Stanley P. Sander

Atmospheric bromine and ozone perturbations in the lower stratosphere

The role of bromine compounds in the photochemistry of the natural and perturbed stratosphere has been reexamined using an expanded reaction scheme and the results of recent laboratory studies of several key reactions. The most important finding is that through the reaction BrO + CIO → Br + Cl + O2, there is a synergistic effect between bromine and chlorine which results in an efficient catalytic destruction of ozone in the lower stratosphere. One-dimensional photochemical model results indicate that BrO is the major bromine species throughout the stratosphere, followed by BrONO2, HBr, HOBr and Br. We show from the foregoing that bromine is more efficient than chlorine as a catalyst for destroying ozone, and discuss the implications for stratospheric ozone of possible future growth in the industrial and agricultural use of bromine. Bromine concentrations of 20 pptv (2 × 10^−11), as suggested by recent observations, can decrease the present-day integrated ozone column density by 2.4%, and can enhance ozone depletion from steady-state chlorofluoromethane release at 1973 rates by a factor of 1.1–1.2.

Precision requirements for space-based XCO 2 data

Precision requirements are determined for space-based column-averaged CO_2 dry air mole fraction (X_(CO)_2) data. These requirements result from an assessment of spatial and temporal gradients in (X_(CO)_2) the relationship between (X_(CO)_2) precision and surface CO_2 flux uncertainties inferred from inversions of the (X_(CO)_2) data, and the effects of (X_(CO)_2) biases on the fidelity of CO_2 flux inversions. Observational system simulation experiments and synthesis inversion modeling demonstrate that the Orbiting Carbon Observatory mission design and sampling strategy provide the means to achieve these (X_(CO)_2) data precision requirements.

Validation of Ozone Monitoring Instrument nitrogen dioxide columns

[1] We review the standard nitrogen dioxide (NO2) data product (Version 1.0.), which is based on measurements made in the spectral region 415–465 nm by the Ozone Monitoring Instrument (OMI) on the NASA Earth Observing System-Aura satellite. A number of ground- and aircraft-based measurements have been used to validate the data product’s three principal quantities: stratospheric, tropospheric, and total NO2 column densities under nearly or completely cloud-free conditions. The validation of OMI NO2 is complicated by a number of factors, the greatest of which is that the OMI observations effectively average the NO2 over its field of view (minimum 340 km 2 ), while a ground-based instrument samples at a single point. The tropospheric NO2 field is often very inhomogeneous, varying significantly over tens to hundreds of meters, and ranges from 10 16 cm � 2 over urban and industrial areas. Because of OMI’s areal averaging, when validation measurements are made near NO2 sources the OMI measurements are expected to underestimate the ground-based, and this is indeed seen. Further, we use several different instruments, both new and mature, which might give inconsistent NO2 amounts; the correlations between nearby instruments is 0.8–0.9. Finally, many of the validation data sets are quite small and span a very short length of time; this limits the statistical conclusions that can be drawn from them. Despite these factors, good agreement is generally seen between the OMI and ground-based measurements, with OMI stratospheric NO2 underestimated by about 14% and total and tropospheric columns underestimated by 15–30%. Typical correlations between OMI NO2 and ground-based measurements are generally >0.6.

The rotational spectrum and structure of chlorine peroxide

The products of the ClO self‐reaction have been studied in a flowing chemical reactor using submillimeter wave spectroscopy. The complete spectrum between 415 to 435 GHz has been measured as well as selected transitions in the range 285 to 415 GHz. The major products have been identified as the ClO dimer (Cl2O2) and chlorine dioxide (OClO). The observed rotational b‐type spectra of the most abundant isotopic species35 ClOO35Cl and 37ClOO35Cl have been analyzed. The observed nuclear spin statistics for the main species, the relative abundance of the lesser species, and the structure determination demonstrate unambigiously that the ClO dimer must possess identical chlorine atoms in a peroxide structure. The rotational constants as well as a complete set of quartic centrifugal distortion constants have been determined. Structural parameters for the vibronic ground state have been calculated: rOO=142.59(21) pm, rClO=170.44(4) pm, ∠ClOO=110.07(1)° and dihedral angle=81.03(1)°. Rotational transitions in the fir...

Rate of Gas Phase Association of Hydroxyl Radical and Nitrogen Dioxide

Honing in on HONO2 Modeling air pollution requires knowledge of all the interrelated reactions occurring in the atmosphere. Among the most significant is the formation of nitric acid (HONO2) from OH and NO2 radicals. One sticking point in the study of this reaction has been the uncertainty in how often radicals link through an O-O rather than an O-N bond. Mollner et al. (p. 646) measured the partitioning coefficient, as well as the overall consumption rate of the radicals, with an array of highly sensitive spectroscopic techniques in the laboratory. The measurements yielded a well-defined rate constant for nitric acid formation, which was applied to the prediction of ozone levels in atmospheric simulations of the Los Angeles basin. Laboratory measurements of a critical atmospheric rate constant should improve predictions of tropospheric ozone formation. The reaction of OH and NO2 to form gaseous nitric acid (HONO2) is among the most influential in atmospheric chemistry. Despite its importance, the rate coefficient remains poorly determined under tropospheric conditions because of difficulties in making laboratory rate measurements in air at 760 torr and uncertainties about a secondary channel producing peroxynitrous acid (HOONO). We combined two sensitive laser spectroscopy techniques to measure the overall rate of both channels and the partitioning between them at 25°C and 760 torr. The result is a significantly more precise value of the rate constant for the HONO2 formation channel, 9.2 (±0.4) × 10−12 cm3 molecule−1 s−1 (1 SD) at 760 torr of air, which lies toward the lower end of the previously established range. We demonstrate the impact of the revised value on photochemical model predictions of ozone concentrations in the Los Angeles airshed.

Absorption cross section of BrO between 312 and 385 nm AT 298 and 223 K

Abstract The absolute UV cross section of BrO at 338.1 ± 0.1 nm, the peak of the (7,0) band of the A( 2 Π)←X( 2 Π) transition, was measured at 298±2 and 223±4 K to be (1.71±0.14) × 10 −17 and (2.21±0.16) × 10 −17 cm 2 , respectively, using the technique of flash photolysis-ultraviolet absorption. The spectral resolution for these measurements was 0.18 nm. The absorption spectra of BrO in the wavelength range 312–385 nm were measured at 298±2 and 223±4 K using a flow tube reactor coupled to a diode array spectrometer. Using the (7,0) band cross sections, the absorption cross sections in the above wavelength range were calculated.

Inhalation anaesthetics and climate change

BACKGROUND Although the increasing abundance of CO(2) in our atmosphere is the main driver of the observed climate change, it is the cumulative effect of all forcing agents that dictate the direction and magnitude of the change, and many smaller contributors are also at play. Isoflurane, desflurane, and sevoflurane are widely used inhalation anaesthetics. Emissions of these compounds contribute to radiative forcing of climate change. To quantitatively assess the impact of the anaesthetics on the forcing of climate, detailed information on their properties of heat (infrared, IR) absorption and atmospheric lifetimes are required. METHODS We have measured the IR spectra of these anaesthetics and conducted calculations of their contribution to radiative forcing of climate change recognizing the important fact that radiative forcing is strongly dependent on the wavelength of the absorption features. RESULTS Radiative efficiencies of 0.453, 0.469, and 0.351 W m(-2) ppb(-1) and global warming potentials (GWPs) of 510, 1620, and 210 (100 yr time horizon) were established for isoflurane, desflurane, and sevoflurane, respectively. CONCLUSIONS On the basis of the derived 100 yr GWPs, the average climate impact per anaesthetic procedure at the University of Michigan is the same as the emission of ∼22 kg CO(2). We estimate that the global emissions of inhalation anaesthetics have a climate impact which is comparable with that from the CO(2) emissions from one coal-fired power plant or 1 million passenger cars.

Measuring rate constants for reactions of the simplest Criegee intermediate (CH2OO) by monitoring the OH radical.

While generating the CH2OO molecule by reacting CH2I with O2, significant amounts of the OH radical were observed by laser-induced fluorescence. At least two different processes formed OH. A fast process was probably initiated by a reaction of vibrationally hot CH2I radicals. The second process appeared to be associated with the decay of the CH2OO molecule. The addition of molecules known to react with CH2OO increased the observed decay rates of the OH signal. Using the OH signals as a proxy for the CH2OO concentration, the rate constant for the reaction of hexafluoroacetone with CH2OO was determined to be (3.33 ± 0.27) × 10(-11) cm(3) molecule(-1) s(-1), in good agreement with the value measured by Taatjes et al.1 The rate constant for the reaction of SO2 with CH2OO, (3.53 ± 0.29) × 10(-11) cm(3) molecule(-1) s(-1), showed no pressure dependence over the range of 50-200 Torr and was in agreement with the value at 4 Torr reported by Welz et al.

High-resolution Fourier-transform ultraviolet–visible spectrometer for the measurement of atmospheric trace species: application to OH

A compact, high-resolution Fourier-transform spectrometer for atmospheric near-ultraviolet spectroscopy has been installed at the Jet Propulsion Laboratorys Table Mountain Facility (34.4 degrees N, 117.7 degrees W, elevation 2290 m). This instrument is designed with an unapodized resolving power near 500,000 at 300 nm to provide high-resolution spectra from 290 to 675 nm for the quantification of column abundances of trace atmospheric species. The measurement technique used is spectral analysis of molecular absorptions of solar radiation. The instrument, accompanying systems designs, and results of the atmospheric hydroxyl column observations are described.

Pressure dependence and metastable state formation in the photolysis of dichlorine monoxide (Cl2O)

The photodissociation of dichlorine monoxide (Cl2O) was studied using broadband flash photolysis to investigate the influence of variations in the photolysis wavelength domain, bath gas pressure and bath gas identity on the yield and temporal dependence of the ClO product. ClO yields were independent of bath gas pressure when the photolysis spectral band extended to 200 nm (quartz cutoff) but for photolysis restricted to wavelengths longer than about 250 nm, ClO yields decreased with increasing bath gas pressure and there was a pressure‐dependent delay in the formation of ClO. Under these conditions, a weak, highly structured absorption spectrum was observed in the range 16 600–26 000 cm−1 with a lifetime on the order of 500 ms. A portion of the spectrum could be analyzed (22 000–26 000 cm−1) which showed progressions having differences of 283, 443, and 505 cm−1. Ab initio calculations were performed to evaluate vertical excitation energies and oscillator strengths from the lowest‐energy singlet (X 1A1) o...