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 100μm. 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, kγ = kγ,Rayleigh + kγ,ozone + kγ,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 kγ,aer ∝ -α log γ. The coefficient α 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 dawn, making it unlikely that the aerosols were locally generated dust. Figure 2 illustrates the change of the U, B and V-band extinction coefficients during the course of the night. The 23h00 arrival time of the aerosol cloud and its intensification just before dawn can clearly be seen on the graph. Such events can be interpreted in conjunction with meteorological data to estimate the generation and transport of the aerosols. References 1. Andrews, P.J., Williams, I.P. 1989, The Observatory, 109, 15. 2. Cousins, A.W.J. 1985, Mon. Not. Astr. Soc. South Africa, 44, 10. 3. Garstang, R.H. 1991, Publs Astr. Soc. Pacific, 103, 1109. 4. Hayes, D.S. & Latham, D.W. 1975, Astrophys. J., 197, 593. 5. Kidger, M.R. 1988, The Observatory, 108, 226. 6. Kilkenny, D. 1995, The Observatory, 115, 25. 7. Krisciunas, K., Schaefer, B.E. 1991, Publs Astr. Soc. Pacific, 103, 1033. 8. Spencer Jones, J.H. 1980, Mon. Not. Astr. Soc. South Africa, 39, 89. 9. Stickland, D.J., Lloyd, C., Pike, C.D. & Walker, E.N. 1987, The Observatory, 107, 74. 10. Whittet, D.C.B., Bode, M.F. & Murdin, P. 1987, Vistas in Astronomy,30, 135. On Apr 9, 3:26 pm, "Alvia Gaskill" <[email protected]> wrote: > First, Alan must not have read the article because it addresses corrections > for optical interference (Sec. 6). Second, most astronomy is not based on > visible light. In looking at over 250 web pages on Google for Pinatubo > astronomy, I found almost nothing on problems with the Pinatubo event. > Wouldn't you think if there had been problems, this would have been > mentioned before? > > ----- Original Message ----- > From: "Eugene I. Gordon" <[email protected]> > To: <[email protected]>; <[email protected]>; > > <[email protected]> > Sent: Thursday, April 09, 2009 5:17 PM > Subject: RE: [geo] Re: Wouldn't stratospheric aerosols ruin astronomical > observations? > > > The answer to the astronomy issue is a slam dunk. Too damn bad! There is > > far > > too much at stake to worry that astronomy takes a temporary hit. In any > > case the clouds need not be permanent. > > > -----Original Message----- > > From: [email protected] > > [mailto:[email protected]] On Behalf Of Alvia Gaskill > > Sent: Thursday, April 09, 2009 2:39 PM > > To: [email protected]; [email protected] > > Subject: [geo] Re: Wouldn't stratospheric aerosols ruin astronomical > > observations? > > > The effect of aerosols can be corrected for as indicated in the attached > > paper. Note also that periodic dust storms create localized extinction > > coefficients of around 0.25, similar to that from Pinatubo. Since > > Pinatubo > > levels would not be needed for a long time, the actual extinction would be > > much less. Some of the observational problems from Pinatubo were also due > > to transient ash from the eruption and not the aerosol. > > > ----- Original Message ----- > > From: "Alan Robock" <[email protected]> > > To: <[email protected]> > > Cc: "Climate Intervention" <[email protected]> > > Sent: Thursday, April 09, 2009 1:17 PM > > Subject: [geo] Wouldn't stratospheric aerosols ruin astronomical > > observations? > > >> Dear All, > > >> As some of you know, I published a paper last year: > > >> Robock, Alan, 2008: 20 reasons why geoengineering may be a bad idea. > >> Bull. Atomic Scientists, 64, No. 2, 14-18, 59, doi:10.2968/064002006. > > >>http://climate.envsci.rutgers.edu/pdf/20Reasons.pdf > > >> which also produced a roundtable discussion: > > >>http://www.thebulletin.org/web-edition/roundtables/has-the-time-come-g > >> eoengineering > > >> Since then, I have been evaluating these reasons and two of them seem > >> to not be of concern, excess acid deposition and cost. Our two papers > >> on these results, now under review, are: > > >> Kravitz, Ben, Alan Robock, Luke Oman, Georgiy Stenchikov, and Allison B. > >> Marquardt, 2009: Sulfuric acid deposition from stratospheric > >> geoengineering with sulfate aerosols. Submitted to J. Geophys. Res. > > >>http://climate.envsci.rutgers.edu/pdf/AcidDepositionJGRsubmitted.pdf > > >> Robock, Alan, Allison B. Marquardt, Ben Kravitz, and Georgiy > >> Stenchikov, > >> 2009: The practicality of geoengineering. Submitted to Geophys. Res. > >> Lett. > > >>http://climate.envsci.rutgers.edu/pdf/practicality8NoFig3.pdf > > >> But I have also been giving talks on the subject and two days ago a > >> member of an audience suggested another reason why geoengineering > >> (with stratospheric aerosols) may be a bad idea: > > >> It would ruin Earth-based optical astronomy! > > >> With the tremendous investment in equipment, and mountain-top > >> observatories to get above most of the junk in the atmosphere, not to > >> mention sophisticated signal processing algorithms to remove the > >> remaining atmospheric influence, how could astronomers stay silent and > >> allow permanent clouds that would block their seeing? > > >> 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 --~--~---------~--~----~------------~-------~--~----~ 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 -~----------~----~----~----~------~----~------~--~---
