactic cosmic rays is a very significant process throughout the altitude range of 100 to 400 km. An approximate form of the Boltzmann equation for cosmic ray transport has been used to obtain local ionization rates. Models of both ion and neutral chemistry have been employed to compute electron and ion density profiles for three different values of the H 2 /CH 4 abundance ratio. The peak electron density is of the order 10 3 cm −3 . The most abundant positive ions are C 2 H 9 + and C 3 H 9 + , while the predicted densities of the negative ions H − and CH 3 − are very small ( −4 that of the positive ions). It is suggested that inclusion of the ion chemistry is important in the computation of the H and CH 3 density profiles in the lower ionosphere.

Predicted electrical conductivity between 0 and 80 km in the Venusian atmosphere

Abstract Calculations of the space charge, ion density, and conductivity in the Venus atmosphere were made. The presence of the cloud particles on Venus causes a profound reduction in the calculated values of the ion density and conductivity compared to the values that are obtained without consideration of the cloud particles. When the cloud particles are included in the calculations, the results for the ion density and conductivity are approximately the same as those of the terrestrial atmosphere at the same pressure-altitude. Because the particles span such a large range of sizes and are abundant over a substantial range of pressure, the space charge varies strongly with altitude and particle size. Differential settling of the particles is expected to produce weak electric fields in the clouds.