Tropospheric Ozone Amounts Research Articles

Long-Term Tropospheric Ozone Data Analysis 1997–2019 at Giordan Lighthouse, Gozo, Malta

Long-term data analysis of the hourly ozone volume fractions in the middle of the Mediterranean Seawas carried out covering a period of 22 years. It was noticed that the amount of ozone during this period very rarely exceeded the recommended upper limit value of 80 ppb and that the amount of tropospheric ozone in the area is rather low. Fourier data analysis shows the presence of only a seasonal cycle in ozone concentrations. Statistical analysis of the data is showing a slightly negative trend in ozone concentrations of −0.46 ± 0.08 ppb/year for average values and a slightly higher negative trend of −0.54 ± 0.11 ppb/year for the 95th percentile values. These results obtained through simple linear regression were confirmed using the more appropriate Mann–Kendall test. The possible quadratic trend was not observed for the whole series of data. Air mass trajectories were calculated for those days in the year with the highest pollution, indicating that during those days horizontal air transfer, in most cases, brings the air mass from the North and from Sicily in Southern Italy.

Estimation of Stratospheric Intrusions During Indian Cyclones

AbstractDeep convection associated with tropical cyclones leads to stratosphere‐troposphere exchange (STE), which affects the upper‐tropospheric ozone concentrations in the vicinity of the cyclones. This study estimates the ozone enhancements over India due to the North Indian Ocean (NIO) cyclones‐driven STE. Indicators such as stratospheric fraction and potential vorticity calculated using the reanalysis data sets suggest that roughly 70% of the cyclones show anomalously high stratospheric intrusions. Aircraft observations over different locations across India also show elevated ozone concentrations in the mid‐to‐upper troposphere on cyclone days. Further, ozone and stratospheric ozone tracer concentrations from Goddard Earth Observing System‐Chemistry simulations and the Copernicus Atmosphere Monitoring Service reanalysis data sets show up to 40 ppb of excess upper tropospheric ozone over India, of which stratospheric ozone accounts for roughly 60%. Stratospheric intrusion due to the Bay of Bengal and the Arabian Sea cyclones affected the upper tropospheric ozone amounts over North and South India, respectively. The stratospheric ozone was observed to propagate downwards into the troposphere, often reaching ∼600 hPa and, in some cases, even the surface.

Update and evaluation of the ozone dry deposition in Oslo CTM3 v1.0

Abstract. High concentrations of ozone in ambient air are hazardous not only to humans but to the ecosystem in general. The impact of ozone damage on vegetation and agricultural plants in combination with advancing climate change may affect food security in the future. While the future scenarios in themselves are uncertain, there are limiting factors constraining the accuracy of surface ozone modeling also at present: the distribution and amount of ozone precursors and ozone-depleting substances, the stratosphere–troposphere exchange, as well as scavenging processes. Removal of any substance through gravitational settling or by uptake by plants and soil is referred to as dry deposition. The process of dry deposition is important for predicting surface ozone concentrations and understanding the observed amount and increase of tropospheric background ozone. The conceptual dry deposition velocities are calculated following a resistance-analogous approach, wherein aerodynamic, quasi-laminar, and canopy resistance are key components, but these are hard to measure explicitly. We present an update of the dry deposition scheme implemented in Oslo CTM3. We change from a purely empirical dry deposition parameterization to a more process-based one which takes the state of the atmosphere and vegetation into account. We examine the sensitivity of the scheme to various parameters, e.g., the stomatal conductance-based description of the canopy resistance and the choice of ozone surface resistance, and evaluate the resulting modeled ozone dry deposition with respect to observations and multi-model studies. Individual dry deposition velocities are now available for each land surface type and agree generally well with observations. We also estimate the impact on the modeled ozone concentrations at the surface. We show that the global annual total ozone dry deposition decreases with respect to the previous model version (−37 %), leading to an increase in surface ozone of more than 100 % in some regions. While high sensitivity to changes in dry deposition to vegetation is found in the tropics and the Northern Hemisphere, the largest impact on global scales is associated with the choice of prescribed ozone surface resistance over the ocean and deserts.

Tropospheric ozone in CCMI models and Gaussian process emulation to understand biases in the SOCOLv3 chemistry–climate model

Previous multi-model intercomparisons have shown that chemistry-climate models exhibit significant biases in tropospheric ozone compared with observations. We investigate annual-mean tropospheric column ozone in 15 models participating in the SPARC/IGAC (Stratosphere-troposphere Processes and their Role in Climate/International Global Atmospheric Chemistry) Chemistry-Climate Model Initiative (CCMI). These models exhibit a positive bias, on average, of up to 40-50% in the Northern Hemisphere compared with observations derived from the Ozone Monitoring Instrument and Microwave Limb Sounder (OMI/MLS), and a negative bias of up to ~30% in the Southern Hemisphere. SOCOLv3.0 (version 3 of the Solar-Climate Ozone Links CCM), which participated in CCMI, simulates global-mean tropospheric ozone columns of 40.2 DU - approximately 33% larger than the CCMI multi-model mean. Here we introduce an updated version of SOCOLv3.0, "SOCOLv3.1", which includes an improved treatment of ozone sink processes, and results in a reduction in the tropospheric column ozone bias of up to 8 DU, mostly due to the inclusion of N2O5 hydrolysis on tropospheric aerosols. As a result of these developments, tropospheric column ozone amounts simulated by SOCOLv3.1 are comparable with several other CCMI models. We apply Gaussian process emulation and sensitivity analysis to understand the remaining ozone bias in SOCOLv3.1. This shows that ozone precursors (nitrogen oxides (NOx), carbon monoxide, methane and other volatile organic compounds) are responsible for more than 90% of the variance in tropospheric ozone. However, it may not be the emissions inventories themselves that result in the bias, but how the emissions are handled in SOCOLv3.1, and we discuss this in the wider context of the other CCMI models. Given that the emissions data set to be used for phase 6 of the Coupled Model Intercomparison Project includes approximately 20% more NOx than the data set used for CCMI, further work is urgently needed to address the challenges of simulating sub-grid processes of importance to tropospheric ozone in the current generation of chemistry-climate models.

Překročení kritických úrovní ozonu na stanici monitoringu čistoty ovzduší v areálu BZA MZLU v Brně v letech 2001 až 2006

Cumulative effects of agricultural crops exposition, forests and other ecosystems for ozone concentration above the threshold is characterized by exposition index AOT. According to experst for EHK, the recent level is 40 ppb. Very dry summer in 2006 and 2003 created good conditions for photochemical reaction as well as ozone transport in Central Europe. Such years can be taken as an example in connection with expected concentration of ground ozone (as a result of global warming). Amount of ozone in troposphere is measured in Botanic garden and MZLU arboretum areas since 2001 by analyzator 41M–LCD. Ozone course correlates with course of air temperature from end of May till end of August. Coefficient of correlation for 2006 was 0,872 and year 2003 was 0,777, which is statistically significant. Similar course was found for summer months of 2004 (0.703), a bit lower as 2006. There were exceeded imission limits for vegetation protection and AOT40 in 2003 and 2004. There were the highest values for vegetation protection in 2006, 2003 and 2004. Immision limits exceeded 90 days in 2006, 61 days in 2003 and 53 days in 2004.

First comparison of OMI‐DOAS total ozone using ground‐based observations at a megacity site in East Asia: Causes of discrepancy and improvement in OMI‐DOAS total ozone during summer

AbstractThis study compares, for the first time, the total ozone columns derived from the Ozone Monitoring Instrument‐Differential Optical Absorption Spectroscopy (OMI‐DOAS algorithm) (TOCs‐OMI) with those obtained from ground‐based Brewer and Dobson spectrophotometers (TOCs‐Ground) in Seoul, a megacity in northeast Asia, over the 3 years between 2008 and 2010. We found a seasonal mean underestimation of 2.68% (maximum 18.33% on a single day) in TOCs‐OMI when compared with TOCs‐Ground from Seoul, particularly during the summer seasons (June, July, and August) of our study period: 20 of the 30 days when this underestimation of TOCs‐OMI was greatest occurred during the summer. The causes of such large underestimations in summer TOCs‐OMI were investigated, and we found that the ghost column densities (GCDs) used in the current OMI‐DOAS algorithm did not fully account for the tropospheric ozone amounts below the cloud top in Seoul, particularly during the summer season when surface ozone is enhanced due to active photochemical reactions. We propose the use of New TOCs‐OMI based on New GCDs that were calculated using ozonesonde data for the limited number of days when such data were available. The mean bias errors (MBE) against the TOCs‐Ground of the New TOCs‐OMI and original TOCs‐OMI were −0.60% and −2.16%, respectively, which demonstrates the greater accuracy of the New TOCs‐OMI. To increase the amount of New TOCs‐OMI data available for comparison with the TOCs‐Ground data, the regression equation for the relationship between the ozonesonde data and OMI‐DOAS cloud pressure data was used to increase the availability of New GCD data for each measurement date that TOCs‐OMI data were available for. This procedure reduced the MBE of the original TOCs‐OMI by 1.29%, 1.67%, and 1.29% in June, July, and August, respectively. The present study demonstrates that the underestimation of GCDs is one of the major causes of the underestimation of TOCs‐OMI during the summer season and suggests that improvements could be achieved using the proposed correction for these GCDs.

Snow-sourced bromine and its implications for polar tropospheric ozone

Abstract. In the last two decades, significant depletion of boundary layer ozone (ozone depletion events, ODEs) has been observed in both Arctic and Antarctic spring. ODEs are attributed to catalytic destruction by bromine radicals (Br plus BrO), especially during bromine explosion events (BEs), when high concentrations of BrO periodically occur. However, neither the exact source of bromine nor the mechanism for sustaining the observed high BrO concentrations is completely understood. Here, by considering the production of sea salt aerosol from snow lying on sea ice during blowing snow events and the subsequent release of bromine, we successfully simulate the BEs using a global chemistry transport model. We find that heterogeneous reactions play an important role in sustaining a high fraction of the total inorganic bromine as BrO. We also find that emissions of bromine associated with blowing snow contribute significantly to BrO at mid-latitudes. Modeled tropospheric BrO columns generally compare well with the tropospheric BrO columns retrieved from the GOME satellite instrument (Global Ozone Monitoring Experiment). The additional blowing snow bromine source, identified here, reduces modeled high latitude lower tropospheric ozone amounts by up to an average 8% in polar spring.

IASI observations of seasonal and day-to-day variations of tropospheric ozone over three highly populated areas of China: Beijing, Shanghai, and Hong Kong

Abstract. IASI observations of tropospheric ozone over the Beijing, Shanghai and Hong Kong areas during one year (2008) have been analysed, demonstrating the capability of space-borne infrared nadir measurements to probe seasonal and even day-to-day variations of lower tropospheric ozone (0–6 km partial columns) on the regional scale of highly populated areas. The monthly variations of lower tropospheric ozone retrieved from IASI clearly show the influence of the Asian summer monsoon that brings clean air masses from the Pacific during summer. They exhibit indeed a sharp ozone maximum in late spring and early summer (May–June) followed by a summer minimum. The time periods and the intensities of the maxima and of the decreases are latitude-dependent: they are more pronounced in Hong Kong and Shanghai than in Beijing. Moreover, IASI provides the opportunity to follow the spatial variations of ozone over the surroundings of each megacity as well as its daily variability. We show here that the large lower tropospheric ozone amounts (0–6 km partial columns) observed with IASI are mainly downwind the highest populated areas in each region, thus possibly suggesting the anthropogenic origin of the large ozone amounts observed. Finally, an analysis of the mean ozone profiles over each region – for selected days with high ozone events – in association with the analysis of the meteorological situation shows that the high ozone amounts observed during winter are likely related to descents of ozone-rich air from the stratosphere, whereas in spring and summer the tropospheric ozone is likely enhanced by photochemical production in polluted areas and/or in air masses from fire plumes.

The variability of free tropospheric ozone over Beltsville, Maryland (39N, 77W) in the summers 2004–2007

Ozone profiles are often used to investigate day-to-day and year-to-year variability in origins of free tropospheric ozone. With this in mind, more than 50 ozonesonde launches were conducted in Beltsville, MD, during the summers of 2004 through 2007. Budgets of free tropospheric ozone were calculated for each ozone profile in the four summers using a laminar identification (LID) method and unusual episodes were analyzed with respect to meteorological variables. The laminar method showed that stratosphere-to-troposphere transport (ST) accounted for greater than 50% of the free tropospheric ozone column on 17% of days sampled, a more pronounced influence than regional convective and lightning (RCL) sources. The ST origins were confirmed with trajectories, and tracers (water vapor and potential vorticity). The amount of free tropospheric ozone from ST and RCL sources varied from year-to-year (up to 13%) and can be explained by differences in mean meteorological patterns. On average, almost 30% of the free tropospheric column was attributed to ST influence, about twice as much as RCL, although the LID method may not capture weeks-old lightning influences as in a chemical model. The prevalence of ST ozone in summertime Beltsville soundings was similar to six sounding sites in the IONS-04 campaign [Thompson, A.M., et al., 2007b. Intercontinental Transport Experiment Ozonesonde Network Study (IONS, 2004): 1. Summertime upper tropospheric/lower stratosphere ozone over northeastern North America. J. Geophys. Res. 112, D12S12; Thompson, A.M., et al., 2007c. Intercontinental Transport Experiment Ozonesonde Network Study (IONS, 2004): 2. Tropospheric ozone budgets and variability over northeastern North America. J. Geophys. Res. 112, D12S13.] and to statistics from a 30 year climatology of European soundings [Collette, A., Ancellet, G., 2005. Impact of vertical transport processes on the tropospheric ozone layering above Europe. Part II: Climatological analysis of the past 30 years. Atmos. Environ. 39, 5423–5435]. The Beltsville record also demonstrated the value of soundings for air quality forecasting in an urban area. The 22 nighttime soundings collected over Beltsville in 2004–2007 can be divided into distinct polluted and unpolluted subsets, the former 20 ppbv higher in residual layer ozone (1 km) than the latter. These distinctions propagated to daytime differences of 10 ppbv at the surface in the Washington, DC, area, with the high-ozone residual layers leading to non-attainment of the National Ambient Air Quality Standard for ozone. More frequent ozone observations aloft appear essential for better understanding ozone variability and for enabling air quality modelers to achieve more accurate ozone forecasts.

Tropospheric ozone distributions over Europe during the heat wave in July 2007 observed from infrared nadir spectra recorded by IASI

First partial tropospheric ozone columns (0–6 km) derived from radiances observed by the IASI instrument aboard the MetOp‐A platform over Europe during summer 2007 are presented. They were retrieved using an altitude‐dependent regularization method. Comparison with measurements from balloon sondes shows excellent agreement. Space‐borne observations show large lower tropospheric ozone amounts over South‐Eastern Europe during the heat wave period, which are also displayed by simulations with a regional chemistry‐transport model CHIMERE.

Long-term changes in tropospheric ozone

Tropospheric ozone changes are investigated using a selected network of surface and ozonesonde sites to give a broad geographic picture of long-term variations. The picture of long-term tropospheric ozone changes is a varied one in terms of both the sign and magnitude of trends and in the possible causes for the changes. At mid latitudes of the S.H. three time series of ∼20 years in length agree in showing increases that are strongest in the austral spring (August–October). Profile measurements show this increase extending through the mid troposphere but not into the highest levels of the troposphere. In the N.H. in the Arctic a period of declining ozone in the troposphere through the 1980s into the mid-1990s has reversed and the overall change is small. The decadal-scale variations in the troposphere in this region are related in part to changes in the lowermost stratosphere. At mid latitudes in the N.H., continental Europe and Japan showed significant increases in the 1970s and 1980s. Over North America rises in the 1970s are less than those seen in Europe and Japan, suggesting significant regional differences. In all three of these mid latitude, continental regions tropospheric ozone amounts appear to have leveled off or in some cases declined in the more recent decades. Over the North Atlantic three widely separated sites show significant increases since the late-1990s that may have peaked in recent years. In the N.H. tropics both the surface record and the ozonesondes in Hawaii show a significant increase in the autumn months in the most recent decade compared to earlier periods that drives the overall increase seen in the 30-year record. This appears to be related to a shift in the transport pattern during this season with more frequent flow from higher latitudes in the latest decade.

Tropospheric bromine chemistry and its impacts on ozone: A model study

An off‐line three‐dimensional tropospheric chemical transport model, parallel–Tropospheric Off‐Line Model of Chemistry and Transport (p‐TOMCAT), has been extended by incorporating a detailed bromine chemistry scheme that contains gas‐phase reactions and heterogeneous reactions on both cloud particles and background aerosols. Bromine emission from bromocarbon photo‐oxidation and from sea‐salt bromine depletion and bromine removal through dry and wet deposition are included. Using this model, tropospheric bromine chemistry and ozone budgets are studied. The zonal mean of the inorganic gas‐phase bromine compounds (Brx) is calculated to be high (4–8 pptv) in the lower troposphere of the midlatitudes to high latitudes in each hemisphere, with decreasing trends with altitude (down to ∼2–3 pptv in the upper troposphere). The lowest Brx (<2 pptv) is over low latitudes, corresponding to small sea‐salt Br emission and a high rate of precipitation scavenging. A mean lifetime of ∼5 days is obtained for the tropospheric Brx. Sea‐salt emission plays the dominant role in total Brx in the lower troposphere while organic Br‐containing compounds are important in upper layers. High daytime BrO mixing ratios (>1 pptv) are found over the high‐latitude ocean surface, corresponding to high tropospheric column BrO values of up to 1.6 × 1013 molecules/cm2 in the monthly mean. The addition of bromine chemistry to the model leads to a reduction in tropospheric ozone amounts by 4–6% in the Northern Hemisphere and up to ∼30% in the Southern Hemisphere high latitudes. The net ozone loss depends not only on total Brx, but also on solar irradiance, especially at high latitudes. The hydrolysis reaction of bromine nitrate, which occurs on cloud and aerosol surfaces (BrONO2 + H2Oaq → HOBr + HNO3), has a significant influence on ozone chemistry through its effect on NOx as well as on reactive BrO and Br.

Influence of stratospheric airmasses on tropospheric vertical O<sub>3</sub> columns based on GOME (Global Ozone Monitoring Experiment) measurements and backtrajectory calculation over the Pacific

Abstract. Satellite based GOME (Global Ozone Measuring experiment) data are used to characterize the amount of tropospheric ozone over the tropical Pacific. Tropospheric ozone was determined from GOME data using the Tropospheric Excess Method (TEM). In the tropical Pacific a significant seasonal variation is detected. Tropospheric excess ozone is enhanced during the biomass burning season from September to November due to outflow from the continents. In September 1999 GOME data reveal an episode of increased excess ozone columns over Tahiti (18.0° S; 149.0° W) (Eastern Pacific) compared to Am. Samoa (14.23° S; 170.56° W) and Fiji (18.13° S; 178.40° E), both situated in the Western Pacific. Backtrajectory calculations show that none of the airmasses arriving over the three locations experienced anthropogenic pollution (e. g. biomass burning). Consequently other sources of ozone have to be considered. One possible process leading to an increase of tropospheric ozone is stratosphere-troposphere-exchange. An analysis of the potential vorticity along trajectories arriving above each of the locations reveals that airmasses at Tahiti are subject to enhanced stratospheric influence, compared to Am. Samoa and Fiji. As a result this study shows clear incidents of transport of airmasses from the stratosphere into the troposphere.

Tropospheric ozone over the North Pacific from ozonesonde observations

As part of the Transport and Chemical Evolution over the Pacific (TRACE‐P) mission, ozonesondes were used to make ozone vertical profile measurements at nine locations in the North Pacific. At most of the sites there is a multiyear record of observations. From locations in the western Pacific (Hong Kong; Taipei; Jeju Island, Korea; and Naha, Kagoshima, Tsukuba, and Sapporo, Japan), a site in the central Pacific (Hilo, Hawaii), and a site on the west coast of the United States (Trinidad Head, California) both a seasonal and event specific picture of tropospheric ozone over the North Pacific emerges. Ozone profiles over the North Pacific generally show a prominent spring maximum throughout the troposphere. This maximum is tied to the location of the jet stream and its influence on stratosphere‐troposphere exchange and the increase in photochemical ozone production through the spring. Prominent layers of enhanced ozone in the middle and upper troposphere north of about 30°N seem to be more closely tied to stratospheric intrusions while biomass burning leads to layers of enhanced ozone in the lower and upper troposphere at Hong Kong (22°N) and Taipei (25°N). The lower free tropospheric layers at Hong Kong are associated with burning in SE Asia, but the upper layer may be associated with either equatorial Northern Hemisphere burning in Africa or SE Asian biomass burning. In the boundary layer at Taipei very high mixing ratios of ozone were observed that result from pollution transport from China in the spring and local urban pollution during the summer. At the ozonesonde site near Tokyo (Tsukuba, 36°N) very large enhancements of ozone are seen in the boundary layer in the summer that are characteristic of urban air pollution. At sites in the mid and eastern Pacific the signature of transport of polluted air from Asia is not readily identifiable from the ozonesonde profile. This is likely due to the more subtle signal and the fact that from the ozone profile and meteorological data by themselves it is difficult to identify such a signal. During the TRACE‐P intensive campaign period (February–April 2001), tropospheric ozone amounts were generally typical of those seen in the long‐term records of the stations with multiyear soundings. The exception was the upper troposphere over Hong Kong and Taipei where ozone amounts were lower in 2001.

Ozone profile differences between Europe and New Zealand: Effects on surface UV irradiance and its estimation from satellite sensors

Substantial differences in summertime surface UV irradiance between New Zealand and Europe have been reported previously. The main contributors to these differences are differences in total column ozone, differences in Sun‐Earth separation between the Southern Hemisphere summer (December/January) and the Northern Hemisphere summer (June/July), and differences in tropospheric pollution. Differences due to higher Northern Hemisphere aerosol extinction have been well documented. Here we present a climatology of ozone profiles measured by balloon‐borne sensors launched from Lauder, New Zealand, and from Hohenpeissenberg, Germany, to show that there are also substantial differences in the shapes of ozone profiles, especially during the summer months. A radiative transfer model is then used to investigate the extent to which these profile shape differences contribute to observed UV differences. It is found that the differences in the vertical distribution of ozone amplify existing Northern and Southern Hemisphere differences in UV. Effects can be significant in the UV‐B region, but the sign of the effect depends on the solar zenith angle. Consequently, the effect on daily doses of biologically harmful UV radiation is suppressed somewhat, and the differences are responsible for differences in erythemally weighted UV of only a percent or two, which is small compared with other effects. While these effects are relatively small, differences in the vertical profile of ozone can affect the accuracy of satellite retrievals of total column ozone. This analysis shows that the lower tropospheric ozone amounts at Lauder may lead to underestimations of summertime UV from satellite by ∼4% at that site.

Southern Hemisphere Additional Ozonesondes (SHADOZ) 1998–2000 tropical ozone climatology 2. Tropospheric variability and the zonal wave‐one

The first view of stratospheric and tropospheric ozone variability in the Southern Hemisphere tropics is provided by a 3‐year record of ozone soundings from the Southern Hemisphere Additional Ozonesondes (SHADOZ) network (http://croc.gsfc.nasa.gov/shadoz). Observations covering 1998–2000 were made over Ascension Island, Nairobi (Kenya), Irene (South Africa), Réunion Island, Watukosek (Java), Fiji, Tahiti, American Samoa, San Cristóbal (Galapagos), and Natal (Brazil). Total, stratospheric, and tropospheric column ozone amounts usually peak between August and November. Other features are a persistent zonal wave‐one pattern in total column ozone and signatures of the quasi‐biennial oscillation (QBO) in stratospheric ozone. The wave‐one is due to a greater concentration of free tropospheric ozone over the tropical Atlantic than the Pacific and appears to be associated with tropical general circulation and seasonal pollution from biomass burning. Tropospheric ozone over the Indian and Pacific Oceans displays influences of the waning 1997–1998 El Niño, seasonal convection, and pollution transport from Africa. The most distinctive feature of SHADOZ tropospheric ozone is variability in the data, e.g., a factor of 3 in column amount at 8 of 10 stations. Seasonal and monthly means may not be robust quantities because statistics are frequently not Gaussian even at sites that are always in tropical air. Models and satellite retrievals should be evaluated on their capability for reproducing tropospheric variability and fine structure. A 1999–2000 ozone record from Paramaribo, Surinam (6°N, 55°W) (also in SHADOZ) shows a marked contrast to southern tropical ozone because Surinam is often north of the Intertropical Convergence Zone (ITCZ). A more representative tropospheric ozone climatology for models and satellite retrievals requires additional Northern Hemisphere tropical data.

A case study of extreme tropospheric ozone contamination in the tropics using in‐situ, satellite and meteorological data

Since 1992, more than 150 radiosoundings have been performed from Reunion Island in the Indian Ocean in Southern tropics. On August 25, 1995, tropospheric ozone amounts of over 200 ppbv through the entire troposphere were encountered. Typical values during that period do not usually reach more than 80 ppbv. In order to explain these disturbances, we performed an analysis using satellite and ECMWF data. Results indicate that several mechanisms are working together, including photochemical effect of biomass burning associated with convection, and stratosphere‐troposphere exchange near the subtropical jet stream. Based on results shown in this paper, it is difficult to estimate the relative contribution of biomass burning and stratospheric intrusions. Quantitative estimates should stem from other chemical component measurements and from a more detailed dynamical analysis.

Tropical tropospheric ozone from total ozone mapping spectrometer by a modified residual method

A modified residual method for deriving time‐averaged tropical tropospheric ozone (TTO) from total ozone mapping spectrometer (TOMS) has been refined [cf Kim et al., 1996] and applied to a 2‐year period, 1991–1992. This period was selected because of the availability of a unique sonde record at tropical Atlantic and near‐Atlantic sites: Natal, Brazil; Ascension Island; and Brazzaville. The essentials of the modified‐residual method are (1) Fourier analysis to identify the latitude range for use of the method; (2) use of the wave‐like pattern to estimate stratospheric ozone column and a background tropospheric ozone column, which is defined as the amount of ozone expected in the absence of chemical pollution; (3) assumption that the wave‐one feature is tropospheric; and (4) the stratospheric and background tropospheric ozone amounts consistent with the constraint of the three station ozonesonde measurements for total tropospheric ozone. An excess ozone (pollution) signal is also derived from TOMS total ozone. Parameters derived from the modified residual method are evaluated with independent data to verify the quality of the TTO. Derived tropical stratospheric column ozone for 1991–1992 agrees very well with the corresponding UARS Microwave Limb Sounder and Stratospheric Aerosol and Gas Experiment records (to within 5 Dobson units (DU) for the latter). Background tropospheric ozone, which exhibits a seasonally in 1991–1992, has a 24±4.8 DU mean value at wave maximum (∼0° longitude) and a 15.2±6.0 DU mean value at 180° longitude. The Atlantic value is remarkably similar to the climatological 26 DU previously inferred from subtropical sondes [Kim et al., 1996; Komhyr et al., 1989]. The ∼5 DU standard deviation is the accuracy of the modified‐residual method and is the best that can be achieved given the 15‐day averaging of the sondes (1σ = 5.3 DU). The wave‐like pattern in 1991–1992 has a semiannual periodicity with zero wave amplitude in June and December.

Global tropospheric and total ozone monitoring with a double-etalon Fabry–Perot interferometer II Feasibility analysis

The feasibility of observing global tropospheric and total ozone (O(3)) fields with a double-etalon Fabry-Perot interferometer (FPI) has been assessed. The FPI provides high spectral resolution and high-throughput capabilities that enable observation in pressure-broadened wings of strong O(3) lines while minimizing the impact of undesirable signal contributions (i.e., from the terrestrial surface and interfering species). A retrieval technique has been implemented and is demonstrated for a tropical atmosphere possessing enhanced tropospheric ozone amounts. An error analysis assessing the impact on retrieved O(3) amounts from the most significant uncertainties associated with this particular measurement has been performed, and findings for a tropical atmosphere are presented. Results show the proposed instrumentation to enable a good measurement of absolute ozone amounts and an even better determination of relative changes.

Estimation of the amount of tropospheric ozone in a cloudy sky by ground-based Fourier-transform infrared emission spectroscopy

The problem of retrieving minor concentrations of constituents by ground-based Fourier-transform infrared emission spectroscopy is addressed by means of the concept of differential optical emission spectroscopy in analogy to the concept of differential optical absorption spectroscopy. Using the prominent nu3 ozone feature at 1043 cm(-1), we show that the strength of the spectral signature depends not only on the amount of ozone but also on the atmospheric thermal structure. This dependence can be described by a rather accurate approximation, which was used to construct a simple diagram to estimate the amount of column ozone between the instrument site and a cloud deck as well as to determine the detection limit. The detection limit is shown to depend on cloud base height. For a given thermal lapse rate it was found that the lower the detection limit, the higher the cloud base altitude. However, as shown in a case study with variable cloud base height, the concept fails for semitransparent clouds. Multiple scattering of the emitted radiation within the clouds yielded a path enhancement that simulated an enhanced amount of constituent. The path enhancement was estimated to be 2.4-4 km at 1000 cm(-1) for low-level clouds, equivalent to an enhancement factor of 6-21. The multiple scattering effect has considerable consequences for ground-based as well as for nadir satellite retrieval techniques in cloudy skies.