Person:

Daube, Bruce

The Total Carbon Column Observing Network (TCCON) produces precise measurements of the column average dry-air mole fractions of (CO_2), (CO), (CH_4), (N_2O) and (H_2O) at a variety of sites worldwide. These observations rely on spectroscopic parameters that are not known with sufficient accuracy to compute total columns that can be used in combination with in situ measurements. The TCCON must therefore be calibrated to World Meteorological Organization (WMO) in situ trace gas measurement scales. We present a calibration of TCCON data using WMO-scale instrumentation aboard aircraft that measured profiles over four TCCON stations during 2008 and 2009. These calibrations are compared with similar observations made in 2004 and 2006. The results indicate that a single, global calibration factor for each gas accurately captures the TCCON total column data within error.

An automated Fourier Transform Spectroscopic (FTS) solar observatory was established in Darwin, Australia in August 2005. The laboratory is part of the Total Carbon Column Observing Network, and measures atmospheric column abundances of (CO_2) and (O_2) and other gases. Measured (CO_2) columns were calibrated against integrated aircraft profiles obtained during the TWP-ICE campaign in January–February 2006, and show good agreement with calibrations for a similar instrument in Park Falls, Wisconsin. A clear-sky low airmass relative precision of 0.1% is demonstrated in the (CO_2) and (O_2) retrieved column-averaged volume mixing ratios. The 1% negative bias in the FTS (X_{CO_2}) relative to the World Meteorological Organization (WMO) calibrated in situ scale is within the uncertainties of the NIR spectroscopy and analysis.

Rates for large-scale vertical transport of air in the Tropical Tropopause Layer (TTL) were determined using high-resolution, in situ observations of CO2 concentrations in the tropical upper troposphere and lower stratosphere during the NASA Tropical Composition, Cloud and Climate Coupling (TC4) campaign in August 2007. Upward movement of trace gases in the deep tropics was notably slower in TC4 than during the Costa Rica AURA Validation Experiment (CR-AVE), in January 2006. Transport rates in the TTL were combined with in situ measurements of chlorinated and brominated organic compounds from whole air samples to determine chemical loss rates for reactive chemical species, providing empirical vertical profiles for 24-h mean concentrations of hydroxyl radicals (OH) and chlorine atoms in the TTL. The analysis shows that important short-lived species such as CHCl3, CH2Cl2, and CH2Br2 have longer chemical lifetimes than the time for transit of the TTL, implying that these species, which are not included in most models, could readily reach the stratosphere and make significant contributions of chlorine and/or bromine to stratospheric loading.

Measurements of reactive nitrogen oxides (NOₓ and NO{sub y}) and ozone (O₃) were made in the planetary boundary layer (PBL) above a taiga woodland in northern Quebec, Canada, during June-August, 1990, as part of NASA Arctic Boundary Layer Expedition (ABLE) 3B. Levels of nitrogen oxides and O₃ were strongly modulated by the synoptic scale meteorology that brought air from various regions to the site. Industrial pollution from the Great Lakes region of the US and Canada appears to be a major source for periodic elevation of NOₓ, NO{sub y} and O₃. We find that NO/NO₂ ratios at this site at midday were approximately 50% those expected from a simple photochemical steady state between NOₓ and O₃, in contrast to our earlier results from the ABLE 3A tundra site. The difference between the taiga and tundra sites is likely due to much larger emissions of biogenic hydrocarbons (particularly isoprene) from the taiga vegetation. Hydrocarbon photooxidation leads to relatively rapid production of peroxy radicals, which convert NO to NO₂, at the taiga site. Ratios of NOₓ to NO{sub y} were typically 2-3 times higher in the PBL during ABLE 3B than during ABLE 3A. This is probably the result of high PAN levels and suppressed formation of HNO₃ from NO₂ due to high levels of biogenic hydrocarbons at the ABLE 3B site.

The Shenandoah Cloud and Photochemistry Experiment (SCAPE) was conducted during September 1990 in the rural continental atmosphere at a mountain top site (1014 m) in Shenandoah National Park, Virginia. We report here the extensive set of trace gas measurements performed during clear sky periods of SCAPE, with particular focus on the carboxylic acids, formic, acetic, and pyruvic. Median mixing ratios were 5.4 and 2.1 parts per billion by volume (ppbv) for formic and acetic acid, respectively, and they did not exhibit the diurnal variation characteristic of low-elevation sites. Mixing ratios of formic acid often approached or exceeded 10 ppbv, which are the largest values yet reported for the nonurban troposphere. Over the rural eastern United States, formic and acetic acid appear to have significant nonphotochemical sources. Secondary production from suspected pathways appears to be relatively unimportant. The observed lack of correlation between formic and acetic acid with peroxide species argues against a significant source from permutation reactions of peroxy radicals. In addition, model calculations using the SCAPE data indicate minimal production of carboxylics from olefin/O3 oxidation reactions. The tight correlation (r2 = 0.88) between mixing ratios of formic and acetic acid is strongly suggestive of a commonality in their sources. The seasonal cycle of carboxylic acids in the atmosphere and precipitation over the eastern United States is evidence that combustion emissions are not a principal source of these species. It appears that direct biogenic emissions from vegetation and soils cannot be ruled out as important sources. In particular, the correlation between the seasonal variation of formic and acetic acid and the ambient temperature is consistent with a soil microbial source. Similar conclusions were reached for pyruvic acid, with its mixing ratio ranging 4–266 parts per trillion by volume (pptv) (median = 63) and most likely supported by biogenic emissions and possibly photochemical sources.

As part of the Shenandoah Cloud and Photochemistry Experiment (SCAPE), we measured formaldehyde (HCHO), glyoxal (CHOCHO), and methylglyoxal (CH3C(O)CHO) concentrations in air and cloudwater at Pinnacles (elevation 1037 m) in Shenandoah National Park during September 1990. Mean gas-phase concentrations of HCHO and CHOCHO were 980 and 44 pptv, respectively. The concentration of CH3C(O)CHO rarely exceeded the detection limit of 50 pptv. Mean cloudwater concentrations of HCHO and CHOCHO were 9 and 2 μM, respectively; the mean CH3C(O)CHO concentration was below its detection limit of 0.3 μM. The maximum carbonyl concentrations were observed during stagnation events with high O3, peroxides, and CO. Outside of these events the carbonyls did not correlate significantly with O3, CO, or NOy. Carbonyl concentrations and concentration ratios were consistent with a major source for the carbonyls from isoprene oxidation. Oxidation of CH4 supplies a significant background of HCHO. The carbonyl concentrations were indistinguishable in two size fractions of cloudwater having a cut at d=18 μm. Gas- and aqueous-phase concentrations of HCHO from samples collected during a nighttime cloud event agree with thermodynamic equilibria within a factor of 2. Samples collected during a daytime cloud event show HCHO supersaturation by up to a factor of 4. Positive artifacts in the cloudwater samples due to hydrolysis of hydroxymethylhydroperoxide (HOCH2OOH) could perhaps account for this discrepancy.

High-accuracy continuous measurements of greenhouse gases (CO2 and CH4) during the BARCA (Balanço Atmosférico Regional de Carbono na Amazônia) phase B campaign in Brazil in May 2009 were accomplished using a newly available analyzer based on the cavity ring-down spectroscopy (CRDS) technique. This analyzer was flown without a drying system or any in-flight calibration gases. Water vapor corrections associated with dilution and pressure-broadening effects for CO2 and CH4 were derived from laboratory experiments employing measurements of water vapor by the CRDS analyzer. Before the campaign, the stability of the analyzer was assessed by laboratory tests under simulated flight conditions. During the campaign, a comparison of CO2 measurements between the CRDS analyzer and a nondispersive infrared (NDIR) analyzer on board the same aircraft showed a mean difference of 0.22±0.09 ppm for all flights over the Amazon rain forest. At the end of the campaign, CO2 concentrations of the synthetic calibration gases used by the NDIR analyzer were determined by the CRDS analyzer. After correcting for the isotope and the pressure-broadening effects that resulted from changes of the composition of synthetic vs. ambient air, and applying those concentrations as calibrated values of the calibration gases to reprocess the CO2 measurements made by the NDIR, the mean difference between the CRDS and the NDIR during BARCA was reduced to 0.05±0.09 ppm, with the mean standard deviation of 0.23±0.05 ppm. The results clearly show that the CRDS is sufficiently stable to be used in flight without drying the air or calibrating in flight and the water corrections are fully adequate for high-accuracy continuous airborne measurements of CO2 and CH4

We present an evaluation of aircraft observations of the carbon and greenhouse gases CO2, CH4, N2O, and CO using a direct-absorption pulsed quantum cascade laser spectrometer (QCLS) operated during the HIPPO and CalNex airborne experiments. The QCLS made continuous 1 Hz measurements with 1σ Allan precisions of 20, 0.5, 0.09, and 0.15 ppb for CO2, CH4, N2O, and CO, respectively, over > 500 flight hours on 79 research flights. The QCLS measurements are compared to two vacuum ultraviolet (VUV) CO instruments (CalNex and HIPPO), a cavity ring-down spectrometer (CRDS) measuring CO2 and CH4 (CalNex), two broadband non-dispersive infrared (NDIR) spectrometers measuring CO2 (HIPPO), two onboard gas chromatographs measuring a variety of chemical species including CH4, N2O, and CO (HIPPO), and various flask-based measurements of all four species. QCLS measurements are tied to NOAA and WMO standards using an in-flight calibration system, and mean differences when compared to NOAA CCG flask data over the 59 HIPPO research flights were 100, 1, 1, and 2 ppb for CO2, CH4, N2O, and CO, respectively. The details of the end-to-end calibration procedures and the data quality assurance and quality control (QA/QC) are presented. Specifically, we discuss our practices for the traceability of standards given uncertainties in calibration cylinders, isotopic and surface effects for the long-lived greenhouse gas tracers, interpolation techniques for in-flight calibrations, and the effects of instrument linearity on retrieved mole fractions.

We determined methane (CH4) emissions from Alaska, USA using airborne measurements from the Carbon Arctic Reservoirs Vulnerability Experiment (CARVE). Atmospheric sampling was conducted between May and September 2012, and analyzed using a customized version of the Polar Weather Research and Forecast model linked to a Lagrangian particle dispersion model (Stochastic Time-Inverted Lagrangian Transport Model). We estimated growing season CH4 fluxes of 8 2 mg CH4 m 2 d 1 averaged over all of Alaska, corresponding to fluxes from wetlands of 56+22 13 20 mg CH4 m 2 d 1 if we assumed that wetlands are the only source from the land surface (all uncertainties are 95% confidence intervals from a bootstrapping analysis). Fluxes roughly doubled from May to July, then decreased gradually in August and September. Integrated emissions totaled 25 2:1 0:5 Tg CH4 for Alaska from May to September 2012, close to the average (2.3, range 0.7-6 Tg CH4) predicted by various land surface models and inversion analyses for the growing season. Methane emissions from boreal Alaska were larger than from the North Slope; the monthly regional flux estimates show no evidence of enhanced 30 emissions during early spring or late fall, although these bursts may be more localized in time and space than can be detected by our analysis. These results provide an important baseline to which future studies can be compared.