Bryan P. Wert

Intercomparison of six ambient [CH2O] measurement techniques

From May 29 to June 3, 1995 a blind intercomparison of six ambient formaldehyde measurement techniques took place at a field site near the National Center for Atmospheric Research in Boulder, Colorado. The continuous measurement methods intercompared were tunable diode laser absorption spectroscopy, (TDLAS); coil/2,4-dinitrophenylhydrazine, (CDNPH); 1,3-cyclohexanedione-diffusion scrubber (CHDDS); and the coil enzyme method (CENZ). In addition, two different cartridge methods were compared: silica gel-2,4-dinitrophenylhydrazine (DPNH) systems and a C-18-DNPH system. The intercomparison was conducted with spiked zero air (part 1) and ambient air (part 2). The CH2O standards for part 1 were calibrated by several independent methods and delivered to participants via a common glass manifold with potential trace gas interferants common to ambient air (O3, SO2, NO2, isoprene, H2O). The TDLAS system was used to confirm the absolute accuracy of the standards and served as a mission reference for part 1. The ambient phase lasted 44 hours with all participants sampling from a common glass tower. Differences between the ambient [CH2O] observed by the TDLAS and the other continuous methods were significant in some cases. For matched ambient measurement times the average ratios (±1σ) [CH2O]measured/[CH2O]TDLAS were: 0.89±0.12 (CDNPH); 1.30±0.02 (CHDDS); 0.63±0.03 (CENZ). The methods showed similar variations but different absolute values and the divergences appeared to result largely from calibration differences (no gas phase standards were used by groups other than NCAR). When the regressions of the participant [CH2O] values versus the TDLAS values, (measured in part 1), were used to normalize all of the results to the common gas phase standards of the NCAR group, the average ratios (±1σ), [CH2O]corrected/[CH2O]TDLAS for the first measurement period were much closer to unity: 1.04±0.14 (CDNPH), 1.00±0.11 (CHDDS), and 0.82±0.08 (CENZ). With the continuous methods used here, no unequivocal interferences were seen when SO2, NO2, O3, and isoprene impurities were added to prepared mixtures or when these were present in ambient air. The measurements with the C-18 DNPH (no O3 scrubber) and silica gel DNPH cartridges (with O3 scrubber) showed a reasonable correlation with the TDLAS measurements, although the results from the silica cartridges were about a factor of two below the standards in the spike experiments and about 35% below in the ambient measurements. Using the NCAR gas-phase spike data to calibrate the response of the silica gel cartridges in the ambient studies, the results are the same within statistical uncertainty. When the same gas phase calibration was used with the C-18 cartridges, the results showed a positive bias of about 35%, presumably reflecting a positive ozone interference in this case (no ozone scrubber used). The silica DNPH cartridge results from the second participant were highly scattered and showed no significant correlation with the TDLAS measurements.

Tunable diode laser absorption spectrometer for ground-based measurements of formaldehyde

We describe here a sensitive tunable diode laser absorption spectrometer (TDLAS) which was employed for ambient measurements of formaldehyde (HCHO) during the 1993 Idaho Hill/Fritz Peak Photochemistry Experiment. This system incorporated many new features and approaches including a novel astigmatic Herriott sampling cell which achieves a 100-m pathlength in a 3-L volume. We also describe procedures and tests carried out to ensure high accuracy, including the verification of HCHO standards by means of four techniques. During the field campaign, ambient HCHO measurements were acquired with an average 1σ measurement precision of 0.17 ppbv employing 1–5 min integration times. When combined with a maximum systematic uncertainty of 10%, ambient HCHO concentrations around 1.5 ppbv were measured with an average total (random plus systematic) 1σ uncertainty of 15% during the field campaign. In the intervening 2 years since the field experiment, additional features have been implemented for continuous unattended operation and improved performance. Rapid background subtraction now routinely allows HCHO measurements to be acquired with replicate precisions of 0.040 to 0.056 ppbv employing a 5-min integration period. This corresponds to routine minimum detectable absorbances of 1.2 to 1.6×10−6 in an actual mobile laboratory field environment.

Difference-frequency-based tunable absorption spectrometer for detection of atmospheric formaldehyde

High-sensitivity detection of formaldehyde (CH2O) at 3.5315 micrometers (2831.64 cm-1) is reported with a diode-laser-pumped, fiber-coupled, periodically poled LiNbO3 spectroscopic source. This source replaced the Pb-salt diode laser Dewar assembly of an existing tunable diode-laser absorption spectrometer designed for ultrasensitive detection of CH2O. Spectra are recorded with 2f-modulation spectroscopy and zero-air rapid background subtraction. Initial measurements reported here, determined from multiple measurements of a flowing 7.7 parts per billion by volume (ppbv, parts in 10(9)) CH2O in air mixture, indicate replicate precisions as low as 0.24 ppbv.

High-precision CO 2 isotopologue spectrometer with a difference-frequency-generation laser source

A precision laser spectrometer for the detection of CO(2) isotopes is reported. The spectrometer measures the fundamental absorption signatures of (13)C and (12)C isotopes in CO(2) at 4.32 microm using a tunable mid-IR laser source based on difference-frequency generation. The spectrometer attains a precision of up to 0.02 per thousand for 150 s of averaging. An overall accuracy of 0.05 per thousand was obtained when sampling various calibrated reference gases.

Airborne tunable diode laser measurements of formaldehyde

Abstract Accurate measurements of formaldehyde (CH 2 O) in the atmosphere are essential to further our understanding of various atmospheric cycles involving hydrogen and carbon-containing species. Comparisons among independent measurements of this gas and between measurements and model calculations have raised numerous questions regarding the veracity of both endeavors. The present paper describes a long-term effort by our group to develop and employ tunable diode laser absorption spectroscopy (TDLAS) for highly accurate measurements of this gas on both ground-based and aircraft platforms. A highly sensitive and selective TDLAS system, which has successfully flown on three different aircraft campaigns, will be described. Many new hardware and software features, which have been implemented, now make it possible to detect ambient CH 2 O concentrations as low as 55 parts-per-trillion employing a 20-s integration time. This paper will also discuss the many aspects associated with high accuracy and its verification, including a brief discussion of our aircraft sampling system and inlet surface effects.

Airborne measurements of tropospheric formaldehyde by tunable diode laser absorption spectroscopy

A ground based tunable diode laser absorption spectrometer (TDLAS) developed at NCAR for the measurement of formaldehyde (HCHO) has been modified for use aboard tropospheric aircraft. Measurements of HCHO are essential to comprehensive investigations of atmospheric oxidation processes, and aircraft platforms provide the advantage of vertically and spatially resolved measured. Initial deployment of the aircraft system occurred during the spring and summer of 1996 as part of the NARE and STERAO campaigns. Data coverage exceeded 95 percent out of a sum total of 175 flight hours. Sensitivities achieved during STERAO were approximately 40-60 pptv for 4.5 min of measurement and 80- 120 pptv for 55s; NARE sensitivities were slightly worse. For both campaigns, post-flight fitting of background spectra indicated periodic outgassing and contamination of the background matrix air. Analysis of data collected during the May 1995 SOS intercomparison suggests that background subtraction largely nullifies any outgassing effect. Background matrix gas HCHO concentrations were determined by fitting background spectra and were then used to correct the associated ambient data sets. Finally, fits of the difference of successive backgrounds appear to approximate measurement replicate precisions and are more informative than calculated fit precisions.

Enhancement of a tunable diode laser tropospheric trace gas measurement system

Rapid and accurate ambient measurements of the tropospheric trace gas formaldehyde (CH2O) have been made by the NCAR low altitude tunable diode laser absorption spectrometer on both aircraft and ground based platforms. Field sensitivities of 80 - 120 pptv in 1 minute (40 - 60 pptv in 5 min) were typical of the first aircraft version of the instrument, providing good resolution for studying formaldehydes role in the oxidative mechanisms of the troposphere. Recently the instrument has been modified to provide simultaneous detection of a second tropospherically interesting molecule, hydrogen peroxide (H2O2), as well as enhanced measurement precision and instrument stability. The optic assembly of the new Dual Channel Airborne Laser System (DCALS) has been designed to be more mechanically stable and better thermally conditioned. Other improvements include measures to mitigate optical noise, stabilize cell pressure, and minimize sample perturbation. Measurements of formaldehyde by DCALS at a ground site during the 1999 Southern Oxidants Study show improved sensitivities of 30 - 100 pptv in 1 minute, and much better long term instrument stability.

Airborne tunable diode laser measurements of formaldehyde during the 1997 North Atlantic Regional Experiment

Accurate measurements of formaldehyde (CH2O), a trace gas found throughout the atmosphere, are important for furthering our understanding of hydrocarbon oxidation processes in the atmosphere. During the 1997 North Atlantic Regional Experiment numerous trace gases, including CH2O, were measured onboard a WP3 aircraft operated by the National Oceanic and Atmospheric Administration to study continental transport and photochemistry over remote regions of the North Atlantic Ocean. A highly sensitive tunable diode laser absorption spectrometer was employed in acquiring ambient CH2O measurements on 10 different flights during this campaign. A second instrument, based on chemical derivatization of ambient CH2O with DNPH, was also operated on the WP3 aircraft. This paper will briefly summarize the aircraft TDLAS system employed and discuss the level of agreement obtained between both instruments. This will be followed by a brief discussion of the results, and concludes with a preliminary comparison of the measurements with a 0-dimensional box model constrained by the measurements of other species during the campaign.

Airborne tunable diode laser measurements of trace atmospheric gases

Highly sensitive and accurate measurements of numerous trace gases are required to further our understanding of atmospheric processes. Tunable diode laser systems, which offer many advantages in this regard, can be designed for reliable field measurements on both ground-based and aircraft platforms. The present paper describes the long term effort at the National Center for Atmospheric Research (NCAR) to develop, employ, and validate a highly sensitive tunable diode laser absorption spectrometer for the measurement of various trace gases, including formaldehyde and carbon monoxide. This system was successfully employed on three recent aircraft campaigns. The present paper describes the aircraft instrument along with hardware and software features incorporated for high sensitivity, with particular emphasis on major modifications to the NCAR aircraft system over the past year.

Airborne measurements of formaldehyde employing a high-performance tunable diode laser absorption system

Formaldehyde (CH2O) is a ubiquitous component of both the remote atmosphere as well as the polluted urban atmosphere. This important gas-phase intermediate is a primary emission product from hydrocarbon combustion sources as well as from oxidation of natural hydrocarbons emitted by plants and trees. Through its subsequent decomposition, formaldehyde is a source of reactive hydrogen radicals, which control the oxidation capacity of the atmosphere. Because ambient CH2O concentrations attain levels as high as several tens of parts-per-billion (ppbv) in urban areas to levels as low as tens of parts-per-trillion (pptv) in the remote background atmosphere, ambient measurements become quite challenging, particularly on airborne platforms. The present paper discusses an airborne tunable diode laser absorption spectrometer, which has been developed and refined over the past 6 years, for such demanding measurements. The results from a recent study will be presented.