Research Projects

Project at a Glance

Project Status: complete

Title: Studies of dry acid deposition in the South Coast Air Basin: tunable diode laser absorption spectrometer measurements of HNO3, HCHO and NO2 and luminol NO2 measurements in Claremont, California, September 1985.

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

Contractor: Unisearch Associates

Contract Number: A4-148-32


Research Program Area: Atmospheric Processes

Topic Areas: Acid Deposition


Abstract:

The major components of acid deposition, nitric acid (HNO3) and sulfuric acid (H2SO4) are formed by the oxidation of the primary pollutants NOX, and SOX. In the South Coast Air Basin HNO3 is particularly important because of the higher ratios of NO to SO emissions relative to those in central and eastern U.S.A. In addition, due to the relatively dry climate in the Basin, dry deposition is thought to contribute the majority of the acid deposition. HNO3 is also important because it is one of the final oxidation products in the formation of photochemical (urban) smog. Measurements of gas phase HNO3 concentrations are therefore needed for understanding both of these problems but are largely lacking. In September 1985, the Air Resources Board organized an intercomparison of methods for measuring gas phase HNO3 in ambient air.

Tunable diode laser absorption spectrometry (TDLAS) affords an attractive method for such measurements. It is a passive method applicable to virtually all gases of atmospheric interest. Because of its very high resolution the method provides unequivocal identification of the gas of interest with no interferences from other gases. It therefore can serve as a standard against which other, less definitive methods can be compared. It provides real time measurements with a time resolution of less than one minute and detection limits in the sub ppbv range.

Two TDLAS systems were used in the intercomparision study, one provided by Unisearch Associates and the other by the Atmospheric Environment Service of Canada (AES). Comparisons were made with methods using filter packs, denuders, chemiluminescence and Fourier Transform InfraRed (FTIR) spectroscopy.


The Unisearch TDLAS system was also used to measure formaldehyde (HCHO). Formaldehyde mixing ratio measurements provide information on the hydrocarbon oxidation level of the ambient atmosphere. Nitrogen dioxide (NO2) was measured by the AES TDLAS system and by the LUMINOX , LMA-3 which is a new, small, simple and extremely sensitive instrument for NO2 measurements. The importance of these measurements is the role that NO2 plays as the direct precursor to gas phase HNO3.

The measurement period was between September 11-19, 1985. Data was obtained over 5 minute averaging periods and also reduced to one hour averages. The data coverage of these one hour averages were better than 90% for HNO3 over this period, 94% for NO2 by the TDLAS method and 98% for the LUMINOX instrument. The coverage for HCHO by the Unisearch TDLAS was 89%. Virtually complete coverage was obtained for the 5 daily integration periods used for intercomparison with the other techniques.

Agreement between the HNO3 measurements obtained by the 2 TDLAS systems were within the combined experimental errors with the largest deviations occurring when the ambient concentrations were changing rapidly.

The diurnal behavior of the HNO3 concentrations were quite regular. The maximum concentrations occurred between 15:00 and 18:00 each day. Maximum concentrations were observed on September 14 when values as high as 18 ppbv were observed for five-minute averages.

The measurements of NO2 made by the TDLAS and the LUMINOX methods agreed within the combined uncertainties over most of the range of concentrations encountered. The LUMINOX instrument did, however, give somewhat higher values at high NO2 concentrations and somewhat lower values at low concentrations. The effect at low concentrations have subsequently been traced to a small non-linear response of the Luminol instrument at low concentrations. The instrument has now been modified to correct this difficulty.

The measurements made with the LUMINOX instrument were 35-50% higher than the TDLAS values at periods around 22:00 on two evenings when the concentrations were more than 120 ppbv. Since the LUMINOX instrument does not provide the same specificity as the TDLAS method, the most likely explanation is that the LUMINOX instrument is also responding to some other constituent(s) present in the highly polluted air at night.

Both instruments indicate the same diurnal behavior with the NO2 mixing ratio exhibiting two maxima during each 24-hour period; one centered at about 10:00 and the other centered at 22:00.

The HCHO concentrations generally maximize during periods of high pollution as indicated by maxima in the NO2 mixing ratio. As with NO2 and HNO3 the maximum HCHO was observed during the period September 12 through 14. After that time the HCHO levels were much lower and continued to decrease until the end of the measurement period. The mixing ratios do not show strong diurnal dependence but there is some indication for maxima near local noon.


 

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