P. Hamill

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...

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...

Soot aerosol in the lower stratosphere: Pole-to-pole variability and contributions by aircraft

A NASA ER-2 high-altitude research aircraft intercepted the exhaust wake of a supersonic Concorde aircraft in the stratosphere near New Zealand on October 8, 1994. Black carbon (soot) aerosol (BCA) was sampled by wire impactors during the first five of 12 short-duration wake intercepts. BCA concentration in Concorde exhaust at 16.3 km altitude was 0.2 particles cm−3, the size distribution peaked at a geometric mean radius of 0.09 μm, and the mass loading was 2.0±1.4 ng m−3. With a plume dilution factor (DF) of 1.0×10−5, determined by the ratio of CO2 measured in the plume (above the ambient stratospheric background level) to CO2 in the engine exhaust plane, the Concorde BCA emission index was EI(BCA)=0.07±0.05 g BCA per kg fuel burned. Applying this EI to estimates of aircraft fuel burned by the current subsonic fleet in the stratosphere yields average stratospheric BCA loadings of 0.5 ng m−3, commensurate with observations in the northern stratosphere. Applying the Concorde EI to fuel consumption by a projected future fleet suggests a twofold-threefold increase of stratospheric BCA by the year 2015. A strong gradient in BCA concentration exists between the northern and the southern hemispheres, indicating interhemispheric mixing times longer than stratospheric residence times.

Stratospheric Aerosol Modification by Supersonic Transport and Space Shuttle Operations—Climate Implications

Abstract We have estimated the potential effects on stratospheric aerosols of supersonic transport emissions of sulfur dioxide gas and submicron soot granules, and space shuttle rocket emissions of aluminum oxide particulates. Recently, exhaust particles from large aircraft and rocket engines have been characterized experimentally, and we have adopted new data where appropriate. We use an interactive particle-gas model of the stratospheric aerosol layer to calculate changes due to exhaust emissions. We also employ an accurate radiation transport model to compute the effect of aerosol changes on the earths average surface temperature. Our major conclusions are as follows. The release of large numbers of small particles (soot or aluminum oxide) into the stratosphere should not lead to corresponding significant increase in the concentration of large, optically active aerosols. On the contrary, the increase in large particles is severely limited by the total mass of sulfate available to make large particles ...

New evidence of size and composition of polar stratospheric cloud particles

A NASA Ames ER-2 aircraft encountered polar stratospheric cloud particles on July 28 and July 30, 1994 during the ASHOE/MAESA deployment. Stratospheric particles were collected by impaction techniques on specially treated substrates. For the first time, Nitron- nitrate reaction spots were detected on the ice crystal replicas, indicating the presence of NO 3 - ions on/in ice. Because the reaction spots were detected only on very small crystals (r<1 μm) and never on larger crystals, suggest that the amount of NO 3 - coating sufficient to initiate reactions will accumulate during ice crystal evaporation in ice subsaturated air. This may slow down the evaporation rate and enable ice crystals to survive longer in subsaturated environment. That provides an explanation of why on both days ice crystals were replicated at temperatures above the frost point, when their appearance and size suggest substantial evaporation.

Heterogeneous Chemistry of Polar Stratospheric Clouds and Volcanic Aerosols

The chemistry of the stratosphere is strongly influenced by the presence of small particles composed of sulfuric acid, nitric acid and other materials. The ubiquitous background stratospheric aerosol layer is composed of sulfuric acid droplets, while the clouds observed in the polar winter stratosphere (the polar stratospheric clouds, or PSCs) are composed of nitric acid ices. Chemical reactions can occur efficiently on the particle surfaces, and in solution in the case of liquid droplets. Such reactions affect the concentrations of chlorine and nitrogen species in the lower stratosphere, and play a critical role in ozone depletion. Indeed, the “ozone hole” has been shown to be initiated by “heterogeneous” reactions occurring on PSC particles. The origins and properties of sulfate aerosols, PSCs and other observed stratospheric particles are surveyed. Anthropogenic influences on these aerosols are discussed. The heterogeneous chemistry of polar stratospheric clouds, and the chemical processing of air in contact with such clouds, are illustrated using detailed model simulations. The injection of sulfur and chlorine into the stratosphere by volcanic eruptions is also investigated. HCl scavenging in volcanic eruption plumes is quantified based on an analysis of the dynamics, physical chemistry and microphysics of eruption columns. It is shown that very little chlorine is likely to enter the stratosphere in volcanic plumes because of efficient HCl absorption in supercooled water that condenses on sulfuric acid aerosols. The possible role of sulfate aerosols — both of volcanic and background origin — as a medium for heterogeneous chemical reactions is assessed. It is argued that the sulfate aerosols can produce significant chemical pertubations in regions of the atmosphere where temperatures drop below about 200 K. The potential contribution of sulfate aerosols to ozone depletion at high latitudes is discussed. Outstanding scientific issues concerning stratospheric aerosols and their chemical effects are summmarized.