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Representing Tropospheric Aerosols and Their Climatic Effects in Global Models


Adams, Peter Jonathan (2002) Representing Tropospheric Aerosols and Their Climatic Effects in Global Models. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/10CM-8W55.


In order to better understand and quantify the direct and indirect effect of aerosols on climate, an earlier general circulation model (GCM) simulation of tropospheric sulfate has been extended by incorporating aerosol thermodynamics and microphysics. The thermodynamic simulation allows the prediction of nitrate, ammonium, and aerosol water concentrations. It is estimated that nitrate contributes as much to total aerosol mass as sulfate on regional scales in parts of Europe and North America. The direct radiative forcing associated with the sulfate-nitrate-ammonium-water mixture is estimated to be —1.14 W M^(-2) for the present day. Based on a future emissions scenario, this could increase to as much as —2.13 W^(-2) by the year 2100, an increase that results from increased nitrate concentrations. Although currently a minor contributor to aerosol direct radiative forcing, nitrate is predicted to exceed sulfate in its contribution by the end of the century for this emissions scenario. It is also found that direct radiative forcing estimates are highly sensitive to aerosol behavior at relative humidity above 90%, highlighting the shortcomings of global models in their treatment of aerosol water uptake under partly cloudy conditions. The microphysical simulation allows the prediction of tropospheric aerosol number concentrations and size distributions, key parameters in determining the indirect effect of aerosols on clouds. A two-moment sectional algorithm is used to simulate the microphysical processes of condensation/evaporation and coagulation. It has been tested by performing a simulation of sulfate microphysics. Predicted aerosol number concentrations generally agree with observations to within 25%. The microphysical simulation also reproduces key features of the tropospheric aerosol such as increasing number concentrations with altitude and land-sea contrasts in cloud condensation nuclei concentrations. It is found that there are important uncertainties in the source rates of new particles to the atmosphere, whether from in situ nucleation or emissions of particulates, that can significantly impact predicted aerosol number and cloud condensation nuclei concentrations.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:(Chemical Engineering and Environmental Science and Engineering)
Degree Grantor:California Institute of Technology
Division:Chemistry and Chemical Engineering
Major Option:Chemical Engineering
Minor Option:Environmental Science and Engineering
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Seinfeld, John H.
Thesis Committee:
  • Seinfeld, John H. (chair)
  • Flagan, Richard C.
  • Hoffmann, Michael R.
  • Yung, Yuk L.
Defense Date:16 July 2001
Record Number:CaltechTHESIS:10122010-125450691
Persistent URL:
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:6137
Deposited By: Benjamin Perez
Deposited On:13 Oct 2010 15:12
Last Modified:12 Nov 2021 23:48

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