Bruce Henry

Airborne intercomparison of vacuum ultraviolet fluorescence and tunable diode laser absorption measurements of tropospheric carbon monoxide

During the fall 1997 North Atlantic Regional Experiment (NARE 97), two separate intercomparisons of aircraft-based carbon monoxide measurement instrumentation were conducted. On September 2, CO measurements were simultaneously made aboard the National Oceanic and Atmospheric Administration (NOAA) WP-3 by vacuum ultraviolet (VUV) fluorescence and by tunable diode laser absorption spectroscopy (TDLAS). On September 18, an intercomparison flight was conducted between two separate instruments, both employing the VUV fluorescence method, on the NOAA WP-3 and the U.K. Meteorological Office C-130 Hercules. The results indicate that both of the VUV fluorescence instruments and the TDLAS system are capable of measuring ambient CO accurately and precisely with no apparent interferences in 5 s. The accuracy of the measurements, based upon three independent calibration systems, is indicated by the agreement to within 11% with systematic offsets of less than 1 ppbv. In addition, one of the groups participated in the Measurement of Air Pollution From Satellite (MAPS) intercomparison [Novelli et al., 1998] with a different measurement technique but very similar calibration system, and agreed with the accepted analysis to within 5%. The precision of the measurements is indicated by the variability of the ratio of simultaneous measurements from the separate instruments. This variability is consistent with the estimated precisions of 1.5 ppbv and 2.2 ppbv for the 5 s average results of the C-130 and the WP-3 instruments, respectively, and indicates a precision of approximately 3.6% for the TDLAS instrument. The excellent agreement of the instruments in both intercomparisons demonstrates that significant interferences in the measurements are absent in air masses that ranged from 7 km in the midtroposphere to boundary layer conditions including subtropical marine air and continental outflow with embedded urban plumes. The intercomparison of the two VUV instruments that differed widely in their design indicates that the VUV fluorescence technique for CO measurements is not particularly sensitive to the details of its implementation. These intercomparisons help to establish the reliability of ambient CO measurements by the VUV fluorescence technique.

The reaction probability of N2O5 with sulfuric acid aerosols at stratospheric temperatures and compositions

The authors have measured the rate of reaction of N{sub 2}O{sub 5} with H{sub 2}O on monodisperse, submicrometer H{sub 2}SO{sub 4} particles in a low-temperature flow reactor. Measurements were carried out at temperatures between 225 K and 293 K on aerosol particles with sizes and compositions comparable to those found in the stratosphere. At 273 K, the reaction probability was found to be 0.103{+-}0.006, independent of H{sub 2}SO{sub 4} composition from 64 to 81 wt %. At 230 K, the reaction probability increased from 0.077 for compositions near 60% H{sub 2}SO{sub 4} to 0.146 for compositions near 70% H{sub 2}SO{sub 4}. Intermediate conditions gave intermediate results except for low reaction probability of about 0.045 at 260 K on aerosols with about 78% H{sub 2}SO{sub 4}. The reaction probability did not depend on particle size. These results imply that the reaction occurs essentially at the surface of the particle. A simple model for this type of reaction that reproduces the general trends observed is presented. The presence of formaldehyde did not affect the reaction rate. 53 refs., 12 figs., 4 tabs.

HO2/OH and RO2/HO2 ratios during the Tropospheric OH Photochemistry Experiment: Measurement and theory

Ambient concentrations of the hydroxyl (OH), hydroperoxyl (HO2), and total peroxy (ΣRO2) radicals were measured as part of the Tropospheric OH Photochemistry Experiment at Idaho Hill, Colorado, during August and September of 1993. OH radicals were measured using ion-assisted mass spectroscopy and low-pressure laser-induced fluorescence (LIF) detection techniques. HO2 was measured using chemical conversion and LIF detection of OH. ΣRO2 radicals were measured using a chemical amplifier technique. The simultaneous measurements of these key species provide an opportunity to test our present understanding of the fast photochemistry of the troposphere. Measured HO2/OH ratios were typically between 15 and 80, and agreed well with predictions under conditions where NO mixing ratios were greater than 100 pptv. However, under clean conditions the measured ratio was a factor of 3–4 lower than predicted. The RO2/HO2 ratio was typically a factor of 4–15 larger than predicted by present theories of tropospheric chemistry. A steady state model was used in an attempt to analyze the discrepancies between the measured HO2/OH and RO2/HO2 ratios with present theories of hydrocarbon oxidation in the troposphere.

Isoprene and its oxidation products, methacrolein and methylvinyl ketone, at an urban forested site during the 1999 Southern Oxidants Study

Isoprene (ISOP) and its oxidation products, methacrolein (MACR) and methyl vinyl ketone (MVK), were measured at an urban forested site in Nashville, Tennessee, as part of the 1999 Southern Oxidants Study (SOS). Hourly observations were performed at Cornelia Fort Airpark for a 4 week period between June 13 and July 14. At the midday photochemical peak (1200 local standard time, LST), average mixing ratios of isoprene, MACR, and MVK were 410 parts per trillion by volume (pptv), 240 pptv, and 430 pptv, respectively. Median isoprene, MACR, and MVK mixing ratios were 400 pptv, 200 pptv, and 360 pptv, respectively, at 1200 LST. An emissions inventory calculation for Davidson County, encompassing Nashville, suggests that MACR and MVK were produced predominately from isoprene oxidation rather than direct combustion emissions. The observations are compared with results from two chemical models: a simple sequential reaction scheme and a one-dimensional (1-D) numerical box model. The daytime ratios of MVK/ISOP and MACR/ISOP varied in a systematic manner and can be reproduced by the analytical solution of the sequential reaction scheme. Air masses with more photochemically aged isoprene were observed during SOS 1999 at Cornelia Fort (0.3-1.6 hours) compared to the SOS 1990 canopy study at Kinterbish (0.1-0.6 hours). This is consistent with the proximity of the tower inlets to the forest canopies during both campaigns. Isoprene had a chemical lifetime of 20 min at the average observed midday HO mixing ratio of 8 x 10 6 molecules/cm 3 . As a result, significant conversion of isoprene to its oxidation products was observed on the timescale of transport from the dense forest canopies surrounding Nashville. The systematic diurnal behavior in the MVK/MACR ratio can also be simulated with a 1-D photochemical box model. General agreement between the observations of MACR and MVK during SOS 1999 with the two chemical models suggests we have a comprehensive understanding of the first few stages of isoprene oxidation in this urban forested environment.

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.

Versatile integrated tunable diode laser system for high precision : application for ambient measurements of OCS

A versatile and integrated tunable diode laser system for high precision measurements of the important sulfur gas carbonyl sulfide is described. We explicitly address some of the-major factors affecting tunable diode laser measurement precision as well as accuracy and have implemented a number of new features for increased system control and versatility. The system described herein provides the capability for measuring ambient concentrations of this gas with a precision in the range from +/-0.3 to +/-1% over time periods of many hours. Such a precision provides us with an important new capability for measuring true spatial and temporal variations of carbonyl sulfide in the atmosphere.

Comparison of tunable diode laser and long-path ultraviolet/visible spectroscopic measurements of ambient formaldehyde concentrations during the 1993 OH Photochemistry Experiment

Two different spectroscopic techniques for measuring atmospheric concentrations of formaldehyde were compared during a 6-week field study in the mountains 17 km west of Boulder, Colorado, in August and September 1993. A long-path ultraviolet/visible (UV/Vis) absorption spectrometer and an IR tunable diode laser absorption spectrometer (TDLAS) were the two instruments employed. The former measured ambient formaldehyde levels over a 10.3 km open path (20.6 km total path) extending between Fritz Peak Observatory and Idaho Hill, while the latter measured in situ levels at the Idaho Hill site. In addition to utilizing different spectral regions, both instruments employed different sampling and calibration approaches. Because of the closer proximity to anthropogenic sources the long-path UV/Vis instrument generally detected higher formaldehyde concentrations than the TDLAS system at all times during the day. Averaged over the entire study for all wind regimes, the former resulted in formaldehyde concentrations 15% higher than the latter. The mean and median formaldehyde concentration measured by both instruments was around 1.5 ppbv and individual 5-min averages varied from a maximum of about 5 ppbv down to levels 0.6 ppbv. However, during periods of strong westerly low where anthropogenic and meteorological influences are minimized, both techniques were in agreement to within 5%. This regime, which constitutes 19% of the total mutual data set, resulted in a continental background formaldehyde concentration of 0.92 ± 0.16 ppbv. The present instrument comparison thus bridges the gap in formaldehyde instrument comparison studies between those in the background free troposphere and those in polluted urban regimes. In addition to providing further confidence in both measurement techniques, the present comparison study also provided a valuable data set necessary for advancing our understanding of tropospheric oxidation mechanisms. A set of guidelines for future comparisons of long-path and in situ measurements of formaldehyde at this site are also discussed.

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.

Intercomparison of tunable diode laser and gas filter correlation measurements of ambient carbon monoxide

Abstract An intercomparison that involved a standards intercomparison, interferant spiking tests and simultaneous ambient measurements was carried out between two CO measurement systems: a tunable diode laser absorption spectrometer (TDLAS) and a gas filter correlation, non-dispersive infrared absorption instrument (GFC). Both the TDLAS and the GFC techniques responded to CO. No major interferences were found for the TDLAS system; tested species included H 2 O, O 3 and OCS. The GFC instrument exhibited no interference from H 2 O or O 3 , but only a relatively high upper limit could be placed on the O 3 interference. For CO measurements in ambient air at levels from 100 to 1500 ppbv, the results from the two instruments agreed within their combined uncertainties. On average the GFC technique was 6% higher than the TDLAS system, and there was no systematic, constant offset. The precision of the GFC instrument was about 10%, and the precision of the TDLAS system was better than 4%.

Tunable diode laser ratio measurements of atmospheric constituents by employing dual fitting analysis and jump scanning

We present two new approaches in tunable diode laser absorption spectroscopy (TDLAS) for measuring the ratio of two components with high precision. These techniques, dual line fitting analysis and jump scanning, greatly extend the versatility and applicability of TDLAS. Three important applications for these approaches are discussed. In addition, we demonstrate the capability to quantify features that are different in amplitude by a factor of 22.4 with a precision of 0.3%. Such a precision is also achieved for features that are different in amplitude by a factor of 12.9 and separated by 0.2363 cm(-1). Both jump scanning and dual line fitting analysis are being used on a routine basis in a laboratory kinetics study to measure simultaneously the concentrations of H(2)O and NO, whose features are separated by 0.4866 cm(-1).