Pamela M. Chu

Consistency of ozone and nitrogen oxides standards at tropospherically relevant mixing ratios.

Abstract The absolute accuracy and long‐term precision of atmospheric measurements hinge on the quality of the instrumentation and calibration standards. To assess the consistency of the ozone (O3) and nitrogen oxides (NOx) standards maintained at the National Institute of Standards and Technology (NIST), these standards were compared through the gas‐phase titration of O3 with nitric oxide (NO). NO and O3 were monitored using chemiluminescence and UV absorption, respectively. Nitrogen dioxide (NO2) was monitored directly by laser‐induced fluorescence and indirectly by catalytic conversion to NO, followed by chemiluminescence. The observed equivalent loss of both NO and O3 and the formation of NO2 in these experiments was within 1% on average over the range of 40–200 nmol mol−1 of NO in excess O3, indicating that these instruments, when calibrated with the NIST O3 and NO standards and the NO2 permeation calibration system, are consistent to within 1% at tropospherically relevant mixing ratios of O3. Experiments conducted at higher initial NO mixing ratios or in excess NO are not in as good agreement. The largest discrepancies are associated with the chemiluminescence measurements. These results indicate the presence of systematic biases under these specific conditions. Prospects for improving these experiments are discussed.

Trends in microwave spectroscopy for the detection of chemical agents

Recent developments in microwave spectroscopy have encouraged researchers to develop this technique for analytical applications such as environmental monitoring, industrial process control, and homeland defense. This paper presents a general overview of microwave spectroscopy with a focus on aspects relevant for detecting chemical warfare agents (CWAs) and their surrogates. In particular, the high spectral resolution of microwave methods provides exceptional selectivity which is critical for detecting and identifying CWAs given the complex environments and numerous interferents that may obscure measurements by instruments with poor specificity. Ongoing efforts to develop a microwave spectral database of CWAs and improve the quantitative capabilities of Fourier transform microwave spectrometers are discussed. Additionally, future improvements to achieve a field-deployable sensor are presented.

On-Demand Generation of a Formaldehyde-in-Air Standard

The feasibility of using catalytic conversion of methanol to formaldehyde to produce standard amount of substance fractions of formaldehyde was examined. The conversion efficiencies of several catalysts were measured as a function of temperature, balance gas, catalyst bed length, and methanol amount of substance fraction in an effort to identify conditions which yield high and consistent conversion of methanol to formaldehyde. The highest observed conversion rate was (97 ± 4) % using a molybdenum catalyst, where the error is the 2σ uncertainty. The conversion efficiency was found to be consistent over repeated cycles and over a long lifetime test, suggesting that a molybdenum catalyst is a viable candidate for a standard formaldehyde generator, particularly for low formaldehyde amount of substance fractions (< 15 μmol/mol).

Evaluation of Fourier-transform microwave spectroscopy as a tool for quantitative analysis: signal stability considerations

There is a continuing need for improved analytical techniques to measure the concentration of trace gases for monitoring hazardous air pollutants, industrial emissions, chemical-warfare agent release, etc. Methods of analysis that can conclusively identify several analytes in a mixture are particularly desired. Towards this end, the use of Fourier-transform microwave (FTMW) spectroscopy as a quantitative analytical technique has been proposed. The high spectral resolution of FTMW provides a quick and unambiguous method for identifying multiple analytes in the gas phase. A small-scale FTMW spectrometer has recently been constructed for use in quantitative analysis. Prior to the present investigation, however, the use of this spectrometer in quantitative work has not been rigorously evaluated. This work summarizes efforts to identify and categorize sources of signal instability in the FTMW spectrometer. Methods employed to minimize these effects will also be discussed.