Roland Neuber

Chemical depletion of Arctic ozone in winter 1999/2000

During Arctic winters with a cold, stable stratospheric circulation, reactions on the surface of polar stratospheric clouds (PSCs) lead to elevated abundances of chlorine monoxide (ClO) that, in the presence of sunlight, destroy ozone. Here we show that PSCs were more widespread during the 1999/2000 Arctic winter than for any other Arctic winter in the past two decades. We have used three fundamentally different approaches to derive the degree of chemical ozone loss from ozonesonde, balloon, aircraft, and satellite instruments. We show that the ozone losses derived from these different instruments and approaches agree very well, resulting in a high level of confidence in the results. Chemical processes led to a 70% reduction of ozone for a region ∼1 km thick of the lower stratosphere, the largest degree of local loss ever reported for the Arctic. The Match analysis of ozonesonde data shows that the accumulated chemical loss of ozone inside the Arctic vortex totaled 117 ± 14 Dobson units (DU) by the end of winter. This loss, combined with dynamical redistribution of air parcels, resulted in a 88 ± 13 DU reduction in total column ozone compared to the amount that would have been present in the absence of any chemical loss. The chemical loss of ozone throughout the winter was nearly balanced by dynamical resupply of ozone to the vortex, resulting in a relatively constant value of total ozone of 340 ± 50 DU between early January and late March. This observation of nearly constant total ozone in the Arctic vortex is in contrast to the increase of total column ozone between January and March that is observed during most years.

Temperature histories in liquid and solid polar stratospheric cloud formation

Polar stratospheric clouds (PSCs) have been observed by balloonborne backscatter sondes from Alert, Thule, Heiss Island, Scoresbysund, Sodankyla, Sondre Stromfjord, and Ny Alesund during winters 1989, 1990, 1995, and 1996 in 30 flights. The observations can be categorized into two main groups: type 1a and type 1b PSC particles. Type 1b PSCs show the characteristics expected from liquid ternary solution (HNO3/H2SO4/H2O) particles, consistent with model simulations. Type 1a PSCs are observed at all temperatures below the condensation temperature TNAT of nitric acid trihydrate (NAT), consistent with solid NAT composition. Air parcel trajectories have been calculated for all observations to provide synoptic temperature histories of the observed particles. A number of cases have been identified, where the particles have experienced temperatures close to or above the sulfuric acid tetrahydrate melting temperatures within 20 days prior to observation. This assures a knowledge of the physical phase (liquid) of the particles at this time, prior to observation. The subsequent synoptic temperature histories, between melting and the time of observation, show pronounced differences for type 1a and type 1b PSC particles, indicating the qualitative temperature conditions, necessary to generate solid type 1a PSCs. The temperature histories of type 1b particles show smoothly, in most cases monotonie, decreasing temperatures. The temperature can apparently decrease to the frost point without causing the particles to freeze. The type 1b PSC particles are mostly observed shortly after entering a cold region. The observed type 1a particles have spent several days at temperatures close to or below TNAT prior to observation, often associated with several synoptic temperature oscillations around TNAT, and the particles are observed in aged clouds. It appears that the PSC particles may freeze, if they experience synoptic temperatures below TNAT with a duration of at least 1 day, possibly accompanied by several temperature oscillations. However, liquid particles that experience a smooth cooling, even to very low temperatures, or single smooth cooling/heating below TNAT without synoptic temperature fluctuations do not seem to freeze.

Vertical ozone distribution in the marine atmosphere over the central Atlantic Ocean (56°S – 50°N)

Abstract. The vertical ozone distribution over the Atlantic Ocean has been measured in situ by shipborne ozone soundings during three RV Polarstern meridional transects in January/February 1993, October/November 1993, and May/June 1994. The low ozone column densities measured by Nimbus 7 and Meteor 3 satellites in 1993 could be confirmed by our investigations. We observed distinct differences in the vertical distribution pattern of tropospheric ozone between the northern and the southern hemisphere: The ozone mixing ratio gradients were flat in the northern hemisphere and ozone mixing ratios in the free troposphere never did exceed 80 parts per billion by volume (ppbv) up to the tropopause, while the southern hemisphere exhibited a pronounced vertical gradient. Extremely dry air masses with enhanced ozone amounts up to 120 ppbv have been found in the tropical free troposphere of the southern hemisphere between 0° and 20°S. The vertical ozone stratification in the troposphere of the southern hemisphere was dominated by this large-scale feature. Photochemical ozone production as a consequence of the emissions of natural fires or intrusions of stratospheric air masses are the most probable sources for these ozone-rich layers. On the basis of our results, a stringent differentiation between both alternatives could not be given.

Multiwavelength lidar measurements of stratospheric aerosols above Spitsbergen during winter 1992/93

Within the period of December 1992 to March 1993 lidar investigations of stratospheric aerosols were performed at Ny-Alesund, Spitsbergen (79°N, 12°E). Backscatter signals at wavelengths of 353, 532 and 1064 nm and depolarization signals at 532 nm in altitudes ranging from the tropopause up to 30 km were analyzed. Throughout the whole measurement period we observed an aerosol layer of volcanic origin in the lower stratosphere. Depolarization profiles suggest that the volcanic aerosol layer consisted mainly of liquid droplets. Comparison with model calculations indicate median particle radii between 0.1 and 0.2 µm. Surface densities exceeded approximately 40 µm²/cm³ in the lower part of the layer around 12 km. In January 1993 polar stratospheric clouds (PSCs) were frequently observed at altitudes up to 22 km. We analyzed the backscatter and depolarization data with respect to the temperature history of backward trajectories reaching Ny-Alesund. Signatures for micron sized crystalline PSC particles appear for cooling rates of −1 to −3 K day−1. Larger cooling rates of −4 to −10 K day −1 produced submicron sized aerosols, presumably supercooled droplets, characterized by enhanced backscatter ratios and reduced depolarization.

Aircraft lidar observations of an enhanced type Ia polar stratospheric clouds during APE-POLECAT

Polar stratospheric clouds (PSCs) which do not fit into the standard type Ia/Ib scheme were measured by the airborne lidar OLEX (Ozone Lidar Experiment) on board the Deutsches Zentrum fur Luft- und Raumfhart (DLR) Falcon during the Airborne Polar Experiment and Polar stratospheric clouds, Leewaves, Chemistry Aerosol and Transport (APE-POLECAT) campaign. In contrast, the standard classification is satisfied by almost all observations for four winters at Ny Alesund, Spitsbergen, which is one of the most comprehensive data sets of ground station lidar measurements presently available. The cloud observed by the Falcon south of Spitsbergen on December 31, 1996, was a 400-km long type I cloud with backscatter ratio S = 2.5 and aerosol depolarization δA = 15%, which is clearly distinct from the Ny Alesund 4 year record. Using a combination of microphysical and optical modeling, we investigate the possible evolution of this cloud assuming either in situ freezing of ternary HNO3/H2SO4/H2O droplets as nitric acid trihydrate, or the formation of the clouds in mountain waves over the east coast of Greenland, as suggested by a mountain wave model. Best agreement with the observations was obtained by assuming mountain-wave-induced cloud formation, which yields nitric acid trihydrate particles with much higher total mass than achieved by assuming synoptic-scale freezing. Our analysis suggests that this rare type of PSC, which we term type Ia-enh, is characterized by nitric acid hydrate particles rather close to thermodynamic equilibrium, while the more common type Ia PSCs appear to contain much less mass than representative of equilibrium.

Ozone differential absorption lidar algorithm intercomparison

An intercomparison of ozone differential absorption lidar algorithms was performed in 1996 within the framework of the Network for the Detection of Stratospheric Changes (NDSC) lidar working group. The objective of this research was mainly to test the differentiating techniques used by the various lidar teams involved in the NDSC for the calculation of the ozone number density from the lidar signals. The exercise consisted of processing synthetic lidar signals computed from simple Rayleigh scattering and three initial ozone profiles. Two of these profiles contained perturbations in the low and the high stratosphere to test the vertical resolution of the various algorithms. For the unperturbed profiles the results of the simulations show the correct behavior of the lidar processing methods in the low and the middle stratosphere with biases of less than 1% with respect to the initial profile to as high as 30 km in most cases. In the upper stratosphere, significant biases reaching 10% at 45 km for most of the algorithms are obtained. This bias is due to the decrease in the signal-to-noise ratio with altitude, which makes it necessary to increase the number of points of the derivative low-pass filter used for data processing. As a consequence the response of the various retrieval algorithms to perturbations in the ozone profile is much better in the lower stratosphere than in the higher range. These results show the necessity of limiting the vertical smoothing in the ozone lidar retrieval algorithm and questions the ability of current lidar systems to detect long-term ozone trends above 40 km. Otherwise the simulations show in general a correct estimation of the ozone profile random error and, as shown by the tests involving the perturbed ozone profiles, some inconsistency in the estimation of the vertical resolution among the lidar teams involved in this experiment.

Arctic Study of Tropospheric Aerosol and Radiation (ASTAR) 2000: Arctic haze case study

The ASTAR 2000 (Arctic Study of Tropospheric Aerosol and Radiation) campaign ran from 12 March until 25 April 2000 with extensive flight operations in the vicinity of Svalbard (Norway) from Longyearbyen airport (78.25°N, 15.49°E). It was a joint Japanese (NIPR Tokyo)–German (AWI Bremerhaven/Potsdam) airborne measurement campaign using AWI aircraft POLAR 4 (Dornier 228-101). Simultaneous ground-based measurements were done at the international research site Ny-Ålesund (78.95°N, 11.93°E) in Svalbard, at the German Koldewey station, at the Japanese Rabben station and at the Scandinavian station at Zeppelin Mountain (475 m above sea level). During the campaign 19 profiles of various aerosol properties were measured. In general, the Arctic spring aerosol in the vicinity of Svalbard had significant temporal and vertical variability. A strong haze event occurred between 21 and 25 March in which the optical depth from ground-based observation was 0.18, which was significantly greater than the background value of 0.06. Airborne measurements on 23 March during this haze event showed a high aerosol layer with an extinction coefficient of 0.03 km−1 or more up to 3 km and a scattering coefficient from 0.02 in the same altitude range. From the chemical analyses of airborne measurements, sulfate, soot and sea salt particles were dominant, and there was a high mixing ratio of external soot particles in some layers during the haze event, whereas internal mixing of soot in sulfate was noticeable in some layers for the background condition. We argue that the high aerosol loading is due to direct transport from anthropogenic source regions. In this paper we focus on the course of the haze event in detail through analyses of the airborne and ground-based results.

Nonequilibrium coexistence of solid and liquid particles in Arctic stratospheric clouds

Observations of polar stratospheric clouds (PSCs) from Ny Alesund, Spitsbergen, have been examined to quantify the occurrence of solid particles. The polarized backscatter ratio was found to be a more sensitive indicator of the presence of non-spherical (solid) particles in a PSC than the aerosol depolarization, which approaches zero for clouds containing a large volume of liquid droplets. The analysis corroborates our previous finding that type Ia PSCs cannot be composed of nitric acid trihydrate particles in equilibrium with the gas phase, which would lead to a much too high backscatter ratio. Conversely, type Ib PSCs, previously thought to be composed solely of liquid droplets, often contain a very small fraction of solid particles. These clouds develop a high parallel backscatter, arising from Mie scattering by droplets, and a nonvanishing perpendicular signal due to the few solid particles. Other type Ib PSCs appear to contain only liquid particles. The mixed liquid/solid clouds are observed on the smallest spatial and temporal scale resolvable by the lidar instrument (30 m, l min), implying that their properties are not a result of spatial averaging of different cloud types. Comparison of the observations with optical calculations shows that such nonequilibrium particle distributions are to be expected, as temperature changes are sufficiently rapid to prevent the particles from assuming equilibrium sizes. The observed optical characteristics of type Ia and type Ib clouds can be reproduced in a model by assuming that a very small fraction of the particles are composed of nitric acid hydrate, with the majority being binary H2SO4/H2O or ternary HNO3/H2SO4/H2O droplets.

Temperature dependence of ternary solution particle volumes as observed by lidar in the Arctic stratosphere during winter 1992/1993

Multiwavelength lidar measurements of stratospheric aerosols performed at the Arctic Network for the Detection of Stratospheric Change station on Spitsbergen during winter 1992/1993 are analyzed. Altitude profiles of particle median radius and volume density are derived for measurements with aerosol depolarization smaller than 0.01. Below an altitude corresponding to 450 K potential temperature the Pinatubo aerosol layer dominated the stratospheric aerosol content with volume densities of more than 5 μm 3 cm -3 , whereas above 450 K, volume densities were close to background values of 0.1 μm 3 cm -3 . However, at all altitude levels between 350 and 550 K, volume densities consistently increased by a factor of 2-30 when temperatures approached the frost point. The observations are compared to results from thermodynamic model calculations at altitude levels of 400, 440, and 480 K. Good agreement between the observed and theoretically derived temperature dependencies of volume density suggests that nondepolarizing polar stratospheric cloud particles, as well as volcanic aerosols, at low temperatures are composed of a ternary liquid solution of sulfuric and nitric acid. At all altitude levels, model results indicated more than 90% HNO 3 gas phase depletion as temperatures approached the frost point. A mean profile of total H 2 SO 4 volume mixing ratio is derived, decreasing from about 4 parts per billion by volume (ppbv) at 350 K to about 0.5 ppbv above 450 K.

Polar stratospheric cloud threshold temperatures in the 1995-1996 arctic vortex

Balloon-borne backscattersondes have been used to study the relationship between particle scattering and ambient temperature near the vertical edge of arctic polar stratospheric clouds (PSCs) as well as to delineate the cloud type occurrence probability as a function of temperature. The observed typical threshold temperatures as a function of altitude are about1°K warmer than the temperature TSTS expected for rapid growth of supercooled ternary solution aerosols. A more descriptive analysis shows that the threshold temperatures occur over a definable range of temperatures and tend to cluster near, but somewhat warmer than, TSTS. Considering the experimental and theoretical uncertainties, this difference may not be significant. The probability of type Ib PSC occurrence shows a dramatic increase at TSTS±1°K, while for type Ia PSCs the probability is roughly constant at 10% for temperatures below the formation point of nitric acid trihydrate (TNAT).