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Project Status: complete

Title: Determination of acidity in ambient air. Phase II

Principal Investigator / Author(s): Appel, B R

Contractor: Air and Industrial Hygiene Laboratory, California Department of Health

Contract Number: A4-074-32

Research Program Area: Atmospheric Processes

Topic Areas: Acid Deposition, Ambient Air Quality Stds


A laboratory and field study was conducted as part of a continuing assessment of methodologies for measurement of gaseous and particulate acids in the atmosphere. The acidic species included the gases nitric acid (HN03), nitrous acid (HONO), hydrochloric acid (HCI) and the particulate phase species, sulfuric acid. Nitric acid measurement by the automated tungstic acid technique (TAT) was evaluated for interferences due to NO, NO2 and HONO. Loss of nitric acid in passage through various types of Teflon tubing as well as within a variety of cyclones was also studied. The annular denuder method (ADM) for HN03 was compared to the denuder difference method (DDM). An effort was made to assess NO2 interference in HONO measurement with the ADM. Measurement of HCI by the DDM strategy used for HN03 was briefly investigated. A semi continuous method for particulate S, permitting discrimination between H2SO, and ammonium sulfates, was assembled and evaluated. The degree of interference in such H2SO, measurements due to organo-sulfur species (hydroxymethane sulfonic acid (HMSA) and bishydroxymethyl sulfone) was assessed and a technique to improve the selectivity of the H2SO, measurement was proposed and evaluated. Atmospheric sampling was done at a site chosen to provide elevated particulate S and low NH3levels to compare the semi-continuous particulate S monitor to filter collection techniques. The TAT for HN03 was not subject to significant interference from HONO or N02, alone, or in mixtures with NO in humidified air. Thus the cause of the large positive errors experienced in nighttime HN03 measurements with the TAT remains unknown. Loss of laboratory-generated HN03 was severe within PTFE Teflon tubing and within cyclones providing relatively long retention times.

With laboratory-generated HN03, the ADM gave about 10% lower recovery than the DDM. However, when HN03 at very low concentrations in ambient air was sampled in parallel through equivalent inlets, the ADM results were notably higher than those by the DDM. Nitrous acid measurements with the ADM are subject to positive errors which may involve HONO formed within the inlet as well as a very low percentage retention of N02, Hydrochloric acid can be measured with the DDM with at least an 80% accuracy.

In atmospheric sampling done in Torrance, CA, the semi-continuous monitor for sulfate salts and H2SO4 provided agreement within 20% with S04 measured by filter collection. However, no H2SO4 was detected. HMSA responded indistinguishably from H2SO4, but NH3 addition allowed discrimination Of H2S04, and the sulfone. Using aerosol mixtures, NH4NO3, and (NH4)2SO4 provided significant negative errors in H2SO4 measurement.


For questions regarding this research project, including available data and progress status, contact: Research Division staff at (916) 445-0753

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