Showing papers by "James Walega published in 2007"
TL;DR: The first airborne deployment and performance tests of a mid-IR difference frequency spectrometer system for highly sensitive measurements of formaldehyde are discussed and the instrument's performance characteristics spanning more than 300 flight hours during three consecutive airborne field missions MIRAGE, IMPEX and TexAQS are demonstrated.
Abstract: We discuss the first airborne deployment and performance tests of a mid-IR difference frequency spectrometer system for highly sensitive measurements of formaldehyde. 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 distributed feedback (DFB) fiber laser at 1083 nm in a periodically poled LiNbO3 (PPLN) crystal. Advanced LabVIEWTM software for lock-in, dual-beam optical noise subtraction, thermal control and active wavelength stabilization, renders a sensitivity of ~20 pptv (Absorbance ~7*10-7) for 30s of averaging. The instrument’s performance characteristics spanning more than 300 flight hours during three consecutive airborne field missions MIRAGE, IMPEX and TexAQS operating on two airborne platforms, NCAR’s C-130 and NOAA’s P-3 aircraft are demonstrated.
74 citations
TL;DR: In this paper, the variability in the upper troposphere is driven by local photochemical production and transport via regional convection and long-range pathways, suggesting transport mechanisms are important factors to include in photochemical models simulating H2O2, CH3OOH, CH2O, and H Ox.
Abstract: [1] Hydrogen peroxide (H2O2), methyl hydroperoxide (CH3OOH), and formaldehyde (CH2O) were measured over North America and the North Atlantic during the INTEX-NA, TOPSE, and SONEX aircraft campaigns. An overview of H2O2, CH3OOH, and CH2O across the geographic and temporal range of these campaigns is presented. H2O2, CH3OOH, and CH2O mixing ratios and variability were larger during INTEX-NA compared to TOPSE and SONEX. Mean H2O2, CH3OOH, and CH2O were 1390, 440, and 480 pptv, respectively, more than two times higher than TOPSE measurements and an order of magnitude higher than SONEX measurements. This is attributed to higher solar radiation levels and the more polluted conditions of INTEX-NA. Mixing ratios and variability decreased with altitude for all three gases and on all three campaigns, except for CH3OOH during TOPSE. The impact of convection on H2O2, CH3OOH, and CH2O is also discussed. Using the ratio H2O2/CH3OOH, convectively influenced air parcels were found to be enhanced in CH3OOH, CH2O, CO, NO, and NO2 while H2O2 and HNO3 were depleted by wet removal. Biomass burning was also shown to increase H2O2, CH3OOH, and CH2O mixing ratios up to 1.5, 2, and 1 ppbv, respectively, even after 4–5 days of transit. Results from this study show considerable variability in H2O2, CH3OOH, and CH2O throughout the North American and North Atlantic troposphere. The variability in the upper troposphere is driven by local photochemical production and transport via regional convection and long-range pathways, suggesting transport mechanisms are important factors to include in photochemical models simulating H2O2, CH3OOH, CH2O, and HOx.
59 citations
TL;DR: In this paper, the combined use of a novel multipass cell and a sample modulation scheme based on the Stark effect in molecular spectra is used to suppress time dependent background signals, which in general limit spectrometer performance during measurements.
Abstract: The combined use of a novel multipass cell and a sample modulation scheme based on the Stark effect in molecular spectra is used to suppress time dependent background signals, which in general limit spectrometer performance during measurements. A rapid background subtraction scheme, in which the external electric field was turned off on alternate scans, as well as a double modulation experiment show drift free, white noise limited characteristics up to integration times of more than 1000 s. This exceeds the generally obtained spectrometer stability by about one order of magnitude.
7 citations
06 May 2007
TL;DR: In this article, an airborne, difference-frequency generation mid-IR spectrometer for ultra sensitive measurements of formaldehyde at 3.5 mum is described, yielding sensitivities of ~20 pptv (absorbance ~7*10-7).
Abstract: An airborne, difference-frequency generation mid-IR spectrometer for ultra sensitive measurements of formaldehyde at 3.5 mum is described. The system performance is assessed during three airborne field missions, yielding sensitivities of ~20 pptv (absorbance ~7*10-7).
3 citations