Tropospheric Ozone Mixing Ratios Research Articles

Effect of ozone exposure on the foraging behaviour of Bombus terrestris

Tropospheric ozone (O3) mixing ratios have increased substantially since preindustrial times and high O3 peaks are increasingly common. Plant-pollinator interactions are central to natural ecosystem functioning and food production systems but could be negatively affected by unfavourable environmental conditions such as elevated O3. Ecosystem functioning is threatened by O3, which can degrade floral volatile organic compounds (VOCs) used by pollinators as olfactory cues during foraging. It can also exert oxidative stress on VOC-emitting plants and receiving organisms, potentially disturbing the sending and receiving of VOC signals. The aim of this study was to determine the effects of elevated ozone on the foraging behaviour of Bombus terrestris on three species of the Brassicaceae, with a particular focus on bumblebee choices and the mechanisms underpinning differences observed. Moreover, the study was designed to fill a gap between observations in small-scale laboratory experiments and large-scale modelling through empirical observations in polytunnels that represent a medium-large-scale artificial environment. Using 10 × 3 × 2 m polytunnels the effects of O3 on pollinator foraging parameters on Sinapis alba, Sinapis arvensis and Raphanus raphanistrum were assessed. Significant effects of elevated O3 (100 ± 10 ppb) on the time taken for the first bee to alight on a flower and the cumulative amount of time spent on flowers was observed. To further investigate the underlying mechanisms, a laboratory test was conducted to determine the effects of ozone on the VOC blend composition of S. alba flowers. Synthetic VOC blends representing O3-altered and unaltered profiles were reconstituted and utilized in polytunnel and olfactometry experiments. The results indicated that a reduction of olfaction-mediated orientation, probably via VOC-degradation or direct effects of O3 on bees, was responsible for the altered foraging parameters of B. terrestris, suggesting that the presence of elevated O3 could have negative effects on the foraging efficiency of important pollinator species.

Evaluating stratospheric ozone and water vapour changes in CMIP6 models from 1850 to 2100

Abstract. Stratospheric ozone and water vapour are key components of the Earth system, and past and future changes to both have important impacts on global and regional climate. Here, we evaluate long-term changes in these species from the pre-industrial period (1850) to the end of the 21st century in Coupled Model Intercomparison Project phase 6 (CMIP6) models under a range of future emissions scenarios. There is good agreement between the CMIP multi-model mean and observations for total column ozone (TCO), although there is substantial variation between the individual CMIP6 models. For the CMIP6 multi-model mean, global mean TCO has increased from ∼ 300 DU in 1850 to ∼ 305 DU in 1960, before rapidly declining in the 1970s and 1980s following the use and emission of halogenated ozone-depleting substances (ODSs). TCO is projected to return to 1960s values by the middle of the 21st century under the SSP2-4.5, SSP3-7.0, SSP4-3.4, SSP4-6.0, and SSP5-8.5 scenarios, and under the SSP3-7.0 and SSP5-8.5 scenarios TCO values are projected to be ∼ 10 DU higher than the 1960s values by 2100. However, under the SSP1-1.9 and SSP1-1.6 scenarios, TCO is not projected to return to the 1960s values despite reductions in halogenated ODSs due to decreases in tropospheric ozone mixing ratios. This global pattern is similar to regional patterns, except in the tropics where TCO under most scenarios is not projected to return to 1960s values, either through reductions in tropospheric ozone under SSP1-1.9 and SSP1-2.6, or through reductions in lower stratospheric ozone resulting from an acceleration of the Brewer–Dobson circulation under other Shared Socioeconomic Pathways (SSPs). In contrast to TCO, there is poorer agreement between the CMIP6 multi-model mean and observed lower stratospheric water vapour mixing ratios, with the CMIP6 multi-model mean underestimating observed water vapour mixing ratios by ∼ 0.5 ppmv at 70 hPa. CMIP6 multi-model mean stratospheric water vapour mixing ratios in the tropical lower stratosphere have increased by ∼ 0.5 ppmv from the pre-industrial to the present-day period and are projected to increase further by the end of the 21st century. The largest increases (∼ 2 ppmv) are simulated under the future scenarios with the highest assumed forcing pathway (e.g. SSP5-8.5). Tropical lower stratospheric water vapour, and to a lesser extent TCO, shows large variations following explosive volcanic eruptions.

Impacts of the atmospheric apparent heat source over the Tibetan Plateau on summertime ozone vertical distributions over Lhasa

The Tibetan Plateau (TP) is an area sensitive to climate change, where the ozone distribution affects the atmospheric environment of the TP and its surrounding regions. The relatively low total column ozone over the TP in boreal summer and its spatiotemporal variations have received extensive attention. In this study, five-year balloon-borne measurements of ozone over Lhasa in boreal summer are used to investigate the influences of the apparent heat source (Q1) on the ozone vertical structure over the plateau. The mechanisms for the above processes are also explored. The results show that the tropospheric ozone mixing ratio over Lhasa decreases when the total atmospheric Q1 in the troposphere over the TP is relatively high. Strengthened ascending motions are accompanied by enhanced Q1 over the main TP region. Consequently, the tropospheric ozone mixing ratio over Lhasa decreases when Q1 is higher in summer, which is attributed to the upward transport of the ozone-poor surface air.摘要青藏高原(TP)是一个对气候变化敏感的地区, 其上空的臭氧分布影响着青藏高原及其周边地区的大气环境.北半球夏季青藏高原上空臭氧柱总量相对较低的现象, 及其时空变化受到广泛关注.本研究利用北半球夏季5年的拉萨上空臭氧的气球测量数据, 研究高原上空大气视热源(Q1)对臭氧垂直分布的影响并探讨了该过程的机制.结果表明, 当TP上空对流层整体的Q1相对较高时, 拉萨上空对流层臭氧浓度下降.大气更强的上升运动伴随着TP主体区域上空的Q1的增大.因此, 当夏季Q1较高时, 由于近地表低浓度臭氧空气向上输送, 拉萨上空的对流层臭氧浓度下降.

Net ozone production and its relationship to nitrogen oxides and volatile organic compounds in the marine boundary layer around the Arabian Peninsula

Abstract. Strongly enhanced tropospheric ozone (O3) mixing ratios have been reported in the Arabian Basin, a region with intense solar radiation and high concentrations of O3 precursors such as nitrogen oxides (NOx) and volatile organic compounds (VOCs). To analyze photochemical O3 production in the marine boundary layer (MBL) around the Arabian Peninsula, we use shipborne observations of NO, NO2, O3, OH, HO2, HCHO, the actinic flux, water vapor, pressure and temperature obtained during the summer 2017 Air Quality and Climate in the Arabian Basin (AQABA) campaign, and we compare them to simulation results from the ECHAM-MESSy Atmospheric Chemistry (EMAC) general circulation model. Net O3 production rates (NOPRs) were greatest over both the Gulf of Oman and the northern Red Sea (16 ppbv d−1) and over the Arabian Gulf (32 ppbv d−1). The NOPR over the Mediterranean, the southern Red Sea and the Arabian Sea did not significantly deviate from zero; however, the results for the Arabian Sea indicated weak net O3 production of 5 ppbv d−1 as well as net O3 destruction over the Mediterranean and the southern Red Sea with values of −1 and −4 ppbv d−1, respectively. Constrained by HCHO∕NO2 ratios, our photochemistry calculations show that net O3 production in the MBL around the Arabian Peninsula mostly occurs in NOx-limited regimes with a significant share of O3 production occurring in the transition regime between NOx limitation and VOC limitation over the Mediterranean and more significantly over the northern Red Sea and Oman Gulf.

Meteorological factors affecting lower tropospheric ozone mixing ratios in Bangkok, Thailand

This paper examines the influence of meteorological conditions in ozone mixing ratio measured at the Thai Meteorological Department (TMD) in Bangkok, Thailand. In addition to surface wind speed and direction, surface ozone concentrations, ozonesondes and CALIPSO Lidar images were collected during the study period extending from 01/01/2014 to 30/04/2015. Surface ozone concentrations show a strong seasonality, with maximum in the dry months of December to April and minimum during the wet southwest (SW) monsoon period extending from May to October. High ozone concentrations are related to biomass burning in the northeast highland regions of the country and neighboring Myanmar and southern China. These precursors travel in a southerly direction towards Bangkok in a well-defined aerosol layer which may be at ground level or at elevated heights. The growth of the daytime mixed layer scavenges some of the upper level aerosols, although local maxima in ozone concentrations at 1–2km are a frequent feature at Bangkok. There is an evidence of fumigation in the Gulf of Thailand and a return flow via the southerly sea breezes.

Bimodal distribution of free tropospheric ozone over the tropical western Pacific revealed by airborne observations

AbstractA recent airborne field campaign over the remote western Pacific obtained the first intensive in situ ozone sampling over the warm pool region from oceanic surface to 15 km altitude (near 360 K potential temperature level). The new data set quantifies ozone in the tropical tropopause layer under significant influence of convective outflow. The analysis further reveals a bimodal distribution of free tropospheric ozone mixing ratio. A primary mode, narrowly distributed around 20 ppbv, dominates the troposphere from the surface to 15 km. A secondary mode, broadly distributed with a 60 ppbv modal value, is prominent between 3 and 8 km (320 K to 340 K potential temperature levels). The latter mode occurs as persistent layers of ozone‐rich drier air and is characterized by relative humidity under 45%. Possible controlling mechanisms are discussed. These findings provide new insight into the physical interpretation of the “S”‐shaped mean ozone profiles in the tropics.

Tropical upper tropospheric ozone enhancements due to potential vorticity intrusions over Indian sector

Influence of potential vorticity (PV) intrusions at 13.5°N over and near Indian sector (50°E–90°E) on tropical upper tropospheric ozone mixing ratio (OMR) variations is demonstrated based on two case studies. Increase of ECMWF (European Centre for Medium-range Weather Forecasting) reanalysis (ERA)-interim OMR in the upper troposphere (200–500hPa) is observed during the intrusion events consistently in both cases. The OMR also shows similar tongue like structure as PV and it even follows the spatial shift of the PV tongue. In addition, the enhancements in the upper tropospheric OMR during the intrusion events are confirmed using microwave limb sounder (MLS) ozone data at 216hPa. It is suggested that the existence of strong downdrafts, associated with the ageostrophic circulation due to jet stream, which is inferred from longitude-height cross-section of ERA-interim vertical velocity could bring the ozone further down, though high PV tongue remains only at higher level (above 400hPa). The importance of these results lies in demonstrating the role of PV intrusion events on the enhancement of tropical upper tropospheric ozone over Indian sector, where the impact of the PV intrusions is not well understood when compared to that over Pacific and Atlantic sectors.

Examination of the atmospheric conditions associated with high and low summer ozone levels in the lower troposphere over the eastern Mediterranean

Abstract. In order to evaluate the observed high rural ozone levels in the eastern Mediterranean area during summertime, vertical profiles of ozone measured in the period 1994–2008 in the framework of the MOZAIC project (Measurement of Ozone and Water Vapor by Airbus in Service Aircraft) over the eastern Mediterranean basin (Cairo, Tel Aviv, Heraklion, Rhodes, Antalya) were analyzed, focusing in the lower troposphere (1.5–5 km). At first, vertical profiles collected during extreme days with very high or very low tropospheric ozone mixing ratios have been examined together with the corresponding back-trajectories. Also, the average profiles of ozone, relative humidity, carbon monoxide, temperature gradient and wind speed corresponding to the 7% highest and the 7% lowest ozone mixing ratios for the 1500–5000 m height layer for Cairo and Tel Aviv have been examined and the corresponding composite maps of geopotential heights at 850 hPa have been plotted. Based on the above analysis, it turns out that the lower-tropospheric ozone variability over the eastern Mediterranean area is controlled mainly by the synoptic meteorological conditions, combined with local topographical and meteorological features. In particular, the highest ozone concentrations in the lower troposphere and subsequently in the boundary layer are associated with large-scale subsidence of ozone-rich air masses from the upper troposphere under anticyclonic conditions while the lowest ozone concentrations are associated with low pressure conditions inducing uplifting of boundary-layer air, poor in ozone and rich in relative humidity, to the lower troposphere.

Differences in the variability of measured and simulated tropospheric ozone mixing ratios over the Paso del Norte Region

The objective of this study is to present differences in the variability of observed and ozone-mixing ratios simulated by a three-dimensional atmospheric chemical model using two chemical mechanisms. In this study the Comprehensive Air Quality Model with Extensions is used to make ozone simulations with the Carbon Bond mechanism, versions 4 and 5. The Paso del Norte region is used as a test-bed for these simulations. The shared variance between the simulations and measurements is typical for air quality models ranging from 0.51 to 0.86 for both mechanisms. The smallest mean normalized gross error is about 31 % with CB4 but the normalized bias is over 30 % as well. Boundary conditions, emissions and other factors affect the levels of ozone of the simulated mixing ratios and therefore error and bias but these factors have a much less affect on the simulated ozone variability. The differences in the ozone variability of the measurements and the simulations are very large and different for the two chemical mechanisms. There are many more instances of low ozone mixing ratios in the measurements than in the simulated ozone. One possible explanation is that these differences are due to problems associated with comparing point measurements with grid averages. A more disturbing possibility is that the bias could be due to the procedures used in the development and testing of air quality modeling systems. Air quality mechanisms are evaluated against environmental chamber data where the chemistry occurs at high concentrations and this may lead to a systematic positive bias in ozone simulations.

Multi-site tropospheric ozone measurements across the North Tropical Atlantic during the summer of 2010

Ozone soundings are launched during two Intensive Observing Periods (IOPs) from Dakar, Senegal; Sao Vicente, Cape Verde; and St James, Barbados to investigate ozone variability across the North Tropical Atlantic during June/July and August/September 2010. Two objectives of the campaign are to compare background tropospheric ozone mixing ratios and its variability associated with the Saharan Air Layer (SAL) and African Easterly Waves (AEWs) at sites located in the Eastern and Western Tropical Atlantic Ocean. During IOP1 (June–July), reduced ozone mixing ratios are found in the SAL with elevated ozone levels at the SAL's base and above it. During IOP I, the tropospheric column ozone (TCO) is higher at Cape Verde (27.5 DU) when compared to Barbados (19.8 DU). During IOP II (August–September) ozone-rich air is found above 500 hPa prior to the passage of AEW or developing tropical cyclones. The observed larger mixing ratios of middle/upper tropospheric ozone are most prominent at Dakar prior to the passage of an AEW, which we attribute to lightning NOx. During IOP II the tropospheric column ozone is highest at Dakar (30.5 DU) when compared to Cape Verde (20.2 DU) and Barbados (17.2 DU).

The influence of the SAL and lightning on tropospheric ozone variability over the Northern Tropical Atlantic: Results from Cape Verde during 2010

We present results for multiple Saharan Air Layer (SAL) events and their relationship to tropospheric ozone mixing ratios that were observed in the Eastern Atlantic during the summer of 2010 using ozonesondes. In particular, 5 SAL events are sampled during 2010 in Cape Verde indicting a reduction of ozone mixing ratios throughout much of the SAL layer, except near the base of the SAL. In this layer of enhanced ozone mixing ratio, we find increases of 20–30 ppb in some cases between non‐SAL and SAL conditions. In addition, ozone concentrations are enhanced above the SAL layer, with trajectories suggesting enhancements by lightning from the middle/upper troposphere with outflow from Africa. Additional aircraft measurements are required to examine the chemical and aerosol distributions from the Marine Boundary Layer (MBL) through the upper troposphere to determine the heterogeneous chemical processes related to reduced ozone mixing ratio, and further quantify elevated ozone mixing ratios at the base of the SAL and above the SAL.

Tropospheric bromine chemistry: implications for present and pre-industrial ozone and mercury

Abstract. We present a new model for the global tropospheric chemistry of inorganic bromine (Bry) coupled to oxidant-aerosol chemistry in the GEOS-Chem chemical transport model (CTM). Sources of tropospheric Bry include debromination of sea-salt aerosol, photolysis and oxidation of short-lived bromocarbons, and transport from the stratosphere. Comparison to a GOME-2 satellite climatology of tropospheric BrO columns shows that the model can reproduce the observed increase of BrO with latitude, the northern mid-latitudes maximum in winter, and the Arctic maximum in spring. This successful simulation is contingent on the HOBr + HBr reaction taking place in aqueous aerosols and ice clouds. Bromine chemistry in the model decreases tropospheric ozone mixing ratios by <1–8 nmol mol−1 (6.5% globally), with the largest effects in the northern extratropics in spring. The global mean tropospheric OH concentration decreases by 4%. Inclusion of bromine chemistry improves the ability of global models (GEOS-Chem and p-TOMCAT) to simulate observed 19th-century ozone and its seasonality. Bromine effects on tropospheric ozone are comparable in the present-day and pre-industrial atmospheres so that estimates of anthropogenic radiative forcing are minimally affected. Br atom concentrations are 40% higher in the pre-industrial atmosphere due to lower ozone, which would decrease by a factor of 2 the atmospheric lifetime of elemental mercury against oxidation by Br. This suggests that historical anthropogenic mercury emissions may have mostly deposited to northern mid-latitudes, enriching the corresponding surface reservoirs. The persistent rise in background surface ozone at northern mid-latitudes during the past decades could possibly contribute to the observations of elevated mercury in subsurface waters of the North Atlantic.

Are there urban signatures in the tropospheric ozone column products derived from satellite measurements?

Abstract. In view of the proposed geostationary satellite missions to monitor air quality from space, it is important to first assess the capability of the current suite of satellite instruments to provide information on the urban scale pollution. We explore the possibility of detecting urban signatures in the tropospheric column ozone data derived from Total Ozone Mapping Spectrometer (TOMS)/Solar Backscattered Ultraviolet (SBUV) and Ozone Monitoring Instrument (OMI)/Microwave Limb Sounder (MLS) satellite data. We find that distinct isolated plumes of tropospheric ozone near several large and polluted cities around the world may be detected in these data sets. The ozone plumes generally correspond with the tropospheric column NO2 plumes around these cities as observed by the Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) instrument. Similar plumes are also seen in tropospheric mean ozone mixing ratio distribution after accounting for the surface and tropopause pressure variations. The total column ozone retrievals indicate fairly significant sensitivity to the lower troposphere over the polluted land areas, which might help explain these detections. These results indicate that ultraviolet (UV) measurements may, in principle, be able to capture the urban signatures and may have implications for future missions using geostationary satellites.

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.

Accurate satellite-derived estimates of the tropospheric ozone impact on the global radiation budget

Abstract. Estimates of the radiative forcing due to anthropogenically-produced tropospheric O3 are derived primarily from models. Here, we use tropospheric ozone and cloud data from several instruments in the A-train constellation of satellites as well as information from the GEOS-5 Data Assimilation System to accurately estimate the radiative effect of tropospheric O3 for January and July 2005. Since we cannot distinguish between natural and anthropogenic sources with the satellite data, our derived radiative effect reflects the unadjusted (instantaneous) effect of the total tropospheric O3 rather than the anthropogenic component. We improve upon previous estimates of tropospheric ozone mixing ratios from a residual approach using the NASA Earth Observing System (EOS) Aura Ozone Monitoring Instrument (OMI) and Microwave Limb Sounder (MLS) by incorporating cloud pressure information from OMI. We focus specifically on the magnitude and spatial structure of the cloud effect on both the short- and long-wave radiative budget. The estimates presented here can be used to evaluate the various aspects of model-generated radiative forcing. For example, our derived cloud impact is to reduce the radiative effect of tropospheric ozone by ~16%. This is centered within the published range of model-produced cloud effect on unadjusted ozone radiative forcing.

Surface ozone mixing ratio increase with altitude in a transect in the Catalan Pyrenees

Tropospheric ozone mixing ratios and their phytotoxicity and NO 2 mixing ratios were measured along an altitudinal gradient at the Meranges valley in the Catalan Pyrenees. Biweekly measurements using Radiello passive samplers were taken along a transect of seven stations ranging from 1040 to 2400 m ASL from May to December 2004. As well, at each station biweekly evaluations were made of the visual symptoms of ozone damage in Bel-W3 and Bel-B tobacco cultivars. Whereas ozone mixing ratios increased with altitude, NO 2 mixing ratios decreased from the valley floor upwards. Ozone damage rates were found to vary with time and space depending on local environmental and meteorological conditions, although the highest ozone damage to foliage was found in the stations at greatest altitude, especially wherever altitudinal micrometeorological conditions enhanced plant sensitivity.

Enhanced view of the “tropical Atlantic ozone paradox” and “zonal wave one” from the in situ MOZAIC and SHADOZ data

Ozone vertical profiles from the Measurements of Ozone from Airbus In‐service Aircraft (MOZAIC) program over Africa are used to complement pictures of the wave‐one pattern and the “tropical Atlantic paradox” identified through soundings in the Southern Hemisphere Additional Ozonesondes (SHADOZ) network. The Atlantic paradox refers to a greater tropospheric ozone column amount over the South Atlantic than the North Atlantic during the West African biomass burning season. SHADOZ and MOZAIC data from 1998–2002 and 1997–2004, respectively, are used to show that these two phenomena are linked. The combined data are used to address the following: Does the (continental) MOZAIC data modify the appearance of the paradox? Do lower tropospheric MOZAIC data lead to new conclusions about ozone in the wave‐one maximum region? During December, January, and February (DJF), the lower troposphere over Africa exhibits a higher ozone signal in the burning hemisphere, that is, north of the equator, so the “paradox” does not appear over the African continent. The MOZAIC data set over Africa highlights another component of the wave‐one feature when the tropospheric ozone mixing ratio is viewed in zonal cross section. The lower troposphere makes a nonnegligible contribution to the regionally higher ozone column during the biomass burning periods of each hemisphere (DJF) for West Africa and June, July, and August (JJA) for the central Africa region. A southern preference for the wave‐one character, previously deduced from satellite data, is confirmed with a stronger maximum in September, October, and November (SON). Both the paradox and wave‐one phenomena are consistent with a view that the African continent is a major source of biomass burning and lightning emissions.

Seasonal characteristics of tropospheric ozone production and mixing ratios over East Asia: A global three‐dimensional chemical transport model analysis

We examine seasonal and geographical distributions of tropospheric ozone production and mixing ratios over East Asia with a global three‐dimensional chemical transport model called Model of Ozone and Related Tracers, version 1 (MOZART 1). Net ozone production within the East Asian boundary layer exhibits three distinct seasonal cycles depending on region (north of 20°N, 5–20°N and south of 5°N). North of 20°N, net ozone production over East Asia from spring through autumn is found to have a maximum extending from 25°N–40°N and from central eastern China to Japan, resulting from the strong emission and transport of anthropogenic O3 precursors. In winter, maximum O3 production in this region occurs between 20°N and 30°N. This is a region of long‐range transport. Over the Indochina peninsula, between 5°N and 20°N, net O3 production is controlled by the seasonal cycle between wet and dry seasons and has a maximum at the end of the dry season due to emissions from biomass burning. South of 5°N, in the true tropics, O3 mixing ratios are relatively constant throughout the year and do not exhibit a seasonal cycle. A spring‐summer maximum of net O3 production is found throughout the troposphere in East Asia. We estimate an annual net O3 production in East Asia of 117 Tg/yr. Both model results and analysis of measurements of O3/CO correlations over East Asia and Japan show strong variability as a function of both photochemical activity and seasonal meteorology, and indicate ozone export off the coast of East Asia in spring. An upper estimate of O3 export from East Asia to the Pacific Ocean in the mid‐1980s of 3.3 Gmol/d (58 Tg/yr) is obtained.

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.

Measurement of tropospheric ozone by thermal emission spectroscopy

A new technique has been developed for the remote sensing of tropospheric ozone. In this technique the thermal radiation from ozone is measured against the cold background blackbody emission of clouds using ground-based FTIR spectroscopy. Preliminary measurements obtained in the springtime at Peterborough, Ontario, using this new technique, have indicated an average tropospheric ozone mixing ratio of 50 ± 7 ppbv within a 2.7 km layer from the surface for air flowing from a heavily industrialized and highly urbanized region. Ozone concentrations of this magnitude are in accordance with the amounts obtained for moderately polluted, springtime air using traditional in situ measurement techniques. The greenhouse radiation flux associated with this amount of ozone was estimated to be 0.61 W m −2 ± 13%. Since there are few techniques available for the remote sensing of tropospheric air pollutants, this new technique could be a potentially valuable development.