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Project Status: complete

Report Published May 1986:


Title: Absolute measurements of nitric acid by kilometer pathlength FT-IR spectroscopy and their intercomparison with other measurement methods.

Principal Investigator / Author(s): Winer, Arthur M

Contract Number: A5-051-32


Topic Areas: Acid Deposition, Monitoring


Abstract:

Measurements of ambient nitric acid (HNO3) and ammonia (NH3) concentrations were conducted using a Fourier transform infrared (FT-IR) spectrometer interfaced to an open-path, multiple reflection optical system. These measurements provided benchmark data for gaseous HNO3 and NH3 during a field study held at Claremont, California, September 11-19, 1985, which compared current analytical methods for determining nitrogenous species concentrations in the atmosphere. Signal averaging for ~5 minutes at a pathlength of 1150 meters and spectral resolution of 0.125 cm-1 afforded detection sensitivities of approximately 4 ppb for HNO3 and 1.5 ppb for NH3.

The most reliable FT-IR measurements of HNO3 for comparison purposes were those obtained during September 14 and 17. Nitric acid concentrations were above the FT-IR detection limit most of the daytime hours during the smog episode of September 14 when O3 peaked at > 0.2 ppm; the highest HNO3 concentration of 26 ppb was recorded at ~3:45 p.m.

The higher FT-IR detection sensitivity for NH3 afforded a more precise and complete set of data than those obtained for HNO3. Background NH3 levels were generally 2-4 ppb but concentration "spikes" as high as 84 ppb were measured when the wind direction was from nearby agricultural sources. Hourly average concentrations of HNO3 and NH3 are reported, along with the calculated average concentrations for the sampling periods designated for the majority of the other measurement methods.


 

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