The atmosphere is mostly troposphere, by mass. The troposphere is also the grimiest part of the atmosphere, even relative to mass. The contribution of stratospheric aerosols to interfering with astronomical observation seems likely to be negligible -- as it apparently was in the case of Pinatubo.
But if not, tough luck, obviously. It's not going to be done unless the trillion-dollar considerations dictate so, and all earth-based astronomy could be replaced with space-based observations for a tithe of the amounts at stake in climate change. On Apr 9, 7:40 pm, Alan Robock <[email protected]> wrote: > Dear Dan, > > Yes, again this shows how astronomical observations can characterize > aerosols, but it does not say that observations of faint objects will > not be severely impacted by a permanent aerosol cloud. > > You will note that this article also mentions the issue of the growth of > the size of the Pinatubo aerosols over time. With a permanent cloud, > additional SO2 injections may make existing particles larger rather than > produce more particles of a size more effective at scattering solar > radiation. This means that while we might have a handle on putting SO2 > into the stratosphere, we still do not know how to make aerosols of the > most effective size. This is an area of ongoing research by our group, > led by Rich Turco. > > Alan > > Alan Robock, Professor II > Director, Meteorology Undergraduate Program > Associate Director, Center for Environmental Prediction > Department of Environmental Sciences Phone: +1-732-932-9800 x6222 > Rutgers University Fax: +1-732-932-8644 > 14 College Farm Road E-mail: [email protected] > New Brunswick, NJ 08901-8551 USA http://envsci.rutgers.edu/~robock > > On Thu, 9 Apr 2009, DW wrote: > > > This is apparently (as one might imagine) a fairly well explored > > topic. One good reference which talks about Pinatubo as well as > > Saharan dust events which affect observatories in the Canaries, and > > even marine aerosols which result from sea spray is at (graphs did not > > copy): > > >http://www.saao.ac.za/~wgssa/as5/winkler.html > > > An excerpt > > > Aerosols and their effect on optical radiation > > Aerosols may be defined as particles suspended in the atmosphere, and > > the term is generally used to denote units larger than molecules. > > Aerosol diameters typically range from about 10-4 to 100 m. > > > Apart from their use as tracers in atmospheric circulation studies, > > aerosols have more recently been recognised as important contributors > > to weather phenomena and climate change. This is partly due to their > > role as nuclei on which water droplets can grow, and also partly > > because of their effect on the global radiation balance. > > > Three processes determine the concentration and particle size > > distribution of an aerosol ensemble: > > > * The injection of aerosol into the atmosphere from ground level > > through a variety of mechanisms described below; > > * The growth of particles through the coalescing of smaller > > particles; > > * The deposition of airborne particles on the ground through > > precipitation. > > > The composition, shape, size and refractive properties of aerosol > > particles are often determined by their mode of generation. It is > > convenient to categorise aerosols accordingly: > > > (a) Volcanic ash: Propelled skywards in the course of volcanic > > eruptions, these sulphur-rich aerosols are occasionally lifted as high > > as the stratosphere, where they have typical lifetimes of several > > years, much longer than their tropospheric counterparts. Recent such > > events include the eruptions of Agung (1963), El Chichon (1982) and > > Pinatubo (1991). Characteristically, the aerosols get dissipated > > throughout the stratosphere within a few months. The particles then > > coalesce until they become too large to be supported and fall to the > > ground. > > (b) Pyrogenic aerosols: These are in essence the smoke from > > forest and savannah fires. High concentrations of these aerosols are > > usually recorded over sub-Saharan Africa during and just after the dry > > season. > > (c) Windborn sand and dust: Such aerosols are usually generated > > in arid regions and tend to be rich in silicates. Significant > > generation of dust also occurs in wetter areas following the ploughing > > season or even through traffic on dirt roads. > > (d) Maritime aerosols: These result from the uplifting of sea > > spray through wind. These particles characteristically have high > > abundances of sodium chloride. Though prevalent over the oceans, these > > aerosols can be transported far inland. > > (e) Biogenic emissions: Biogenic processes are more commonly > > responsible for trace gas generation, which may contribute to the > > formation of aerosols. They also produce airborne microscopic > > organisms such as pollen. > > (f) Industrial and other anthropogenic emissions: Aerosols > > originating in this fashion include the emissions from coal burning > > power stations, dust generated by opencast mining operations and > > domestic wood and coal burning. > > > Aerosols contribute to the attenuation of incoming starlight, which in > > turn implies that their concentration may be estimated by measuring > > the degree of extinction in the atmosphere. Extinction in the > > wavelength range 350-800 nm may be due to Rayleigh scattering, > > stratospheric ozone or aerosols, > > k = k ,Rayleigh + k ,ozone + k ,aer > > > where k is the standard astronomical extinction coefficient, defined > > as > > > k = 2.5 (log Intensity above atmosphere > > -log Intensityon ground) > > for a star at the zenith. > > > The Rayleigh extinction is almost constant at any particular location > > and altitude, while ozone only affects specific parts of the spectrum. > > Outside these spectral regions any variations in the extinction are > > thus due to changes in the aerosol concentration or characteristics. > > > Extinction by aerosols is largely the result of Mie scattering, and > > its dependence on wavelength may be described by the following > > relation [4]: > > log k ,aer - log . > > > The coefficient ranges from 0 for very large particles to 4 for very > > small particles. > > Examples of cases where astronomical extinction > > measurements facilitated atmospheric research in Africa > > Properties of Saharan dust and its transportation to the Canary > > Islands > > > Saharan dust is occasionally transported as far as the Canary Islands > > in the northern hemisphere summer months. It manifests itself as an > > almost fog-like haze at the various astronomical sites on the > > archipelago, such as the Roque de los Muchachos observatory on La > > Palma. Through the measurement of the extinction during such events it > > has been possible to not only monitor the passage and density of the > > dust clouds, but also to determine the colour dependence of the > > aerosol opacity (and hence particle size distribution) of Saharan > > dust. Stickland et al (1987) found that the aerosol opacity at La > > Palma is independent of wavelength to a good approximation[9]. This > > confirmed the theoretical work of several authors[10], who showed that > > the refractive properties of typical Saharan dust grains are expected > > to be colour-neutral. Kidger[5] and Andrews & Williams[1] found a > > small wavelength dependence on the extinction coefficients in the > > infrared and optical regimes respectively, which is likely to be the > > result of mixing of the type of grains modelled by Whittet, Bode & > > Murdin with smaller particles[10]. > > > The Pinatubo ash-cloud and its evolution > > > The volcanic eruption of Mount Pinatubo in the Philippines in 1991 > > injected huge quantities of volcanic ash into the stratosphere. Within > > a couple of months these volcanic aerosols became distributed around > > the globe. The development of the volcanic ash clouds over the South > > African Astronomical Observatory in Sutherland can be traced by > > plotting the measured extinction coefficients[6]. The study showed > > that enhanced aerosol concentrations persisted for several years. It > > also illustrated the patchy nature of the stratospheric ash clouds. > > > Figure 1 shows the volcanic ash extinction coefficients calculated by > > Kilkenny as a function of wavelength for two high-extinction events > > following the eruption. These were obtained by subtracting the > > Sutherland "normal" (i.e. pre-Pinatubo clear day) values from the > > measured extinction coefficients. Note that the value of a i.e. the > > slope of the graph) is much smaller on 26 September 1992 than on 10 > > September 1991. This illustrates the change in the particle size > > distribution in the intervening period - the smaller particles that > > had dominated the distribution soon after the eruption had coalesced > > into bigger units a year later. > > > The brown haze in Cape Town > > > Where telescopes equipped with photometers exist in urban areas, the > > extinction measurements may be utilised to study pollutants. The city > > with the largest available extinction value database in Africa is > > probably Cape Town, as a result of the extensive standard star work by > > Cousins at the South African Astronomical Observatory headquarters. > > Cousins has described extinction coefficient behaviour as a function > > of meteorological conditions[2]. He has been able to detect maritime > > aerosols and the "brown haze", which is caused by domestic fires in > > the Cape Flats. Future measurements of the extinction at the site will > > provide the opportunity to monitor the severity of brown haze type > > pollution as a result of further urbanisation and electrification. > > > Passage of pyrogenic aerosol clouds over Sutherland > > > During the winter months an anti-cyclonic air circulation pattern > > frequently develops over southern Africa. > > > The late winter months are a period of intense woodland burning in the > > belt just to the south of the Intertropical Convergence Zone, centred > > on Zambia and including neighbouring countries. > > > The pyrogenic aerosols thus placed into circulation are frequently > > transported southward and form layers of haze over the subcontinent, > > occasionally moving as far south as Sutherland. On the night of 29-30 > > September 1997, an aerosol cloud passed over Sutherland observatory, > > and the extinction was measured regularly throughout the night. > > Brownish haze was spotted above the horizons at > > ... > > read more » --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "geoengineering" group. To post to this group, send email to [email protected] To unsubscribe from this group, send email to [email protected] For more options, visit this group at http://groups.google.com/group/geoengineering?hl=en -~----------~----~----~----~------~----~------~--~---
