ALBERT

Description: Simultaneous in situ measurements of stratospheric ClO and HCl have been made for the first time, during numerous flights of the ER-2 aircraft covering latitudes 24-90 deg N from October 1991 through March 1992. The ClO/HCl ratio is identified as a key indicator of heterogeneous processing both outside and within the Arctic polar vortex. For ClO mixing ratios below about 120 pptv, remarkably constant ClO/HCl values of about 15% characterize the lower stratosphere. The observed values are significantly higher than those derived from a 2-D model using either gas phase photochemistry alone (2%), or including heterogeneous sulfate chemistry (5-10%). During the Arctic early spring, after conversion of HCl into reactive chlorine has taken place, the vortex edge is poorly defined by ClO levels. Loss of HCl and its slow recovery following low-temperature polar heterogeneous chemistry distinguishes HCl as a new and unique dynamical tracer of polar stratospheric clouds (PSC)-processed air.

Description: Back trajectory analysis of Arctic and Antarctic aircraft data reveals that high ClO concentrations are associated with predicted polar stratospheric cloud (PSCs) encounters. The ClO concentrations within the Arctic and Antarctic polar vortices vary widely but appear to be inversely related to parcel solar exposure since the last PSC interaction. These results imply that production of NO(x) from HNO3 photolysis and reaction with OH is the mechanism for the loss of chlorine radicals through the reformation of chlorine nitrate. Highly denitrified air parcels show no change in ClO with solar exposure. The recovery process is quantitatively duplicated using a model of chemistry along trajectories. Although PSC processing is the primary mechanism for producing elevated ClO amounts, back trajectories apparently unperturbed by PSC's also show slightly elevated ClO levels in 1992 compared to Arctic 1989 and Antarctic 1987 measurements presumably due to the presence of Pinatubo aerosol.

Description: Measurements of lower stratospheric ClO taken during a NASA ER-2 flight between Moffett Field, CA (37 N, 122 W) and Great Slave Lake, Canada (61 N, 116 W) on 13 February 1988 are reported. Northbound, the aircraft was flown at about 20 km altitude from 39 N to 56 N, at 18 km from there to 58 N, in a descent to 15 km at 60 N, and in a rise and turn at the northernmost point. The southbound leg was flown in a gradual climb from 20 km to 21.5 km. On this day, the central position of the Arctic polar vortex, as determined by an NMC analysis of heights and temperatures at the 50 mb and 70 mb levels, was approximately 79 N, 100 W. Because the vortex was located on the North American side of the pole, the aircraft was able to reach a point slightly inside the maximum horizontal wind region where wind speeds were 80 to 90 knots. The general pattern for the observed ClO is that it increased with both latitude and altitude, and attained a maximum of about 55 pptv at 61 N latitude and 20.5 km altitude. This value is about 20 times smaller than the maxima observed over Antarctica, but is comparable to those seen just outside the chemical containment vessel located inside the Antarctic Polar vortex. On the other hand, in a comparison with northern midlatitude data taken on this and three other February flights, ClO mixing ratios observed north of 55 N latitude are 2 to 5 times larger at all flight altitudes (15 to 20 km). Possible reasons are discussed for this enhancement over midlatitude and the evidence is considered for whether or not the instruments sampled Arctic polar vortex air. A second feature of the data is the strong positive correlation between ClO and O3 during the entire flight.

Description: Bromine monoxide was observed in situ at approximately 18 km altitude during nine flights of the NASA ER-2 aircraft from Punta Arenas, Chile (54 altitude) to 72 S latitude over the Palmer Peninsula, Antarctica. The first flight for the BrO detection system was on 28 August. Here, the results from the flights over Antarctica and from the ferry flights from Punta Arenas to Moffett Field, CA (37 N latitude are reported. A key question concerning BrO, then, is how it is distributed with respect to the chemical containment vessel defined by elevated ClO mixing ratios. This question is answered with greatest statistical significance if the data are averaged into five regions: outside the vessel, aircraft heading south; inside the vessel on the same potential temperature surface; in the dive region; inside the vessel on a given potential temperature surface, aircraft heading north; and outside the vessel on the same surface. The result is that the BrO distribution inside the chemical containment vessel was different from that found outside. Inside, the BrO mixing ratio was (5.0 plus or minus 1.1) pptv between the 400 K and 460 K potential temperature surfaces, decreasing only slightly with potential temperature, and was less than 3.6 pptv below the 4 00 K surface. The abundance of BrO inside the chemical containment vessel showed no discernible temporal trend during the course of the nine flights. Outside the vessel, the BrO mixing ratio was (4.7 plus or minus 1.3) pptv near the 450 K surface, but decreased to (2.8 plus or minus 1.0) pptv near the 420 K surface.

Description: Results from a series of 12 ER-2 aircraft flights into the Antarctic polar vortex are summarized. These in situ data define the spatial and temporal distribution of ClO as the aircraft flew at an altitude of approx. 18 km from Punta Arenas (54 deg S latitude) to the base of the Palmer Peninsula (72 deg S latitude), executed a rapid descent to approx. 13 km, turned north and climbed bach to approximately 18 km, returning to Punta Arenas. A general pattern in the ClO distribution is reported: mixing ratios of approximately 10 ppt are found at altitude in the vicinity of 55 deg S increasing to 50 ppt at 60 degrees S. In the vicinity of 65 deg S latitude a steep gradient in the ClO mixing ratio is observed. At a fixed potential temperature, the ClO mixing ratio through this sharp transition increases by an order of magnitude within a very few degrees of latitude, thus defining the edge of the chemical containment vessel. From the edge of that containment vessel to the southern extension of the flights, 72 deg S, a dome of slowly increasing ClO best describes the distribution. Conclusion are drawn from the data.

Description: Results are presented for an experimental application of the laminar flow-through/resonance technique, which allows direct observations of the absolute concentration of atoms and radicals in the earth's upper atmosphere. Atomic resonance fluorescence was used in conjunction with a parachute-borne flow-through module deployed from a balloon at a height of 40 km to determine the concentration of ground-state atomic oxygen at 40 to 25 km in the atmosphere. The equipment is described, and the measurements and experimental uncertainties are plotted. It is shown that the observed absolute concentration of ground-state atomic oxygen is similar to that predicted by model calculations, and it is concluded that this technique provides a simple and direct means of measuring atomic and radical concentrations in the part-per-trillion range in unperturbed stratospheric samples.

Description: In order to test photochemical theories linking chlorofluorocarbon derivatives to O3 depletion at high latitudes in the springtime, several related atmospheric species, including O3, ClO, and BrO were measured in the lower stratosphere. The flight path extended to the center of the polar jet associated with but outside of the Arctic vortex, in which the abundance of O3 was twice its midlatitude value, whereas BrO levels were five parts per trillion (pptv) by volume between 18 and 21 km, and 2.4 pptv below that altitude. The ClO mixing ratio was as much as 65 pptv at 60 N latitude at an altitude of 20 km, and was enhanced over midlatitude values by a factor of three to five at altitudes above 18 km and by as much as a factor of 40 at altitudes below 17 km. Levels of ClO and O3 were highly correlated on all measured distance scales, and both showed an abrupt change in character at 54 N latitude. The ClO abundance north of 54 N was probably caused by low NO2 levels in the flight path.

Description: Balloon profiles of chlorine monoxide (ClO), nitric oxide (NO), and ozone (O3) were measured on March 11, 1992 from 100 to 10 mb over Greenland (67.0 deg N, 50.6 deg W). Measurements from SAGE II indicate that the aerosol surface area in the region was enhanced by sulfur from the eruption of Mt. Pinatubo, reaching 50 times background near 20 km. Concentrations of ClO were enhanced and concentrations of NO were suppressed relative to low aerosol conditions consistent with the effects of hydrolysis of N2O5 on the surface of sulfuric acid aerosols. The data are consistent with a value of 2 x 10(exp -4) for the reaction probability of the heterogeneous hydrolysis of ClONO2, indicating a minor role for this reaction at as temperature of 220 K. At these temperatures, we find no evidence for the catastrophic loss of ozone predicted to occur under conditions of enhanced aerosol surface area.

Description: The spatial and temporal distributions of ClO in the latitude belt from 54 to 72 deg S during the later stages of austral winter were obtained from measurements aboard the ER-2 aircraft flying at the altitude of about 18 km into the Antarctic polar vortex. Mixing ratios of about 10 parts per trillion by volume (pptv) were found in the vicinity of 55 deg S, increasing to 50 pptv at 60 deg S. A steep gradient of the ClO mixing ratio occurred in the vicinity of 65 deg S, where, at a fixed potential temperature, the value of the ClO mixing ratio increased by an order of magnitude within a very few degrees of latitude, defining the edge of the 'chemically perturbed region' (CPR). At the southern extension of the flight track, peak mixing ratios increased sharply from 800 pptv on August 23 to 1100 pptv in early September, and then increased more slowly to 1200 pptv on September 22. This corresponds to enhancements 500 times the typical mid-latitude values.

Description: Observations of ClO from 20 to 60 deg N made before and after the eruption of Mt. Pinatubo are compared for changes which may result from increased sulfate aerosol surface area. Using ozone as a vertical coordinate and examining data at similar latitudes and seasons, elevated abundances of ClO are found at low latitudes (20 to 30 deg N), an effect which decreases with increasing latitude. For the flights compared, there appear to be no differences, within the uncertainty of the measurements, at latitudes poleward of 40 deg N. These results are consistent with the idea that the hydrolysis of N2O5 on sulfate aerosols becomes saturated at moderate aerosol loadings.