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Title:Speciated Local Aerosol Characteristics and Radiative Forcing at a Rural Midwestern Site
Author(s):Dillner, Ann Marie
Doctoral Committee Chair(s):Larson, Susan M.
Department / Program:Civil Engineering
Discipline:Civil Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Physics, Atmospheric Science
Engineering, Environmental
Abstract:In this research, physical and chemical properties of ambient aerosols were measured at a rural perturbed mid-latitude site (Bondville, IL) and used to calculate the aerosol optical properties and the resulting direct radiative forcing. Size-segregated aerosol samples were collected during the summer of 1997 using three parallel MOUDIs operating at ambient relative humidity. Two sample sets were used to obtain sulfate, organic carbon (OC), elemental carbon (EC), carbonate and total aerosol mass. The third sample set was used to obtain the size-specific and wavelength-dependent extinction efficiency of EC.
The measured submicrometer mass concentration was 11.4 +/- 4.0 mug m-3. Ammonium sulfate comprised nearly half of the submicrometer aerosol and OC plus EC comprised 25%.
Water content for ammonium sulfate and OC was estimated using both Kohler theory and parameterized water uptake curves from the literature. Water content for internally mixed aerosols was determined using a ZSR method. Aerosol optical properties (extinction efficiency, asymmetry parameter, single scatter albedo) were calculated from measured size distributions and wavelength dependent refractive indexes for each species and for internal and external mixtures using Mie theory.
A technique, utilizing transmission measurements through extracts of size segregated ambient aerosol samples, was developed to obtain the extinction efficiency of EC. Measured EC extinction efficiencies ranged from 7.3 to 1.7 m2 g-1 at 550 nm, depending on particle diameter. Normalized direct aerosol radiative forcing (W g-1 ) was calculated using the Column Radiation Module (CRM) of the National Center for Atmospheric Research (NCAR) Community Climate Model (CCM3). Aerosol optical properties, used in the model, were assumed to be uniform throughout the lowest one kilometer of the atmosphere. The normalized forcing due to ammonium sulfate was -340 +/- 10 W g-1. OC was 1/3 larger and residue was 1/3 smaller. EC within an internally mixed aerosol was shown to suppress forcing.
This research supports the view that species other than ammonium sulfate, namely EC, OC and dust-like aerosol, have significant radiative effects. Although the mass percentage of EC may be low at rural sites, ignoring the presence of EC aerosol leads to an over estimation of the aerosol forcing.
Issue Date:2000
Description:162 p.
Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2000.
Other Identifier(s):(UMI)AAI9971067
Date Available in IDEALS:2015-05-13
Date Deposited:2000

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