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Project Status: complete

Title: Determination of sulfuric acid, total particle-phase acidity and nitric acid in ambient air

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

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

Contract Number: A6-209-30

Topic Areas: Acid Deposition, Ambient Air Quality Stds, Monitoring


A study was conducted to evaluate measurement methods for sulfuric acid, strong acids in particulate matter, and nitric acid in ambient air. Selective extraction of H2SO4 with benzaldehyde and titrimetry for strong acids were evaluated and compared using. Laboratory generated mixtures of £ 0.3 mm H2SO4 aerosols on clean and atmospheric particulate-loaded filters. Filter media were selected based on filtration efficiency and acid recovery with £ 0.3 mm H2SO4 aerosols. Nitric acid collection by filter methods (NaCl - impregnated cellulose and nylon filters) were compared at varying HrfO3 concentrations, relative humidity levels and flow rates. Quantitation of nitric acid, as nitrate, employed an automated copper-cadmium reduction plus diazotization procedure. An ammonia denuder was employed in field sampling together with removal of non-respirable particles to increase the stability of H2S04 following collection.

The presence of atmospheric particulate matter was shown to sharply reduce recoveries of laboratory generated H2SO4 but recoveries of total strong acid usually remained > 60%. Anhydrous benzaldehyde was found to extract NH4HSO4 to a substantial degree. Laboratory generated nitric acid was collected with high efficiency by both nylon and NaCl-impregnated filters while nitrogen dioxide was not. retained by these filters at 90% R.H. The ammonia denuder was shown to remove ammonia with > 99% efficiency.

Samples collected in Pittsburg, CA showed good correlation between sulfuric acid and particulate strong aicid measurements, As much as 0.6 IJg/m3 H2SO4 and 1.6 mg/m3 acidity. expressed as H2SO4. was found. However. based on recovery studies we believe these represent lower limit values. Excellent agreement Was found between nitric acid measurement by nylon and NaCl on cellulose filter collection with concentrations up to 4 mg/m3. Simultaneous gas phase ammonia measurements were made to assist in interpreting the particulate sample results. Glass fiber filters were shown to collect quantitatively. both particulate nitrate and gas phase nitric acid. This is consistent with earlier studies of artifact particulate nitrate formation.


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

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