Stratospheric O3 Research Articles

Quantifying Arctic ozone loss during the 2004–2005 winter using satellite observations and a chemical transport model

During the last decade, much attention has been placed on quantifying and modeling Arctic stratospheric O3 loss. At issue in particular is the reliability of models for simulating the loss under variable dynamical conditions in the Arctic region. This paper describes inferred O3 loss calculations for the 2004–2005 Arctic winter using data from four solar occultation satellite instruments, as well as the Earth Observing System Microwave Limb Sounder (EOS MLS). O3 loss is quantified with the “Chemical Transport Model (CTM) passive subtraction” approach, using a passive O3 tracer field from the SLIMCAT CTM. The 2004–2005 Arctic winter was moderately active dynamically, but was still one of the coldest Arctic winters on record, with prime conditions for O3 loss. Loss estimates inferred from all of the different satellite instruments peaked in mid‐March at 450 K between 2–2.3 ppmv, slightly less than similar estimations for the cold 1999–2000 winter. The SLIMCAT CTM was also used to simulate O3 for the 2004–2005 winter. In March, near 450 K, the model O3 was 0.3 ppmv (∼10–15%) lower than the observations, leading to a maximum O3 loss that was 10–15% larger than that inferred from observations, using the passive subtraction approach. Modeled loss maximized around the same time as that inferred from observations. Although some discrepancies between the observed and modeled O3 remain, the level of agreement presented here shows that the SLIMCAT CTM was able to satisfactorily simulate O3 and polar O3 loss during the dynamically active 2004–2005 Arctic winter.

Validation of Global Ozone Monitoring Experiment ozone profiles and evaluation of stratospheric transport in a global chemistry transport model

This paper presents a validation of Global Ozone Monitoring Experiment (GOME) ozone (O3) profiles which are used to evaluate stratospheric transport in the chemistry transport model (CTM) Tracer Model version 5 (TM5) using a linearized stratospheric O3 chemistry scheme. A comparison of GOME O3 profile measurements with independent O3 sonde measurements at midlatitudes shows an excellent agreement. Differences are smaller than 5%, well within the uncertainty of the O3 sonde measurements. Within the tropics, the GOME O3 profile differences are larger, with a clear lower stratospheric negative O3 bias with compensating positive biases in the troposphere and higher stratosphere. The TM5 model with linearized O3 chemistry simulates realistic lower and middle stratospheric spatial and temporal O3 variations on both short (daily) and long (seasonal) timescales. Model stratospheric O3 is significantly overestimated in the extratropics and slightly underestimated in the tropics, as is also shown in a comparison with Total Ozone Mapping Spectrometer total O3 column measurements. This model bias predominantly occurs in the lower stratosphere and is present throughout the year, albeit with seasonal variations: The bias is larger during local winter compared with local summer. The particular spatial and seasonal variations of the model bias suggest a too fast meridional stratospheric transport in TM5, which agrees with earlier found shortcomings of using winds from data assimilation systems. The model results are very sensitive to the data assimilation method in the numerical weather prediction that provides the model wind fields. A large reduction (up to 50% of the bias) in modeled lower stratospheric midlatitude O3 was found when winds from four‐dimensional instead of three‐dimensional data assimilation were used. Previous work has shown that using different forecast periods was important for improving the age of air. Model results differed with different forecast periods (up to 3 days), although the effect was mainly confined to high‐latitude lower stratospheric O3. Apparently, using different forecast periods is more important for age‐of‐air calculations than for stratospheric O3 calculations. A positive bias in the extratropical lower stratosphere of about 20% remained, possibly related to the lack of heterogeneous polar stratospheric O3 destruction in TM5.

EOS Microwave Limb Sounder observations of upper stratospheric BrO: Implications for total bromine

This paper describes new total stratospheric inorganic bromine (Bry) abundance estimates inferred from the first global observations of upper stratospheric BrO, made by the EOS Microwave Limb Sounder on the Aura satellite. Our ‘best estimate’ of total upper stratospheric bromine loading (based on JPL‐2002 kinetics with the addition of a BrONO2 + O reaction) is 18.6 ± 5.5 pptv, for the period September 2004 to August 2005, from 55° S to 55° N. This implies a contribution of 3.0 ± 5.5 pptv from sources other than long lived CH3Br and halons. The possibility of such other sources has been raised by balloon, aircraft and satellite observations of BrO in the lower and middle stratosphere. These upper stratospheric observations provide new information to help resolve the current uncertainty in stratospheric bromine loading. The abundance of bromine, particularly in the lower stratosphere, is a significant factor in the budget of stratospheric O3.

Inter-comparison of stratospheric O<sub>3</sub> and NO<sub>2</sub> abundances retrieved from balloon borne direct sun observations and Envisat/SCIAMACHY limb measurements

Abstract. Stratospheric O3 and NO2 abundances measured by different remote sensing instruments are inter-compared: (1) Line-of-sight absorptions and vertical profiles inferred from solar spectra in the ultra-violet (UV), visible and infrared (IR) wavelength ranges measured by the LPMA/DOAS (Limb Profile Monitor of the Atmosphere/Differential Optical Absorption Spectroscopy) balloon payload during balloon ascent/descent and solar occultation are examined with respect to internal consistency. (2) The balloon borne stratospheric profiles of O3 and NO2 are compared to collocated space-borne skylight limb observations of the Envisat/SCIAMACHY satellite instrument. The trace gas profiles are retrieved from SCIAMACHY spectra using different algorithms developed at the Universities of Bremen and Heidelberg and at the Harvard-Smithsonian Center for Astrophysics. A comparison scheme is used that accounts for the spatial and temporal mismatch as well as differing photochemical conditions between the balloon and satellite borne measurements. It is found that the balloon borne measurements internally agree to within ±10% and ±20% for O3 and NO2, respectively, whereas the agreement with the satellite is ±20% for both gases in the 20 km to 30 km altitude range and in general worse below 20 km.

Fall vortex ozone as a predictor of springtime total ozone at high northern latitudes

Abstract. Understanding the impact of atmospheric dynamical variability on observed changes in stratospheric O3 is a key to understanding how O3 will change with future climate dynamics and trace gas abundances. In this paper we examine the linkage between interannual variability in total column O3 at northern high latitudes in March and lower-to-mid stratospheric vortex O3 in the prior November. We find that these two quantities are significantly correlated in the years available from TOMS, SBUV, and POAM data (1978-2004). Additionally, we find that the increase in March O3 variability from the 1980s to years post-1990 is also seen in the November vortex O3, i.e., interannual variability in both quantities is much larger in the later years. The cause of this correlation is not clear, however. Interannual variations in March total O3 are known to correspond closely with variations in winter stratospheric wave driving consistent with the effects of varying residual circulation, temperature, and chemical loss. Variation in November vortex O3 may also depend on dynamical wave activity, but the dynamics in fall are less variable than in winter and spring. We do not find significant correlations of dynamic indicators for November such as temperature, heat flux, or polar average total O3 with the November vortex O3, nor with dynamical indicators later in winter and spring that might lead to a connection to March. We discuss several potential hypotheses for the observed correlation but do not find strong evidence for any considered mechanism. We present the observations as a phenomenon whose understanding may improve our ability to predict the dependence of O3 on changing dynamics and chemistry.

Chemical effects in 11‐year solar cycle simulations with the Freie Universität Berlin Climate Middle Atmosphere Model with online chemistry (FUB‐CMAM‐CHEM)

The impact of 11‐year solar cycle variations on stratospheric ozone (O3) is studied with the Freie Universität Berlin Climate Middle Atmosphere Model with interactive chemistry (FUB‐CMAM‐CHEM). To consider the effect of variations in charged particle precipitation we included an idealized NOx source in the upper mesosphere representing relativistic electron precipitation (REP). Our results suggest that the NOx source by particles and its transport from the mesosphere to the stratosphere in the polar vortex are important for the solar signal in stratospheric O3. We find a positive dipole O3 signal in the annual mean, peaking at 40–45 km at high latitudes and a negative O3 signal in the tropical lower stratosphere. This is similar to observations, but enhanced due to the idealized NOx source and at a lower altitude compared to the observed minimum. Our results imply that this negative O3 signal arises partly via chemical effects.

Mapping tropospheric ozone profiles from an airborne ultraviolet-visible spectrometer

We present a novel technique for retrieving ozone (O3) profiles and especially tropospheric O3 from airborne UV/visible spectrometer measurements. This technique utilizes radiance spectra from one down-looking and two up-looking (85 degrees and 75 degrees) directions, taking advantage of the O3 absorption structure in the Huggins (300-340-nm) and Chappuis (530-650-nm) bands. This technique is especially sensitive to tropospheric O3 below and < or =8 km above the aircraft with a vertical resolution of 2-6 km and is sensitive to lower and middle stratospheric O3 with a vertical resolution of 8-15 km. It can measure tropospheric O3 at spatial resolutions of 2 km x 2 km or higher and is therefore well suited for regional air-quality studies and validation of satellite measurements.

Definitions and sharpness of the extratropical tropopause: A trace gas perspective

Definitions of the extratropical tropopause are examined from the perspective of chemical composition. Fine‐scale measurements of temperature, ozone, carbon monoxide, and water vapor from approximately 70 aircraft flights, with ascending and descending tropopause crossings near 40°N and 65°N, are used in this analysis. Using the relationship of the stratospheric tracer O3 and the tropospheric tracer CO, we address the issues of tropopause sharpness and where the transitions from troposphere to stratosphere occur in terms of the chemical composition. Tracer relationships indicate that mixing of stratospheric and tropospheric air masses occurs in the vicinity of the tropopause to form a transition layer. Statistically, this transition layer is centered on the thermal tropopause. Furthermore, we show that the transition is much sharper near 65°N (a region away from the subtropical jet) but spans a larger altitude range near 40°N (in the vicinity of the subtropical jet). This latter feature is consistent with enhanced stratosphere‐troposphere exchange and mixing activity near the tropopause break.

NOGAPS-ALPHA model simulations of stratospheric ozone during the SOLVE2 campaign

Abstract. This paper presents three-dimensional prognostic O3 simulations with parameterized gas-phase photochemistry from the new NOGAPS-ALPHA middle atmosphere forecast model. We compare 5-day NOGAPS-ALPHA hindcasts of stratospheric O3 with satellite and DC-8 aircraft measurements for two cases during the SOLVE II campaign: (1) the cold, isolated vortex during 11-16 January 2003; and (2) the rapidly developing stratospheric warming of 17-22 January 2003. In the first case we test three different photochemistry parameterizations. NOGAPS-ALPHA O3 simulations using the NRL-CHEM2D parameterization give the best agreement with SAGE III and POAM III profile measurements. 5-day NOGAPS-ALPHA hindcasts of polar O3 initialized with the NASA GEOS4 analyses produce better agreement with observations than do the operational ECMWF O3 forecasts of case 1. For case 2, both NOGAPS-ALPHA and ECMWF 114-h forecasts of the split vortex structure in lower stratospheric O3 on 21 January 2003 show comparable skill. Updated ECMWF O3 forecasts of this event at hour 42 display marked improvement from the 114-h forecast; corresponding updated 42-hour NOGAPS-ALPHA prognostic O3 fields initialized with the GEOS4 analyses do not improve significantly. When NOGAPS-ALPHA prognostic O3 is initialized with the higher resolution ECMWF O3 analyses, the NOGAPS-ALPHA 42-hour lower stratospheric O3 fields closely match the operational 42-hour ECMWF O3 forecast of the 21 January event. We find that stratospheric O3 forecasts at high latitudes in winter can depend on both model initial conditions and the treatment of photochemistry over periods of 1-5 days. Overall, these results show that the new O3 initialization, photochemistry parameterization, and spectral transport in the NOGAPS-ALPHA NWP model can provide reliable short-range stratospheric O3 forecasts during Arctic winter.

Vertical profile and origin of wintertime tropospheric ozone over China during the PEACE‐A period

During the Pacific Exploration of Asian Continental Emission (PEACE) phase A mission in January 2002, we launched ozonesondes in subtropical southeast China at Hong Kong (114.17°E, 22.31°N), middle latitude northeast China at Beijing (116.47°E, 39.81°N), and northwest China at Xining (101.45°E, 36.43°N) in order to study long‐range ozone (O3) transport from Eurasia, tropospheric O3 sources in China, and O3 outflow to the Pacific. Tropospheric O3 showed a complex vertical distribution with average tropospheric O3 columns of 39 ± 4, 23 ± 3, and 30 ± 6 DU in Hong Kong, Beijing, and Xining, respectively, which accounted for 17 ± 2%, 7 ± 1%, and 10 ± 1% of the total O3 column. The lower tropospheric and boundary layer (BL) O3 over Xining and especially Beijing exhibited low values, suggesting negligible O3 formation in middle latitudes of China during the winter season. The results also revealed frequent propagation of enhanced O3 layers from the lower stratosphere to the upper troposphere over Xining and especially Beijing, suggesting that stratospheric O3 is an important source of O3 in the upper troposphere of northern China. This “natural” O3 is transported downwind by the prevailing westerly wind and acts as a source of O3 to the east Asian coast and northwestern Pacific. We observed elevated O3, with a maximum mixing ratio up to 111 ppbv, at 1.5 km in the upper BL over Hong Kong. The elevated O3 was resulted from transport of pollutants from northwest‐central or southwest China and regional O3 formation and accumulation in south China including the Pearl River Delta and Hong Kong. We also observed enhanced O3 (&gt;95 ppbv) in the middle and upper troposphere over Hong Kong in air masses transported along the subtropical jet from tropical and subtropical East Africa, south Asia, and Southeast Asia. The O3 enhancements were most likely due to intrusion of stratospheric O3 into the troposphere in the Indo‐Burmese region of tropical Southeast Asia, where substantial downward motion had been observed.

A global climate‐chemistry model study of present‐day tropospheric chemistry and radiative forcing from changes in tropospheric O3 since the preindustrial period

We present simulations of present‐day tropospheric O3 and of changes in its concentrations and the associated radiative forcing since the industrial revolution. Numerical experiments were conducted using a global tropospheric climate‐chemistry model (GCCM) developed by incorporating the University of Oslo (UiO) photochemical module (a reduced tracer scheme that lumps nonmethane volatile organic compounds (NMVOCs) to simulate the CO‐NOx‐HOx‐O3 system) into State University of New York at Albany–Community Climate Model 3 (SUNYA CCM3). The GCCM was run with emissions of pollutants corresponding to two periods, the early 1990s and the preindustrial period, and the simulated tropospheric O3 is used for off‐line calculations of radiative forcing. The study consists of two parts: an evaluation of the GCCM‐simulated present‐day distributions of tropospheric chemical species with sensitivity experiments to test the effects of NO production by lightning and stratospheric O3 influx on the simulated results, and the radiative forcing due to O3 changes since the industrial revolution. The model can reproduce the temporal and spatial variations of O3 precursors, including CO and NOx (NO + NO2), in the Northern Hemisphere (NH), although there are biases such as the larger CO concentrations in the Southern Hemisphere (SH) and the smaller NOx concentrations in the tropical middle‐upper troposphere. The simulated OH concentrations are sensitive to the NO production by lightning. The simulated concentrations of tropospheric OH have a global and annual average of about 9.8 × 105 molecules/cm3 (corresponding to a tropospheric CH4 lifetime due to OH of 8.6 years), when the annual production of NO by lightning is about 6 TgN/yr. Compared with O3 sonde climatology, the model can simulate the seasonal variations of lower to middle tropospheric O3 within the observed interannual variations when the stratospheric O3 influx is about 600 Tg/yr, although detailed biases exist in the middle troposphere during fall to winter and in the continental boundary layer during summer. The present‐day tropospheric O3 burden is calculated to be about 376 Tg (or 34 DU) with a net tropospheric chemical production of about 513 Tg/yr. Using realistic stratospheric O3 concentrations instead of a prescribed O3 influx as the upper boundary condition enhances the model biases in the extratropical upper‐middle tropospheric O3 concentrations. The model calculates a globally and annually averaged increase in tropospheric O3 burden of about 13 DU since the preindustrial period, if the production of NO by lightning is kept the same for both the present‐day and preindustrial periods. The annually and globally averaged instantaneous radiative forcing due to the O3 changes is about 0.54 W m−2, with forcing larger than 1 W m−2 occurring over most subtropical to middle‐latitude areas in the NH during summer. An assumed 50% reduction in NO production by lightning in the preindustrial period results in an increase of tropospheric O3 change by 10%, with the associated radiative forcing enhanced by 11%.

Future changes in stratosphere‐troposphere exchange and their impacts on future tropospheric ozone simulations

We assess future climate change impacts on stratosphere‐troposphere exchange (STE) and their influences on tropospheric O3, using a chemistry coupled climate model. Tropospheric O3 distribution and budget were predicted decadally for 1990 to 2100 with emission changes (for O3 precursors) and climate change specified by the IPCC SRES‐A2 scenario. Our simulations show increases in stratospheric O3 transport to the troposphere as a result of enhancement in the tropospheric and stratospheric circulation with climate change in the model. With emission changes only, net stratospheric O3 input to the troposphere was simulated to decrease by ∼20% during 1990–2100, but to increase by ∼80% with including climate change also. Simulated increases in net cross‐tropopause O3 transport are most significant particularly after 2050.

Sensitivity studies of the recent new data on O(&amp;lt;sup&amp;gt;1&amp;lt;/sup&amp;gt;&amp;lt;i&amp;gt;D&amp;lt;/i&amp;gt;) quantum yields in O&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt; Hartley band photolysis in the stratosphere

Abstract. The production yields of excited oxygen O(1D) atoms from the near ultraviolet O3 photolysis are essential quantities for atmospheric chemistry calculations because of its importance as major sources of hydroxyl (OH) radicals and nitric oxide (NO). Recently, new O(1D) quantum yields from O3 photolysis between 230 and 305 nm in the Hartley band region were reported, which are almost independent of the photolysis wavelength (0.88-0.93) and smaller than NASA/JPL-2000 recommendations (0.95 between 240 and 300 nm). In order to assess consequences of the new data of O(1D) quantum yields on the stratospheric chemistry, the changes in stratospheric chemical partitioning and O3 concentration are examined using a one-dimensional atmospheric model. Our steady state model simulations for 40° N in March indicate that the smaller O(1D) quantum yields result in increases of stratospheric O3 (up to ~2% in the upper stratosphere), which are attributed to the changes in HOx, NOx, and ClOx abundance and their catalyzed O3 loss rates.

Characteristics of a tropospheric ozone profile and implications for the origin of ozone over subtropical China in the spring of 2001

During the Transport and Chemical Evolution Over the Pacific (TRACE‐P) period in the spring of 2001 we launched an ozonesonde at three Chinese sites: Kunming (102.68°E, 25.03°N), Hong Kong (114.17°E, 22.31°N), and Linan (119.75°E, 30.30°N). The sites extend from subtropical southwestern China close to the Southeast (SE) Asian border, to the southeastern Asian coast, and to the edge of the middle latitudes of central eastern China, respectively. The aims of the study are to provide ozonesonde data within the source regions of the Chinese mainland, to investigate the source of tropospheric ozone (O3), and to investigate to what extent SE Asian biomass burning emissions impact both tropospheric O3 over the subtropical Chinese mainland and O3 outflows to the Pacific. The results show that there are substantial variations in vertical O3 distributions over these sites, with low O3 values in the upper troposphere of Hong Kong, high O3 values in the middle and upper troposphere of Linan, and frequent O3 enhancements in the lower troposphere of Hong Kong and Kunming. The low values in the upper troposphere over Hong Kong in the spring of 2001 were not usually observed from 1993 to 2000 and are the result of the transport of O3‐depleted air from the intertropical convergence zone of equatorial SE Asian regions following the eastern Asia local Hadley circulation. Such transport processes do not affect the higher latitude at the edge of the middle latitude of Linan, where stratospheric O3 is the major contributing source to middle and upper tropospheric O3. The O3 enhancements over the lower troposphere of Kunming and Hong Kong are caused by SE Asian biomass burning emissions. Such enhancements are frequently observed over Hong Kong, less often over Kunming, and scarcely ever over Linan. Our analysis shows that biomass burning emissions from SE Asia in the spring of 2001 mainly affected the southern parts of the subtropical Chinese region.

Changes in tropospheric ozone between 2000 and 2100 modeled in a chemistry‐climate model

We present results using a coupled chemistry/climate model to study changes in tropospheric ozone between 2000 and 2100. We assess changes first due to the increased emissions of NOx and VOCs (using the IPCC SRES scenario A2) and then due to both emission changes and the anticipated climate change with doubled CO2. In 2100, with the scenarios used, there is a substantial calculated increase in tropospheric O3, but in contrast to earlier studies, the increase is larger for doubled CO2. The increases are most pronounced in the extratropical middle and upper troposphere; changes in circulation and a chemically induced increase in lower stratospheric O3, mainly due to reduced temperatures there, both enhance stratosphere/troposphere exchange. These changes in the lower stratosphere are crucial to our results.

A comparison of climate response to different radiative forcings in three general circulation models: towards an improved metric of climate change

In order to review, and possibly refine, the concept of radiative forcing as a suitable metric for climate change, the responses of three general circulation models to distinct forcing scenarios are compared. CO2, solar radiation, and O3 are added in different locations, whilst keeping the globally averaged radiative forcing constant at 1 Wm–2. The three models react differently to the forcings, as feedback mechanisms such as sea-ice albedo and clouds behave differently in each model. However, we find that their climate sensitivities λ (defined as the ratio of the globally averaged surface temperature change to the radiative forcing), normalised by the climate sensitivity for a control case (e.g. CO2 added globally), match each other to within 30% in most experiments. Moreover, the models indicate generic deviations of λ from the case of global CO2 perturbations: upper tropospheric O3 increases generally produce lower values of λ, while lower stratospheric O3 perturbations lead to higher values of λ, as found in some previous work. λ tends to be higher for extratropical forcings than tropical forcings; a phenomenon which can be partially accounted for by a new explanation based on the variation of the outgoing longwave radiation with latitude. Our results suggest that if the radiative forcing associated with some perturbation is multiplied by some factor accounting for the efficiency of that mechanism, then such modified forcings can be compared more robustly than the forcings themselves.

Stratospheric influence on the northern North American free troposphere during TOPSE: 7Be as a stratospheric tracer

We use 7Be, with HNO3 and O3, to identify air masses sampled from the NCAR C‐130 during TOPSE that retained clear evidence of stratospheric influence. A total of 43 such air masses, spread fairly evenly across the February to May sampling period, and 40°N–86°N latitude range, were encountered. South of 55°N, nearly all clear stratospheric influence was restricted to altitudes above 6 km. At higher latitudes stratospherically influenced air masses were encountered as low as 2 km. Approximately 12% of all TOPSE sampling time at altitudes above 2 km was spent in stratospherically impacted air, above 6 km this increased to more than half of the time. Because it is not certain how much of this stratospherically influenced air irreversibly injected mass (and chemical compounds) into the troposphere, we estimate the stratospheric fraction of O3 in high latitude TOPSE samples based on a linear relationship to 7Be and compare it to in situ O3. This analysis indicates that the stratospheric source can account for a dominant fraction (&gt;85%) of in situ O3 throughout TOPSE, but that the stratospheric contribution was nearly constant through the 4 month campaign. In February and March the 7Be based estimates of stratospheric O3 account for 10–15% more O3 than was measured, but by April and May there is up to about 10% more O3 than expected from the stratospheric source. This trend suggests that a seasonal transition from O3 depletion to photochemical production in the high latitude North American troposphere is the major cause of the springtime increase in O3.

Thermal and dynamical changes of the stratosphere since 1979 and their link to ozone and CO2 changes

This study examines which part of the observed stratospheric thermal and dynamical changes since 1979 can be attributed to the observed stratospheric ozone (O3) losses and CO2 increases. Further, the processes are studied that lead to temperature and circulation changes when stratospheric O3 and CO2 are modified. We compared results from simulations of the Freie Universität Berlin Climate Middle Atmosphere Model (FUB CMAM) using observed O3 and CO2 changes with observed trends of stratospheric temperature and circulation for the period 1979–2000 from FUB data and National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalyses. The observed O3 decrease leads in the FUB CMAM to a global mean stratospheric cooling, which is enhanced in the upper stratosphere by the imposed CO2 increase. While the model is able to reproduce the observed stratospheric cooling in the upper stratosphere, it underestimates the observed trends in the lower stratosphere, particularly in middle latitudes and during Northern Hemisphere (NH) spring. The observed intensification and increased lifetimes of the polar vortices in spring are captured by the model but with smaller magnitude than observed. It is suggested that the observed upper stratospheric temperature trends during the past two decades in low to middle latitudes are caused by radiative effects due to the O3 and CO2 changes, while the cooling of the polar stratosphere in winter is enhanced by changes in dynamical heating. However, in northern midlatitudes and in Arctic spring, other effects than O3 and CO2 changes must be considered to fully explain the observed changes in the lower stratosphere.

Origin of the springtime tropospheric ozone maximum over east China at LinAn in 2001

Tropospheric ozone (O3) profiles over the east China coast at LinAn (30.30°N, 119.75°E) were measured by electrochemical concentration cell ozonesondes in the spring of 2001. The measurement revealed that extremely high O3 with mixing ratios up to 1200 ppbv sometimes penetrated deeply into 8.5–16 km above sea level as a result of lowering of the tropopause across the East Asia strong subtropical jet stream. In addition, high O3 in the 75–150 ppbv range penetrated into the upper and middle troposphere following the extreme O3 regimes and caused an overall high O3 mixing ratio in these regions. The occurrence of the high O3 regimes followed the propagation of dry air with low relative humidity and high potential vorticity suggesting that the O3 is of stratospheric origin. High O3 episodes with O3 mixing ratios in the 75–100 ppbv range were observed in the lower troposphere and especially in the boundary layer. Trajectory analysis suggested that the O3-rich air masses had passed through the industrialized and urbanized zones of the Pearl River Delta region of south China, northeast China coast and Yangtze River Delta region of east China. The O3 is probably of anthropogenic origin resulting from photochemical formation from pollutants emitted from these regions. Our analysis thus revealed that over the east China coast at LinAn, stratospheric O3 is still the predominant source of O3 in the middle and upper troposphere, while anthropogenic sources caused a high O3 pollution episode in the boundary layer.

On the origin of tropospheric O&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt; over the Indian Ocean during the winter monsoon: African biomass burning vs. stratosphere-troposphere exchange

Abstract. This study investigates the origin of a commonly observed feature in the O3 profiles: mid tropospheric O3 maxima (300--500 hPa) over the tropical Indian Ocean. A comparison and analysis of model simulations, using a 3-D global climate-chemistry model, and measured O3 profiles from the INDOEX campaign is presented. European Centre for Medium-Range Weather Forecast (ECMWF) meteorological analyses have been assimilated into the 3-D model to represent actual meteorology. The model realistically simulates the observed mid-tropospheric O3 maxima. The analysis of the model simulations shows that the major source of the mid-tropospheric O3 maxima is advection of polluted air masses from continental biomass burning areas over Africa, with generally only a small contribution of stratospheric O3. Previous studies hinted at stratosphere-troposphere exchange (STE) along the subtropical jet (STJ) as the primary source of the mid-tropospheric O3 maxima over the Indian Ocean. Analysis of the model simulations shows that the mechanism causing the mid-tropospheric transport of African biomass burning pollution and stratospheric air masses are frontal zones or waves passing along the subtropical jets, causing advection of tropical air masses in the prefrontal (equatorward) zone. Furthermore, the frontal zones or waves also cause STE at the poleward side of the STJ. The model simulations also indicate that the contribution of STE in general is minor compared to advection and in situ tropospheric production of O3 for the mid-tropospheric O3 budget over the Indian Ocean region.