Kristie A. Boering

Evaluation of transport in stratospheric models

We evaluate transport characteristics of two- and three-dimensional chemical transport models of the stratosphere by comparing their simulations of the mean age of stratospheric air and the propagation of annually periodic oscillations in tracer mixing ratio at the tropical tropopause into the stratosphere to inferences from in situ and satellite observations of CO2, SF6, and water vapor. The models, participants in the recent NASA “Models and Measurements II” study, display a wide range of performance. Most models propagate annual oscillations too rapidly in the vertical and overattenuate the signal. Most models also significantly underestimate mean age throughout the stratosphere, and most have at least one of several unrealistic features in their mean age contour shapes. In the lower stratosphere, model-to-model variation in N2O, NOy, and Cly is well correlated with variation in mean age, and the magnitude of NOy and Cly variation is large. We conclude that model transport inaccuracies significantly affect simulations of important long-lived chemical species in the lower stratosphere.

A high-precision fast-response airborne CO2 analyzer for In situ sampling from the surface to the middle stratosphere

Abstract Two in situ CO2 analyzers have been developed for deployment on the NASA ER-2 aircraft and on stratospheric balloons. The ER-2 instrument has had more than 150 flights during 21 deployments from 1992 to 2000, resulting in a dataset with nearly pole-to-pole coverage that includes data from all seasons in both hemispheres except austral summer. In-flight calibrations show that the typical long-term (i.e., flight-to-flight) precision of the instruments is better than ±0.1 ppmv. The flight standards are traceable to standards held by the Scripps Institute of Oceanography and the National Oceanic and Atmospheric Administrations Climate Monitoring and Diagnostics Laboratory. The balloon instrument has had eight balloon flights since September 1996, providing the first in situ observations of CO2 above ∼21 km. In addition, the balloon instrument has been flown on board a Cessna Citation II aircraft for sampling between the surface and 10 km. In this paper, the instrumentation and calibration procedures...

Observations of the anomalous oxygen isotopic composition of carbon dioxide in the lower stratosphere and the flux of the anomaly to the troposphere

[1] Measurements of the triple oxygen isotopic composition of stratospheric CO 2 in whole air samples from the NASA ER-2 aircraft show anomalous enrichments in 17 O and 18 O. The compact correlation of the isotope anomaly (defined as Δ 17 O = δ 17 O - 0.516 x δ 18 O) with simultaneous N 2 O measurements demonstrates that Δ 17 O CO2 is a long-lived tracer with a stratospheric source. These characteristics, and an isotopic link to O 3 production, make Δ 17 O CO2 potentially useful as a tracer of integrated stratospheric chemistry and transport. The Δ 17 O CO2 :N 2 O correlation is also used to estimate a net Δ 17 O CO2 flux to the troposphere of 3.6 ± 0.9 x 10 15 ‰ mol CO 2 yr -1 . This flux is required to predict and understand the CO 2 and O 2 isotope anomalies in the troposphere and their use as tracers of gross carbon exchanges between the atmosphere and biosphere on interannual to glacial-interglacial time scales.

Can N2O stable isotopes and isotopomers be useful tools to characterize sources and microbial pathways of N2O production and consumption in tropical soils

Nitrous oxide (N2O) is an important greenhouse gas in which the main sources are tropical rainforest and agricultural soils. N2O is produced in soils by microbial processes, which are enhanced by the application of nitrogenous fertilizers. The soil N2O bulk isotopic composition (δ 15Nbulk and δ 18O) and the “site-specific,” or intramolecular, 15N isotopic composition, i.e., the 15N/14N ratio at the cenral (α) or terminal (β) nitrogen position, expressed in this study as δ 15N α and δ 15N β could help identify both the sources (natural and anthropogenic) and microbial pathways of N2O production and consumption prior to emission.We report new isotope measurements of soil N2O emissions and from soil air collected during the rainy season in a mature tropical forest (Tapajos National Forest, Para, Brazil) and in a tropical agricultural corn field (“Fundo Tierra Nueva,” Guarico State, Venezuela). The statistically different δ 15Nbulk emission weighted average between the mature forest (−18.0‰ ± 4.0‰, n = 6) and agricultural corn field (−34.3‰ ± 12.4‰, n = 17) suggest that theδ 15Nbulk data are useful for distinguishing N2O fluxes from fertilized agricultural and natural “background” soils. They also demonstrate that the site-specific δ 15N measurements have the potential to provide a new tool to differentiate between the production and consumption N2O microbiological processes in soils. This study further demonstrates that the observed correlations (or lack thereof) between δ 15N α ,δ 15N β , and δ 18O can be used to estimate the relative proportion of N2O that would have been emitted to the air but was consumed via reduction of N2O to N2 within the soil.

Strong Negative Temperature Dependence of the Simplest Criegee Intermediate CH2OO Reaction with Water Dimer.

The kinetics of the reaction of CH2OO with water vapor was measured directly with UV absorption at temperatures from 283 to 324 K. The observed CH2OO decay rate is second order with respect to the H2O concentration, indicating water dimer participates in the reaction. The rate coefficient of the CH2OO reaction with water dimer can be described by an Arrhenius expression k(T) = A exp(-Ea/RT) with an activation energy of -8.1 ± 0.6 kcal mol(-1) and k(298 K) = (7.4 ± 0.6) × 10(-12) cm(3) s(-1). Theoretical calculations yield a large negative temperature dependence consistent with the experimental results. The temperature dependence increases the effective loss rate for CH2OO by a factor of ~2.5 at 278 K and decreases by a factor of ~2 at 313 K relative to 298 K, suggesting that temperature is important for determining the impact of Criegee intermediate reactions with water in the atmosphere.

UV absorption spectrum of the C2 Criegee intermediate CH3CHOO

The UV spectrum of CH3CHOO was measured by transient absorption in a flow cell at 295 K. The absolute absorption cross sections of CH3CHOO were measured by laser depletion in a molecular beam to be (1.06 ± 0.09) × 10(-17) cm(2) molecule(-1) at 308 nm and (9.7 ± 0.6) × 10(-18) cm(2) molecule(-1) at 352 nm. After scaling the UV spectrum of CH3CHOO to the absolute cross section at 308 nm, the peak UV cross section is (1.27 ± 0.11) × 10(-17) cm(2) molecule(-1) at 328 nm. Compared to the simplest Criegee intermediate CH2OO, the UV absorption band of CH3CHOO is similar in intensity but blue shifted by 14 nm, resulting in a 20% slower photolysis rate estimated for CH3CHOO in the atmosphere.

Unimolecular Decomposition Rate of the Criegee Intermediate (CH3)2COO Measured Directly with UV Absorption Spectroscopy

The unimolecular decomposition of (CH3)2COO and (CD3)2COO was measured by direct detection of the Criegee intermediate at temperatures from 283 to 323 K using time-resolved UV absorption spectroscopy. The unimolecular rate coefficient kd for (CH3)2COO shows a strong temperature dependence, increasing from 269 ± 82 s(-1) at 283 K to 916 ± 56 s(-1) at 323 K with an Arrhenius activation energy of ∼6 kcal mol(-1). The bimolecular rate coefficient for the reaction of (CH3)2COO with SO2, kSO2, was also determined in the temperature range 283 to 303 K. Our temperature-dependent values for kd and kSO2 are consistent with previously reported relative rate coefficients kd/kSO2 of (CH3)2COO formed from ozonolysis of tetramethyl ethylene. Quantum chemical calculations of kd for (CH3)2COO are consistent with the experiment, and the combination of experiment and theory for (CD3)2COO indicates that tunneling plays a significant role in (CH3)2COO unimolecular decomposition. The fast rates of unimolecular decomposition for (CH3)2COO measured here, in light of the relatively slow rate for the reaction of (CH3)2COO with water previously reported, suggest that thermal decomposition may compete with the reactions with water and with SO2 for atmospheric removal of the dimethyl-substituted Criegee intermediate.

Large and unexpected enrichment in stratospheric 16O13C18O and its meridional variation.

The stratospheric CO2 oxygen isotope budget is thought to be governed primarily by the O(1D)+CO2 isotope exchange reaction. However, there is increasing evidence that other important physical processes may be occurring that standard isotopic tools have been unable to identify. Measuring the distribution of the exceedingly rare CO2 isotopologue 16O13C18O, in concert with 18O and 17O abundances, provides sensitivities to these additional processes and, thus, is a valuable test of current models. We identify a large and unexpected meridional variation in stratospheric 16O13C18O, observed as proportions in the polar vortex that are higher than in any naturally derived CO2 sample to date. We show, through photochemical experiments, that lower 16O13C18O proportions observed in the midlatitudes are determined primarily by the O(1D)+CO2 isotope exchange reaction, which promotes a stochastic isotopologue distribution. In contrast, higher 16O13C18O proportions in the polar vortex show correlations with long-lived stratospheric tracer and bulk isotope abundances opposite to those observed at midlatitudes and, thus, opposite to those easily explained by O(1D)+CO2. We believe the most plausible explanation for this meridional variation is either an unrecognized isotopic fractionation associated with the mesospheric photochemistry of CO2 or temperature-dependent isotopic exchange on polar stratospheric clouds. Unraveling the ultimate source of stratospheric 16O13C18O enrichments may impose additional isotopic constraints on biosphere–atmosphere carbon exchange, biosphere productivity, and their respective responses to climate change.

The hydrogen isotopic composition of water vapor entering the stratosphere inferred from high‐precision measurements of δD‐CH4 and δD‐H2

The hydrogen isotopic composition of water vapor entering the stratosphere provides an important constraint on the mechanisms for dehydration of air ascending through the tropical tropopause layer. We have inferred the annual mean hydrogen isotopic composition of water vapor entering the stratosphere (or δD-H_(2)O_0) for the mid to late 1990s based on high-precision measurements of the hydrogen isotopic compositions of stratospheric H_2 and CH_4 from whole air samples collected on the NASA ER-2 aircraft between 1996 and 2000 and remote observations of δD-H_2O from the FIRS-2 far infrared spectrometer. We calculate an annual mean value for δD-H_(2)O_0 of −653 (+24/−25)‰ relative to Vienna standard mean ocean water (VSMOW). Previous inferences from balloon-borne and spacecraft remote-sensing observations are ∼20‰ lighter than the value from this analysis. We attribute the difference to an underestimation of deuterium in the molecular H_2 reservoir in earlier work. This precise and more accurate value for the annual mean δD-H_(2)O_0 will be useful as a 1990s benchmark for detecting future changes in the details of the dehydration of air due to the impact of climate change on convection intensity, cloud microphysics, or tropical tropopause layer temperatures. In addition, we report a value for the total deuterium content in the three main stratospheric hydrogen reservoirs HDO, HD, and CH_(3)D of 1.60 (+0.02/−0.03) ppbv.

Unexpected variations in the triple oxygen isotope composition of stratospheric carbon dioxide

We report observations of stratospheric CO2 that reveal surprisingly large anomalous enrichments in 17O that vary systematically with latitude, altitude, and season. The triple isotope slopes reached 1.95 ± 0.05(1σ) in the middle stratosphere and 2.22 ± 0.07 in the Arctic vortex versus 1.71 ± 0.03 from previous observations and a remarkable factor of 4 larger than the mass-dependent value of 0.52. Kinetics modeling of laboratory measurements of photochemical ozone–CO2 isotope exchange demonstrates that non–mass-dependent isotope effects in ozone formation alone quantitatively account for the 17O anomaly in CO2 in the laboratory, resolving long-standing discrepancies between models and laboratory measurements. Model sensitivities to hypothetical mass-dependent isotope effects in reactions involving O3, O(1D), or CO2 and to an empirically derived temperature dependence of the anomalous kinetic isotope effects in ozone formation then provide a conceptual framework for understanding the differences in the isotopic composition and the triple isotope slopes between the laboratory and the stratosphere and between different regions of the stratosphere. This understanding in turn provides a firmer foundation for the diverse biogeochemical and paleoclimate applications of 17O anomalies in tropospheric CO2, O2, mineral sulfates, and fossil bones and teeth, which all derive from stratospheric CO2.