יום רביעי, 4 ביוני 2008

סמינר: Identifying Absorption Enhancement Effects in Aerosol Particle-Laden Filters by Laser Heating

Cary Presser

National Institute of Standards and Technology, Gaithersburg

ההרצאה תתקיים ביום רביעי כ"ט סיון תשס"ח (02.07.2008)
שעה 16:30
בנין אוירונוטיקה חדר 241
כבוד קל יוגש לפני ההרצאה

תקציר


Aerosols affect directly the atmospheric radiative loading through both scattered and absorbed solar and infrared radiation, as well as indirect effects on cloud cover and albedo (fraction of solar radiation reflected by a surface or object). Atmospheric aerosol optical properties are dependent on particle/droplet size distribution, number concentration, and chemical composition. Depending of the mixture composition and type of mixing, absorption can be influenced significantly. Although quantitative values of the radiative forcing are available for different aerosols, the estimated uncertainties can be as large as a factor of 2. These large uncertainties in forcing by aerosols preclude meaningful model evaluation by comparison with global satellite observations.

Several laser- and filter-based techniques are used to measure atmospheric aerosol optical properties, with one widely used filter-based method based on thermal-optical analysis (TOA). The TOA relies on distinguishing BC from organic carbon (OC) of carbonaceous particles collected on quartz-fiber filters through a process of OC thermal desorption of volatile gaseous species, OC thermal pyrolysis, and oxidation of solid matter.

Over the years, there have been several studies carried out to elucidate various issues associated with the operation of the filter-based techniques. In particular, one issue that was studied was the effect of the filter substrate on light absorption (referred to as absorption enhancement). This effect is attributed to absorption of backscattered laser light by particles discretely distributed over and embedded within the quartz-fiber filter that on first pass is not absorbed by the particles. As a result, a substantial increase occurs in the mean absorption of particles, which influences determination of OC and BC concentrations during different segments of the temperature protocol.

A laser-heating approach was used to measure the absorptivity of quartz filters laden with a variety of carbonaceous-based materials in helium and at sample temperatures that were relevant to thermal optical analysis. Substance absorptivity is determined by perturbing the sample temperature from a pre-selected steady state, and monitoring the thermal response during decay back to steady state, along with thermal energy conservation. Results were obtained for clean quartz filters and for filters contaminated with aerosol particles from three different cities (Atlanta, Seattle, and Los Angeles). Absorption enhancement effects were investigated by comparing the absorption coefficient for stacked filters with different orientations. The absorption coefficient, in general, decreased with steady-state sample temperature. The bsorption coefficient of two individual filters was less than the same two filters stacked together. The highest values were obtained for two stacked filters with the particle side facing one another.

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