High Ozone Mixing Ratios Research Articles

Heterogeneous ozonation kinetics of 4-phenoxyphenol in the presence of photosensitizer

Abstract. In this work we have quantitatively measured the degradation of 4-phenoxyphenol adsorbed on silica particles following oxidative processing by gas-phase ozone. This was performed under dark conditions and in the presence of 4-carboxybenzophenone under simulated sunlight irradiation of the particles surface. At the mixing ratio of 60 ppb which corresponds to strongly polluted ozone areas, the first order of decay of 4-phenoxyphenol is k1=9.95×10−6 s−1. At a very high ozone mixing ratio of 6 ppm the first order rate constants for 4-phenoxyphenol degradation were the following: k1=2.86×10−5 s−1 under dark conditions and k1=5.58×10−5 s−1 in the presence of photosensitizer (4-carboxybenzophenone) under light illumination of the particles surface. In both cases, the experimental data follow the modified Langmuir-Hinshelwood equation for surface reactions. The Langmuir-Hinshelwood and Langmuir-Rideal mechanisms for bimolecular surface reactions are also discussed along with the experimental results. Most importantly, the quantities of the oligomers such as 2-(4-Phenoxyphenoxy)-4-phenoxyphenol and 4-[4-(4-Phenoxyphenoxy)phenoxy]phenol formed during the heterogeneous ozonolysis of adsorbed 4-phenoxyphenol were much higher under solar light irradiation of the surface in comparison to the dark conditions.

Increasing interannual and altitudinal ozone mixing ratios in the Catalan Pyrenees

Interannual, seasonal, daily and altitudinal patterns of tropospheric ozone mixing ratios, as well as ozone phytotoxicity and the relationship with NO x precursors and meteorological variables were monitored in the Central Catalan Pyrenees (Meranges valley and Forest of Guils) over a period of 5 years (2004–2008). Biweekly measurements using Radiello passive samplers were taken along two altitudinal transects comprised of thirteen stations ranging from 1040 to 2300 m a.s.l. Visual symptoms of ozone damage in Bel-W3 tobacco cultivars were evaluated biweekly for the first three years (2004–2006). High ozone mixing ratios, always above forest and vegetation protection AOT40 thresholds, were monitored every year. In the last 14 years, the AOT40 (Apr–Sept.) has increased significantly by 1047 μg m −3 h per year. Annual means of ozone mixing ratios ranged between 38 and 67 ppb v (38 and 74 ppb v during the warm period) at the highest site (2300 m) and increased at a rate of 5.1 ppb v year −1. The ozone mixing ratios were also on average 35–38% greater during the warm period and had a characteristic daily pattern with minimum values in the early morning, a rise during the morning and a decline overnight, that was less marked the higher the altitude. Whereas ozone mixing ratios increased significantly with altitude from 35 ppb v at 1040 m–56 ppb v at 2300 m (on average for 2004–2007 period), NO 2 mixing ratios decreased with altitude from 5.5 ppb v at 1040 m–1 ppb v at 2300 m. The analysis of meteorological variables and NO x values suggests that the ozone mainly originated from urban areas and was transported to high-mountain sites, remaining aloft in absence of NO. Ozone damage rates increased with altitude in response to increasing O 3 mixing ratios and a possible increase in O 3 uptake due to more favorable microclimatic conditions found at higher altitude, which confirms Bel-W3 as a suitable biomonitor for ozone concentrations during summer time. Compared to the valley-bottom site the annual means of ozone mixing ratios are 37% larger in the higher sites. Thus the AOT40 for the forest and vegetation protection threshold is greatly exceeded at higher sites. This could have substantial effects on plant life at high altitudes in the Pyrenees.

Severe ozone air pollution in the Persian Gulf region

Abstract. Recently it was discovered that over the Middle East during summer ozone mixing ratios can reach a pronounced maximum in the middle troposphere. Here we extend the analysis to the surface and show that especially in the Persian Gulf region conditions are highly favorable for ozone air pollution. We apply the EMAC atmospheric chemistry-climate model to investigate long-distance transport and the regional formation of ozone. Further, we make use of available in situ and satellite measurements and compare these with model output. The results indicate that the region is a hot spot of photochemical smog where European Union air quality standards are violated throughout the year. Long-distance transports of air pollution from Europe and the Middle East, natural emissions and stratospheric ozone conspire to bring about relatively high background ozone mixing ratios. This provides a hotbed to strong and growing indigenous air pollution in the dry local climate, and these conditions are likely to get worse in the future.

Satellite observations of high nighttime ozone at the equatorial mesopause

Measurements by the Sounding of the Atmosphere using Broadband Emission Radiometry instrument enable the characterization of the seasonal variation of ozone and temperature in the upper mesosphere. These are the first global measurements that resolve both the structure of the secondary ozone maximum at night and the temperature over all seasons of the year. The average nighttime mixing ratios at the altitude of the maximum vary with latitude and season. Analysis shows that the highest mixing ratios are clustered near the equator during equinoxes. The high ozone mixing ratios are observed in exactly the place and time at which the diurnal tide is largest. The diurnal tidal phase is such that coldest temperatures at 95 km occur near midnight. The high ozone is coincident with regions that have both low temperature and low amounts of atomic hydrogen. We focus particularly on ozone mixing ratios in the range of 18–50 ppmv; these occur intermittently in the equinoctial tropics on days when the night temperature is particularly cold. The occurrence of ozone maxima over 20 ppmv was unexpected but is shown in this paper to be consistent with theory and is a result of large‐amplitude diurnal tides. The same seasonal and latitudinal characteristics are seen in ozone density measured by Global Ozone Monitoring by Occultation of Stars.

A 10-year study of background surface ozone concentrations on the island of Gozo in the Central Mediterranean

A 10-year study of surface ozone mixing ratios in the Central Mediterranean was conducted based on continuous ozone measurements from 1997 to 2006 by a background regional Global Atmospheric Watch (GAW) station on the island of Gozo. The mean annual maximum mixing ratio is of the order of 66 ppbv in April–May with a broad secondary maximum of 64 ppbv in July–September. No long-term increase or decrease in the background level of surface ozone could be observed over the last 10 years. This is contrary to observations made in the Eastern Mediterranean, where a slow decrease in the background ozone mixing ratio was observed over the past 7 years. Despite the very high average annual ozone mixing ratio exceeding 50 ppbv—in fact, the highest average background ozone mixing ratio ever measured in Europe—, the diurnal O 3 max/O 3 min index of <1.40 indicates that the island of Gozo is a good site for measuring background surface ozone. However, frequent photosmog events from June to September during the past 10 years with ozone mixing ratios exceeding 90 ppbv indicate that the Central Mediterranean is prone to long-range transport of air pollutants from Europe by northerly winds. This was particularly evident during the so-called “August heatwave” of the year 2003 when the overall ozone mixing ratio was 4.6 ppbv higher than the average of all other 9 months of August since 1997. Air mass back-trajectory analysis of the August 2003 photosmog episodes on Gozo confirmed that ozone pollution originated from the European continent. Regression analysis was used to analyse the 10-year data set in order to model the behaviour of the ozone mixing ratio in terms of the meteorological parameters of wind speed, relative humidity, global radiation, temperature, month of year, wind sector, atmospheric pressure, and time of day (predictors). Most of these predictors were found to significantly affect the ozone mixing ratios. From March to November, the monthly average of the AOT40 threshold value for the protection of crops and vegetation against ozone was constantly exceeded on Gozo during the past 10 years.

Influence of sea-salt activated chlorine and surface-mediated renoxification on the weekend effect in the South Coast Air Basin of California

High ozone mixing ratios are a serious concern of public health. While ozone concentrations are high on weekdays due to anthropogenic emissions, they are often higher on weekends. This phenomenon has been named the weekend effect. This study uses the University of California, Irvine-California Institute of Technology (UCI-CIT) air quality model to assess the weekend effect in the South Coast Air Basin (SoCAB) of California. The weekend effect is reproduced by the model using an emissions inventory that includes representative weekday and weekend emissions. Additionally, this study modifies the Caltech Atmospheric Chemistry Mechanism (CACM), used in the UCI-CIT model, by introducing new heterogeneous reactions involving nitrogen oxides and chlorine. Eight modeling scenarios that include the nitrogen oxide renoxification and heterogeneous/multiphase chlorine reactions are presented to quantify how these reactions impact the weekend effect. The renoxification reaction and chlorine chemistry are found to increase ozone levels during weekdays and weekend days. However, increases in weekdays are generally larger than these increases that occur in the weekend. As a result, renoxification and chlorine chemistry lead to a net decrease in the average weekend effect intensity. The influence of renoxification on the weekend effect depends on the reaction probability ( P), and the impact on the weekend effect is significant for P larger than 0.1. The influence of chlorine chemistry on the weekend effect depends strongly on the sea-salt source function that activates the chlorine chemistry. An amplification factor of 10 for the sea-salt source function, which produces the best agreement with observed chlorine levels in the SoCAB, leads to a basin-wide overall decrease of 29% in the weekend effect intensity with respect to the base case.

Long‐range transport and tropospheric ozone variability in the western Mediterranean region during the Intercontinental Transport of Ozone and Precursors (ITOP‐2004) campaign

Two ground‐based ozone lidars have been operated at Observatoire de Haute Provence (OHP) for 11 days during the Intercontinental Transport of Ozone and Precursors (ITOP‐2004) measurement campaign. Ozone and scattering ratio vertical profiles have been measured from the boundary layer up to the tropopause. At first order, tropospheric ozone temporal variability is due to local pollution within the planetary boundary layer and to stratosphere‐troposphere exchange. Remaining ozone rich layers within the free troposphere are related to long‐range transport processes. Transport pathways are discussed combining Lagrangian particle dispersion modeling analysis and upstream airborne in situ measurements of the chemical composition of these air masses. High ozone and CO mixing ratios measured within polluted plumes aboard the aircraft correspond to ozone and aerosol layers seen by the lidar. Most of these layers have their origin in North America where they are uplifted either by forest fires or by warm conveyor belts in the vicinity of frontal regions. During the campaign, these polluted and thin (&lt;1 km) layers remain coherent and are transported in a Lagrangian manner over the Atlantic Ocean. The layers observed above OHP in the lower free troposphere exhibit ozone mixing ratio 50% larger than background values. With an ozone content of 3 to 6 Dobson Units, these layers increase by 5 to 10% the background tropospheric ozone column.

Seasonal variation of the surface ozone and its precursor gases during 2001–2003, measured at Anantapur (14.62°N), a semi-arid site in India

A systematic analysis of surface ozone and its precursor gases observed in a semi-arid region centered at Anantapur is presented. The analysis is based on ozone and its precursor gas (NO x and CO) data for the rural station Anantapur, which is about 200 and 400 km from the two nearby major cities, Bangalore to the south and Hyderabad to the north, respectively. Ozone levels were highest during the winter and summer period and lowest during the monsoon. The rate of increase in O 3 is greatest (about 4.7 ppbv/h) around 0900 h local time, whereas the magnitude of the maximum rate of decrease during the evening is considerably smaller (about − 3.0 ppbv/h). This feature distinguishes the urban and rural sites since the magnitudes of the rates during morning and evening are more similar at urban sites. A high annual average ozone mixing ratio is observed at Anantapur (35.9 ppbv) in comparison with Gadanki (25 ppbv), Pune (16 ppbv), Ahmedabad (13 ppbv) and Delhi (11 ppbv). At Anantapur, annual averages of oxides of nitrogen (NO x ) and CO are observed as 3.9 ± 0.6 ppbv and 436 ± 64 ppbv, respectively.

Modeling the effects of VOC and NOX emission sources on ozone formation in Houston during the TexAQS 2000 field campaign

Abstract A meteorological and chemical modeling system is used to determine the effect of ethene and propene point source emission rates on the magnitude and distribution of ozone in the vicinity of Houston. The model performance is evaluated using surface and airborne meteorological and chemical measurements made as part of the 2000 Texas Air Quality Study. A simulation that employed the reported mobile, area, biogenic, and point source emissions produced ozone mixing ratios as high as 120 ppb and distributions of nitrogen oxides that were similar to measurements at most locations, but the model underestimated ozone mixing ratios greater than 140 ppb that were located just downwind of petrochemical facilities. When the point source emission rates of ethene and propene were increased by a factor of 10, the simulated peak ozone levels were in better agreement with surface, aircraft, and lidar observations. The magnitude of the simulated ethene and olefin concentrations were in better agreement with canister samples aloft as well; however, there was still a large amount of scatter in the results. While the highest ozone mixing ratios were produced just downwind of large point source emissions of VOCs, sensitivity simulations also showed that reductions in anthropogenic emissions of NO x would be needed to reduce ozone mixing ratios over a larger area.

Stratospheric ozone reaches new minima above McMurdo Station, Antarctica, between 1998 and 2001

Ozonesonde measurements were performed annually in late winter and early spring from McMurdo Station, Antarctica, between 1998 and 2001, continuing a record starting in 1986. Mean column ozone during the same 47 day period in all four years are at least 11% below the average during the same period of the 1986–1997 record in the total column and at least 24% below the 1986–1997 record in the 12–20 km column. Average column ozone of 2000 and 2001 is also clearly below the minima observed between 1986 and 1997, with 2001 the lowest on record. These observations of lowest ozone are consistent with predictions and observations of peaking halogen abundance in the lower stratosphere around 2000 and a very slow decline thereafter. Such low ozone values are a combined consequence of stratospheric halogens and a continual and cold polar vortex. Man‐released halogens mainly affect the 12–20 km ozone column, while the ozone column above and below the 12–20 km column stays about constant. Several kilometer deep ozone‐depleted layers with mixing ratios below 0.1 ppmv appeared about 10 days earlier in 2001 than in 1998. Integrated ozonesonde measurements are compared to TOMS and Dobson spectrophotometer data with generally very good agreement. Measurement differences are largest when the vortex edge is close to McMurdo and decrease with increasing distance to the vortex edge region. The analysis of the vortex edge location at 400 K and 550 K shows that nearly all measurements were performed inside the polar vortex at 400 K, but extravortex air was sampled on occasion above 550 K. The earlier departure of the polar vortex edge above 550 K south of McMurdo led in 2000 to very high column ozone, both in the total column and in the 12–20 km column, which occasionally exceeds maximum observed values since 1986. Synchronous temperature measurements show correlations between high temperature, high ozone mixing ratio, and extravortex air.

Intercontinental transport and its influence on the ozone concentrations over central Europe: Three case studies

In this paper we report on a detailed analysis of the first clear observation of ozone import from North America in the free troposphere over central Europe in May 1996 and of two more recent cases of transatlantic transport. The analysis is based on calculations with the FLEXTRA trajectory model and the FLEXPART tracer model and data from Measurements of Ozone and Water Vapor by Airbus In‐Service Aircraft (MOZAIC) flights and North American surface sites. Lidar measurements carried out under conditions of developing anticyclonicity at Garmisch‐Partenkirchen, Germany, consistently yield peak ozone mixing ratios in the middle and upper troposphere between 80 and 110 ppb during the warm season. These layers are traced back to North America by FLEXTRA trajectories. The prevailing transport pathway involves a warm conveyor belt exporting high amounts of ozone, formed during a prefrontal high‐ozone episode, from the central part of the eastern United States to the upper troposphere. The polluted air may enter central Europe above an airstream descending from the stratosphere, thus inverting the normal atmospheric stratification, with stratospheric air at low levels and boundary layer air in the upper troposphere. However, the first two cases studied in this paper show that the export from the eastern United States may arrive over Europe at times prior to the onset of the anticyclonic conditions. Analysis of these cases reveals the complexity of the air export from North America. The ozone maxima occurring during the observational periods are traced back to the planetary boundary layer (PBL) in other source areas in the United States. Some of them are related to entrainments even from source areas beyond North America, either in the stratosphere or in Asia. The third case is quite different in its advection pathway, with almost straight and horizontal advection at 6 to 8 km above sea level starting from the Great Lakes area. A positive correlation between ozone and aerosol in that layer verifies the presence of PBL air. However, the very high ozone mixing ratio (up to 130 ppb) and an anticorrelation of ozone and humidity suggest the entrainment of another component from long‐range transport in the upper troposphere from beyond North America. In two of the case studies the export from the PBL was presumably influenced by large‐scale convection.

Vertical and latitudinal distributions of tropospheric ozone over the western Pacific: Case studies from the PACE aircraft missions

Three Pacific atmospheric chemistry experiments (PACE I, II, and III) were conducted each in January 1994, October 1994, and July 1995. The objective was to investigate the latitudinal distribution, transportation, and possible sources of tropospheric ozone over the western Pacific. Measurements of ozone using UV absorption method were taken by an aircraft flying at a maximum altitude of 6000 m between the Southern Hemisphere (SH) and the Northern Hemisphere (NH) (38°S–38°N) and within the longitudinal range of 120°E–150°E. The PACE aircraft missions first provide the middle troposphere (5000–6000 m) ozone distributions in three different seasons over this region in relatively short periods of time (within 9 days). On the basis of one‐minute average data, ozone mixing ratios in the middle troposphere were significantly higher in the NH midlatitudes (39–92 ppbv) and the SH midlatitudes (30–71 ppbv) than in the tropics (10–35 ppbv) during three different seasons. In particular, an air mass with low ozone mixing ratios (2–28 ppbv) extended from 7°S to 29°N in July during PACE III, in contrast to the air mass with higher ozone mixing ratios (39–57 ppbv) observed in the NH midlatitudes (&gt;21°N) in January during PACE I. Several episodes of increased ozone were observed during the PACE missions. In the SH subtropics (16°S–22°S), photochemical ozone production in a biomass‐burning smoke, probably emitted from the northwest part of Australia, caused relatively high ozone mixing ratios (maximum 89 ppbv) at 5200 m during PACE II. In contrast, ozone transport over a long distance from the upper troposphere south of Africa brought about a maximum ozone mixing ratio of 64 ppbv in the same geographical region above 4500 m during PACE III. Large‐scale circulation coupled with a typhoon was found to impact ozone transport from the NH midlatitudes to the tropics. An air mass with ozone mixing ratios averaging about 50 ppbv at 5200 m, which originated over the Asian continent in the NH midlatitudes, was transported to the tropical western Pacific region (9°N–14°N) by a circulation coupled with a typhoon during PACE II. Summer observations of tropopause folding over the western Pacific are rare, yet high potential vorticity (PV) value (&gt;1 PV unit) above 600 hPa level at 35°N shows that the tropopause folding beneath the subtropical jet occurred in the NH midlatitudes over the western Pacific in summer during PACE III. In this measurement, the increased ozone (maximum 92 ppbv) combined with decreased water vapor mixing ratio (&lt;3.0 g/kg) and large potential temperature gradient (0.083 K/m) was observed above 3800 m. These facts suggest stratospheric influenced air descended to 3800 m. The source region of the high ozone observed over the tropical region (5°S–9°S) during PACE II has not been confirmed, but it is thought to have originated from biomass burning and might be influenced by circulation associated with El Nino.

POAM III observations of the anomalous 2002 Antarctic ozone hole

The 2002 southern hemisphere winter was marked by unusually large wave activity, culminating with an unprecedented major warming in late September. This led to an ∼250 DU increase in column ozone near the pole as measured by the Polar Ozone and Aerosol Measurement (POAM) III instrument. POAM measurements of unusually high ozone mixing ratio throughout most of the stratosphere resulted primarily from air from outside the polar vortex being transported to the POAM measurement latitude. In the altitude region where chemical ozone loss from chlorine catalyzed chemistry occurs (below 600 K potential temperature) the 2002 ozone loss was similar to previous years up to the time of the major warming. The ozone loss diminished after this time (about 1 week earlier than usual), resulting in up to 20% less chemical ozone loss within the vortex than in previous winters. This corresponds to partial column ozone values below 600 K inside the polar vortex that were ∼25 DU larger in 2002 than in previous years.

Rapid changes in ozone mixing ratios at Cerro Tololo (30°10′S, 70°48′W, 2200 m) in connection with cutoff lows and deep troughs

Rapid changes in surface ozone mixing ratios (up to 15 ppbv over 12 hours) at the Cerro Tololo (CT) station (70°W, 30°S, 2200 m) over the period 1996–2000 are analyzed. These changes explain most of the wintertime variability of the ozone data at CT. Since the wintertime data show no significant diurnal cycle, local circulations and in situ photochemical processes cannot account for the sudden changes in ozone. Rather, a synoptic‐scale forcing of the changes is described on the basis of composite analyses of reanalysis data. Synoptic configurations associated with sudden changes in ozone were separated into W and D cases, according to the local evolution of humidity. W cases are characterized by humidity values in excess of 4 hPa lasting one or two days before the ozone maximum. D cases show no significant change in humidity before the ozone maximum. Both in W and in D cases, stratospheric air appears to be advected from polar latitudes to midlatitudes and subtropical latitudes during the development of cutoff lows and deep troughs. The origin of the air arriving at CT during a W and a D case study was assessed using mesoscale numerical simulations. The model calculations for the W case show a deep stratospheric intrusion reaching the lower troposphere in subtropical latitudes during the amplification and mature stages of a cutoff low. For D cases the stratospheric intrusion is mostly confined to midlatitudes and to an oceanic area far from the site of study. Analyses of trajectories show that in both cases, high ozone mixing ratios at the surface occur in connection with air parcels having a stratospheric history.

Persistence of stratospheric ozone layers in the troposphere

On 20th March 1994 at 12Z two dry layers with high ozone mixing ratio were encountered in the free troposphere by an ozonesonde ascent at Aberystwyth (52.4°N, 4.1°W). The layers resembled tropopause folds, but were not associated with any significant synoptic development, and did not have enhanced static stability. We use isentropic trajectory analysis, with winds taken from ECMWF analyses, to look for the origin of the layers. The analysis suggests that the two layers had different source regions. The lower layer spent 10 days over the Atlantic before reaching Northern Europe, and appears to have originated in the break-up of a stratospheric streamer between the 5th and 9th of March. The upper layer seems to have originated over the Western USA where the trajectories passed through a region of low Richardson number above the Rocky Mountains. In both cases, the low-water-vapour/high-ozone air mass had been advected with little mixing in the troposphere for at least 10 days. We argue that the air must be of stratospheric origin, since photo-chemistry cannot generate large amounts of ozone at this time of year and the trajectories do not, in any case, point to a boundary-layer origin. If, as the analysis suggests, the upper layer reached the troposphere by mixing across the jet then this may be evidence for stratosphere–troposphere exchange (STE) taking place other than by folding of the tropopause. The persistence of layers with anomalous chemical content in the troposphere for so long indicates that an accurate model of tropospheric transport and mixing is needed to assess the chemical impact of STE on tropospheric chemistry, in addition to a representation of the dynamical behaviour near to the tropopause.

Tropospheric ozone above the southwestern Indian Ocean is strongly linked to dynamical conditions prevailing in the tropics

A large ozone profile data set obtained at Réunion Island (21°S, 55°E) since 1992 is presented and linked to observed dynamical conditions in the region. The average seasonal variation of column integrated tropospheric ozone reaches 25 Dobson Units and peaks at 55 Dobson Units during the month of October. Two thermal inversions and high values of ozone mixing ratio during biomass burning activity in southeastern Africa are observed. The marine boundary layer (MBL) is characterized by high relative humidity values and low ozone mixing ratios. The decrease in the water vapor content above 2 km is strongly linked to seasonally evolving dynamical conditions. The first inversion largely hinders vertical exchanges, and low water vapor values in the free troposphere favor the photochemical production of ozone. The second thermal inversion is linked to the zonal wind inversion. Above this second inversion a large ozone enhancement is observed that occurs in parallel the biomass‐burning season in southeastern Africa and Madagascar. The season extents from August to December and peaks in October. The ozone maximum concomitantly results from the transport of ozone and its precursors westward from the biomass‐burning zones. High ozone values recorded in the upper troposphere during austral winter are likely to originate from additional stratosphere‐troposphere exchanges. Winter anticyclonic conditions further favor the photochemical production of ozone. The low tropospheric ozone mixing ratios observed across the entire troposphere during austral summer could result from the heterogeneous destruction of ozone and from less favorable conditions for its photochemical production, because of the development of deep convection.

Atlantic subtropical potential vorticity barrier as seen by Measurements of Ozone by Airbus In‐Service Aircraft (MOZAIC) flights

The existence and the seasonal variability of the Atlantic subtropical potential vorticity barrier controlling the stratosphere‐troposphere exchanges between the lowermost extratropical stratosphere and the upper equatorial troposphere are investigated using Measurements of Ozone by Airbus In‐Service Aircraft (MOZAIC). The methodology is based on the relationship between strong gradients of potential vorticity and of ozone mixing ratio situated on the cyclonic‐shear side of the subtropical jet stream. Episodes of high ozone mixing ratio sampled along MOZAIC flight tracks over the subtropical Atlantic with mixing ratio exceeding 100 ppbv on length scales larger than 200 km on flight levels between 11 and 12 km are studied. A total of 154 high ozone episodes is extracted from the MOZAIC database over the period August 1994 to April 1997. All these high ozone episodes are observed north of 15°N and have lengths ranging from subsynoptic to synoptic scales. It is shown that this barrier effect at 15°N over the central/eastern Atlantic fits with the southernmost latitude of the subtropical jet stream during the period of interest. South of the subtropical jet stream within the latitude band where the Intertropical Convergence Zone oscillates, tens of ozone‐rich transients (high‐ozone episodes with length scales smaller than 80 km) are sampled within the upper equatorial Atlantic troposphere (9 to 12 km). At present, the origin of these tropical ozone‐rich transients is still not clear. Some outlooks are given to investigate the possibility that some of the ozone‐rich transients may be interpreted as the final result of tropopause foldings and small scale mixing processes. The seasonal variability of the subtropical barrier is captured when determining the subtropical tropopause break (STB) point for each flight, that is, the southernmost latitude of the southernmost high‐ozone episode of a flight, and classifying STB points on a monthly basis. A sinusoidal evolution of STB points appears, reflecting the northernmost (southernmost) position of the dynamical barrier in summer (winter) boreal months. This seasonal variability clearly agrees with that of the position of the subtropical jet stream as derived from mean isotach analysis.

Tropospheric water-vapour and ozone cross-sections in a zonal plane over the central equatorial Pacific Ocean

Tropospheric water-vapour and ozone measurements, using calibrated balloon-borne sensors, are reported from the Central Equatorial Pacific Experiment (CEPEX). the sensors were launched from the Research Vessel Vickers along 2°S latitude between 156°E (west of the international date line) and 155°W (east of the date line). These measurements are combined with those from water-vapour sondes launched over the western Pacific warm pool, during the Coupled Ocean-Atmosphere Response Experiment (TOGA-COARE). Taking the two experiments CEPEX and TOGA-COARE together, the sensors included frost-point hygrometers, Humicap-A Väisälä sondes, Humicap-H Väisälä sondes and electrochemical ozone-sondes. Taken together, the CEPEX and TOGA-COARE data provide over 150 vertical profiles of water vapour within the troposphere in varied conditions of convective activity ranging from disturbed to suppressed. the primary motivation behind the present analyses is to understand the role of tropical deep convection in the vertical distribution of water-vapour. With this in mind, the profiles have been analysed in relation to occasions of recent deep convection and occasions when convection was suppressed. We employ three different criteria to identify the profiles influenced by deep convection: brightness temperature in the infrared-window channel of the Japanese Geostationary Meteorological Satellite (GMS); ozone as a quasi-conservative tracer for deep convection; and using water vapour itself, that is the wettest versus the driest soundings. Irrespective of the criteria used, we report here that the atmosphere, while under the influence of active deep convection, was found to have relative humidities in excess of 75% over most of the troposphere between the surface and about 14 km. the sondes were launched routinely over a period of 45 days (between CEPEX and TOGA-COARE), without biasing the sample towards convectively disturbed conditions. A feature of the convectively disturbed profile is a distinct minimum in relative humidity at about 700 hPa, where it was as low as 65%. the low relative humidity was accompanied by relatively high ozone mixing ratios, which raises the possibility of long-range transport of dry sub-tropical air into the warm, convectively disturbed, regions of the equatorial Pacific Ocean. Inspection of the analysed fields, and the wind fields from the sondes, supports this assertion. It then follows that the omnipresent minimum of moist static energy and minimum relative humidity at 700 hPa in the inner tropics may be the result of long-range, inclined, transport of dry air from non-convective regions. This detection suggests a linkage between the large-scale circulation, deep convection and the thermodynamic structure within the equatorial troposphere. The results presented here demonstrate the applicability of ozone as a quasi-conservative tracer of transport in the context of deep convection. The ozone-based criterion is used to diagnose recent deep convection, independent of the GMS satellite observations, and allows one to follow the evolution of relative humidity and of water-vapour mixing ratio after the dissipation of the cloud anvil to optically thin conditions. We show that the troposphere dries to low humidity soon after anvil dissipation. This observation leads to the hypothesis that moistening of the atmosphere, away from the core of Cb convection, occurs by evaporation of precipitation falling out of the anvils. After anvil dissipation, the ensuing subsidence in clear air causes the relative humidity and the water mixing ratio to decrease.

Tropospheric water‐vapour and ozone cross‐sections in a zonal plane over the central equatorial Pacific Ocean

AbstractTropospheric water‐vapour and ozone measurements, using calibrated balloon‐borne sensors, are reported from the Central Equatorial Pacific Experiment (CEPEX). the sensors were launched from the Research Vessel Vickers along 2°S latitude between 156°E (west of the international date line) and 155°W (east of the date line). These measurements are combined with those from water‐vapour sondes launched over the western Pacific warm pool, during the Coupled Ocean‐Atmosphere Response Experiment (TOGA‐COARE). Taking the two experiments CEPEX and TOGA‐COARE together, the sensors included frost‐point hygrometers, Humicap‐A Väisälä sondes, Humicap‐H Väisälä sondes and electrochemical ozone‐sondes. Taken together, the CEPEX and TOGA‐COARE data provide over 150 vertical profiles of water vapour within the troposphere in varied conditions of convective activity ranging from disturbed to suppressed. the primary motivation behind the present analyses is to understand the role of tropical deep convection in the vertical distribution of water‐vapour. With this in mind, the profiles have been analysed in relation to occasions of recent deep convection and occasions when convection was suppressed.We employ three different criteria to identify the profiles influenced by deep convection: brightness temperature in the infrared‐window channel of the Japanese Geostationary Meteorological Satellite (GMS); ozone as a quasi‐conservative tracer for deep convection; and using water vapour itself, that is the wettest versus the driest soundings. Irrespective of the criteria used, we report here that the atmosphere, while under the influence of active deep convection, was found to have relative humidities in excess of 75% over most of the troposphere between the surface and about 14 km. the sondes were launched routinely over a period of 45 days (between CEPEX and TOGA‐COARE), without biasing the sample towards convectively disturbed conditions.A feature of the convectively disturbed profile is a distinct minimum in relative humidity at about 700 hPa, where it was as low as 65%. the low relative humidity was accompanied by relatively high ozone mixing ratios, which raises the possibility of long‐range transport of dry sub‐tropical air into the warm, convectively disturbed, regions of the equatorial Pacific Ocean. Inspection of the analysed fields, and the wind fields from the sondes, supports this assertion.It then follows that the omnipresent minimum of moist static energy and minimum relative humidity at 700 hPa in the inner tropics may be the result of long‐range, inclined, transport of dry air from non‐convective regions. This detection suggests a linkage between the large‐scale circulation, deep convection and the thermodynamic structure within the equatorial troposphere.The results presented here demonstrate the applicability of ozone as a quasi‐conservative tracer of transport in the context of deep convection.The ozone‐based criterion is used to diagnose recent deep convection, independent of the GMS satellite observations, and allows one to follow the evolution of relative humidity and of water‐vapour mixing ratio after the dissipation of the cloud anvil to optically thin conditions. We show that the troposphere dries to low humidity soon after anvil dissipation. This observation leads to the hypothesis that moistening of the atmosphere, away from the core of Cb convection, occurs by evaporation of precipitation falling out of the anvils. After anvil dissipation, the ensuing subsidence in clear air causes the relative humidity and the water mixing ratio to decrease.

Polyphenylenesulfide, noxon® an ozone scavenger for the analysis of oxygenated terpenes in air

During sampling, oxygenated terpenes may undergo decomposition through reaction with atmospheric ozone. We have studied their ozonolytic decomposition during preconcentration on Tenax. The saturated. terpenoids 1,8-cineole, bornyl acetate nopinone and pinonaldehyde are practically unaffected by ozone in the range of 8 to 120 ppbv. Compounds which contain one or more C-C double bonds are decomposed in the order: linalool ≈ citronellal ≈ 6-methyl-5-hepten-2-one > citral > 4-acetyl-1-methyl-cyclohexane > 3-(1-methylethenyl)-6-oxo-heptanal > myrtenal ≈ 2-methyl-3-buten-2-ol. The degree of decomposition varies from 0 to 5% for the least reactive to 80 to 90% for the most reactive compounds. A broad range of material was investigated as potential ozone scavengers. By using the polymer noXon (polyphenylenesulfide) all the investigated compounds could be sampled with quantitative recoveries even at high ozone mixing ratios (95–110 ppbv). This ozone scrubber was tested for sampling of terpene oxidation products on Tenax and dinitrophenylhydrazine impregnated C 18-silicagel cartridges. Recoveries from 85 to 110% were obtained for all investigated compounds. The method was used for the analysis of oxidation products of terpenes in ambient air in three campaigns. Attention was focused on nopinone from β-pinene, pinonaldehyde from α-pinene, 3-(1-methylethenyl)-6-oxo-heptanal and 4-acetyl-1-methyl-cyclohexane from limonene, and 5-(1-methylethyl)-bicyclo[3.1.0] hexan-2-one from sabinene. Nopinone was the only product which could be frequently detected in ratios from 0 to 90% of the measured β-pinene concentrations. Pinonaldehyde was encountered only once (30% of α-pinene) while the other products were not found. These data have to be seen as a first attempt to measure terpene oxidation products in the troposphere.