HR: 1340h
AN: A33B-1180    [Abstracts]
TI: Cavity Attenuation Phase Shift Detection of Trace Atmospheric Species: Nitrogen Dioxide
AU: * Freedman, A
EM: af@aerodyne.com
AF: Aerodyne Research, Inc., 45 Manning Road, Billerica, MA 01821-3976, United States
AU: Herndon, S C
EM: herndon@aerodyne.com
AF: Aerodyne Research, Inc., 45 Manning Road, Billerica, MA 01821-3976, United States
AU: Kebabian, P L
EM: pkebab@aerodyne.com
AF: Aerodyne Research, Inc., 45 Manning Road, Billerica, MA 01821-3976, United States
AU: Wood, E C
EM: ezrawood@aerodyne.com
AF: Aerodyne Research, Inc., 45 Manning Road, Billerica, MA 01821-3976, United States
AB: We present details of an apparatus capable of measuring optical extinction (i.e., scattering and/or absorption) which employs one variant of cavity enhanced detection, specifically cavity attenuated phase shift spectroscopy (CAPS). It uses a near confocal arrangement of two high reflectivity (R~0.9999) mirrors in tandem with an enclosed cell 26 cm in length, a light emitting diode (LED) and a vacuum photodiode detector. Square wave modulated light from the LED passes through the sample cell and is detected as a distorted waveform which is characterized by a phase shift with respect to the initial modulation. The amount of that phase shift is a function of fixed instrument properties - cell length, mirror reflectivity, and modulation frequency – and of the presence of a scatterer or absorber (air, particles, trace gases, etc.) within the cell. The monitor is enclosed within a standard rack-mounted instrumentation box, weighs 10 kg and uses 70 W of electrical power including a vacuum pump. The instrument has demonstrated a detection limit of 0.02 Mm^-1 (1σ) in ten seconds integration time and a baseline drift of less than ± 0.1 Mm^-1 over a 24 hour period. We also present an intercomparison of measurements of ambient nitrogen dioxide concentrations using this sensor, a quantum cascade laser-based infrared absorption spectrometer and a standard hot metal/chemiluminescence-based NOx analyzer. Measurements using the quantum cascade-laser and CAPS monitor agree well within the inter- calibration error. The chemiluminescence analyzer data falls outside the expected error band, a fact that is ascribed to the presence of known chemical interferences for the chemiluminescence-based analyzer.
DE: 0305 Aerosols and particles (0345, 4801, 4906)
DE: 0345 Pollution: urban and regional (0305, 0478, 4251)
DE: 0394 Instruments and techniques
DE: 3394 Instruments and techniques
SC: Atmospheric Sciences [A]
MN: 2007 Fall Meeting