Showing papers by "James Walega published in 2006"

Formaldehyde Distribution over North America: Implications for Satellite Retrievals of Formaldehyde Columns and Isoprene Emission

Abstract: Formaldehyde (HCHO) columns measured from space provide constraints on emissions of volatile organic compounds (VOCs). Quantitative interpretation requires characterization of errors in HCHO column retrievals and relating these columns to VOC emissions. Retrieval error is mainly in the air mass factor (AMF) which relates fitted backscattered radiances to vertical columns and requires external information on HCHO, aerosols, and clouds. Here we use aircraft data collected over North America and the Atlantic to determine the local relationships between HCHO columns and VOC emissions, calculate AMFs for HCHO retrievals, assess the errors in deriving AMFs with a chemical transport model (GEOS-Chem), and draw conclusions regarding space-based mapping of VOC emissions. We show that isoprene drives observed HCHO column variability over North America; HCHO column data from space can thus be used effectively as a proxy for isoprene emission. From observed HCHO and isoprene profiles we find an HCHO molar yield from isoprene oxidation of 1.6 +/- 0.5, consistent with current chemical mechanisms. Clouds are the primary error source in the AMF calculation; errors in the HCHO vertical profile and aerosols have comparatively little effect. The mean bias and 1Q uncertainty in the GEOS-Chem AMF calculation increase from <1% and 15% for clear skies to 17% and 24% for half-cloudy scenes. With fitting errors, this gives an overall 1 Q error in HCHO satellite measurements of 25-31%. Retrieval errors, combined with uncertainties in the HCHO yield from isoprene oxidation, result in a 40% (1sigma) error in inferring isoprene emissions from HCHO satellite measurements.

Ultra-high-precision mid-IR spectrometer II: system description and spectroscopic performance

Abstract: The development and spectroscopic performance evaluation of an ultra-sensitive, mid-IR spectrometer is reported. The laser system is based upon difference-frequency generation (DFG) at ∼3.5 μm by mixing a DFB diode laser at 1562 nm and a DFB fiber laser at 1083 nm using a periodically poled LiNbO3 crystal. DFG radiation was coupled to a 100 m optical path length astigmatic Herriott cell. Sensitive and selective spectroscopic detection of formaldehyde was performed with second-harmonic detection using Peltier-cooled HgCdTe detectors. By applying computer lock-ins, dual-beam optical noise subtraction, focus matching, thermal stabilization, active wavelength control, and advanced signal processing a sensitivity corresponding to an absorbance ∼1.6×10-7 is achieved for 260 s of averaging.

Advances in hardware, system diagnostics software, and acquisition procedures for high performance airborne tunable diode laser measurements of formaldehyde

Abstract: A tunable diode laser absorption spectroscopy (TDLAS) instrument was deployed onboard a DC-8 aircraft as part of the International Chemical Transport Experiment – North America (INTEX-NA) during the summer of 2004 to quantify atmospheric formaldehyde (CH2O) concentrations. A number of improvements, both software and hardware, are discussed and include the laser tuning waveform, spectral wavelength centering, and optical stabilization. In addition, the impact of perturbations to the instrument in flight is reviewed and a number of advanced TDLASdata-acquisition and processing concepts are introduced to identify the presence of optical perturbations in flight to objectively eliminate such perturbed data, assess the validity of the fitting routine in the presence of perturbed data, provide various diagnostic measures to elucidate system behavior, and assess the efficacy of various opto-mechanical improvements implemented to reduce the magnitude of such perturbations. The concepts specific to our TDLASmeasurements of CH2O should have broader and more universal applicability to measurement of other trace gases and possibly other methods of detection.

Tunable diode laser and difference frequency generation absorption spectrometers for highly sensitive airborne measurements of trace atmospheric constituents

Abstract: Enhancing our understanding of atmospheric processes and transformations require a suite of ever more sensitive, selective, versatile, and fast instruments that can measure trace atmospheric constituents at and below mixing ratios of 100-parts-per-trillion on airborne platforms. Instruments that can carry out such measurements are very challenging, as airborne platforms vibrate, experience accelerations, and undergo large swings in cabin temperatures and pressures. These challenges notwithstanding, scientists and engineers at the National Center for Atmospheric Research (NCAR) in collaboration with Rice University have long been employing mid-infrared absorption spectroscopy to acquire atmospheric measurements of important trace gases like formaldehyde on a variety of airborne platforms. The present paper will discuss two very recent airborne formaldehyde instruments employing tunable diode laser and difference frequency generation mid-IR laser sources. Both instruments employ second-harmonic absorption spectroscopy utilizing astigmatic multipass Herriott cells. This paper will discuss the performance of both instruments during recent airborne campaigns, focusing on the many steps necessary for minimizing the various aircraft perturbations. Prospects for the detection of other trace gases will also be presented.

Progress on DFG Laser Sources and Its Application to Ultra-Sensitive Trace Gas Detection

Abstract: Tunable mid-IR laser sources based upon fiber optic pumped difference-frequency generation (DFG) have recently made significant progress. Several advances in the design of DFG sources and detection schemes permit ultra-sensitive detection of Amin~1×E-7.