Comment on: “The measurement of tropospheric OH radicals by laser-induced fluorescence spectroscopy during the POPCORN Field Campaign” by Hofzumahaus et al. and “Intercomparison of tropospheric OH radical measurements by multiple folded long-path laser absorption and laser induced fluorescence” by Brauers et al.

Calibration of laser induced fluorescence (LIF) instruments that measure OH is challenging because it is difficult to reliably introduce a known amount of this reactive radical into a measurement apparatus. In a recent paper, Hofzumahaus et al., [1996] describe a novel and seemingly simple technique to accomplish this goal: they dissociate trace quantities of water vapor in air with a low pressure mercury (Hg) lamp to produce low concentrations (10^5 - 10^9 cm^(-3)) of OH (R1)

Twilight observations suggest unknown sources of HO_x

Measurements of the concentrations of OH and HO_(2) (HO_(x)) in the high-latitude lower stratosphere imply the existence of unknown photolytic sources of HO_(x). The strength of the additional HO_(x) source required to match the observations depends only weakly on solar zenith angle (SZA) for 80° < SZA < 93°. The wavelengths responsible for producing this HO_(x) must be longer than 650 nm because the flux at shorter wavelengths is significantly attenuated at high SZA by scattering and absorption. Provided that the sources involve only a single photon, the strength of the bonds being broken must be < 45 kcal mole^(−1). We speculate that peroxynitric acid (HNO_4) dissociates after excitation to an unknown excited state with an integrated band cross section of 2-3 × 10^(−20) cm^(2) molecule^(−1) nm (650 < λ < 1250 nm)

Observed OH and HO_2 in the upper troposphere suggest a major source from convective injection of peroxides

ER-2 aircraft observations of OH and HO_2 concentrations in the upper troposphere during the NASA/STRAT campaign are interpreted using a photochemical model constrained by local observations of O_3, H_2O, NO, CO, hydrocarbons, albedo and overhead ozone column. We find that the reaction Q(^(1)D) + H_2O is minor compared to acetone photolysis as a primary source of HO_x (= OH + peroxy radicals) in the upper troposphere. Calculations using a diel steady state model agree with observed HO_x concentrations in the lower stratosphere and, for some flights, in the upper troposphere. However, for other flights in the upper troposphere, the steady state model underestimates observations by a factor of 2 or more. These model underestimates are found to be related to a recent (< 1 week) convective origin of the air. By conducting time-dependent model calculations along air trajectories determined for the STRAT flights, we show that convective injection of CH_3OOH and H_2O_2 from the boundary layer to the upper troposphere could resolve the discrepancy. These injections of HO_x reservoirs cause large HO_x increases in the tropical upper troposphere for over a week downwind of the convective activity. We propose that this mechanism provides a major source of HO_x in the upper troposphere. Simultaneous measurements of peroxides, formaldehyde and acetone along with OH and HO_2 are needed to test our hypothesis

The lady vanishes: what's missing from the stem cell debate

Most opponents of somatic cell nuclear transfer and embryonic stem cell technologies base their arguments on the twin assertions that the embryo is either a human being or a potential human being, and that it is wrong to destroy a human being or potential human being in order to produce stem cell lines. Proponents’ justifications of stem cell research are more varied, but not enough to escape the charge of obsession with the status of the embryo. What unites the two warring sides in ‘the stem cell wars’ is that women are equally invisible to both: ‘the lady vanishes’. Yet the only legitimate property in the body is that which women possess in their reproductive tissue and the products of their reproductive labour. By drawing on the accepted characterisation in law of property as a bundle of rights, and on a Hegelian model of contract as mutual recognition, we can lessen the impact of the tendency to regard women and their eggs as merely receptacles and women’s reproductive labour as unimportant

Quantitative constraints on the atmospheric chemistry of nitrogen oxides: An analysis along chemical coordinates

In situ observations Of NO_2, NO, NO_y, ClONO_2, OH, O_3, aerosol surface area, spectrally resolved solar radiation, pressure and temperature obtained from the ER-2 aircraft during the Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) experiments are used to examine the factors controlling the fast photochemistry connecting NO and NO_2 and the slower chemistry connecting NO_x and HNO_3. Our analysis uses “chemical coordinates” to examine gradients of the difference between a model and precisely calibrated measurements to provide a quantitative assessment of the accuracy of current photochemical models. The NO/NO_2 analysis suggests that reducing the activation energy for the NO+O_3 reaction by 1.7 kJ/mol will improve model representation of the temperature dependence of the NO/NO_2 ratio in the range 215–235 K. The NO_x/HNO_3 analysis shows that systematic errors in the relative rate coefficients used to describe NO_x loss by the reaction OH + NO_2 → HNO_3 and by the reaction set NO_2 + O_3 → NO_3; NO_2 + NO_3 → N_(2)O_5; N_(2)O_5 + H_(2)O → 2HNO_3 are in error by +8.4% (+30/−45%) (OH+NO_2 too fast) in models using the Jet Propulsion Laboratory 1997 recommendations [DeMore et al., 1997]. Models that use recommendations for OH+NO2 and OH+HNO_3 based on reanalysis of recent and past laboratory measurements are in error by 1.2% (+30/−45%) (OH+NO_2 too slow). The +30%/−45% error limit reflects systematic uncertainties, while the statistical uncertainty is 0.65%. This analysis also shows that the POLARIS observations only modestly constrain the relative rates of the major NO_x production reactions HNO3 + OH → H_(2)O + NO_3 and HNO_3 + hν → OH + NO_2. Even under the assumption that all other aspects of the model are perfect, the POLARIS observations only constrain the rate coefficient for OH+HNO_3 to a range of 65% around the currently recommended value

NO(y) partitioning from measurements of nitrogen and hydrogen radicals in the upper troposphere

Recent studies using NO, NO(y), OH and HO2 (HO(X)) observations have postulated acetone and convection of peroxides as significant sources of HO(X) in the upper troposphere (UT). This work focuses on the effect these additional HO(X) sources have on the modeled NO(y) partitioning and comparisons of the modeled NO(x)/NO(y) ratio to observations. The measured NO(x)/NO(y) ratio is usually much higher than predicted regardless of the presence of acetone in the model. The exception occurs for air parcels having low NO(y) and O3 values. For these air parcels the measured NO(x)/NO(y) ratio is much lower than the calculated ratio unless acetone is included in the model. In all cases acetone increases the fraction of NO(y) that is peroxy acetyl nitrate (PAN) from typical values of much less than 0.1 to values as high as 0.35. Including acetone also reduces the scatter in a comparison between modeled and observed NO(x)/NO(y) ratios

Inorganic chlorine partitioning in the summer lower stratosphere: Modeled and measured [ClONO_2]/[HCl] during POLARIS

We examine inorganic chlorine (Cl_y,) partitioning in the summer lower stratosphere using in situ ER-2 aircraft observations made during the Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) campaign. New steady state and numerical models estimate [ClONO_2]/[HCl] using currently accepted photochemistry. These models are tightly constrained by observations with OH (parameterized as a function of solar zenith angle) substituting for modeled HO_2 chemistry. We find that inorganic chlorine photochemistry alone overestimates observed [ClONO_2]/[HCl] by approximately 55–60% at mid and high latitudes. On the basis of POLARIS studies of the inorganic chlorine budget, [ClO]/[ClONO_2], and an intercomparison with balloon observations, the most direct explanation for the model-measurement discrepancy in Cl_y, partitioning is an error in the reactions, rate constants, and measured species concentrations linking HCl and ClO (simulated [ClO]/[HCl] too high) in combination with a possible systematic error in the ER-2 ClONO_2 measurement (too low). The high precision of our simulation (±15% 1σ for [ClONO_2]/[HCl], which is compared with observations) increases confidence in the observations, photolysis calculations, and laboratory rate constants. These results, along with other findings, should lead to improvements in both the accuracy and precision of stratospheric photochemical models

Ozone destruction and production rates between spring and autumn in the Arctic stratosphere

In situ measurements of radical and long-lived species were made in the lower Arctic stratosphere (18 to 20 km) between spring and early autumn in 1997. The measurements include O_3, ClO, OH, HO_2, NO, NO_2, N_(2)O, CO, and overhead O_3. A photochemical box model constrained by these and other observations is used to compute the diurnally averaged destruction and production rates of O3 in this region. The rates show a strong dependence on solar exposure and ambient O_3. Total destruction rates, which reach 19%/month in summer, reveal the predominant role of NO_x and HO_x catalytic cycles throughout the period. Production of O_3 is significant only in midsummer air parcels. A comparison of observed O_3 changes with destruction rates and transport effects indicates the predominant role of destruction in spring and an increased role of transport by early autumn

A comparison of observations and model simulations of NO\u3csub\u3ex\u3c/sub\u3e/NO\u3csub\u3ey\u3c/sub\u3e in the lower stratosphere

Extensive airborne measurements of the reactive nitrogen reservoir (NOy) and its component nitric oxide (NO) have been made in the lower stratosphere. Box model simulations that are constrained by observations of radical and long-lived species and which include heterogeneous chemistry systematically underpredict the NOx (= NO + NO2) to NOy ratio. The model agreement is substantially improved if newly measured rate coefficients for the OH + NO2 and OH + HNO3 reactions are used. When included in 2-D models, the new rate coefficients significantly increase the calculated ozone loss due to NOx and modestly change the calculated ozone abundances in the lower stratosphere. Ozone changes associated with the emissions of a fleet of supersonic aircraft are also altered. Copyright 1999 by the American Geophysical Union