R. D. Diab

Ozone over southern Africa during SAFARI-92/TRACE A

Characteristics of total O3 in southern Africa and over the adjacent Atlantic during the IGAC/STARE/SAFARI-92/TRACE A (International Global Atmospheric Chemistry/South Tropical Atlantic Regional Experiment/Southern African Fire Atmospheric Research Initiative/Transport and Atmospheric Chemistry near the Equator-Atlantic) field experiments are described. Most of the analysis is based on data from the Nimbus 7/total ozone mapping spectrometer (TOMS) gridded O3 data archive (version 6.0), which is used to examine O3 in terms of seasonal and interannual variability. Total O3 column variability is compared to the tropospheric O3 column derived from balloonborne ozonesondes at four fixed SAFARI-92/TRACE A sites (Ascension Island, Brazzaville, Okaukuejo, and Irene) from September 1 to October 23, 1992. All of these sites except Okaukuejo had regular ozonesonde launches from 1990 to 1992. Total O3 and integrated tropospheric O3 at the sounding sites showed the expected September–October maxima over southern Africa and the adjacent Atlantic Ocean. Statistical analysis of the TOMS record for 1979–1992 allows disaggregation of components contributing to total O3 variability: Signals due to semiannual and annual cycles and the quasi-biennial oscillation are identified at the sounding sites. The tropospheric O3 column estimated from integrated sondes (to ∼16 km) at the four sites ranged from 24 to 62 Dobson units (DU) (mean, 45 DU) and averaged 15% of total O3 at Irene (14 launches) and 19% of total O3 at Ascension (20 launches). Tropospheric O3 was higher at Ascension and Brazzaville than at the sites south of 15°S because transport from biomass burning regions was more direct at these sites. This transport is seen in Hovmoller (time-longitude) plots of total O3. A comparison of 1990–1992 integrated tropospheric O3 amounts with the annual total ozone cycle shows that tropospheric ozone variations may account for all of the annual signal at Ascension (8°S) and Brazzaville (4°S) but only 30–40% of the seasonal total O3 variation at Irene (26°S). Hovmoller plots of daily TOMS O3 over southern Africa and the Atlantic show easterly flow of local O3 maxima at 0°–10°S and westerly movement from 30°–40°S. At 0°–10°S the continent-ocean total O3 gradient and Ascension and Brazzaville O3 soundings are used to estimate a photochemical O3 formation rate of 1–2 ppbv O3/d over the Atlantic. This agrees with model calculations of moderately aged biomass burning emissions from SAFARI-92/TRACE A [Jacob et al., this issue; Thompson et al., 1996, this issue].

Vertical ozone distribution over southern Africa and adjacent oceans during SAFARI‐92

A set of four ozonesonde stations located at Ascension Island, Brazzaville, Okaukuejo, and Irene, operational during the TRACE A and SAFARI-92 experiments has provided an opportunity to investigate the vertical distribution of ozone over southern Africa and adjacent oceans. All stations display a springtime maximum in tropospheric ozone. Enhanced tropospheric ozone, which occurs between June and September at Brazzaville and between July and October at Ascension Island, is linked to dry season biomass burning. The influence of tropical biomass burning is delayed until September at Okaukuejo when a sharp increase in tropospheric ozone is experienced. The biomass burning influence at Irene is less because of its more southerly location. A general tropospheric enhancement is observed at all stations. It is manifest as an enriched layer in the upper troposphere at Okaukuejo (9-12 km) and Brazzaville (11-14 km) and in the lower troposphere (2-8 km) at Ascension Island. At Ascension Island lower tropospheric ozone values are about 20 parts per billion by volume greater than elsewhere and the tropospheric component here accounts for about 18% of the total column ozone. A series of tethersonde soundings conducted at hourly intervals at Okaukuejo revealed ozone to be well mixed in the lower boundary layer during the day, but to display marked vertical stratification at night.

Ultraviolet Radiation Exposure of Children and Adolescents in Durban, South Africa¶

Abstract The solar ultraviolet radiation (UVR) exposure of 30 children and adolescents in three age groups (4–6 years, 7–9 years and 13–14 years) was measured for 1 week in late summer (February–March) in Durban, South Africa, using UVR-sensitive polysulfone film badges (PSFB) attached to the lapel region of the body. The mean and median values for all ages over the study period were 2.0 and 1.2 standard erythemal dose (SED) units, respectively, where 1 SED = 100 J·m−2. Individual PSFB doses were analyzed as a function of age, gender and behavior. No significant statistical differences were found between different age groups; however, there was a statistical difference between males and females, with males generally receiving higher PSFB doses. Subjects completed UVR exposure journals documenting their time outdoors, shade versus sun conditions, nature of their activities, clothing worn and their use of sunscreen for each day of the study. Activity patterns were noted as the most important factor influencing individual UVR dose. Ambient erythemal UVR was measured by a Yankee Environmental Systems UVB pyranometer, and a relationship between ambient UVR and individual UVR dose was derived. On average, subjects received a dose of 4.6% of the total daily erythemal UVR. Based on this factor, the potential dose of an individual over a full annual cycle was estimated. Accordingly, there were 139 days during the year when an individual with skin type I (light skin) would be likely to experience minimal erythema and 97 and 32 days for individuals with skin types II and III, respectively.

Health risk assessment of kerosene usage in an informal settlement in Durban, South Africa

In Durban, South Africa studies have shown that more than seven out of ten households in low-income metropolitan areas rely on kerosene for domestic purposes, leading to widespread problems of poor indoor air quality. In light of the known health effects of kerosene usage, this study aimed to quantify the health risk for people living in a densely populated informal settlement known as Cato Crest within the Durban metropolitan area. The pollutants investigated included nitrogen dioxide, benzene and toluene. Nitrogen dioxide is known to affect both respiratory and immune systems, benzene is carcinogenic while toluene has a neurological health end point. All three pollutants are harmful when inhaled. The United States Environmental Protection Agency (US EPA) health risk assessment (HRA) framework was applied. Information on the exposure patterns of residents in Cato Crest were acquired through questionnaires in which data on fuel use, building structure, cooking habits and time-activity patterns were collected. Air quality monitoring of nitrogen dioxide and volatile organic compounds was also conducted in the households. The time-activity pattern survey revealed that the exposure periods of individuals in Cato Crest were far greater than the default exposure periods used by the US EPA. The results of the HRA showed that the residents of Cato Crest may experience significant health risks as a result of kerosene usage in their homes. Exposure to 1-h nitrogen dioxide concentration is not likely to produce adverse health effects, whereas exposure over a 24-h period indicates a potential health risk to sensitive individuals in two of the households when US EPA exposure values are used and in all of the households when locally derived exposure values are used. Benzene poses a health risk to sensitive individuals in 50% of the households when local exposure parameters are used, whereas there is no health risk associated with exposure to toluene.

TOWARDS AN INVERSION CLIMATOLOGY OF SOUTHERN AFRICA: PART II, NON-SURFACE INVERSIONS IN THE LOWER ATMOSPHERE

ABSTRACT Radiosonde data for the years 1969–1972 have been processed to determine non-surface inversion conditions over Southern Africa. Results are presented to show, first, the spatial and point characteristics of mean non-surface inversions by season and by month, and secondly, the nature of inversion occurrence within six layers of the lower atmosphere.

Dynamical study of a tropical cut-off low over South Africa, and its impact on tropospheric ozone

The structure and evolution of an intense tropical cut-off low (COL) occurring over South Africa is documented, by combining meteorological and ozone data from a number of different sources, such as in situ radiosoundings, Measurement of ozone and water vapor by airbus in-service aircraft data, satellite (Meteosat) and modelled European Center for Medium-Range Weather Forecast data. The COL extends to latitude 10°S, with a lifetime of approximately 2 weeks, and a horizontal size of about 10°. A distinguishing feature of this case study is that it becomes detached from the stratospheric reservoir in both the vertical and horizontal planes, in an irreversible way and as such is different in structure to a mid-latitude or a polar COL. Consequently, even though tropical COLs may occur infrequently, it is likely that the tropospheric ozone enhancement induced by this irreversible intrusion could be significant.

Total ozone and synoptic weather relationships over southern Africa and surrounding oceans

Relationships between Total Ozone Mapping Spectrometer (TOMS) total ozone and synoptic weather systems for a region bounded by longitudes 10 deg W - 50 deg E and latitudes 0 deg - 50 deg S are presented. Least squares regression analysis revealed a generally poor but negative relationship between total ozone and the heights of the 500, 300, and 100 hPa geopotential surfaces at nine selected stations for the period 1987 to 1988. The relationship strengthens with increasing latitude and suggests a physical link with the midlatitude cyclone which facilitates the injection of stratospheric ozone rich air into the troposphere. In the tropics the link with middle to upper tropospheric meteorological parameters is weak. Analysis of daily relationships between gridded TOMS ozone and gridded European Centre for Medium-Range Weather Forecasts (ECMWF) geopotential height data over the study area showed considerable variations, indicative of varying synoptic weather situations. Case studies of a cutoff low sequence and a subtropical anticyclone system were selected as examples of synoptic situations which accounted for the highest and lowest correlations, respectively. The contrasting dynamics operating in these systems are highlighted.

A comparison of anticyclone and trough influences on the vertical distribution of ozone and meteorological conditions during SAFARI-92

The vertical distribution of ozone recorded over northern Namibia during SAFARI-92 is examined as a function of prevailing atmospheric circulation. Two dominant synoptic weather types, namely, anticyclonic flow and flow associated with a westerly trough, are identified and composite ozone profiles produced for each weather type. In both cases an elevated, enriched ozone layer (9–12 km), where ozone concentrations exceed 80 ppbv on average, and 100–120 ppbv on individual days, is present. Under anticyclonic conditions, ozone increases steadily from surface values of around 40 ppbv to about 60–80 ppbv throughout the troposphere. In contrast, the westerly trough is characterized by a midtropospheric (4–8 km) ozone minimum, where values are 20 ppbv lower. This gives rise to a double ozone maximum in the troposphere. The elevated, enriched layer is thought to be a pervasive layer equatorward of 20°S and to result from biomass burning.

Towards An Inversion Climatology Of Southern Africa: Part I, Surface Inversions

Abstract Radiosonde data for the years 1969-1972 have been processed to determine surface inversion conditions over Southern Africa. Results are presented to show the actual and probable frequency of occurrence, depth and strength of surface inversions at midnight, early morning and midday for the year as a whole and for summer, autumn, winter and spring.

Atmospheric transport and photochemistry of ozone over central Southern Africa during the Southern Africa Fire‐Atmosphere Research Initiative

Vertically integrated back and forward trajectories for the 300-200, 700-500 and surface-800 hPa levels are calculated using Pretoria as point of origin for the Southern Africa Fire-Atmosphere Research Initiative (SAFARI) period September-October 1992. The transport fields are then combined to show both horizontal and vertical transport of air to and from Pretoria at the different levels. Air transport patterns in the vertical are linked to the occurrence of absolutely stable layers which are also evident in the 16 ozonesonde profiles recorded at Pretoria during SAFARI. The coherence of the stratification based on dynamical and ozone analysis permits the use of mean ozone profiles with air volume fluxes to interpret the ozone in terms of photochemistry and transport within stable layers. Extensive recirculation across the meridional plane at Pretoria implies that advection of ozone is slow and that photochemistry is responsible for the observed vertical structure over central southern Africa in September and October 1992. Requisite ozone formation rates are supported by model analysis of ozone and ozone precursors measured from SAFARI and Transport and Atmospheric Research Chemistry near the Equator-Atlantic aircraft.