Processes of long-term relaxation of stratospheric aerosol layer in Northern Hemisphere midlatitudes after a powerful volcanic eruption

Multiyear lidar measurements of characteristics of stratospheric aerosol layer, made at midlatitude observatories in Tomsk (56.5°N, 85.0°E) and Minsk (53.9°N, 27.5°E), are analyzed and used to study the processes of long-term relaxation of the aerosol-perturbed stratosphere after powerful volcanic eruptions to background state. The absence of significant seasonal variations of vertical stratification of stratospheric aerosol and exponential altitudinal decrease of aerosol backscattering coefficient are proposed as criteria of background state of stratospheric aerosol layer for Northern Hemisphere midlatitudes.     

Monitoring of ozone in a marine environment in Tenerife (Canary Islands)

In the Aguere Valley (in the oceanic boundary layer at Tenerife, 28°N, 16°W, 580 m a.s.l.) the ozone levels were monitored for ambient air quality assessment. Although precursors are emitted in this area, the strong correlation between ozone levels and wind velocity indicates that ozone is transported into the valley from the ocean. The inland ozone supply along the valley is induced by an orographic channelling effect of the northern oceanic air masses. The highest ozone concentrations are mostly recorded during the nocturnal stage under the influence of fresh oceanic air masses, and during high wind speed events. The seasonal cycle is characterised by elevated ozone mixing ratios in the spring (nighttime levels >45 ppbv) and low mixing ratios in the summer (nighttime levels in the range 20–35 ppbv). Back-trajectory analysis shows that the ozone monitored in the Aguere Valley is associated with long-range transport processes. High ozone events in the spring are associated with transport from upper tropospheric levels, both over the North Atlantic-high latitudes (>45°N) and Europe. This downward transport was observed in the western edge of upper tropospheric cyclones, which suggests that the upper tropospheric/low stratospheric ozone sources play a significant role. In summer, ozone is mainly transported from the North Atlantic-high latitudes (>45°N) and from mid- to low-tropospheric levels. In autumn and winter, the high ozone concentrations are transported from sources located a few km above the North Atlantic-high latitudes (>45°N) and over Europe. The Central-North Atlantic (<45°N) and North Africa are not significant sources of ozone. The high spring and lower summer ozone events in the Aguere Valley agree with other North Atlantic ozone observation in the oceanic boundary layer. However, this behaviour contrasts with the high ozone events frequently recorded at Izaña BAPMoN station (located in the free troposphere in Tenerife) during the summer, which have been attributed in the literature to downward transport from upper levels. An intensification of the inversion layer that separates the oceanic boundary layer of the free troposphere during the summer in Canary Islands is interpreted as the cause of this different behaviour between ozone in the Aguere Valley and Izaña BAPMoN station.     

Simulations of stratospheric to tropospheric transport during the tropical cyclone Marlene event

Enhanced ozone values observed in the upper troposphere near intense tropical cyclones have raised the question of the role of stratospheric–tropospheric exchange. The dynamical mechanisms involved in the enhanced ozone values of 6 April 1995 observed at Reunion and associated with the tropical cyclone Marlene could not be explained by ECMWF meteorological analysis with 1.125° horizontal resolution. A previous study based on the ECHAM model has demonstrated the impact of biomass burning, but of limited amplitude (<60–80 ppbv max). In this paper, the upper tropospheric ozone enhancement on the periphery of Marlene has been studied with a mesoscale model (MESO-NH). This model is able to reproduce a stratospheric PV filament into the troposphere, crossing the isentropes to the 350 K level. The ageostrophic circulation associated with divergence zones that have induced vertical movements has been shown. Further, the influence of vertical wind shear, evident in both the mesoscale analysis and in the idealized HURRICANE tropical cyclone model, also contributes to our understanding of this downward transport process.     

A lagrangian approach to analyse the tropospheric ozone climatology in the tropics: Climatology of stratosphere–troposphere exchange at Reunion Island

Sixteen years of ozone measurements (1992–2006) at Reunion Island (21°S, 55.5°E) have been processed to detect stratospheric signatures on each single ozone profile.The characterisation method consists in the advection of the potential vorticity (PV) over two to ten days of backtrajectory with the lagrangian trajectory code LACYTRAJ. LACYTRAJ is a Trajectory-Reverse Domain Filling code using the ERA40 ECMWF database and allowing the reconstruction of high resolution advected PV profiles. Correlation between high values of ozone mixing ratio and high PV is interpreted as a stratospheric signature.A climatology of STE events at Reunion has been derived and reveals that STE events occur more frequently during spring (SON) and summer (DJF). The method is tested for a set of PV threshold values (i.e. 1 PVU, 1.5 PVU and 2 PVU) and for a set of duration of backtrajectories (i.e. 2 days, 5 days and 10 days). The number of detected STE is sensitive to PV threshold values and duration criterions. For instance, the number of stratospheric intrusions detected in October with a 1.5 PVU criterion ranges between 25% (2 days of backtrajectories) and 56% (10 days of backtrajectories). The vertical distributions of STE show intrusions covering the whole free troposphere (between 7 and 15 km) and mainly located in the upper troposphere.Finally, results show that an important number of stratospheric intrusions are detected during spring and in the upper troposphere what points at the contribution of the stratospheric source to the tropospheric ozone spring maximum which is strongly influenced by the biomass burning emissions from South Africa and Madagascar.     

The stratospheric ozone layer-an overview

This paper summarises the knowledge on the properties of the stratospheric ozone layer. Dynamic, chemical, and microphysical aspects are reviewed with emphasis on chemistry. The questions addressed are as follows. Do we have a quantitative understanding of the Antarctic ozone hole? What lies behind the trend of slowly decreasing ozone columns over northern mid-latitudes? To what degree was chemistry responsible for the extremely low ozone levels over northern Europe in January 1992? The discovery of the ozone hole in 1985 exposed scientific neglect of the category of fast heterogeneous reactions taking place on particulate matter in the stratosphere. But even now after the wide acceptance of some heterogeneous reactions it is difficult to fully account for the rate at which Antarctic ozone is depleted each year in August. After reviewing the known heterogeneous reactions, possible hitherto unrecognised mechanisms are briefly outlined. The paper also includes a discussion of the chemical reactions which can occur even under relatively warm conditions on the ubiquitous, stratospheric aerosol particles and which could contribute to the observed mid-latitudinal ozone depletion. Finally, the paper underlines the importance of dynamic processes, that is, horizontal transport and vertical adiabatic motion, which appear to be the main cause of the anomalously low northern hemispheric ozone values during the 1991/1992 winter.     

Ozone and meteorological boundary-layer conditions at Summit,Greenland, during 3–21 June 2000

The temporal and spatial distributions of boundary-layer ozone were studied during June 2000 at Summit, Greenland, using surface-level measurements and vertical profiling from a tethered balloon platform. Three weeks of continuous ozone surface data, 133 meteorological vertical profile data and 82 ozone vertical profile data sets were collected from the surface to a maximum altitude of 1400 m above ground.The lower atmosphere at Summit was characterized by the prevalence of strong stable conditions with strong surface temperature inversions. These inversions reversed to neutral to slightly unstable conditions between ∼9.00 and 18.00 h local time with the formation of shallow mixing heights of ∼70–250 m above the surface.The surface ozone mixing ratio ranged from 39 to 68 ppbv and occasionally had rapid changes of up to 20 ppb in 12 h. The diurnal mean ozone mixing ratio showed diurnal trends indicating meteorological and photochemical controls of surface ozone. Vertical profiles were within the range of 37–76 ppb and showed strong stratification in the lower troposphere. A high correlation of high ozone/low water vapor air masses indicated the transport of high tropospheric/low stratospheric air into the lower boundary layer. A ∼0.1–3 ppb decline of the ozone mixing ratio towards the surface was frequently observed within the neutrally stable mixed layer during midday hours. These data suggest that the boundary-layer ozone mixing ratio and ozone depletion and deposition to the snowpack are influenced by photochemical processes and/or transport phenomena that follow diurnal dependencies. With 37 ppb of ozone being the lowest mixing ratio measured in all data no evidence was seen for the occurrence of ozone depletion episodes similar to those that have been reported within the boundary layer at coastal Arctic sites during springtime.     

Measurements of trace gases in the tropical tropopause layer

A unique dataset of airborne in situ observations of HCl, O₃, HNO₃, H₂O, CO, CO₂ and CH₃Cl has been made in and near the tropical tropopause layer (TTL). A total of 16 profiles across the tropopause were obtained at latitudes between 10°N and 3°S from the NASA WB-57F high-altitude aircraft flying from Costa Rica. Few in situ measurements of these gases, particularly HCl and HNO₃, have been reported for the TTL. The general features of the trace gas vertical profiles are consistent with the concept of the TTL as distinct from the lower troposphere and lower stratosphere. A combination of the tracer profiles and correlations with O₃ is used to show that a measurable amount of stratospheric air is mixed into this region. The HCl measurements offer an important constraint on stratospheric mixing into the TTL because once the contribution from halocarbon decomposition is quantified, the remaining HCl (>60% in this study) must have a stratospheric source. Stratospheric HCl in the TTL brings with it a proportional amount of stratospheric O₃. Quantifying the sources of O₃ in the TTL is important because O₃ is particularly effective as a greenhouse gas in the tropopause region.     

The extraordinary events of the major,sudden stratospheric warming,the diminutive antarctic ozone hole,and its split in 2002

GOAL, SCOPE AND BACKGROUND: Great interest in the unprecedented events of the major, sudden stratospheric warming and the ozone hole split over Antarctica in September 25, 2002 motivates a necessity to analyze the current understanding on the dynamics, chemistry and climate impacts that are associated with both events. METHODS: Significant progress in the analysis of the observational data obtained, as well as successful development and application of dynamical modeling, which have been achieved very recently, create a basis for the first survey on the role of the major, sudden stratospheric warming observed in the southern hemisphere and its relationship to the diminutive Antarctic ozone hole and its break up into two parts. RESULTS AND DISCUSSION: Special attention has been paid to assessments of the causes of the major warming event and the future expectations concerning the stratospheric ozone depletion effect. Among the principal results is the fact that, as the polar vortex elongated, it became hydrodynamically unstable, and this insta-, bility affected the upper troposphere and stratosphere. During the major, sudden stratospheric warming, the middle stratospheric vortex split into two pieces; one piece rapidly mixed with extra vortex air, while the other returned to the pole as a much weaker and smaller vortex. The polar night jet was considerably weaker than normal, and was displaced more poleward than has been observed in previous winters, resulting from a series of wave events (propagated from the troposphere) that took place over the course of the winter. Finally, the relative ozone decrease (increase) in the eastern Antarctic is tightly associated with westerly (easterly) zonal wind anomalies near the southern tip of South America, and the unusual behavior of the ozone hole in 2002 therefore appears to be caused by great easterlies in this region. CONCLUSIONS: The main conclusion is that the southern polar vortex and the diminutive ozone hole split into two parts in September 2002, due to the prevalence of very strong planetary waves, led to the appearance of a major, sudden stratospheric warming. Although there is evidence that sea surface temperature anomalies contributed to the excitation of the quite strong planetary waves over Antarctica in 2002, there is not yet a widely approved mechanism supporting that. RECOMMENDATIONS AND OUTLOOK:The appearance of the near-record size of the 2003 ozone hole confirmed that the 'no-ozone-hole' episode observed in the year 2002 does not denote a recovery of the ozone layer. Despite the current successful attempts to get a sufficient understanding for the genesis of both extraordinary events, more observations and further modeling efforts are necessary to more reliably assess the contribution of various dynamic mechanisms to the recently observed tropo-stratospheric surprises.     

Tethered balloon-based soundings of ozone,aerosols, and solar radiation near Mexico City during MIRAGE-MEX

A tethered balloon sampling system was used to measure vertical profiles of ozone, particles, and solar radiation in the atmospheric boundary layer on the northern edge of Mexico City, in March 2006 as part of the Megacity Impact on Regional and Global Environment-Mexico experiment. Several commercial sensors, designed for surface applications, were deployed on a tethered balloon platform.Profiles indicate that for these 3 scalars the boundary layer (surface up to 700 m) was well mixed in the period 10:00–16:00 LST. Good agreement was observed for median surface and balloon ozone and particle number concentrations. For most profiles, the surface deposition of ozone was not significant compared to median profile concentrations. Particle number concentration (0.3, 0.5, 1.0 and 5.0 μm) also showed little variation with attitude. Radiatprofiles showed a monotonic increase in diffuse radiation from the maximum altitude of profiles to the surface. Consequently, it was inferred that surface measurements of these likely were representative of lower boundary layer values during this time period.     

The southern hemisphere ozone hole split in 2002

Among the most important aspects of the atmospheric pollution problem are the anthropogenic impacts on the stratospheric ozone layer, the related trends of the total ozone content drop and the solar ultraviolet radiation enhancement at the Earth's surface level. During September 2002, the ozone hole over the Antarctic was much smaller than in the previous six years. It has split into two separate holes, due to the appearance of sudden stratospheric warming that has never been observed before in the southern hemisphere. The analysis of this unprecedented event is attempted, regarding both the meteorological and photochemical aspects, in terms of the unusual thermal field patterns and the induced polar vortex disturbances.     

Changes in seasonal and diurnal cycles of ozone and temperature in the eastern U.S.

The pollutant tropospheric ozone causes human health problems, and environmental degradation and acts as a potent greenhouse gas. Using long-term hourly observations at five US air quality monitoring surface stations we studied the seasonal and diel cycles of ozone concentrations and surface air temperature to examine the temporal evolution over the past two decades. Such an approach allows visualizing the impact of natural and anthropogenic processes on ozone; nocturnal inversion development, photochemistry, and stratospheric intrusion. Analysis of the result provides an option for determining the duration for a regulatory ozone season. The application of the method provides independent confirmation of observed changes and trends in the ozone and temperature data records as reported elsewhere. The results provide further evidence supporting the assertion that ozone reductions can be attributed to emission reductions as opposed to weather variation. Despite a (～0.5 °C decade^?1) daytime warming trend, ozone decreased by up to 6 ppb decade^?1 during times of maximum temperature in the most polluted locations. Ozone also decreased across the emission reduction threshold of 2002 by 6–10 ppb indicating that emission reductions have been effective where and when it is most needed. Longer time series, and coupling with other data sources, may allow for the direct investigation of climate change influence on regional ozone air pollution formation and destruction over annual and daily time scales.     

What is causing high ozone at Summit,Greenland?

Causes for the unusually high and seasonally anomalous ozone concentrations at Summit, Greenland were investigated. Surface data from continuous monitoring, ozone sonde data, tethered balloon vertical profiling data, correlation of ozone with the radionuclide tracers ⁷Be and ²¹⁰Pb, and synoptic transport analysis were used to identify processes that contribute to sources and sinks of ozone at Summit. Northern Hemisphere (NH) lower free troposphere ozone mixing ratios in the polar regions are ∼20 ppbv higher than in Antarctica. Ozone at Summit, which is at 3212 m above sea level, reflects its altitude location in the lower free troposphere. Transport events that bring high ozone and dry air, likely from lower stratospheric/higher tropospheric origin, were observed ∼40% of time during June 2000. Comparison of ozone enhancements with radionuclide tracer records shows a year-round correlation of ozone with the stratospheric tracer ⁷Be. Summit lacks the episodic, sunrise ozone depletion events, which were found to reduce the annual, median ozone at NH coastal sites by up to ∼3 ppbv. Synoptic trajectory analyses indicated that, under selected conditions, Summit encounters polluted continental air with increased ozone from central and western Europe. Low ozone surface deposition fluxes over long distances upwind of Summit reduce ozone deposition losses in comparison to other NH sites, particularly during the summer months. Surface-layer photochemical ozone production does not appear to have a noticeable influence on Summit's ozone levels.     

Ozone recovery as seen in perspective of the Dobson spectrophotometer measurements at Belsk (52°N, 21°E) in the period 1963–2008

The total ozone monthly means derived from measurements by the Dobson spectrophotometer at Belsk (52°N, 21°E) and satellite observations over central and midlatitudinal Europe are analyzed for the long-term changes between 1995 and 2008. Standard explanatory variables representing physical and chemical processes known to influence the ozone distribution are considered. The potential proxies are: atmospheric loading by the ozone depleting substances characterized by the equivalent effective stratospheric chlorine (EESC) time series, various drivers of ozone dynamical variability including solar cycle, teleconnection patterns, temperature at 50 hPa, and pressure at the tropopause level. The multivariate adaptive regression splines (MARS) methodology is used to find optimal set of the explanatory variables and shape of the anthropogenic trend curve. Following options for the trend curve are examined: proportional to EESC, piecewise linear (with the turning points in 1980 and 1995), and selected from a smooth curve fit to the total ozone time series having “natural variations” removed. Statistical estimates and their uncertainties are calculated using block bootstrapping. The analyses indicate that ozone over Belsk, in central Europe, and in midlatitudinal Europe reaches at least first stage of recovery as defined by the World Meteorological Organization: a statistically significant reduction in the rate of decline. Model using the EESC time series as a proxy for the anthropogenic trend pattern yields even the second stage, i.e., a positive trend in time series that remains after removal dynamical signal from the analyzed data. Substantial seasonal dependent long-term ozone oscillations by the dynamical drivers are revealed causing estimation of the ozone recovery time even more uncertain.     

Impact of vertical transport processes on the tropospheric ozone layering above Europe.: Part II: Climatological analysis of the past 30 years

Using the set of multivariate criteria described in a companion paper, ozone-rich layers detected in tropospheric soundings are clustered according to their stratospheric or boundary layer origin. An additional class for aged tropospheric air masses is also considered. This analysis is exclusively based on the measured physical properties of the layers. The database includes 27,000 ozone profiles collected above 11 European stations—two of which provide measurements since 1970. The seasonal cycle of the tropospheric ozone stratification exhibits a clear summer maximum. This increase is due to aged tropospheric air masses that are more frequently detected, suggesting an enhanced lifetime of layers in summer. In terms of ozone content, the relative impact of stratospheric ozone compared to the other sources is highest in winter while export from the boundary layer presents a uniform seasonal cycle. Altitude and thickness distributions of the layers are consistent with the dynamical processes involved in the layering. Northernmost and southernmost stations are more exposed to stratospheric air intrusions into the free troposphere. Long-term trends show that transport from the tropopause region has increased since the mid 1980s. This trend being concomitant with lower ozone content of such layers, a moderate trend of the transport efficiency from the stratosphere on total tropospheric ozone is observed. The increase of ozone detected in tropospheric layers since the mid 1980s cannot be attributed to any recent export process from either the stratosphere or the boundary layer but rather to enhanced photochemical production in aged air masses or to an increase in the lifetime of the layers.     

Mesoscale modelling of vertical atmospheric transport in the Alps associated with the advection of a tropopause fold – a winter ozone episode

The role of vertical atmospheric transport (VAT) for a winter ozone episode (January 1988) in the Alps, observed both on the mountain crest (3580 m asl) and in the lee of the Alps (209 m asl), is investigated. Numerical simulations were carried out with a doubly nested mesoscale model (14 km horizontal resolution), characterised by a refined orography scheme. A comparison between the modelled vertical profiles with features observed in the aerological soundings over Payerne indicated that the model is able to reproduce the fold in the Alps down to 650 hPa. Frictional processes are found to be responsible for the fragmentation of the lower parts of the fold when it is advected over the mountains, thus enhancing ozone concentrations at the mountain crest with likely cross-tropopause mixing. Vertically propagating gravity waves and strong mesoscale vertical winds (nordfoehn) are responsible for the further downward transport of ozone-rich air to the lower troposphere. This mechanism leads to ozone concentrations up to 70 ppb(v) in the leeside. The study is important because tropospheric ozone trends in the lee of the Alps have been inferred, in the past, from ozone peaks associated with such nordfoehn conditions. It is now shown that nordfoehn may entrain air which has been recently exchanged across the tropopause.     

Daily surface UV exposure and its relationship to surface pollutant measurements

For the past 30 years, the stratospheric ozone layer has decreased in the Northern Hemisphere. The main effect of this ozone decrease was an expected increase in the UV radiation at the Earth's surface, but there has been no clear evidence of an increasing urban trend in surface UV. This study shows that specific air pollutants can reduce the increased surface levels of UV radiation and offers an explanation for why the expected surface UV increases have not been observed, especially in urban regions. A U.S. Environmental Protection Agency (EPA) UV monitoring site at the University of California at Riverside combined with air pollution data from a site operated by the California Air Resources Board in Rubidoux, CA, provided the basis of this study. The 1997 South Coast Ozone Study (SCOS-97) provided three key ingredients: black carbon, PM10 concentrations, and collocated radiometric measurements. The Total Ozone Mapping Spectrometer (TOMS) satellite data were used to provide the stratospheric ozone levels that were included in the statistical model. All of these input parameters would be used to test this study's hypothesis: the expected increase of surface UV radiation, caused by decreases in stratospheric ozone, can be masked by increases in anthropogenic emissions. The values for the pollutants were 7:00 a.m.-5:00 p.m. averages of the instrument's values taken during summer 1997. A statistical linear regression model was employed using the stratospheric ozone, black carbon, PM10, and surface ozone concentrations, and the sin (theta) and cos (theta). The angle theta is defined by theta = 2pi (Julian date/365). This model obtained a coefficient of determination of 0.94 with an uncertainty level (p value) of less than 0.3% for all of the variables in the model except ground-level ozone. The final model, regressed against a data set from a remote, western North Carolina site, resulted in a coefficient of determination of 0.92. The model shows that black carbon can reduce the Diffey-weighted UV levels that reach the surface by as much as 35%, depending on the season.     

The role of halogen species in the troposphere

While the role of reactive halogen species (e.g. Cl, Br) in the destruction of the stratospheric ozone layer is well known, their role in the troposphere was investigated only since their destructive effect on boundary layer ozone after polar sunrise became obvious. During these 'Polar Tropospheric Ozone Hole' events O(3) is completely destroyed in the lowest approximately 1000 m of the atmosphere on areas of several million square kilometres. Up to now it was assumed that these events were confined to the polar regions during springtime. However, during the last few years significant amounts of BrO and Cl-atoms were also found outside the Arctic and Antarctic boundary layer. Recently even higher BrO mixing ratios (up to 176 ppt) were detected by optical absorption spectroscopy (DOAS) in the Dead Sea basin during summer. In addition, evidence is accumulating that BrO (at levels around 1-2 ppt) is also occurring in the free troposphere at all latitudes.In contrast to the stratosphere, where halogens are released from species, which are very long lived in the troposphere, likely sources of boundary layer Br and Cl are autocatalytic oxidation of sea salt halides (the 'Bromine Explosion'), while precursors of free tropospheric BrO and coastal IO probably are short-lived organo-halogen species. At the levels suggested by the available measurements reactive halogen species have a profound effect on tropospheric chemistry: In the polar boundary layer during 'halogen events' ozone is usually completely lost within hours or days. In the free troposphere the effective O(3)-losses due to halogens could be comparable to the known photochemical O(3) destruction. Further interesting consequences include the increase of OH levels and (at low NO(X)) the decrease of the HO(2)/OH ratio in the free troposphere.     

The Impact of Stratospheric Ozone on Tropospheric Air Quality

A background of ozone (O₃), principally of stratospheric origin, is present in the lower free troposphere. Typical mean O₃ levels of 50 ppb, 40 ppb, and 30 ppb are encountered here in spring, summer, and fall, respectively. Maximum hourly O₃ concentrations which are twice these mean values can be expected. Ozone from the free troposphere is routinely brought down to ground level under turbulent atmospheric conditions. Deep and rapid Intrusions of stratospheric air into the lower troposphere are associated with low-pressure troughs and occur regularly. In the mid troposphere, O₃ levels as high as 300 ppb are found within these intrusions. Observational data showing these intrusions, containing high O₃ concentrations, to directly reach ground level are currently lacking. Over the United States, an intrusion was present aloft on 8 9% of the days in 1978. The frequency, however, is somewhat reduced in summer and a northward movement is evident. During 1978, no intrusion occurred south of 30°N between June and August and none south of 40 °N in August.

The hypothesis that low levels of stratospheric O₃ produce disproportionately large amounts of O₃ in the polluted atmosphere cannot be supported from currently known chemistry but should be studied further. The experimental technique involving a ⁷Be/O₃ ratio to estimate the daily stratospheric component of ground level O₃ is unverified and considered to be inadequate for air quality applications. Estimates resulting from such a technique are considered uncertain by a factor of more than three. Specially designed aircraft studies provide the best means to determine quantitatively the impact of stratospheric O₃ on ground level air quality.     

What is the lesson from the unprecedented event over antarctica in 2002

Varotsos (2002a,b), suggested that both the smaller-sized ozone hole over Antarctica and its splitting in two holes in September 2002 occurred due to an unprecedented major sudden stratospheric warming caused by very strong planetary waves propagated in the southern hemisphere. Subsequently, a NASA press release of December 6, 2002, also reported the prevalence of very strong planetary waves in Antarctica. The aim of this Letter is to further discuss the morphology of the Antarctic ozone hole, to detect the causes that allowed the Antarctic stratosphere to exhibit this exceptional warming and to examine what it denotes about its mechanisms. Concerning the morphology, among the principal findings is that the ozone hole split occurred not only in the stratosphere but extended in the lower altitudes (upper troposphere). As to the causes of the major sudden stratospheric warming of 2002, a comparison with the previous warmings in Antarctica since 1964 is made. The smaller-sized Antarctic ozone hole of 2002 is approximately equal to that of 1988 when a strong sudden stratospheric warming occurred. If only the destruction of ozone by chlorofluorocarbons resulted in the delayed sudden stratospheric warmings in Antarctica, then the early sudden stratospheric warmings of 1988 and 2002 would not have occurred, since chlorofluorocarbon loading of the stratosphere has remained relatively stable in recent years. Furthermore, it appears that the El Nino characteristics in 1988 and 2002 are not similar.     

Nocturnal boundary layer characteristics and land breeze development in Houston,Texas during TexAQS II

The nocturnal boundary layer in Houston, Texas was studied using a high temporal and vertical resolution tethersonde system on four nights during the Texas Air Quality Study II (TexAQS II) in August and September 2006. The launch site was on the University of Houston campus located approximately 4 km from downtown Houston. Of particular interest was the evolution of the nocturnal surface inversion and the wind flows within the boundary layer. The land–sea breeze oscillation in Houston has important implications for air quality as the cycle can impact ozone concentrations through pollutant advection and recirculation. The results showed that a weakly stable surface inversion averaging in depth between 145 and 200 m AGL formed on each of the experiment nights, typically within 2–3 h after sunset. Tethersonde vertical winds were compared with two other Houston data sets (High Resolution Doppler Lidar and radar wind profiler) from locations near the coastline and good agreement was found, albeit with a temporal lag at the tethersonde site. This comparison revealed development of a land breeze on three nights which began near the coastline and propagated inland both horizontally and vertically with time. The vertical temperature structure was significantly modified on one night at the tethersonde site after the land breeze wind shift, exhibiting near-adiabatic profiles below 100 m AGL.