Horizontal and vertical graphs of return legs starting at 10 km 129. a) Horizontal and vertical graphs of return legs starting at 2.5,5 and 7.5 km. Horizontal and vertical graphs of return legs starting at 10 km 144. a) Horizontal and vertical graphs of return legs starting at 2.5,5 and 7.5 km.
CHEMISTRY AND DYNAMICS OF
VERTICAL DISTRIBUTION OF
- Background
- Aim and Objectives
- The Scope of this Study
- Introduction
- MOZAIC Data
- The MOZAIC Program
The focus is on the vertical distribution of ozone over Johannesburg (26°S; 28°E) using aircraft profile data derived from the measurement of ozone and water vapor by Airbus In-Service Aircraft (MOZAIC). It consisted of the development and installation of ozone and water vapor equipment on board five A340 aircraft.
I :~~::ASIA
Description of the MOZAIC Instrumentation
- Installation
Static air temperature (SAT, heavy line) and total recycling temperature (TRT, thin line) are shown in the upper panel of the figure (Source: Helten et al., 1998: p 25651). Measurements are also evaluated against the response time of the ozone and water vapor devices (MOZAIC-II: Technical Final Report, 2000; Thouret et al., 1998a).
Total Tropospheric Ozone Methodology
- Definition of Total Tropospheric Ozone
- Computation of Total Tropospheric Ozone
- Data Selection and Processing
The total column ozone is obtained by integrating the ozone profiles between the surface and 12 km. Of the 427 ozone profiles that were available during the period, a total of 381 profiles were finally selected for the calculation of the TTO.
Stable Layer Methodology
- Definition of an Absolutely Stable Layer
- Computation of Absolutely Stable Layers
- Rationale
- TWINSPAN (Two-way indicator species analysis)
- Input Data for TWINSPAN
According to Cosijn and Tyson (1996), the stability structure of the atmosphere over southern Africa strongly influences the accumulation and spread of atmospheric pollutants. The gradient of each segment of the temperature profile (the line between 2 consecutive temperature measurements) was compared to the gradient of the saturated adiabatic drift rate.
Back Trajectory Modelling
- Rationale
- Limitations
Backtrack modeling was performed using the HYSPLIT (version 4) model, which was jointly developed by the National Oceanic and Atmospheric Administration (NOAA) and modified by the Australian Bureau of Meteorology (Draxler, 1998). The meteorological fields used in the HYSPLIT model were four-dimensional gridded fields from the National Center for Environmental Prediction (NCEP).
Introduction
The following sections review the photochemical sources and sinks (section 3.2) and stratospheric exchange processes (section 3.3) that contribute to the tropospheric ozone budget.
Photochemical Processes
- Biogenic Emissions
- Lightning
- Biomass Burning
- Movement of the Tropopause
- Tropopause Folding
Since these emissions are produced close to the surface, their effect on tropospheric ozone production is most prominent in the boundary layer (Diab et al., 1996a). Lightning NOx-derived ozone is still relatively important, especially in tropical regions (Lelieveld and Dentener, 2000) and in the upper troposphere (Ridley et al., 1996; . Zhang et al., 2000). The Northern Hemisphere winter over all continents in the Northern Hemisphere, and most of the continents in the Southern Hemisphere during the winter in the Southern Hemisphere.
In a similar process, the same amount of stratospheric air is returned to the troposphere at middle and higher latitudes. The upward mass flow in the tropical branch of the Hadley cell does not occur in a. The upward movement of the tropopause during spring causes an outflow of ozone-rich air into the troposphere and a subsequent reduction in air mass in the stratosphere (Staley, 1962 ).
Many factors led Baray et al. 2003) to suggest that this COL could have a significant impact on tropospheric ozone over southern Africa since most of the stratospheric ozone remained in the troposphere.
Summary
Introduction
1996) used ozone data measured by the DIAL instrument to determine the inflection point in the ozone profile where ozone concentrations showed a sudden sharp increase, marking the height of the tropopause. 1996a) used a tropopause height of 15 km for Irene, which is at the same latitude as Johannesburg. 1998) used an ozone threshold of 100 ppbv to distinguish between stratospheric and tropospheric air. 2003) also used an approach based on the chemical tropopause, which is defined as the point at which the extrapolation of ozone mixing ratios in the lower stratosphere reaches 100 ppbv.
According to these authors, it is usually within 10–15 hPa of the temperature-based tropopause, except in Irene where a double temperature-based tropopause is sometimes evident. Therefore, in this study, since the TTO values are integrated at an altitude of only 12 km, the values are expected to be smaller than those presented in previous studies. Furthermore, ozone concentrations are generally lowest at night and in the early morning, but increase during the morning after the separation of the overnight inversion layer, resulting in a diurnal variation of surface ozone, with values peaking until late afternoon.
Thus, changes in the time of measurement can have a significant impact on the values (Thouretet et al., 1998).
Previous Research Results on TTO in Southern Africa
A seasonal maximum in integrated tropospheric ozone at Ascension Island (50 DU). l996), the average integrated tropospheric ozone amount in the area was 38 DU for Irene and 53 DU for Ascension Island. Tropospheric ozone is higher at Ascension Island and Brazzaville than the other two stations, as the former was more directly affected by transport from biomass burning, while the latter was removed from burning areas. During SAFARI-2000, ozone measurements taken in Lusaka, Zambia (l5.5°S, 28°E) during the burning season showed that tropospheric ozone in the column exceeded 50 DU and was higher than simultaneous measurements over Nairobi (loS, 38°E, and Irene (25°S, 28°E). 2002) thus concluded that Lusaka was a sink for ozone-rich air originating from local and transboundary sources (i.e., from fire-rich rural areas in neighboring countries).
1996) who detected an increased TTO on the order of 56 DU over the same region and season, but using the airborne DIAL system. 1996) suggested that biomass burning was responsible for as much as 50% of the TTO column across the region. The premise that biomass combustion was responsible for the increase in the mid-tropospheric ozone layer was confirmed by in situ measurements of high concentrations of CO, C~ and CO2. The seasonal cycle of ozone derived from ozone probes and satellite-based data agree well, both peaking in September and both measurements showing the highest integrated.
This study suggested that TOMS satellite-based measurements are a better indicator of integrated tropospheric ozone than ozone probes.
Total Tropospheric Ozone (TTO) over Johannesburg
Daily fluctuations in TTO are expressed by the size of the standard deviation bars shown in Figure 4.1. The lower day-to-day variability in autumn and winter is a reflection of calmer weather at this time. These layers were chosen based on the work of Cosijn and Tyson (1996), who identified it.
To separate events that resulted from an increase in a particular layer, the percentage contribution in each layer was compared to the average percentage contribution of that layer. In the lower troposphere, 22 events showed improvements in the sfc-3 km layer, while 23 events showed improvements in the 3-5 km layer. The number of events showing improvements in the upper troposphere was similar, 20 in the 5-7 km layer and 21 events in the 7-12 km layer.
The extended duration of this event suggests that it is due to an STE event.
Conclusion
- Source Regions of Tropospheric Ozone for the Reduced Tropospheric Ozone Category
- General Characteristics of Ozone Profiles in the Lower Tropospheric Enhancement Category
- General Characteristics of Ozone Profiles in the Considerable Tropospheric Enhancement Category
The variability, as indicated by the standard deviation bars, is highest near the surface and in the upper troposphere. Seasonal trends in the vertical distribution of ozone (Fig. 5.1 a-d) show that there is a large variability over the mean for all seasons, with an increasing trend with altitude. This method was used to explain variations in the vertical distribution of ozone, which are mainly influenced by atmospheric dynamics.
The ozone profiles used in the classification are expected to be recorded at different times of the day and in different seasons. Profiles in Group 2a (Figure 5.5a) show a general upward trend in the lower to mid-tropospheric levels, accompanied by upper enhancement, typically above 8 km. Individual profiles in this group are also characterized by a general trend of increasing ozone in the upper troposphere (app. A), which reflects the lower position of the winter tropopause and the penetration of stratospheric air, which is richer in ozone.
This is reflected in the 'decreased tropospheric' ozone category, in which westerly winds predominate at all tropospheric levels.
Summary
These results suggest a detachment of the atmosphere and emphasize the role of the absolutely stable 5 km layer. The annual mean tropospheric ozone profile during this period showed a sharp increase in ozone within the lowest 2–3 km above sea level. Each category was named according to the structure of the characteristic profiles and defined as follows: single mid-tropospheric peak, stable tropospheric increase, tropospheric ozone depletion, low tropospheric expansion, pronounced stratification, and significant tropospheric increase.
Autumn profiles fell mainly in the 'reduced tropospheric ozone' group, caused by a dominance of westerly flow at all tropospheric levels, while winter profiles fell in both the 'reduced tropospheric ozone' and 'lower tropospheric enhancement' groups. Spring is generally recognized as the season during which tropospheric ozone maximizes, as confirmed by this study. The mean consists of a mixture of tropical and midlatitude regimes, which are likely to vary in relative contribution from year to year as a function of dynamical factors, such as the latitudinal position of the jet stream.
Relating the profiles to the origin of air masses through back-trajectory modeling showed that transboundary sources contribute significantly to the tropospheric ozone enhancement observed at Johannesburg.
Recommendations
Field observations of the vertical distribution of ozone in the troposphere on Réunion Island (southern tropics). Dynamic study of a tropical boundary layer over South Africa, and its impact on tropospheric ozone. Tropospheric ozone derived from TOMS/SBUV measurements during TRACE A. widespread pollution in the Southern Hemisphere derived from satellite analyses.
Model calculations of tropospheric ozone production potential following observed convective events. Journal of Geophysical Research, 95 (D. A case study of extreme tropospheric ozone pollution in the tropics using in-situ, satellite and meteorological data. Stable discontinuities as determinants of the vertical distribution of aerosols and trace gases in the atmosphere.
Where did tropospheric ozone come from over southern Africa and the tropical Atlantic in October 1992. Influence of stratosphere-troposphere exchange on tropospheric ozone over the tropical Indian Ocean during the winter monsoon. DO DO DO DO DO a) Horizontal and vertical plots of return trajectories starting at 2.5, 5 and 8 km.
