Date of Award

Spring 1-1-2011

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Atmospheric & Oceanic Sciences

First Advisor

Owen B. Toon

Second Advisor

Cora E. Randall

Third Advisor

John Cassano

Abstract

Stratospheric aerosols can influence radiative forcing and atmospheric chemistry, yet much remains to be learned about their sources and evolution. To improve understanding of these processes, a sulfate aerosol microphysical sectional model coupled to a climate model (WACCM/CARMA) has been developed. This model includes sulfur emissions, a 63-species chemistry module, and aerosol microphysics (nucleation, coagulation, growth, and deposition). This model was utilized to study stratospheric aerosol under ambient conditions as well as from large volcanic eruptions and hypothetical climate engineering scenarios.

Simulations of ambient aerosol using three nucleation schemes reveal that one theory for ion-induced nucleation from galactic cosmic rays predicts 25% higher nucleation rates in the upper troposphere and lower stratosphere (UTLS) than its related binary homogeneous nucleation scheme, but that the rates predicted by two binary schemes vary by two orders of magnitude. None of the nucleation schemes are superior at matching the limited observations available at the smallest sizes. It is found that coagulation, not nucleation, controls number concentration at sizes greater than approximately 10 nm, suggesting that processes relevant to atmospheric chemistry and radiative forcing in the UTLS are not sensitive to the choice of nucleation schemes. Simulations using all three nucleation schemes compare reasonably well to observations of aerosol size distributions, number concentrations, and mass in the UTLS. The inclusion of van der Waals forces in the coagulation scheme improves comparison to observations in the UTLS.

Simulations of the Mount Pinatubo eruption find stratospheric aerosol mass and aerosol optical depth (AOD) to increase by two orders of magnitude, in agreement with observations, highlighting the eruption’s significant impact on stratospheric aerosol. The model predicts effective radius to triple six months after the eruption and 525 nm AOD to increase to 0.45 three months after the eruption in the tropics, in agreement with observations. In the mid- and high-latitude Southern Hemisphere, the simulated 525 nm AOD is about one-third that of observations 3 months after the eruption, which may be due to the August eruption of Cerro Hudson in Chile, which is not included in the model. Simulated 525 nm AOD spans a narrower range than observations, tapers more quickly, and peaks at 5°N while the data peaks at 5°S. Possible explanations for these differences include the lack of ash, aerosol heating or the Cerro Hudson eruption in the model, unmatched winds for the year 1991, or biases in the data.

Simulations of stratospheric sulfur injection scenarios reveal numerous insights into the efficacy and consequences of hypothetical geoengineering scenarios. Continuous SO2 injection in a narrow region at the equator is found to have limited efficacy at higher injection rates, while broadening the injection region or injecting SO4 particles instead of SO2 gas can increase sulfate burden, in agreement with previous work. Injection of H2SO4 gas does not increase burdens compared to SO2, in disagreement with previous work using a plume model. It is suggested that the results found in the plume model were not due to injecting H2SO4, but rather to converting gases to particles. Considerably more research is needed on plumes to test assumptions made during modeling studies. Additionally, stratospheric geoengineering significantly perturbs tropospheric aerosol mass burden, number, and size distributions at much greater levels than simulated for the eruption of Mount Pinatubo. Tropospheric burdens increase by a factor of two or three, with the majority of the increases occurring at all latitudes in the 100 hPa thick layer just below the tropopause, as well as most of the troposphere at high latitudes. These perturbations could impact upper tropospheric radiative forcing or chemistry, highlighting the need to further study the efficacy and consequences of geoengineering before its employment is seriously considered.

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