Research Projects

Project at a Glance

Project Status: complete

Title: Measurement of atmospheric nitric acid at Azusa by tunable diode laser absorption spectrometry.

Principal Investigator / Author(s): Mackay, Gervase I

Contractor: Unisearch Associates, Inc.

Contract Number: 93-300


Research Program Area: Ecosystem & Multimedia Effects

Topic Areas: Acid Deposition, Monitoring


Abstract:

HNO3 plays an important role in the photochemistry of urban and rural air and its accurate measurement provides a sensitive test for current acid deposition models. Measurements of nitric acid (HNO3) by denuder tube technology are made on a routine basis in the Southern California acid deposition network. Recently, the HNO3 results from a number of these stations have shown variability inconsistent with the expected behavior. It was thought to be prudent, therefore, to compare the measurements with a reference standard air monitoring technique at one of these sites which might help to identify the source of the variability.

Tunable diode laser absorption spectroscopy (TDLAS) offers a number of advantages for making this measurement. 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 the TDLAS 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 Unisearch model TAMS-150 TDLAS was employed for the measurement of HNO3 and had detection limits of better than 0.2 ppbv. Measurements of HNO3by the TDLAS were made during the period 12:00 September 30, 1993 through 24:00 October 30, 1993, inclusive. Data was obtained over 1 minute averaging periods. The results have also been reduced to one-hour average values and Daytime average value (10:00 - 18:00) for comparison with measurements made by the ARB denuder samplers. Data coverage was better than 98 percent.

The diurnal behavior of HNO3 was quite regular. The maximum concentrations generally occurred between 14:00 and 16:00 each day. The HNO3 daily maximum varied between 3 ppbv (during wet weather) and 30 ppbv (hot dry weather) with the median value of about 15 ppbv for the entire period. The averages calculated for the period 10:00 - 18:00 (the denuder sampling period) were on average twice the nighttime (18:00 -- 10:00) average and represented ~75 percent of the total daily HNO3 burden. This indicates that, on most occasions, the denuders should capture a major fraction of the HNO3 produced during a 24-hour period.

There was no evidence for any large particle nitrate ( > 2 m ) dissociation from the particulate matter collected on the inlet filter contributing to the HNO3 signal measured by the TDLAS. While it could not be completely ruled out, dissociation of small nitrate particles ( < 2 m ) (which passed through the filter) in the White cell was not obvious and the results indicate that it could contribute at most 10 percent of the measured signal at any time.


 

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