Research Projects

Project at a Glance

Project Status: complete

Title: Southern California air quality study: tunable diode laser absorption spectrometer measurements of HNO3 at the Claremont "A" site.

Principal Investigator / Author(s): Mackay, Gervase

Contractor: Unisearch Associates Inc.

Contract Number: A732-041


Research Program Area: Atmospheric Processes

Topic Areas: Acid Deposition


Abstract:

During June-July, 1987 the first part of the Southern California Air Quality Study was carried out in the greater Los Angeles area. Measurements were made to address the following issues: ozone (O3), nitrogen dioxide (NO2) and the roles of nitrogen oxides (NOx), PM-10, fine particles, visibility, toxic air contaminants, and atmospheric acidity. Claremont college was chosen as one of the sites for in depth study of these issues.

The tunable diode laser absorption spectrometer (TDLAS) system was employed for the real time measurement s of gas-phase HNO3. TDLAS offers a number of advantages for making these measurements. Its high spectral resolution provides an unequivocal identification of the measured gaseous species free from interferences of other atmospheric constituents. This specificity makes it an ideal instrument to use as a standard against which other, less definitive methods, can be compared. In addition it has high sensitivity and rapid response time so that it can provide real time measurements with detection limits much better than those required for ambient concentrations of the majority of trace constituents under most atmospheric conditions.

The measurement were made with the TAMS-150 which had detection limits of better tha 0.1 parts per billion (ppbv) for 1 minute averaging times. Measurements were made on 32 days during the period June 15th through July 24th 1986. The results have also been reduced to 1 hour average values for comparison with measurements made by other methods. Data coverage was better than 90%.

The diurnal behaviour of HNO3 was quite regular. The maximum concentrations occurred between 12:00 and 18:00 each day. The daily maximum varied considerably during the study from a low of 5 ppbv on July 21th to a high of 24 ppbv on June 25. On average the daytime maxima were of the order of 10 ppbv. Nighttime minima occurred between 22:00 and 06:00 and were generally in the range 0.5 to 1 ppbv.


 

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