HR: 0800h
AN: A31A-0809    [Abstracts]
TI: Diurnal Variations in the Specific Absorption Coefficient: Recent Results from Mexico City
AU: * Baumgardner, D
EM: darrel@servidor.unam.mx
AF: Universidad Nacional Autonoma de Mexico, Ciudad Universitaria Circuito Exterior, Mexico City, DF 04150 Mexico
AU: Kok, G
EM: glkok@dropletmeasurement.com
AF: Droplet Measurement Technologies, 5710B Flatiron Parkway, Boulder, CO 80301 United States
AU: Raga, G B
EM: raga@servidor.unam.mx
AF: Universidad Nacional Autonoma de Mexico, Ciudad Universitaria Circuito Exterior, Mexico City, DF 04150 Mexico
AB: The specific absorption coefficient, σ_a, defined as the absorbance of light per unit path length and per unit of mass concentration, is an important, radiative property of the atmosphere and is also used as a conversion factor when estimating the mass of light absorbing material, usually black carbon, from measurements of the absorption coefficient. The magnitude of σ_a varies over a wide range of values, 2 to 25 m ²g^-1, due to the composition and structure of the light absorbing material. The value of this parameter is usually determined by measuring the absorption coefficient, B_abs with a Particle Soot Absorption Photometer (PSAP) or an Aethalometer and the black carbon (BC) with one of several types of thermal optical techniques. There are two disadvantages to this approach, both associated with the thermo-optical derivations of BC. The first disadvantage is that significant sample times are needed to accumulate sufficient aerosol mass to derive the BC value. Secondly, there is a large uncertainty associated with the BC mass derived from the thermo-optical method. The recent development of the Single Particle Soot Photometer (SP-2) has allowed high resolution estimates of σ_a . The SP2 measures directly the mass of single, light absorbing particles using laser induced incandescence. This mass, when combined with B_abs derived from measurements of light attenuation, leads to an accurate, fast response estimate of σ_a . This technique has been recently applied during a field campaign in Mexico City in April, 2005. An SP-2, PSAP, Nephelometer, and other particle counters measured the physical and optical properties of particles in the southwest sector of Mexico City. Measurements were made 24 hours a day so that the diurnal variations could be monitored. The value of σ_a, derived every five minutes from the measurements of the SP-2 and PSAP, showed clear diurnal variations that reflect the changes in the age of the BC that is emitted by local sources. In the morning the BC is fresh as a result of the peak in traffic and ages with time as the boundary layer develop and photochemical processes presumably alter the characteristics of the BC.
DE: 0305 Aerosols and particles (0345, 4801, 4906)
DE: 0325 Evolution of the atmosphere (1610, 8125)
DE: 0368 Troposphere: constituent transport and chemistry
SC: Atmospheric Sciences [A]
MN: Fall Meeting 2005