The Effect of Aerosols on Radiative Transfer and the Optical Properties of the Atmosphere

Omar, Ali Hassan

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Description

Title

The Effect of Aerosols on Radiative Transfer and the Optical Properties of the Atmosphere

Author(s)

Omar, Ali Hassan

Issue Date

1997

Doctoral Committee Chair(s)

Susan Larson

Department of Study

Civl and Environmental Engineering

Discipline

Civl and Environmental Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

Environmental Sciences

Language

eng

Abstract

In this study we characterized the main physical and chemical properties of a rural mid-latitude continental aerosol using direct measurements and model predictions. To investigate the effect of aerosols on light extinction in the visible wavelengths, we used field measurements of aerosol concentrations, the elastic light scattering and interactive efficiency (ELSIE) model and an open-air integrating nephelometer. The annual-mean species concentrations showed that the most abundant identifiable species is sulfate assumed to be ammonium sulfate (annual-mean concentration 5.72 $\mu$g/m$\sp3$) followed by organic carbon (3.16 $\mu$g/m$\sp3$), ammonium nitrate (1.29 $\mu$g/m$\sp3$) and elemental carbon (0.26 $\mu$g/m$\sp3$). The ELSIE-determined extinction efficiencies for organic carbon ranged from 3.81 m$\sp2$/g at low relative humidities ($$75%), while sulfate extinction efficiencies ranged from 1.78 m$\sp2$/g to 5.78 m$\sp2$/g for the same range of relative humidities. The sulfate-containing particles are the main scattering particulate material at high relative humidities ($>$75%) and the organic carbon particles dominate particulate scattering at low relative humidities ($<$60%). In the intermediate relative humidity range (between 60% and 75%) organic carbon and sulfate species account for a comparable portion of the total light extinction. The radiative-transfer calculations of the direct aerosol forcing at Bondville show that the forcing significantly varies with the season. The soot+sulfate aerosol-forcing was calculated to be negative for the spring, summer and fall seasons. In the winter months, this negative value is small (+0.01 $\pm$ 0.01 Wm$\sp{-2}$) indicating there is complete cancellation of the sulfate forcing in the winter months by the soot component. The highest (negative) forcing in the summer ($-2.22\pm 0.58$ Wm$\sp{-2}$) corresponded to the sulfate concentration maximum. Adding a soot component to the summer burden causes a decrease in the negative forcing of 37%. Unlike the pure sulfate aerosol forcing, there was no negative forcing maximum corresponding to an intermediate solar zenith angle for the partially absorbing Bondville aerosol. The uncertainty in the direct aerosol forcing due to uncertainties in measured aerosol properties was evaluated from the calculations of the MLRCM and Charlson's (1991) reference box model (RBM) using stratified Monte Carlo methods. We used the latin-hypercube sampling (LHS) technique to determine a mean direct forcing due sulfate aerosols of $-0.75\pm 0.33$ Wm$\sp{-2}$. We also found a significant discrepancy (60%) between the nominal forcing and the LHS forcing. This is a sign that the variable covariance and higher-order partial derivatives of the forcing function with respect to the input parameters are large and cannot be ignored. The sensitivity analysis showed that the parameter to which the forcing is most sensitive is the single scattering albedo in the wavelengths from 0.44 to 0.97 $\mu$m.

Graduation Semester

1997

Type of Resource

text

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