Measurement of Organic Nitrates and Selected Trace Gases from Whole Air Samples during TRACE

Measurement of Organic Nitrates and Selected Trace Gases from Whole Air Samples during TRACE-P

Elliot Atlas
Atmospheric Chemistry Division
National Center for Atmospheric Research
Boulder, CO

The goal of the research is to understand the chemical composition of air masses in the TRACE-P study region through targeted analysis of selected organic trace gases and with exploratory studies of chemical composition of whole air samples using gas chromatography/mass spectrometry. This project is a collaborative research project with the Blake/Rowland research group in the Dept. of Chemistry at the University of California, Irvine. Blake et al. describe the whole air sampling procedures from the TRACE P aircraft. In the UCI laboratory, we have collaborated on several of the analytical channels, and provide standardization for organic nitrates and selected other compounds. In addition to the collaboration at UCI, air samples are reanalyzed in the laboratory at NCAR for verification and additional analyses. Since the UCI canisters need to be recycled after analysis, aliquots of selected samples are transferred to NCAR canisters. When recycling is no longer required, UCI sample canisters are shipped to NCAR for the additional analyses. Procedures for sample handling and analysis of canisters by NCAR are given below.

NCAR canisters are cleaned under mild heating (about 80C) by repeated evacuation and flushing with moist air free of organic trace gases (zero air). The 2.6-liter, electropolished stainless steel canisters are stored with 20 psia of wet zero air. Prior to transfer in the UCI laboratory, NCAR canisters are evacuated to <1 torr. The transfer system incorporates an all metal vacuum line with cryogenic filters. To accomplish the transfer, the NCAR canister is connected to the vacuum system along with the UCI canisters. After evacuation of the connecting lines, the NCAR canister is immersed in liquid nitrogen. The valves on the NCAR and UCI canisters are opened and gas is transferred while the flow rate is > 1 lpm.

In the NCAR laboratory, canisters from TRACE-P were analyzed on two analytical systems, both using GC/MS techniques. The system for target compound analysis was a Hewlett Packard 5971 GC/MSD, which was operated in the selected ion mode. Approximately 300 std cc of air was introduced into a 30 m x 0.25 mm id. DB-1 capillary column using cryogenic preconcentration on a glass bead filled sample loop. Target ion signals were recorded over predetermined time windows during the temperature programmed analysis. A calibrated whole air sample was used as a secondary standard, and this was analyzed before and after 3 to 4 sample canisters.

A second GC-MS system was used for full scan MS analysis. A VG TRIO-1000 GC/MS was operated with a 60 m DB-624 column under constant flow conditions. A cryogenic enrichment system for sample preconcentration employed a Teflon water trap (-20C) that was not used in the HP system. Full scan spectra were obtained for semiquantitative analysis of selected compounds, and for comprehensive, but qualitative, identification of compounds from selective regions covered in TRACE-P.

Trace gas standards are based on both laboratory and commercially prepared mixtures. The high concentration mixtures are characterized using gas chromatography with flame ionization and/or atomic emission detection. Comparisons are made to certified reference materials when appropriate. Flow dilution of the high concentration mixtures are used to calibrate working standards near ambient mixing ratios.