Andrew
Dipping into this list after a long time ignoring it. You asked: One thing which I personally am currently unclear on is the optimal microscale mixing ratios required. Has anyone considered the effect of a dense injection regime, e.g. a balloon or slurry pipe, versus a distributed regime, e.g. an aircraft fleet? Simone Tilmes said "I think it is important to point out that there is very likely a limit on how much the Earth's surface could be cooled using sulfate aerosols, due to coagulation processes and fall out of aerosols. Only less than 2 W/m2 reduction of global net surface SW flux was achieved in the study by Heckendorn et al., 2009, using a micro-physical model to consider size distributions of the aerosols. Niemeier et al., 2010, achieved a stronger forcing if injecting particles at 30hPa, which allow them to stay longer in the stratosphere. Though it will be hard to inject particles that high." Small scale mixing is exactly what we focused on in: Jeffrey R. Pierce, Debra K. Weisenstein, Patricia Heckendorn, Thomas Peter and David W. Keith. (2010). Efficient formation of stratospheric aerosol for geoengineering by emission of condensible vapor from aircraft. Geophysical Research Letters, 37, L18805, doi:10.1029/2010GL043975. (PDF with no password needed) The point of this paper is that releasing H2SO4 vapor in plumes that are rapidly mixed allows production of sulfate aerosol with effective size distribution. We coupled a small-scale plume to a global model and found that, in our model, much less sulfate was required to get radiative forcings of a few Wm^-2 that is required with the SO2 method. (E.g., to get 4 Wm^-2 one needs about 8 Mt-S/year with this 'direct aerosol method' versus 20 with SO2.) This method would not work with balloon pipes (assuming reasonable limits on the number of pipes) because dispersal rates must be too big for the H2SO4 vapor route to work. Moreover, we also looked a various SO2 schemes and we do find that reasonably even distribution is important to avoid the big droplet problem. English, Toon and Mill's found similar results with respect to the need for broad dispersal of SO2 http://www.atmos-chem-phys.net/12/4775/2012/acp-12-4775-2012.pdf. At first glance this paper appears to contradict ours on utility of direct H2SO4 injection, but on close inspection I think it shows strong agreement. They found that H2SO4 emitted in dispersed from-that is mixed at the grid-box level-has no advantage. This is exactly what one would expect since from the aerosols microphysics is just like the production of H2SO4 by oxidation of SO2. My understanding is that subsequent e-mail exchanges between English and Pierce have clarified that there is no essential disagreement on this point. Indeed it's not clear if there's any way that one could disperse H2SO4 evenly in a way that matched the simulation of English et al. David -----Original Message----- From: [email protected]<mailto:[email protected]> [mailto:[email protected]]<mailto:[mailto:[email protected]]> On Behalf Of Andrew Lockley Sent: Tuesday, June 05, 2012 7:18 PM To: geoengineering Subject: [geo] Mixing of sulphur precursor gases Hi This paper is well worth reading Heckendorn, P.; Weisenstein, D.; Fueglistaler, S.; Luo, B. P.; Rozanov, E.; Schraner, M.; Thomason, L. W.; Peter, T. (2009). "The impact of geoengineering aerosols on stratospheric temperature and ozone". Environmental Research Letters 4: 045108. Bibcode2009ERL.....4d5108H. doi:10.1088/1748-9326/4/4/045108 The authors consider the temporal and spatial injection regime best suited to attaining well-mixed sulphur particles of the correct size distribution for geoengineering use. They conclude that spatial distribution is helpful, but temporal distribution is unhelpful. I personally would welcome list discussion on whether this conclusion is seen as reliable, and additionally clarification of the processes involved. One thing which I personally am currently unclear on is the optimal microscale mixing ratios required. Has anyone considered the effect of a dense injection regime, e.g. a balloon or slurry pipe, versus a distributed regime, e.g. an aircraft fleet? Heckendorn do not seem to have addressed this issue at all in their paper. It's unclear to me whether the injection density on a scale of 10E1-10E4m would be significant in the formation of aerosols. I'm not aware of any paper which considers this microscale mixing. Any links & comments are appreciated. A -- You received this message because you are subscribed to the Google Groups "geoengineering" group. To post to this group, send email to [email protected]<mailto:[email protected]>. To unsubscribe from this group, send email to [email protected]<mailto:[email protected]>. 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