David M. Wilmouth

UV Dosage Levels in Summer: Increased Risk of Ozone Loss from Convectively Injected Water Vapor

Water In, Ozone Out The danger of stratospheric ozone loss burst into public awareness in the 1980s, when the Antarctic ozone hole was discovered and described. Since then, the specter of ozone depletion in other locations, notably the Arctic, has been identified. Ozone loss is not confined to high latitudes, however, nor is it only the result of the addition of anthropogenic compounds containing chlorine and bromine in the stratosphere, as Anderson et al. (p. 835, published online 26 July; see the Perspective by Ravishankara) now demonstrate. Data from the atmosphere above the continental United States revealed that convective injection of water vapor into the stratosphere affects the free radical chemistry involving the (mostly anthropogenic) chlorine and bromine, thus accelerating ozone loss. This process could become important in the stratospheric ozone budget if the frequency and intensity of these water-injection events, which are most common in the summer, increase as a result of global warming. Convective injection of water vapor into the stratosphere increases the rate of ozone destruction there. The observed presence of water vapor convectively injected deep into the stratosphere over the United States can fundamentally change the catalytic chlorine/bromine free-radical chemistry of the lower stratosphere by shifting total available inorganic chlorine into the catalytically active free-radical form, ClO. This chemical shift markedly affects total ozone loss rates and makes the catalytic system extraordinarily sensitive to convective injection into the mid-latitude lower stratosphere in summer. Were the intensity and frequency of convective injection to increase as a result of climate forcing by the continued addition of CO2 and CH4 to the atmosphere, increased risk of ozone loss and associated increases in ultraviolet dosage would follow.

Chlorine-catalyzed ozone destruction: Cl atom production from ClOOCl photolysis.

Recent laboratory measurements of the absorption cross sections of the ClO dimer, ClOOCl, have called into question the validity of the mechanism that describes the catalytic removal of ozone by chlorine. Here we describe direct measurements of the rate-determining step of that mechanism, the production of Cl atoms from the photolysis of ClOOCl, under laboratory conditions similar to those in the stratosphere. ClOOCl is formed in a cold-temperature flowing system, with production initiated by a microwave discharge of Cl(2) or photolysis of CF(2)Cl(2). Excimer lasers operating at 248, 308, and 352 nm photodissociate ClOOCl, and the Cl atoms produced are detected with time-resolved atomic resonance fluorescence. Cl(2), the primary contaminant, is measured directly for the first time in a ClOOCl cross section experiment. We find the product of the quantum yield of the Cl atom production channel of ClOOCl photolysis and the ClOOCl absorption cross section, (phisigma)(ClOOCl) = 660 +/- 100 at 248 nm, 39.3 +/- 4.9 at 308 nm, and 8.6 +/- 1.2 at 352 nm (units of 10(-20) cm(2) molecule(-1)). The data set includes 468 total cross section measurements over a wide range of experimental conditions, significantly reducing the possibility of a systematic error impacting the results. These new measurements demonstrate that long-wavelength photons (lambda = 352 nm) are absorbed by ClOOCl directly, producing Cl atoms with a probability commensurate with the observed rate of ozone destruction in the atmosphere.

Stratospheric ozone over the United States in summer linked to observations of convection and temperature via chlorine and bromine catalysis

Significance Stratospheric ozone is one of the most delicate aspects of habitability on the planet. Removal of stratospheric ozone over the polar regions in winter/spring has established the vulnerability of ozone to halogen catalytic cycles. Elevated ClO concentrations engendered, in part, by heterogeneous catalytic conversion of inorganic chlorine to free radical form on ubiquitous sulfate−water aerosols, govern the rate of ozone removal. We report here observations of the frequency and depth of penetration of convectively injected water vapor into the stratosphere, triggered by severe storms that are specific to the central United States in summer, and model their effect on lower stratospheric ozone. This effect implies, with observed temperatures, increased risk of ozone loss over the Great Plains in summer. We present observations defining (i) the frequency and depth of convective penetration of water into the stratosphere over the United States in summer using the Next-Generation Radar system; (ii) the altitude-dependent distribution of inorganic chlorine established in the same coordinate system as the radar observations; (iii) the high resolution temperature structure in the stratosphere over the United States in summer that resolves spatial and structural variability, including the impact of gravity waves; and (iv) the resulting amplification in the catalytic loss rates of ozone for the dominant halogen, hydrogen, and nitrogen catalytic cycles. The weather radar observations of ∼2,000 storms, on average, each summer that reach the altitude of rapidly increasing available inorganic chlorine, coupled with observed temperatures, portend a risk of initiating rapid heterogeneous catalytic conversion of inorganic chlorine to free radical form on ubiquitous sulfate−water aerosols; this, in turn, engages the element of risk associated with ozone loss in the stratosphere over the central United States in summer based upon the same reaction network that reduces stratospheric ozone over the Arctic. The summertime development of the upper-level anticyclonic flow over the United States, driven by the North American Monsoon, provides a means of retaining convectively injected water, thereby extending the time for catalytic ozone loss over the Great Plains. Trusted decadal forecasts of UV dosage over the United States in summer require understanding the response of this dynamical and photochemical system to increased forcing of the climate by increasing levels of CO2 and CH4.

Ozone depletion following future volcanic eruptions

While explosive volcanic eruptions cause ozone loss in the current atmosphere due to an enhancement in the availability of reactive chlorine following the stratospheric injection of sulfur, future eruptions are expected to increase total column ozone as halogen loading approaches preindustrial levels. The timing of this shift in the impact of major volcanic eruptions on the thickness of the ozone layer is poorly known. Modeling four possible climate futures, we show that scenarios with the smallest increase in greenhouse gas concentrations lead to the greatest risk to ozone from heterogeneous chemical processing following future eruptions. We also show that the presence in the stratosphere of bromine from natural, very short-lived biogenic compounds is critically important for determining whether future eruptions will lead to ozone depletion. If volcanic eruptions inject hydrogen halides into the stratosphere, an effect not considered in current ozone assessments, potentially profound reductions in column ozone would result.

A case study of convectively sourced water vapor observed in the overworld stratosphere over the United States

On August 27, 2013, during the Studies of Emissions and Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC4RS) field mission, NASAs ER-2 research aircraft encountered a region of enhanced water vapor, extending over a depth of approximately 2 km and a minimum areal extent of 20,000 km2 in the stratosphere (375 K to 415 K potential temperature), south of the Great Lakes (42°N, 90°W). Water vapor mixing ratios in this plume, measured by the Harvard Water Vapor instrument, constitute the highest values recorded in situ at these potential temperatures and latitudes. An analysis of geostationary satellite imagery in combination with trajectory calculations links this water vapor enhancement to its source, a deep tropopause-penetrating convective storm system that developed over Minnesota 20 hours prior to the aircraft plume encounter. High resolution, ground-based radar data reveal that this system was comprised of multiple individual storms, each with convective turrets that extended to a maximum of ~4 km above the tropopause level for several hours. In situ water vapor data show that this storm system irreversibly delivered between 6.6 kt and 13.5 kt of water to the stratosphere. This constitutes a 20 – 25% increase in water vapor abundance in a column extending from 115 hP to 70 hPa over the plume area. Both in situ and satellite climatologies show a high frequency of localized water vapor enhancements over the central U.S. in summer, suggesting that deep convection can contribute to the stratospheric water budget over this region and season.

Stratospheric Ozone Depletion and Recovery

Abstract This chapter provides an overview of the depletion of Earths ozone layer due to human activity and the eventual recovery due to legislation that banned ozone-depleting substances (ODSs) such as chlorofluorocarbons (CFCs) and bromine-bearing halon gases. The importance of ozone in protecting life on Earth is introduced, followed by details on how the release of CFCs and halons led to significant stratospheric ozone losses, as first observed in the mid-1980s. The relevant chemistry in the stratosphere is presented, with particular focus on the processes responsible for severe depletion of polar ozone and modest depletion of midlatitude ozone. The Montreal Protocol and subsequent amendments are shown to have been critical in limiting the loss of ozone, particularly over heavily populated regions. The ozone layer is expected to eventually recover as the abundance of ODSs in the atmosphere declines to preindustrial levels. Although full recovery of the ozone layer is still many decades away due to the long atmospheric lifetimes of CFCs and halons, initial signs of recovery for upper stratospheric ozone are described. Finally, this chapter concludes by showing that future anthropogenic emissions of greenhouse gases actually contribute the largest source of uncertainty for projections of the thickness of Earths ozone layer by the end of the century.

The kinetics of the ClOOCl catalytic cycle: The CIOOCI Catalytic Cycle

We use simultaneous in situ observations of [ClO] and [ClOOCl] obtained in the Arctic polar vortex to evaluate the kinetics of the ClOOCl catalytic cycle. Available laboratory measurements of the ClOOCl absorption cross sections, the ClO + ClO + M reaction rate constant, and the ClO/ClOOCl equilibrium constant are considered, along with compendium evaluations of these kinetic parameters. We show that the most recent (year 2015) recommendations for the kinetics that govern the partitioning of ClO and ClOOCl put forth by the JPL panel are in extremely good agreement with the atmospheric observations of [ClO] and [ClOOCl]. Hence, we suggest that studies of polar ozone loss adopt these most recent recommendations. The most important difference with respect to calculations that rely on older recommendations is the temperature at which loss of O3 by the ClOOCl catalytic cycle terminates. The latest JPL recommendation for the equilibrium constant suggests that ClOOCl is less stable than previously assumed, resulting in an approximate 2 °C downward shift in the termination temperature of polar ozone loss due to the ClOOCl catalytic cycle. Remaining uncertainties in our knowledge of the kinetics that govern the partitioning of ClO and ClOOCl within the activated vortex, and hence the efficiency of O3 loss by the ClOOCl cycle, will be best addressed by future laboratory determinations of the absolute cross section of ClOOCl at the peak (i.e., close to a wavelength of 245 nm) as well as reduced uncertainty in the rate constant of the ClO + ClO + M reaction.