MEASUREMENT OF ORGANIC NITRATES AND
SELECTED HALOCARBONS IN WHOLE AIR SAMPLES
DURING PEM TROPICS B.

Elliot Atlas and Frank Flocke, Principal Investigators
Atmospheric Chemistry Division
National Center for Atmospheric Research

In Collaboration with: Dr. Don Blake and Dr. F. S. Rowland
Department of Chemistry
University of California, Irvine

This project is to measure a series of alkyl nitrates (RONO2) and selected halocarbons in whole air samples collected on board the DC-8 and P-3B aircraft during the PEM-Tropics B mission. Measurements of these trace gases during PEM Tropics A showed that RONO2 and selected halocarbons (including hydrochlorofluorocarbons (HCFCs) and dichloromethane) provided unique characterization of air masses from different source regions. These measurements can be used to test photochemical relationships and mixing processes in the remote atmosphere, and thus contribute substantially to the stated goals of the PEM Tropics B mission.

The photochemical source of alkyl nitrates is from hydrocarbon oxidation and >C2 nitrates have been found to be well correlated with their parental kanes in the remote North Pacific atmosphere and other areas distant from continental sources. However, data from PEM Tropics A confirmed earlier indications of a significant marine source of C1-C3 alkyl nitrates. The source was particularly strong along the equatorial upwelling region and was of sufficient strength to control the atmospheric concentrations of RONO2 over most of the South Pacific. In air masses depleted of nitric acid, organic nitrates may be a significant fraction of the tropospheric odd-nitrogen budget, and modeling studies also indicate that RONO2 provide a small but significant source of NOx in the tropical marine boundary layer.

As shown during PEM Tropics A, HCFC and halocarbon measurements provide valuable diagnostic data to investigate interhemispheric transport and evaluate sources of trace gases with multiple sources, e.g. methane. The HCFCs are emitted almost exclusively in the Northern Hemisphere, with concentrations increasing at a known rate. This emission pattern provides a characteristic gradient between the North and South Hemispheres. Other halocarbons which have been useful as tracers of anthropogenic pollution e.g. methyl chloroform, are becoming homogenized between the North and South Hemispheres as production and emission controls take effect. Thus, HCFCs (as well as some shorter lived anthropogenic gases, such as dichloromethane and perchlorethylene) have become useful and unambiguous tracers of anthropogenic influences in the remote atmosphere. Knowledge of their distributions in the PEM Tropocs region will help in the development and validation of large scale chemistry transport models applied to the PEM Tropics data.

The proposal is a collaborative project with S. Rowland and D. Blake of the University of California at Irvine. Here we propose to continue and extend a successful collaboration begun during PEM Tropics A. We have worked with the UCI group to optimize one channel of the UCI whole-air sample analytical stream for the identification and quantification of C1 - C5 alkyl nitrates, and will continue to provide quality assurance for that measurement. In addition, we will work with the UCI group to transfer analytical protocols used for years at NCAR to establish GC/MS methods for halocarbons, organic nitrates, and other selected trace gases. These techniques employ cryogenic preconcentration, capillary GC separation, and selected ion monitoring by mass spectrometry. Samples will also be analyzed at NCAR with existing systems, including negative ion chemical ionization mass spectrometry. While UCI will be responsible for sample collection, logistics, and their "normal" analytical program, we plan to collaborate in both the analytical protocol for organic nitrate, HFC, and HCFC analyses and subsequent data interpretation.

Elliot Atlas NCAR/Atmospheric Chemistry

Ph: 303-497-1425
Fax: 303-497-1477