Mark S. Zahniser

Astigmatic mirror multipass absorption cells for long-path-length spectroscopy

A multipass absorption cell, based on an astigmatic variant of the off-axis resonator (Herriott) configuration, has been designed to obtain long path lengths in small volumes. Rotation of the mirror axes is used to obtain an effective adjustability in the two mirror radii. This allows one to compensate for errors in mirror radii that are encountered in manufacture, thereby generating the desired reentrant patterns with less-precise mirrors. Acombination of mirror rotation and separation changes can be used to reach a variety of reentrant patterns and path lengths with a fixed set of astigmatic mirrors. The accessible patterns can be determined from trajectories, as a function of rotation and separation, through a general map of reentrant solutions. Desirable patterns for long-path spectroscopy can be chosen on the basis of path length, distance of the closest beam spot from the coupling hole, and tilt insensitivity. We describe the mathematics and analysis methods for the astigmatic cell with mirror rotation and then describe the design and test of prototype cells with this concept. Two cell designs are presented, a cell with 100-m path length in a volume of 3 L and a cell with 36-m path length in a volume of 0.3 L. Tests of low-volume absorption cells that use mirror rotation, designed for fast-flow atmospheric sampling, show the validity and the usefulness of the techniques that we have developed.

Vapor pressures of solid hydrates of nitric acid - Implications for polar stratospheric clouds

Thermodynamic data are presented for hydrates of nitric acid: HNO3�H2O, HNO3�2H2O, HNO3�3H2O, and a higher hydrate. Laboratory data indicate that nucleation and persistence of metastable HNO3�2H2O may be favored in polar stratospheric clouds over the slightly more stable HNO3�3H2O. Atmospheric observations indicate that some polar stratospheric clouds may be composed of HNO3�2H2O and HNO3�3H2O. Vapor transfer from HNO3�2H2O to HNO3�3H2O could be a key step in the sedimentation of HNO3, which plays an important role in the depletion of polar ozone.

Guidance for the Performance Evaluation of Three-Dimensional Air Quality Modeling Systems for Particulate Matter and Visibility

ABSTRACT Guidance for the performance evaluation of three-dimensional air quality modeling systems for particulate matter and visibility is presented. Four levels are considered: operational, diagnostic, mechanistic, and probabilistic evaluations. First, a comprehensive model evaluation should be conducted in at least two distinct geographical locations and for several meteorological episodes. Next, streamlined evaluations can be conducted for other similar applications if the comprehensive evaluation is deemed satisfactory. In all cases, the operational evaluation alone is insufficient, and some diagnostic evaluation must always be carried out. Recommendations are provided for designing field measurement programs that can provide the data needed for such model performance evaluations.

Methane emissions from natural gas infrastructure and use in the urban region of Boston, Massachusetts

Significance Most recent analyses of the environmental impact of natural gas have focused on production, with very sparse information on emissions from distribution and end use. This study quantifies the full seasonal cycle of methane emissions and the fractional contribution of natural gas for the urbanized region centered on Boston. Emissions from natural gas are found to be two to three times larger than predicted by existing inventory methodologies and industry reports. Our findings suggest that natural-gas–consuming regions may be larger sources of methane to the atmosphere than is currently estimated and represent areas of significant resource loss. Methane emissions from natural gas delivery and end use must be quantified to evaluate the environmental impacts of natural gas and to develop and assess the efficacy of emission reduction strategies. We report natural gas emission rates for 1 y in the urban region of Boston, using a comprehensive atmospheric measurement and modeling framework. Continuous methane observations from four stations are combined with a high-resolution transport model to quantify the regional average emission flux, 18.5 ± 3.7 (95% confidence interval) g CH4⋅m−2⋅y−1. Simultaneous observations of atmospheric ethane, compared with the ethane-to-methane ratio in the pipeline gas delivered to the region, demonstrate that natural gas accounted for ∼60–100% of methane emissions, depending on season. Using government statistics and geospatial data on natural gas use, we find the average fractional loss rate to the atmosphere from all downstream components of the natural gas system, including transmission, distribution, and end use, was 2.7 ± 0.6% in the Boston urban region, with little seasonal variability. This fraction is notably higher than the 1.1% implied by the most closely comparable emission inventory.

Application of quantum cascade lasers to high-precision atmospheric trace gas measurements

We review our recent results in development of high-precision laser spectroscopic instrumentation using midinfrared quantum cascade lasers (QCLs). Some of these instruments have been directed at mea- surements of atmospheric trace gases where a fractional precision of 10 −3 or better of ambient concentration may be required. Such high precision is needed in measurements of fluxes of stable atmospheric gases and measurements of isotopic ratios. Instruments that are based on thermoelectrically cooled midinfrared QCLs and thermoelectrically cooled detectors have been demonstrated that meet the requirements of high-precision atmospheric measurements, without the need for cryo- gens. We also describe the design of and results from a new dual QCL instrument with a 200-m path-length absorption cell. This instrument has demonstrated 1-s noise of 32 ppt for formaldehyde (HCHO) and 9 ppt for carbonyl sulfide (OCS). C 2010 Society of Photo-Optical Instrumentation Engineers.

Greenhouse gas budget (CO2, CH4 and N2O) of intensively managed grassland following restoration

The first full greenhouse gas (GHG) flux budget of an intensively managed grassland in Switzerland (Chamau) is presented. The three major trace gases, carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) were measured with the eddy covariance (EC) technique. For CO2 concentrations, an open-path infrared gas analyzer was used, while N2O and CH4 concentrations were measured with a recently developed continuous-wave quantum cascade laser absorption spectrometer (QCLAS). We investigated the magnitude of these trace gas emissions after grassland restoration, including ploughing, harrowing, sowing, and fertilization with inorganic and organic fertilizers in 2012. Large peaks of N2O fluxes (20-50 nmol m(-2) s(-1) compared with a <5 nmol m(-2) s(-1) background) were observed during thawing of the soil after the winter period and after mineral fertilizer application followed by re-sowing in the beginning of the summer season. Nitrous oxide (N2O) fluxes were controlled by nitrogen input, plant productivity, soil water content and temperature. Management activities led to increased variations of N2O fluxes up to 14 days after the management event as compared with background fluxes measured during periods without management (<5 nmol m(-2) s(-1)). Fluxes of CO2 remained small until full plant development in early summer 2012. In contrast, methane emissions showed only minor variations over time. The annual GHG flux budget was dominated by N2O (48% contribution) and CO2 emissions (44%). CH4 flux contribution to the annual budget was only minor (8%). We conclude that recently developed multi-species QCLAS in an EC system open new opportunities to determine the temporal variation of N2O and CH4 fluxes, which further allow to quantify annual emissions. With respect to grassland restoration, our study emphasizes the key role of N2O and CO2 losses after ploughing, changing a permanent grassland from a carbon sink to a significant carbon source.

Entry of gas molecules into liquids

Heterogeneous gas–liquid interactions play a fundamental role in many atmospheric processes. Pivotal for the understanding of such processes is the rate of gas uptake by the relevant liquid. Accurate experimental techniques have been developed to study uptake of gases by liquids. Gas–liquid interactions have already been investigated for more than thirty gas-phase species encompassing two classes of molecules: (1) those that do not react in the liquid but are readily solvated by it and (2) those that have low solubility in the liquid but react in the liquid rapidly to form highly soluble species. Uptake studies of non-reactive gas molecules in water have been explained by a model which treats the water surface as a narrow region of a dense gas-like state within which trace gas–solvent collisions occur. Gas accommodation occurs via nucleation of solvent molecules. Uptake studies for reactive gas molecules indicate that, for some species, reaction rates are much more rapid at the liquid interface than in the bulk liquid. The results of non-reactive and reactive uptake experiments are brought together here. The uptake model is briefly described, and the implication of the model for the kinetic nature of liquid surfaces is examined. Reaction-driven uptake studies which provide clear evidence for reactions at the gas/liquid interface are presented. The nature of interfacial reactions and their connection to the uptake model are examined.

Infrared laser spectrometer with balanced absorption for measurement of isotopic ratios of carbon gases

Measurement of the isotopic compositions of carbon dioxide and methane is a powerful tool for quantifying their atmospheric sources and sinks, which is especially important considering the dramatic increase in these greenhouse gases during the industrial era. Laser absorption spectroscopy is a technique which has demonstrated the high sensitivity needed for isotopic measurement. A significant problem in the spectroscopic measurement of isotopic abundances is the large difference in concentrations of the major and minor isotopic constituents. The measurement of two isotopic species using lines of similar strength but very unequal concentrations leads to low precision, with either the minor constituent having too small an absorption depth, or the major constituent having too great an absorption depth. If lines with unequal strength are chosen to compensate for the absorption depth imbalance, then precision tends to suffer due to the greater temperature sensitivity of the weaker line strength. We describe the development of a compact instrument for isotopic analysis CO2 and CH4 using tunable infrared laser absorption spectroscopy which combines novel optical design and signal processing methods to address this problem. The design compensates for the large difference in concentration between major and minor isotopes by measuring them with pathlengths which differ by a factor of 72 within the same multipass cell. We have demonstrated the basic optical design and signal processing by determining delta13C (CO2) isotopic ratios with precision as small as 0.2/1000 using a lead salt diode laser based spectroscopic instrument.

Uptake of gas phase sulfur species methanesulfonic acid, dimethylsulfoxide, and dimethyl sulfone by aqueous surfaces

Biogenic reduced sulfur species are emitted from the oceans and then oxidized in the marine boundary layer. The gas-liquid interactions of these oxidized species must be understood in order to evaluate the relative contributions to marine boundary layer aerosol levels from anthropogenic and biogenic sources and to assess the overall impact of these aerosols on global climate. A key parameter in understanding these interactions is the mass accommodation coefficient, which is simply the probability that a gas phase molecule enters into a liquid on striking the liquid surface. The mass accommodation coefficients for dimethylsulfoxide, dimethyl sulfone, and methanesulfonic acid into water have been measured as a function of temperature (260–280 K), pH (1–14), and NaCl concentration (0–3.5 M). The experimental method employs a monodispersed train of fast droplets in a low-pressure flow reactor. The mass accommodation coefficients show a negative temperature dependence varying from ∼0.1 to ∼0.2 over the range of temperatures studied. The measured uptake is independent of pH and NaCl concentration in the ranges studied. The mass accommodation coefficients are well expressed in terms of an observed Gibbs free energy ΔGobs# = ΔHobs# - TΔSobs# as α/(1 - α) = exp (−ΔGobs#/RT). The results are discussed in terms of a previously described uptake model. In the marine boundary layer, mass transfer of these species into aerosols will be limited by mass accommodation for aerosols with diameters of less than 2 μm.

Airborne measurements of HCHO and HCOOH during the New England Air Quality Study 2004 using a pulsed quantum cascade laser spectrometer

[1] Atmospheric mixing ratios of formaldehyde and formic acid have been measured from the NOAA WP-3 aircraft during the New England Air Quality Study (NEAQS) of July and August 2004 using a newly developed quantum cascade laser (QCL) spectrometer operating at a wavelength of 5.6 mm. The laser operates in pulsed mode with thermoelectric cooling. The detection is based on direct absorption in a compact 76-m multiple pass absorption cell. The laser is swept over a 0.5 cm 1 spectral region containing multiple lines of both HCHO and HCOOH. Absolute concentrations are retrieved by simultaneous spectral fitting routines with a detection limit (2s) for HCHO of 0.3 parts in 10 9 (ppbv) with an averaging time of 60 s under stable flight conditions. HCHO mixing ratios in the range from 0.3 to 5 ppb were encountered during flight conditions. Some of the highest mixing ratios of HCHO were observed over heavily vegetated areas of Florida during the test flights where the ratios of HCHO to methacrolein and methyl vinyl ketone, measured by proton transfer mass spectroscopy, are consistent with formaldehyde production by isoprene oxidation. The highest mixing ratios of HCOOH, up to 10 ppbv, were observed in an aged forest fire plume encountered over northern Canada, in which ratios of HCOOH/CO are greater than previous observations, while the ratios of HCHO/CO are less than previous reports from forest fire plumes. Observations of HCHO/CO and HCOOH/CO in urban plumes are indicative of a mixture of sources from direct emissions and secondary oxidation of anthropogenic and biogenic hydrocarbons. The ability to measure both HCHO and HCOOH simultaneously is of value in assessing the oxidation mechanisms of atmospheric hydrocarbons and secondary organic aerosol formation and oxidation.