Linnea M. Avallone

Removal of Stratospheric O3 by Radicals: In Situ Measurements of OH, HO2, NO, NO2, ClO, and BrO

Simultaneous in situ measurements of the concentrations of OH, HO2, ClO, BrO, NO, and NO2 demonstrate the predominance of odd-hydrogen and halogen free-radical catalysis in determining the rate of removal of ozone in the lower stratosphere during May 1993. A single catalytic cycle, in which the rate-limiting step is the reaction of HO2 with ozone, accounted for nearly one-half of the total O3 removal in this region of the atmosphere. Halogen-radical chemistry was responsible for approximately one-third of the photochemical removal of O3; reactions involving BrO account for one-half of this loss. Catalytic destruction by NO2, which for two decades was considered to be the predominant loss process, accounted for less than 20 percent of the O3 removal. The measurements demonstrate quantitatively the coupling that exists between the radical families. The concentrations of HO2 and ClO are inversely correlated with those of NO and NO2. The direct determination of the relative importance of the catalytic loss processes, combined with a demonstration of the reactions linking the hydrogen, halogen, and nitrogen radical concentrations, shows that in the air sampled the rate of O3 removal was inversely correlated with total NOx, loading.

Transport out of the lower stratospheric Arctic vortex by Rossby wave breaking

The fine-scale structure in lower stratospheric tracer transport during the period of the two Arctic Airborne Stratospheric Expeditions (January and February 1989; December 1991 to March 1992) is investigated using contour advection with surgery calculations. These calculations show that Rossby wave breaking is an ongoing occurrence during these periods and that air is ejected from the polar vortex in the form of long filamentary structures. There is good qualitative agreement between these filaments and measurements of chemical tracers taken aboard the NASA ER-2 aircraft. The ejected air generally remains filamentary and is stretched and mixed with midlatitude air as it is wrapped around the vortex. This process transfers vortex air into midlatitudes and also produces a narrow region of fine-scale filaments surrounding the polar vortex. Among other things, this makes it difficult to define a vortex edge. The calculations also show that strong stirring can occur inside as well as outside the vortex.

Chlorine chemistry on polar stratospheric cloud particles in the Arctic winter

Simultaneous in situ measurements of hydrochloric acid (HCl) and chlorine monoxide (ClO) in the Arctic winter vortex showed large HCl losses, of up to 1 part per billion by volume (ppbv), which were correlated with high ClO levels of up to 1.4 ppbv. Air parcel trajectory analysis identified that this conversion of inorganic chlorine occurred at air temperatures of less than 196 � 4 kelvin. High ClO was always accompanied by loss of HCI mixing ratios equal to �(ClO + 2Cl2O2). These data indicate that the heterogeneous reaction HCl + ClONO2 → Cl2 + HNO3 on particles of polar stratospheric clouds establishes the chlorine partitioning, which, contrary to earlier notions, begins with an excess of ClONO2, not HCl.

Photochemical evolution of ozone in the lower tropical stratosphere

Rarely does the atmosphere allow direct observation of the photochemical evolution of ozone. In most of the troposphere and lower stratosphere this slow chemistry cannot be understood without including much larger changes caused by the circulation. Yet in the tropical stratosphere, where ozone-poor air of tropospheric origin enters and rises slowly in near isolation, it can be demonstrated that O3 is created by dissociation of O2 at a rate consistent with current theory. The parallel photolytic destruction of the unreactive source gases (for example, N2O and CFCl3) and the consequent evolution of chemically active odd-nitrogen (NOy) and chlorine (Cly) species, however, indicate a small amount of mixing of much older, photochemically aged air from the midlatitude stratosphere into this tropical plume.

In Situ Observations of Aerosol and Chlorine Monoxide After the 1991 Eruption of Mount Pinatubo: Effect of Reactions on Sulfate Aerosol

Highly resolved aerosol size distributions measured from high-altitude aircraft can be used to describe the effect of the 1991 eruption of Mount Pinatubo on the stratospheric aerosol. In some air masses, aerosol mass mixing ratios increased by factors exceeding 100 and aerosol surface area concentrations increased by factors of 30 or more. Increases in aerosol surface area concentration were accompanied by increases in chlorine monoxide at mid-latitudes when confounding factors were controlled. This observation supports the assertion that reactions occurring on the aerosol can increase the fraction of stratospheric chlorine that occurs in ozone-destroying forms.

The seasonal evolution of reactive chlorine in the Northern Hemisphere stratosphere

In situ measurements of chlorine monoxide (ClO) at mid- and high northern latitudes are reported for the period October 1991 to February 1992. As early as mid-December and throughout the winter, significant enhancements of this ozone-destroying radical were observed within the polar vortex shortly after temperatures dropped below 195 k. Decreases in ClO observed in February were consistent with the rapid formation of chlorine nitrate (ClONO2) by recombination of ClO with nitrogen dioxide (NO2) released photochemically from nitric acid (HNO3). Outside the vortex, ClO abundances were higher than in previous years as a result of NOx suppression by heterogeneous reactions on sulfate aerosols enhanced by the eruption of Mount Pinatubo.

The Concordiasi Project in Antarctica

The Concordiasi project is making innovative observations of the atmosphere above Antarctica. The most important goals of the Concordiasi are as follows: To enhance the accuracy of weather prediction and climate records in Antarctica through the assimilation of in situ and satellite data, with an emphasis on data provided by hyperspectral infrared sounders. The focus is on clouds, precipitation, and the mass budget of the ice sheets. The improvements in dynamical model analyses and forecasts will be used in chemical-transport models that describe the links between the polar vortex dynamics and ozone depletion, and to advance the under understanding of the Earth system by examining the interactions between Antarctica and lower latitudes. To improve our understanding of microphysical and dynamical processes controlling the polar ozone, by providing the first quasi-Lagrangian observations of stratospheric ozone and particles, in addition to an improved characterization of the 3D polar vortex dynamics. Techni...

Tracer‐tracer correlations: Three‐dimensional model simulations and comparisons to observations

Calculations of the stratospheric distributions of 12 trace species (N2O, CH4, CFCl3, CF2Cl2, CFCl2CF2Cl, CHF2Cl, CH3Cl, CH3CCl3, CCl4, CH3Br, CF2ClBr, and CF3Br) are performed by using the Goddard Institute for Space Studies/University of California at Irvine (GISS/UCI) three-dimensional chemistry transport model (CTM). Because each of these gases is either an important precursor of ozone-depleting radicals or a significant greenhouse molecule, it is critical that we understand their source strengths and atmospheric lifetimes. In this study, lifetimes against stratospheric loss are determined from the CTM calculations and compared with the currently accepted values. Calculated distributions of these species are compared with observations taken from aircraft platforms at midlatitudes via their correlation with N2O. The sensitivity of the calculated correlations to rate parameters, photolysis cross sections, and lower boundary conditions is explored for several key species. For most of the compounds examined the correlations can be simulated, within the uncertainty of the observations, by using current photochemistry. Finally, the use of correlation diagrams (i.e., scatterplots of one species versus another) as a tool for determining the lifetimes of trace gases on the basis of atmospheric observations is examined in the framework of the theory proposed by Plumb and Ko [1992].

Tests of halogen photochemistry using in situ measurements of ClO and BrO in the lower polar stratosphere

In situ observations of the halogen oxides ClO and BrO made from the NASA ER-2 during the Airborne Arctic Stratospheric Expedition (AASE) I and II missions are used to test current understanding of photochemical parameters. Measurements of ClO obtained during AASE I in the dark perturbed polar vortex are analyzed with respect to temperature to derive the equilibrium expression for the ClO/Cl 2 O 2 system. Assuming photochemical steady state and complete activation of chlorine (ClO + 2Cl 2 O 2 = Cly), observations of ClO made during AASE II are used to derive the photolysis rate of Cl 2 O 2 . The photolysis rate derived from atmospheric observations is compared to J values calculated with a photochemical model and various values for the absorption cross section of Cl 2 O 2 . The photolysis rate calculated with the cross section of Huder and DeMore [1995] is shown to be systematically too small, while those of Burkholder et al. [1990] and Cox and Hayman [1988] are too large to be consistent with atmospheric observations. Observations of BrO made during AASE II indicate that our understanding of the inorganic bromine budget in the polar regions is incomplete. A possible role for the adduct BrOOCI is investigated.

In situ measurements of BrO During AASE II

BrO measured from the NASA ER-2 during AASE II exhibited a mean value (for 20-minute averages) of 5.4±0.3 pptv, with a standard deviation of 3.1 pptv. Ratios of BrO to available inorganic bromine (Bry) show only slight increases in polar regions relative to midlatitudes. A comparison between observed latitudinal and diurnal variations of this same ratio and that calculated by photochemical models shows reasonable agreement in behavior, but significant discrepancies in magnitude. It is unclear whether this difference is due to errors in measurements, models or both.