William A. McClenny

CO 2 Laser Absorption Coefficients for Determining Ambient Levels of O 3 , NH 3 , and C 2 H 4

A carbon dioxide laser source was used to determine absorption coefficients for dilute absorber-air mixtures at wavelengths corresponding to several vibration-rotation lines in each branch of the 00 degrees 1-02 degrees 0 band at 9.4 microm and the 00 degrees 1-10 degrees 0 band at 10.4 microm. For all samples the total pressure was 1 atm and the temperature was 300 K; the concentrations ranged from 10 ppm (parts per million by volume) to 357 ppm for NH(3) and C(2)H(4), and from 10 ppm to 80 ppm for O(3). The absorption coefficients are tabulated, and the use of selected laser lines in monitoring ambient concentrations is discussed.

Optical Remote Measurement of Toxic Gases

Enactment of the Clean Air Act Amendments (CAAA) of 1990 has resulted in increased ambient air monitoring needs for industry, some of which may be met efficiently using open-path optical remote sensing techniques. These techniques include Fourier transform spectroscopy, differential optical absorption spectroscopy, laser long-path absorption, differential absorption lidar, and gas cell correlation spectroscopy. With this regulatory impetus, it is an opportune time to consider applying these technologies to the remote and/or path-averaged measurement and monitoring of toxic gases covered by the CAAA. This article reviews the optical remote sensing technology and literature for that application.

Technique for monitoring ozone precursor hydrocarbons in air at photochemical assessment monitoring stations: Sorbent preconcentration, closed-cycle cooler cryofocusing, and GC-FID analysis

Abstract An automated gas chromatograph (autoGC) system that may be used to collect and analyze both polar and nonpolar volatile organic compounds in ambient air has been evaluated. This system combines the use of dual multiadsorbent traps for sampling 57 min h −1 at ambient temperature, a dry helium purge to remove residual water from the sorbents, thermal desorption of analytes onto a Stirling-cooled trap for refocusing, and GC-flame ionization detection (FID). Method detection limits (MDLs), linearity, cleanliness, precision, and accuracy of the autoGC were determined for a set of 57 ozone precursor hydrocarbons. For most of the compounds tested, MDLs were less than 0.40 ppbv, the FID response was linear over the 5–40-ppbv range, and the trap-to-trap precision was ± 10%. This autoGC was found to be a reliable system that would be suitable for use in field sites such as the Photochemical Assessment Monitoring Stations network, which is being implemented in the United States of America.

Preparing to measure the effects of the NOx SIP call--methods for ambient air monitoring of NO, NO2, NOy, and individual NOz species.

Abstract The capping of stationary source emissions of NOx in 22 states and the District of Columbia is federally mandated by the NOx SIP Call legislation with the intended purpose of reducing downwind O3 concentrations. Monitors for NO, NO2, and the reactive oxides of nitrogen into which these two compounds are converted will record data to evaluate air quality model (AQM) predictions. Guidelines for testing these models indicate the need for semicontinuous measurements as close to real time as possible but no less frequently than once per hour. The measurement uncertainty required for AQM testing must be less than ±20% (±10% for NO2) at mixing ratios of 1 ppbv and higher for NO, individual NOz component compounds, and NOy. This article is a review and discussion of different monitoring methods, some currently used in research and others used for routine monitoring. The performance of these methods is compared with the monitoring guidelines. Recommendations for advancing speciated and total NOy monitoring technology and a listing of demonstrated monitoring approaches are also presented.

Analysis of VOCs in Ambient Air Using Multisorbent Packings for VOC Accumulation and Sample Drying

Abstract Solid multisorbent packings have been characterized for trapping and release efficiency of trace (10-20 ppbv in humidified zero air) volatile organic compounds (VOCs). The use of a two-stage trapping system reduces sample water content typically by more than 95.5% while maintaining a trapping and release efficiency of 100% for 49 VOCs, including eight water-soluble VOCs. Three combinations of primary tube and focusing tube are examined in detail by using an atomic emission detector to monitor hydrogen as an indication of residual water vapor, and to monitor either chlorine, bromine, or carbon for target VOCs. Linearity of response to individual VOCs, the presence of artifacts, and a laboratory monitoring application are also discussed.

FTIR transmission spectrometry for the nondestructive determination of ammonium and sulfate in ambient aerosols collected on teflon filters

Abstract Fourier transform infrared (FTIR) transmission spectrometry has been used to determine ammonium and sulfate in ambient aerosol particles collected on Teflon filters. Integrated absorbance as well as maximum absorbance values for ammonium and sulfate are linearly related to X-ray fluorescence measurements of total sulfur. Apparent detection limits of 1.4 μg m −3 for sulfate and 0.5 μg m −3 for ammonium (for sample volumes of 21.6 m 3 ) were estimated for samples from Topeka, Kansas; Portage, Wisconsin; Steubenville, Ohio; and Charlottesville, Virginia. The apparent detection limits were essentially independent of sample loading over the range of 0–20 μg m −3 of total sulfur. The FTIR transmission measurements of these samples are very precise e.g. repetitive measurements vary ± 0.2% and blank variability is 0.13 μg m −3 and 0.20 μg m −3 in terms of sulfate and ammonium, respectively. Contributions to analytical uncertainty include 5–8% uncertainties in X-ray fluorescence measurements for total sulfur and the influence of unspecified site-dependent sample characteristics. The method offers the significant advantages of nondestructive analysis, no sample preparation, molecular speciation and rapid analysis.

Volatile organic compound concentration patterns at the New Hendersonville monitoring site in the 1995 Southern Oxidants Study in the Nashville, Tennessee, area

Ambient gaseous organic compounds were monitored on an hourly basis with an automated gas chromatograph from June 15 to 27, 1995, at the New Hendersonville site near Nashville. Tennessee. as part of the Southern Oxidants Study (SOS). These compounds and their estimated average contributions to the total were as follows: isoprene (2.1 ppbC), methyl vinyl ketone (1.1 ppbC), methacrolein (0.6 ppbC), α-pinene plus β-pinene (1.3 ppbC), and those nonmethane hydrocarbons, minus isoprene, monitored routinely in the Photochemical Assessment Monitoring Stations (44.6 ppbC). The sum of n-aldehydes from butanal through nonanal averaged 12.6 ppbC, where a conservative estimate of artifact n-aldehydes has been subtracted. Strong diurnal variations in the concentrations of n-aldehydes and the apparent influence of wind shifts on concentration support the existence of n-aldehydes at significant levels in the ambient air. The empirical fit to data as developed by Jobson et al. [1994] was the upper bound to the loci of maximum isoprene concentrations as a function of temperature and the diurnal variation of the ratio of methyl vinyl ketone to methacrolein was similar to that in studies by Montzka et al. [1993] in the rural southeastern Unitcd States. The α-pinene and β-pinene concentrations were proportional and α-pinene concentrations always less than 2 ppbC. In supplemental experiments at a rural site in North Carolina. the α-pinene concentrations showed a distinctive diurnal pattern with high nighttime values and low midday values. Patterns of concentration for the anthropogenic hydrocarbons benzene, toluene, and o-xylene versus wind direction showed the impact of a major toluene-emitting source as well as the influence of emissions from the direction of Nashville.

Long-path FTIR Measurements of Volatile Organic Compounds in an Industrial Setting

A long-path Fourier transform spectrometer was used to make measurements of volatile organic compounds in an industrial complex in New Castle, Delaware as part of the Superfund Innovative Technology Evaluation (SITE) program. These data are compared to data taken with a whole-air canister technique. To provide an effective comparison the canisters were transported along the path for a 32-min period. For each FTIR spectrum, 256 interferograms were co-added over a 4-mln period. Eight individual spectra were then co-added to obtain the comparison data.

Long-path laser monitor of carbon monoxide: system improvements

A system for measurement of long-path absorption of carbon monoxide has been redesigned for improved performance. The diode laser source is frequency stabilized by automatically correcting the laser current and is limited to a single-wavelength output by a tunable étalon. As a result, sensitivity variations of the system to CO are limited to +/-0.7%, and system response is linear with respect to CO concentrations.

Enhanced Performance of Nafion Dryers in Removing Water from Air Samples Prior to Gas Chromatographic Analysis

Nafion tube dryers were tested for applications of sample preparation prior to gas chromatographic analysis. Steady-state and timedependent drying efficiency were measured for four similar dryers using a 50-mL/min sample flow at 95 percent relative humidity at room temperature. A procedure was developed in which water removal efficiency was temporarily enhanced by a factor of 20 as compared with the steady state. This was accomplished by heating the dryer while purging it with a dry sample stream immediately prior to processing the gas sample of interest. The procedure was tested on an automated gas chromatographic system equipped with a cryogenic sample preconcentrator. Data for 15 volatile organic compounds of interest showed no effect of this procedure on sample integrity; some improvement in run-to-run precision was observed.