68 Chapter 3 Comparison of air pollutants levels over the Highveld air pollution hotspots Airborne monitored levels of O3, NOX, SO2 and PM2.5 aerosols will be compared over the Highveld air pollution hotspots; i.e. Secunda, Witbank, Rustenburg and the Vaal Triangle in three seasons studied. The seasonal change in atmospheric loading of these air pollutants over the Highveld air pollution hotspots will be evaluated. Factors that influence the atmospheric loading of these air pollutants, like the prevailing meteorological conditions, and the temporal cycle of the photochemical processes will be considered. Autumn campaign Autumn campaign meteorological overview The surface synoptic conditions over the country during the autumn campaign case study days were dominated by the presence of a surface trough (Figure 3.1). The trough was either situated over the central interior, extending to the southern interior, or centred over the southern or western interior. It brought about partly cloudy to cloudy conditions with showers and thundershowers. Rustenburg weather station on 16/03/2005 measured 0.2 mm of rain between 09h00 and 10h00 (SAST). Witbank weather station recorded daily rainfall of 1.4 mm on 16/03/2005 and 1 mm of rain on 17/03/2005 between 15h00 and 17h00 (SAST). Vereeniging weather station observed 26.2 mm of rain on 16/03/2005 between 0h00 and 10h00 (SAST). The temperatures were generally warm during the autumn campaign (Table 3.1). The winds were generally light winds on the days considered for case studies (Figure 3.3). Figure 3.2 shows the Irene weather observation station temperature profiles which were used to characterise the vertical structure of the lower part of the troposphere. In all the three case studies the lower troposphere was characterised by a low level nocturnal inversion layer in the morning (Figure 3.2(a)), resulting in a shallow mixing layer. In the 69 afternoon the nocturnal inversion was eroded causing a deeper mixing layer (Annegarn et al., 1996b). The lower troposphere was unstable in the afternoon during the three days, except on 18/03/2005 where there was an inversion at 2775 magl (Figure 3.2(b)). The top of the morning surface inversion layer on 16/03/2005 (Figure 3.2(a)) was at 184 magl (above ground level) and at 14?C. Hourly averaged temperatures at Irene in Table 3.1 show that it was eroded at about 06:00 (SAST). The surface winds at Rustenburg on 16/03/2005 by 09:00 (SAST) where above the critical threshold value of 2 ms-1 (Figure 3.3) for nocturnal surface inversion erosion (Hunt et al., 2006). Given the hourly averaged temperature was 16.1 ?C at 09:00 (SAST), Rustenburg was likely monitored after the surface erosion was mixed out. On 17/03/2005 the surface inversion layer top on Figure 3.2(a) was at 276 magl and at 16.4?C. The Irene hourly averaged temperatures in Table 3.1 show that it was eroded between 07:00 and 08:00 (SAST). On 17/03/2005 at Witbank by 09:00 (SAST) the wind was already above the critical threshold value (Figure 3.3) for surface inversion erosion and the hourly averaged temperature was 17.2 ?C at that time (Table 3.1). The monitoring of Witbank was likely done after the surface inversion was mixed out. On 18/03/2005 the surface inversion layer on Figure 3.2(a) was shallow with a top at 92 magl and at 18.4 ?C. Irene hourly averaged temperatures in Table 3.1 show that it mixed out between 08:00 and 09:00 (SAST). The wind speeds at Ermelo and Witbank at 09:00 (SAST) on the 18/03/2005 were above the critical threshold value (Figure 3.3) for surface inversion erosion and the hourly averaged temperature were about 16.0 ?C at that time (Table 3.1). It can be cautiously deduced that this was the case at Secunda which is close to both sites. Secunda was likely monitored after the surface inversion was mixed out. 70 Table 3.1: Hourly averaged surface temperature at Irene and study sites weather stations: from the morning up to the afternoon during the autumn campaign. Site Date Time(SAST) 06:00 07:00 08:00 09:00 10:00 11:00 12:00 13:00 Irene 16/03/2005 Average Temperature (?C) 14.2 14.7 15.8 16.6 16.1 17.5 22.2 23.0 Rustenburg 16/03/2005 Average Temperature (?C) 16.1 16.5 16.7 16.1 18.1 20.4 21.2 22.9 Irene 17/03/2005 Average Temperature (?C) 13.3 13.8 16.7 19.2 21.1 22.0 23.4 23.6 Witbank 17/03/2005 Average Temperature (?C) 12.9 14.2 15.8 17.2 17.9 19.8 21.1 21.7 Irene 18/03/2005 Average Temperature (?C) 12.7 13.6 15.4 19.0 20.3 21.6 23.5 24.7 Ermelo 18/03/2005 Average Temperature (?C) 12.9 13.4 15.0 15.9 17.7 18.6 19.3 19.8 Witbank 18/03/2005 Average Temperature (?C) 12.8 14.0 14.0 15.8 18.0 18.9 20.9 21.7 Figure 3.1: Autumn campaign 14h00 (SAST) surface synoptic charts 3.1(a), 3.1(b) and 3.1(c); 16/03/2005, 17/03/2005 and 18/03/2005 respectively. a ) b c 71 Figure 3.2: Temperature vertical profiles measurements over Irene weather observation station during the autumn campaign. Figure 3.2(a) is a midnight profile and Figure 3.2(b) is an afternoon profile. a b 72 Figure 3.3: Wind speed measurements at the study sites during the autumn campaign. Comparison of air pollutants levels over the Highveld air pollution hotspots during the autumn campaign The Tables 3.2, to 3.6 show statistical analysis of the concentration distribution of O3, NO, NO2, SO2 and PM2.5 aerosols respectively over the four Highveld air pollution hotspots. The data were collected approximately at 167 magl during the autumn campaign. Data from apparent industrial plume penetration were eliminated for this analysis. The Rustenburg site was monitored on 16/03/2005 from 10:22:55 to 11:09:22 (SAST). Witbank and the Vaal Triangle area were monitored on 17/03/2005, Witbank from 09:42:35 to 10:21:20 (SAST) and Vaal Triangle area from 15:17:00 to 15:47:38 (SAST). Secunda was monitored on 18/03/2005 from 10:35:00 to 11:17:00 (SAST). Similar synoptic conditions allowed the comparison of the four air pollution hotspots. 73 Ozone Table 3.2 shows O3 concentration distribution data over Rustenburg, Witbank and Secunda monitored in the morning, and the Vaal Triangle monitored in the afternoon. When comparing the sites monitored in the morning, Secunda was found to have the highest O3 average concentration of 30.00 ppb followed by Rustenburg with an average concentration of 28.46 ppb, then Witbank with an average concentration of 24.41 ppb. Secunda had the most variable O3 concentration spatial distribution, followed by Witbank then Rustenburg, with relative standard deviations of 20.37%, 10.36% and 7.57% respectively. The O3 concentration distribution at Rustenburg was shifted toward higher O3 concentration values in comparison to Secunda and Witbank. It had higher minimum and first quartile O3 concentration values in comparison to Secunda and Witbank. This is due to a generally low NO concentration distribution over Rustenburg as compared to Secunda and Witbank (Table 3.3). NO destroys O3 through NO and O3 titration reaction (Kleinman, 1994; Kley et al., 1994; Poulida et al., 1994; Hobbs et al., 2003; Taubman et al., 2004). The inclusion of the afternoon Vaal Triangle O3 data in Table 3.2 was to show the influence of photochemistry and diurnal evolution of the mixing layer on the diurnal variation of O3 concentration. The Vaal Triangle O3 average concentration of 42.51 ppb and the O3 concentration distribution in general is higher than all the three study areas monitored in the morning. The spatial distribution of O3 concentration was relatively more uniform over the Vaal Triangle with a relative standard deviation of 5.44%. This is because of the turbulent mixing in the mixing layer which is at its peak in the afternoon (Annegarn et al., 1996a; Betts et al., 2002). The relatively stronger winds observed at Vereeniging during the monitoring period (Figure 3.3) supports the turbulent mixing (Hunt et al., 2007). The high O3 concentration over the Vaal Triangle area is due to accumulation of O3 from O3 photochemical production, which reaches its peak in the afternoon (Trainer et al., 1987; Poulida et al., 1994; Annegarn et al., 1996b; Betts et al., 2002; Taubman et al., 2004). 74 Table 3.2: Autumn campaign Highveld hotspots comparison: O3 concentration distribution at approximately 167 m above ground level. Site Min (ppb) 25% (ppb) Median (ppb) 75% (ppb) Max (ppb) Average (ppb) StdDev % Number Rustenburg 24.45 26.74 28.25 30.04 33.24 28.46 7.57 2788 Witbank 16.43 22.91 24.68 26.14 28.70 24.41 10.36 2326 Vaal Triangle 37.87 40.48 42.31 43.91 47.93 42.51 5.44 1839 Secunda 16.37 24.39 31.90 34.24 47.00 30.00 20.37 2521 Nitrogen monoxide Table 3.3 shows NO concentration distribution data over the study sites. For NO levels monitored during the morning flights, Secunda was found to have the highest average concentration of 1.20 ppb followed by Witbank with an average concentration of 0.65 ppb, then Rustenburg with an average concentration of 0.26 ppb. Rustenburg was found to have the most variable NO concentration spatial distribution with the relative standard deviation of 100.86%, followed by Witbank with the relative standard deviation of 90.72%, and then Secunda with a relative standard deviation of 88.45%. The high spatial variability of NO over these sites is due to its short atmospheric life-time (one day) and the uneven distribution of its sources (IPCC, 2001; QEPA, 2006; Seinfeld and Pandis, 2006; USEPA, 2006c; WMO, 2006a). The afternoon NO concentration distribution data over the Vaal Triangle is included in Table 3.3, to show the influence of diurnal variation of the mixing layer height and photochemistry on the diurnal variation of NO concentration. Over the Vaal Triangle no NO was detected in the parts per billion (ppb) measurement scale in the afternoon. NO concentration is normally at its minimum in the afternoon. This is because of its dilution by turbulent mixing in the mixing layer of a larger volume in the afternoon (Annegarn et al., 1996a; Turner, 1996), and maximum photochemical oxidation to produce NO2 during this time of the day (Kley et al., 1994; Poulida et al., 1994; Hobbs et al., 2003, Taubman et al., 2004). The relatively stronger winds monitored at Vereeniging during the flight monitoring period (Figure 3.3) may have enhanced NO concentration dilution through 75 turbulent mixing (Hunt et al., 2007). Leading to the impact of continuous industrial emissions not detectable in ambient air. Table 3.3: Autumn campaign Highveld hotspots comparison: NO concentration distribution at approximately 167 m above ground level. Site Min (ppb) 25% (ppb) Median (ppb) 75% (ppb) Max (ppb) Average (ppb) StdDev % Number Rustenburg 0 0.051 0.21 0.40 1.15 0.26 100.86 2788 Witbank 0 0.055 0.59 0.94 2.89 0.65 90.72 2326 Vaal Triangle 0 0 0 0 0 0 0 1839 Secunda 0 0.38 0.87 1.79 4.33 1.20 88.45 2521 Nitrogen dioxide Table 3.4 shows NO2 concentration distribution data over the study sites. When comparing the sites monitored in the morning, Secunda was found to have the highest NO2 average concentration of 1.48 ppb followed by Witbank with an average concentration of 0.65 ppb, then Rustenburg with an average concentration of 0.17 ppb. Witbank NO2 concentration spatial distribution was most variable with the relative standard deviation of 147.35%, followed by Rustenburg with the relative standard deviation of 94.07%, and then Secunda with a relative standard deviation of 67.20%. The high spatial variability of NO2 over these sites is due to its short atmospheric life-time (one day) and the uneven distribution of its sources (IPCC, 2001; QEPA, 2006; USEPA, 2006c; WMO, 2006a). The afternoon NO2 concentration distribution data over the Vaal Triangle is included in Table 3.4. Its inclusion is to show the influence of diurnal variation of the mixing layer height and photochemistry on the diurnal variation of NO2 concentration. Over the Vaal Triangle no NO2 was detected in the parts per billion (ppb) measurement scale in the afternoon. This is because NO2 concentration is diluted by turbulent mixing in the mixing layer of a larger volume (Annegarn et al., 1996a; Turner, 1996), which is supported by relatively stronger winds observed at Vereeniging (Figure 3.3) and its maximum 76 photochemical consumption in O3 formation in the afternoon (Crutzen et al., 1999; Taubman et al., 2004). Table 3.4: Autumn campaign Highveld hotspots comparison: NO2 concentration distribution at approximately 167 m above ground level. Site Min (ppb) 25% (ppb) Median (ppb) 75% (ppb) Max (ppb) Average (ppb) StdDev % Number Rustenburg 0 0.038 0.095 0.25 0.51 0.17 94.07 2788 Witbank 0.006 0.23 0.63 1.03 1.74 0.65 147.35 2326 Vaal Triangle 0 0 0 0 0 0 0 1839 Secunda 0 0.84 1.28 1.79 4.52 1.48 67.20 2521 Sulphur dioxide Table 3.5 shows SO2 concentration distribution data over the study sites. Secunda was found to have the highest morning SO2 average concentration of 17.02 ppb followed by Witbank with an average concentration of 13.07 ppb, then Rustenburg with an average concentration comparable to that of Witbank of 12.17 ppb. Secunda was found to have the most variable SO2 concentration distribution in space with a relative standard deviation of 71.33%, followed by Witbank with a relative standard deviation of 38.61%, then Rustenburg with a relative standard deviation of 30.23%. Though Rustenburg?s SO2 average concentration is slightly less in comparison to that over Witbank, its SO2 concentration distribution is slightly shifted toward higher SO2 concentrations, except for the third quartile value. The afternoon SO2 concentration distribution data over the Vaal Triangle is included in Table 3.5 for the same reasons as with the other pollutants. The average SO2 concentration of 6.97 ppb over the Vaal Triangle in the afternoon was generally lower than that of all the study areas monitored in the morning. SO2 concentration was less variable in space in comparison to the other study areas, with a relative standard deviation of 23.23%. Like NOX concentrations, SO2 concentrations are at their minimum in the afternoon. This is because of dilution by turbulent mixing in the afternoon mixing 77 layer of a deeper depth (Annegarn et al., 1996a; Turner, 1996; Hunt et al., 2007), supported by relatively stronger winds (Figure 3.3) during the monitoring period, and optimal photochemical oxidation of SO2 to form SO4 2- particles (Brock et al., 2002; Taubman et al., 2004; Springston et al., 2005; WMO, 2006c). The favourable mixing conditions in the afternoon minimises the atmospheric loading of pollutants from continuous industrial emissions. Table 3.5: Autumn campaign Highveld hotspots comparison: SO2 concentration distribution at approximately 167 m above ground level. Site Min (ppb) 25% (ppb) Median (ppb) 75% (ppb) Max (ppb) Average (ppb) StdDev % Number Rustenburg 5.85 9.44 12.07 14.19 28.11 12.17 30.23 2788 Witbank 4.57 9.20 11.39 17.85 27.84 13.07 38.61 2326 Vaal Triangle 4.91 5.73 6.60 7.66 14.05 6.97 23.23 1839 Secunda 2.61 4.40 17.74 25.54 44.28 17.02 71.33 2521 PM2.5 aerosols Table 3.6 shows the PM2.5 aerosols concentration distribution data over the study areas. Witbank was found to have the highest morning average aerosols concentration of 1166.23 cm-3, followed by Secunda with an average concentration of 1052.33 cm-3 then Rustenburg with an average concentration of 872.52 cm-3. The relative standard deviation of aerosols concentration over Secunda of 40.09% indicates aerosol concentrations were more variable in space over Secunda, than over Witbank with a relative standard deviation of 25.56%. The aerosols concentration spatial distribution over Rustenburg was found to be least variable with a relative standard deviation of 11.75%. The afternoon PM2.5 aerosols concentration distribution data over the Vaal Triangle area is included in Table 3.6 for similar reasons as with the other pollutants. The aerosols average concentration of 580.89 cm-3 over the Vaal Triangle in the afternoon was generally lower than that of all the study areas monitored in the morning. The aerosols concentration variability in space was comparable to that found over Witbank in the 78 morning, its relative standard deviation was 26.69%. The concentration distribution of aerosols over Vaal Triangle was generally lower than that over the other study sites, but with a minimum concentration value that was comparable to the one over Witbank, which was monitored in the morning. The general low concentrations may be the result of dilution by turbulent mixing in the afternoon mixing layer of a greater volume (Annegarn et al., 1996a; Turner, 1996), supported by relatively stronger winds observed at Vereeniging (Figure 3.3). Table 3.6: Autumn campaign Highveld hotspots comparison: PM2.5 aerosols concentration distribution at approximately 167 m above ground level. Site Min (#/cm 3 ) 25% (#/cm 3 ) Median (#/cm 3 ) 75% (#/cm 3 ) Max (#/cm 3 ) Average (#/cm 3 ) StdDev % Number Rustenburg 545.00 815.73 864.56 914.05 1471.66 872.52 11.75 2788 Witbank 361.59 963.06 1160.54 1370.16 2487.57 1166.23 25.56 2326 Vaal Triangle 339.12 448.22 540.97 695.47 1252.22 580.89 26.69 1839 Secunda 406.01 664.11 1025.85 1370.87 2507.22 1052.33 40.09 2521 Winter campaign Winter campaign meteorological overview During the winter campaign case study days the synoptic conditions over the country were dominated by either a high pressure system or surface trough. Figure 3.4 shows afternoon 14h00 (SAST) surface synoptic conditions charts during the winter campaign case study days. On the 27/07/2005 and 03/08/2005 the interior was under the influence of high pressure systems, a characteristic winter circulation type (Garstang et al., 1996; Tyson et al., 1996). Ridging from the eastern part of the country on 27/07/2005 (Figure 3.4(a)) and centred over the central interior on 03/08/2007 (Figure 3.4(b)). It brought about clear conditions over the country. The temperatures were generally mild (Table 3.7) and the winds were varying from calmn to moderate winds (Figure 3.6). On 05/08/2005 and 08/08/2005 the interior was under the influence of low pressure troughs, a characteristic summer circulation type (Garstang et al., 1996; Tyson et al., 1996). On 05/08/2005 it was centred over the western interior (Figure 3.4(c)) and on 08/08/2005 it 79 was situated over the eastern interior extending south of the country (Figure 3.4(d)). The temperatures were also mild (Table 3.7) and the winds were generally light winds (Figure 3.6). Figure 3.4: Winter campaign surface synoptic charts 3.4(a) to 3.4(d); 27/07/2005, 03/08/2005, 05/08/2005 and 08/08/2005 respectively. Figure 3.5 shows the Irene weather station temperature profiles during the winter campaign case study days considered. The temperature profiles were used to characterise the vertical structure of the lower section of the troposphere. As it were with autumn campaign case study days, in all the four winter campaign case studies the lower troposphere was characterised by a surface nocturnal inversion layer in the morning. The temperature profiles in the morning during the winter campaign case study days were also characterised by upper level inversions varying in height from about 1000 to a b c d 80 1800 magl (Figure 3.5(a)). In the afternoon the surface nocturnal inversions were eroded resulting a deep mixing layer, however capped by the upper level inversions varying in their heights from 1200 to 1800 magl. It is only on the 27/07/2005 this temporal persistent stable discontinuity was not observed (Figure 3.5(b)). The top of the morning surface inversion layer on Figure 3.5(a) on 27/07/2005 was at 183 m above ground level (magl) and at 12.2 ?C. Hourly averaged temperatures at Irene in Table 3.7 show that it was eroded between 08:00 and 09:00 (SAST). Given Rustenburg was monitored in the afternoon, the site was monitored long after the erosion of the surface inversion layer. On 03/08/2005 the top of the surface nocturnal inversion layer in Figure 3.5(a) was at 184 magl and at 16.4 ?C. Irene hourly averaged temperatures in Table 3.7 show that it was mixed out between 09:00 and 10:00 (SAST). Wind speeds at Ermelo and Witbank on the 03/08/2005 at 08:00 (SAST) were already above 2 m.s-1 the critical threshold value (Figure 3.6) for surface inversion erosion (Hunt et al., 2007), and were averaged at 4.7 m.s-1 and 4 m.s-1 respectively, during the monitoring time. As Secunda is close to both sites, it can be deduced with caution that the winds at Secunda were also strong enough to erode the nocturnal surface inversion. Hence Secunda was likely monitored after the surface inversion was mixed out. On 05/08/2005 the morning surface inversion layer top in Figure 3.5(a) was at 275 magl and at 11 ?C. Hourly averaged temperatures at Irene in Table 3.7 show that it was eroded between 08:00 and 09:00 (SAST). As the Vaal Triangle was monitored in the afternoon, the site was monitored long after the erosion of the nocturnal surface inversion layer. On 08/08/2005 the top of the morning surface inversion layer on Figure 3.5(a) was also at 184 magl and at 16.4 ?C. Irene hourly averaged temperatures in Table 3.7 show that it was also mixed out between 09:00 and 10:00 (SAST). On 08/08/2005 at Witbank by 10:00 (SAST) the wind was already above the critical threshold value (Figure 3.5(a)) for surface inversion erosion (Hunt et al., 2007) and the hourly averaged temperature was 18.0 ?C at that time. The monitoring of Witbank was likely done after the surface inversion was mixed out. 81 Table 3.7: Hourly averaged surface temperature at Irene weather station: from the morning up to the afternoon during the winter campaign. Site Date Time(SAST) 06:00 07:00 08:00 09:00 10:00 11:00 12:00 13:00 Irene 27/07/2005 Average Temperature (?C) 8.7 8.9 12.5 17.4 19.7 20.9 22.3 24.5 Rustenburg 27/07/2005 Average Temperature (?C) 11.9 11.5 15.0 16.8 20.7 22.5 24.2 24.8 Irene 03/08/2005 Average Temperature (?C) 7.0 6.6 10.7 14.9 18.1 20.8 21.7 22.3 Ermelo 03/08/2005 Average Temperature (?C) 9.4 7.5 10.0 13.2 15.7 18.2 19.1 19.9 Witbank 03/08/2005 Average Temperature (?C) 8.6 6.3 12.7 16.4 18.4 19.7 20.5 21.6 Irene 05/08/2005 Average Temperature (?C) 5.1 5.1 8.2 12.7 15.5 16.9 18.6 19.8 Vereeniging 05/08/2005 Average Temperature (?C) 3.9 3.7 9.5 14.9 18.2 19.6 20.5 21.1 Irene 08/08/2005 Average Temperature (?C) 7.5 7.4 10.8 15.9 18.7 20.6 21.6 21.9 Witbank 08/08/2005 Average Temperature (?C) 7.6 7.1 12.2 15.4 18.0 19.3 20.2 22.0 82 Figure 3.5: Temperature vertical profiles measurements over Irene weather observation station during the winter campaign. Figure 3.5(a) is a midnight profile and Figure 3.5(b) is an afternoon profile. a b 83 Figure 3.6: Wind speed measurements at the study sites during the winter campaign. Comparison of air pollutants levels over the Highveld air pollution hotspots during the winter campaign. The Tables 3.8 to 3.12 show statistical analysis of the concentration distribution of O3, NO, NO2, SO2 and PM2.5 aerosols respectively over the four Highveld air pollution hotspots. The data were collected approximately at 167 magl during the winter campaign. Data from apparent industrial plume penetration incidences were eliminated for this analysis. The Rustenburg site was monitored on 27/07/2005 from 14:43:55 to 15:27:00 (SAST). Secunda was monitored on 03/08/2005 from 10:23:20 to 11:19:10 (SAST). The Vaal Triangle area was monitored on 05/08/2005 from 12:56:22 to 13:29:16 (SAST). Witbank was monitored on 08/08/2005 from 10:59:41 to 11:28:20 (SAST). The winter campaign comparison of the four air pollution hotspots is complicated by the different circulation types prevailing on the four case study days and the different times of the day they were monitored. Both these factors influence the levels of air pollutants in the 84 atmosphere. Sites monitored in the morning will be compared, and so will be sites monitored in the afternoon. Ozone Table 3.8 shows O3 concentration distribution data over Secunda, Witbank, Rustenburg and the Vaal Triangle. When comparing the sites monitored in the morning, Secunda and Witbank were found to have comparable O3 average concentrations. The Secunda average concentration was 46.55 ppb and that of Witbank was 46.03 ppb. Both sites were found to have low O3 concentration spatial variation. The O3 concentration relative standard deviations at Witbank and Secunda were 2.43% and 4.79% respectively. The relative uniform distribution of O3 concentration in space over Witbank and Secunda is supported by the relatively strong winds which were prevailing during the monitoring of these sites (Figure 3.6). The afternoon Vaal Triangle and Rustenburg O3 average concentrations were also found to be comparable. The Vaal Triangle O3 average concentration was 56.36 ppb and that of Rustenburg was 53.15 ppb. The relative standard deviations of O3 concentration at Rustenburg and the Vaal Triangle were 13.82% and 8.59% respectively. This indicates that the O3 concentration was more variable in space over Rustenburg than over the Vaal Triangle. The afternoon Rustenburg and the Vaal Triangle average O3 concentrations and O3 concentrations distribution are generally higher than the ones over Secunda and Witbank monitored in the morning. The high afternoon O3 concentration over Rustenburg and the Vaal Triangle are due to a build-up of O3 from O3 photochemical production, which is at its peak in the afternoon (Trainer et al., 1987; Poulida et al., 1994; Annegarn et al., 1996b; Betts et al., 2002; Taubman et al., 2004). 85 Table 3.8: Winter campaign Highveld hotspots comparison: O3 concentration distribution at approximately 167 m above ground level. Site Min (ppb) 25% (ppb) Median (ppb) 75% (ppb) Max (ppb) Average (ppb) StdDev % Number Rustenburg 41.22 46.62 52.29 57.98 73.39 53.15 13.82 2586 Secunda 32.37 46.03 46.81 47.54 58.10 46.55 4.79 2892 Vaal Triangle 46.2 53.52 55.39 59.98 68.70 56.36 8.59 1975 Witbank 40.99 45.17 46.01 46.86 48.86 46.03 2.43 1720 Nitrogen monoxide Table 3.9 shows NO concentration distribution data over the study sites. When comparing the sites monitored in the morning, Witbank was found to have a slightly higher NO average concentration of 0.93 ppb than that over Secunda of 0.74 ppb. The difference in the NO average concentratons of the two sites is likely due to the difference in the height of the mixing layers during the monitoring of the two sites, on 03/08/2005 the mixing height was at 1860 magl and on 08/08/2005 it was at 1280 magl (Figure 3.5(b)). The NO concentration relative standard deviation of 125.10% over Secunda and of 72.44% over Witbank indicates that NO concentration was more variable in space over Secunda than over Witbank. The Vaal Triangle area was found to have a higher afternoon NO average concentration of 3.14 ppb than that over Rustenburg of 0.71 ppb. This can be attributed to the limited depth of the mixing layer by an inversion at 1280 magl on 05/08/2005 and the deeper mixing layer on 27/07/2005 (Figure 3.5(b)). The Vaal Triangle was found to have the NO concentration relative standard deviation of 89.40% and Rustenburg of 77.20%. This indicates that the NO concentration distribution in space was more variable over the Vaal Triangle than over Rustenburg. 86 The morning Witbank NO average concentration and NO concentrations distribution in Table 3.9 are slightly higher than that over Rustenburg monitored in the afternoon. However the morning Secunda NO average concentration was comparable to that over Rustenburg and it?s NO concentrations distribution was slightly lower than that over Rustenburg. It is expected that the NO loading in the morning to be higher than that in the afternoon. This is because air pollutants accumulate in the shallow morning mixing layer and they are diluted in the deeper afternoon mixing layer (Annegarn et al., 1996a; Turner, 1996). The low morning NO concentrations over Secunda and Witbank may be explained by relatively stronger winds which were prevailing during the monitoring of these two sites as compared to winds prevailing during Rustenburg monitoring (Figure 3.6). Though Secunda and Witbank mixing layers were capped by inversions at about 1860 magl and 1280 magl repectively during their monitoring (Figure 3.5(b)). The relatively strong winds were enhancing air pollutants concentrations dilution through turbulent mixing (Hunt et al., 2007). The relatively high afternoon NO average concentration over Rustenburg, which is comparable to the morning average concentrations over Witbank and Secunda could be due to continuous emission sources, emitting throughout the day, which are likely to be industrial sources. The Vaal Triangle high afternoon NO average concentration, which is higher than the average concentrations of the other sites could also be due to continuous sources emitting throughout the day, which are also likely to be industrial sources. The 750-700 hPa persistent stable discontinuity at 1280 magl which limits vertical mixing and the light winds which were prevailing before and during the monitoring of Vaal Triangle, might have contributed to the relatively high afternoon NO levels over Vaal Triangle (Garstang et al., 1996: Freiman and Tyson 2000; Hunt et al, 2007). 87 Table 3.9: Winter campaign Highveld hotspots comparison: NO concentration distribution at approximately 167 m above ground level. Site Min (ppb) 25% (ppb) Median (ppb) 75% (ppb) Max (ppb) Average (ppb) StdDev % Number Rustenburg 0 0.33 0.63 0.97 4.40 0.71 77.20 2586 Secunda 0 0.28 0.49 0.74 6.31 0.74 125.10 2892 Vaal Triangle 0 1.11 1.99 4.02 12.37 3.14 89.40 1975 Witbank 0 0.44 0.78 1.07 3.70 0.93 72.44 1720 Nitrogen dioxide Table 3.10 shows NO2 concentration distribution data over the study sites. Witbank was found to have a higher morning NO2 average concentration of 5.08 ppb than that over Secunda of 1.65 ppb. This may be ascribed to the difference in the depth of the mixing layers during the monitoring of the two sites, on 03/08/2005 the mixing height was at 1860 magl and on 08/08/2005 it was at 1280 magl (Figure 3.5(b)). The NO2 concentration relative standard deviation of 54.36% over Witbank was lower than that of 86.69% over Secunda. This implies the NO2 concentration over Witbank was less variable in space than over Secunda. The Vaal Triangle area was found to have a higher afternoon NO2 average concentration of 19.51 ppb than that over Rustenburg of 6.33 ppb. As is the case with NO, this can be ascribed to the limited depth of the mixing layer by an inversion at 1280 magl on 05/08/2005 and the deeper mixing layer on 27/07/2005 (Figure 3.5(b)). The NO2 concentration relative standard deviation of 79.88% over Rustenburg was higher than that of 66.38% over the Vaal Triangle. This implies the NO2 concentration over the Vaal Triangle was less variable in space than over Rustenburg. 88 The afternoon Vaal Triangle and Rustenburg average NO2 concentration and NO2 concentration distribution in Table 3.10 are higher than that over Secunda and Witbank monitored in the morning. Normally it would be expected that the morning NO2 concentrations over Secunda and Witbank be higher than the afternoon NO2 concentrations over the Vaal Triangle and Rustenburg. This is because of limited vertical dispersion of air pollutants in the shallow morning mixing layer leading to accumulation of air pollutants, and in the afternoon air pollutants are diluted by turbulent mixing in a deeper afternoon mixing layer (Annegarn et al., 1996a; Turner, 1996). Photochemical processes which are optimal in the afternoon, like the oxidation of NO2 by OH radicals to form nitric acid (HNO3) and peroxyacetylnitrate (PAN) (Parrish et al., 1990; IPCC, 2001) and photolysis of NO2 in the formation of O3 (Crutzen et al., 1999; Taubman et al., 2004) reduces NO2 concentration in the afternoon. As in the case of NO concentrations in the morning over Secunda and Witbank, low NO2 concentrations in the morning over these two sites can also be explained by relatively stronger winds which were prevailing during the monitoring of these two sites as compared to winds prevailing during Rustenburg and Vaal Triangle monitoring (Figure 3.6). The high afternoon NO2 concentrations over the Vaal Triangle and Rustenburg could be due to continuous emission sources, emitting throughout the day, which are likely to be industrial sources. As is the case with NO, the occurrence of the 750-700 hPa stable discontinuity at 1280 magl (Figure 3.5(b)) and the light winds which were prevailing before and during the monitoring of Vaal Triangle might have contributed to the high afternoon Vaal Triangle NO2 levels by restricting vertical and horizontal mixing of pollutants (Garstang et al., 1996: Freiman and Tyson 2000; Hunt et al., 2007). 89 Table 3.10: Winter campaign Highveld hotspots comparison: NO2 concentration distribution at approximately 167 m above ground level. Site Min (ppb) 25% (ppb) Median (ppb) 75% (ppb) Max (ppb) Average (ppb) StdDev % Number Rustenburg 0 2.45 4.94 9.09 23.04 6.33 79.88 2586 Secunda 0 0.67 1.26 2.12 8.16 1.65 86.69 2892 Vaal Triangle 4.30 7.27 17.43 27.80 49.44 19.51 66.38 1975 Witbank 0.57 3.38 4.43 5.47 15.10 5.08 54.36 1720 Sulphur dioxide Table 3.11 shows SO2 concentration distribution data over the study sites. When comparing the sites monitored in the morning, Witbank was found to have a slightly higher SO2 average concentration of 7.45 ppb than that over Secunda of 6.65 ppb. As was the case with NO and NO2 the difference may be attributed to the difference in mixing layer heights during the monitoring of the two sites. The relative standard deviation of SO2 concentration of 37.70% over Witbank indicates that SO2 concentration is more variable in space over Witbank than over Secunda with a relative standard deviation of 26.71%. The Vaal Triangle area was found to have a higher afternoon SO2 average concentration of 11.05 ppb than the one over Rustenburg of 4.62 ppb. This can be ascribed to the limited depth of the mixing layer by an inversion at 1280 magl on 05/08/2005 and the deeper mixing layer on 27/07/2005 (Figure 3.5(b)). The SO2 concentration relative standard deviation over Rustenburg of 56.34% was found to be higher than the one of 48.83% over the Vaal Triangle area. This implies the SO2 concentration over the Vaal Triangle was less variable in space than over Rustenburg. The morning Secunda and Witbank SO2 average concentrations and SO2 concentrations distribution in Table 3.11 are generally higher than that over Rustenburg monitored in the afternoon. The morning Secunda and Witbank SO2 concentrations would probably have been much higher if the winds which were prevailing during the monitoring of the two 90 sites were not as strong (Figure 3.6), supporting dilution of pollutants through turbulent mixing (Hunt et al., 2007). The Vaal Triangle afternoon SO2 concentration distribution was generally higher than of all the other study sites, including Secunda and Witbank which were monitored in the morning. This implies that SO2 loading over the Vaal Triangle was generally high throughout the day, since SO2 concentration is normally low in the afternoon due to dilution (Annergarn et al., 1996b). Its high afternoon SO2 average concentration could be due to emission sources with less emitting diurnal cycle, emitting throughout the day, which are likely to be industrial sources. The presence of the 750-700 hPa stable discontinuity at 1280 magl (Figure 3.5(b)) and the light winds which were prevailing before and during the monitoring of Vaal Triangle (Figure 3.6), might also have contributed to the high afternoon SO2 levels. Table 3.11: Winter campaign Highveld hotspots comparison: SO2 concentration distribution at approximately 167 m above ground level. Site Min (ppb) 25% (ppb) Median (ppb) 75% (ppb) Max (ppb) Average (ppb) StdDev % Number Rustenburg 1.14 2.69 3.54 6.31 14.58 4.62 56.34 2586 Secunda 3.64 5.68 6.40 7.05 19.16 6.65 26.71 2892 Vaal Triangle 3.40 6.63 10.18 12.80 26.42 11.05 48.83 1975 Witbank 3.69 5.46 6.51 8.70 19.95 7.45 37.70 1720 PM2.5 aerosols Table 3.12 shows PM2.5 aerosols concentration distribution data over the study sites. Witbank was found to have a higher morning aerosols average concentration of 709.61 cm-3 than that over Secunda of 393.42 cm-3. As is the case with the other pollutants, this is likely due to mixing height difference during the monitoring of these sites (Figure 3.5(b)). The relative standard deviations of aerosols concentration over Witbank of 22.02% and Secunda of 21.22% show that the aerosols concentration variability in space over the two sites was comparable. 91 The Vaal Triangle area was found to have an afternoon aerosols average concentration of 1010.77 cm-3 that is more than twice the average concentration of 482.02 cm-3 over Rustenburg. Despite the general higher concentration distribution over the Vaal Triangle compared to Rustenburg, the Vaal Triangle aerosols concentration was less variable in space than over Rustenburg, it had a relative standard deviation of 21.24% and over Rustenburg it was 63.91%. The morning Witbank aerosols average concentration and concentration distribution in Table 3.12 was generally higher than the one over Rustenburg monitored in the afternoon. But the opposite situation occurred between Secunda monitored in the morning and Rustenburg monitored in the afternoon. The Rustenburg aerosols average concentration was higher than the Secunda average aerosols concentration. The low morning aerosols average concentrations over Secunda and Witbank can be attributed to the relative stronger winds which prevailed during the monitoring of these sites (Figure 3.6) leading to dilution of air pollutants through turbulent mixing (Annegarn et al., 1996a; Turner, 1996; Hunt et al., 2007). The Vaal Triangle afternoon aerosols average concentration and concentration distribution was generally higher than all the other study sites, including Witbank and Secunda which were monitored in the morning. Because aerosols concentrations are normally at their minimum in the afternoon (Annegarn et al., 1996b), this implies that aerosols loading over the Vaal Triangle was generally high on this particular day. The high afternoon aerosols average concentration over the Vaal Triangle must be due to emission sources, emitting throughout the day, which are likely to be industrial sources. As is the case with other pollutants, the occurrence of the 750-700 hPa stable discontinuity at 1280 magl (Figure 3.5(b)) and the light winds which were prevailing before and during the monitoring of Vaal Triangle (Figure 3.6) might have contributed to the high afternoon Vaal Triangle PM2.5 aerosols levels. 92 Table 3.12: Winter campaign Highveld hotspots comparison: PM2.5 aerosols concentration distribution at approximately 167 m above ground level. Site Min (#/cm 3 ) 25% (#/cm 3 ) Median (#/cm 3 ) 75% (#/cm 3 ) Max (#/cm 3 ) Average (#/cm 3 ) StdDev % Number Rustenburg 87.05 202.44 402.99 668.65 1394.70 482.02 63.91 2586 Secunda 165.36 343.51 382.89 426.52 964.07 393.42 21.22 2892 Vaal Triangle 499.02 840.79 1006.73 1161.61 1730.36 1010.77 21.24 1975 Witbank 318.40 598.28 690.07 802.95 1372.15 709.61 22.02 1720 Spring campaign Spring campaign meteorological overview The synoptic conditions over the country during the spring campaign case study days were similar to the ones that were prevailing during the autumn campaign. They were dominated by a surface trough. Figure 3.7 shows similar or comparable afternoon 14h00 (SAST) surface synoptic charts during the spring campaign case study days. On the 20/09/2005 and 23/09/2005 the interior was under the influence of a surface trough, it was situated over the central interior and extending to the southern interior. It brought about partly cloudy to cloudy conditions. The temperatures were generally warm during the spring campaign case study days (Table 3.13), and the winds were varying between calm and fine, and fine and moderate at different stations in different days (Figure 3.8). Figure 3.9 shows the Irene weather station temperature profiles which were used to characterise the vertical structure of the lower part of the troposphere during the spring case study days. In both days of the case studies the lower troposphere was characterised by low level nocturnal inversion layers in the morning (Figure 3.9(a)), resulting a shallow mixing layer. These inversions occurred at the surface and 184 magl on 20/09/2005. On 23/09/2005 the inversion layers occurred at the surface, 184 magl and 641 magl. In the afternoon the nocturnal inversion layers were mixed out resulting a deep mixing layer 93 (Annegarn et al., 1996a). This mixing layer was capped by upper level inversions at 3080 magl on 20/09/2009 and at 1464 magl on 23/09/2005 (Figure 3.9(b)). Figure 3.7: Spring campaign surface synoptic charts. Figure 3.7(a) and Figure 3.7(b) represents the charts on 20/09/2005 and 23/09/2005 respectively. Table 3.13: Hourly averaged surface temperature at Irene and study sites weather stations: from the morning up to the afternoon during the spring campaign. Site Date Time(SAST) 06:00 07:00 08:00 09:00 10:00 11:00 12:00 13:00 Irene 20/09/2005 Average Temperature (?C) 14.2 14.8 19.1 22.0 24.5 26.7 28.4 28.7 Ermelo 20/09/2005 Average Temperature (?C) 12.9 15.4 19.2 22.3 24.8 26.7 26.8 26.4 Witbank 20/09/2005 Average Temperature (?C) 12.0 14.5 17.8 21.2 23.1 25.6 27.7 28.0 Irene 23/09/2005 Average Temperature (?C) 15.0 16.7 19.5 22.1 23.9 26.1 27.0 28.9 Rustenburg 23/09/2005 Average Temperature (?C) 12.6 15.0 20.3 22.3 24.4 26.5 29.2 30.4 The top of the morning surface inversion layer in Figure 3.9(a) on 20/09/2005 was at 92 magl and at 19.8 ?C. Hourly averaged temperatures at Irene in Table 3.13 show that it was eroded between 08:00 and 09:00 (SAST). The wind speed at Witbank at 09:00 (SAST) on the 20/09/2005 was at the critical threshold value and at Ermelo at the same a b 94 time was above the critical threshold value (Figure 3.8) for surface inversion erosion (Hunt et al., 2007). The hourly averaged temperatures at Witbank and Ermelo were 21.2 ?C and 22.3 ?C respectively at 09:00 (SAST) (Table 3.13). It can be deduced with caution that the conditions were similar at Secunda which is close to both sites. Secunda was likely monitored after the surface inversion was mixed out. On 23/09/2005 the morning surface inversion layer top in Figure 3.9(a) was also at 92 magl and at 17.7 ?C. Hourly averaged temperatures at Irene in Table 3.13 show that it was mixed out between 07:00 and 08:00 (SAST). The surface winds at Rustenburg on 23/09/2005 by 09:00 (SAST) were averaged to 1.2 m.s-1 (Figure 3.8), and the temperature was averaged at 22.3 ?C at that time. It was likely that the Rustenburg site was monitored after the surface inversion was mixed out. As the Vaal Triangle and Witbank sites were monitored in the afternoon, both sites were monitored long after the erosion of the nocturnal surface inversion. Figure 3.8: Wind speed measurements at the study sites during the spring campaign. 95 Figure 3.9: Temperature vertical profiles measurements over Irene weather observation station during the spring campaign. Figure 3.9(a) is a midnight profile and Figure 3.9(b) is an afternoon profile. a b 96 Comparison of air pollutants levels over the Highveld air pollution hotspots during the spring campaign. Tables 3.14 to 3.18 show the statistical analysis of the concentration distribution of O3, NO, NO2, SO2 and PM2.5 aerosols respectively over the four Highveld air pollution hotspots. The data were collected approximately 167 m above ground level during the spring campaign. Data from apparent industrial plume penetration were eliminated for this analysis. Secunda and Vaal Triangle sites were monitored on 20/09/2005, the Secunda area from 09:31:56 to 10:13:51 (SAST), and the Vaal Triangle area from 14:02:36 to 14:20:07 (SAST). Rustenburg and Witbank areas were monitored on 23/09/2005, the Rustenburg area from 09:51:07 to 10:53:28 (SAST), and the Witbank area from 14:36:29 to 15:08:07 (SAST). The similar synoptic conditions during the spring campaign monitoring days allowed the comparison of the four air pollution hotspots. The only complication to the comparison is the different times of the day these sites were monitored, because that influences the levels of air pollutants in the atmosphere. Ozone Table 3.14 shows O3 concentration distribution data over Secunda and Rustenburg monitored in the morning, and over Witbank and the Vaal Triangle monitored in the afternoon. Rustenburg was found to have a higher O3 average concentration of 58.80 ppb than that over Secunda of 54.20 ppb. This is due to a generally less NO concentration over Rustenburg as compared to Secunda (Table 3.15), which destroys O3 through NO and O3 titration reaction (Kleinman, 1994; Kley et al., 1994; Poulida et al., 1994; Hobbs et al., 2003; Taubman et al., 2004). The O3 concentration spatial variability was higher over Secunda than Rustenburg. Secunda had O3 concentration relative standard deviation of 17.63% and Rustenburg of 7.78%. 97 Witbank was found to have a higher afternoon O3 average concentration of 71.26 ppb than that over the Vaal Triangle of 63.46 ppb. As the wind at Witbank was stronger than at the Vaal Triangle during the times the sites were monitored (Figure 3.8) and NO concentration was higher at Witbank than at the Vaal Triangle (Table 3.15), the higher O3 concentration at Witbank could be attributed to the inversion at 1464 magl (Figure 3.9(b)). The O3 concentration was less variable in space over both sites, it had low relative standard deviations of 3.31% over Witbank and 1.96% over the Vaal Triangle. Afternoon Witbank and Vaal Triangle O3 average concentrations and O3 concentrations distribution are generally higher than that over Secunda and Rustenburg which were monitored in the morning. The high afternoon O3 concentration over Witbank and the Vaal Triangle are due to a build-up of O3 from O3 photochemical production, which is at its peak in the afternoon (Trainer et al., 1987; Poulida et al., 1994; Annegarn et al., 1996b; Betts et al., 2002; Taubman et al., 2004). Table 3.14: Spring campaign Highveld hotspots comparison: O3 concentration distribution at approximately 167 m above ground level. Site Min (ppb) 25% (ppb) Median (ppb) 75% (ppb) Max (ppb) Average (ppb) StdDev % Number Secunda 20.90 53.05 56.42 59.07 70.92 54.20 17.63 2516 Vaal Triangle 60.97 62.86 63.62 63.86 67.36 63.46 1.96 1052 Rustenburg 27.68 56.42 59.40 61.32 69.36 58.80 7.78 3742 Witbank 63.75 69.50 71.34 72.76 76.93 71.26 3.31 1899 Nitrogen monoxide Table 3.15 shows NO concentration distribution data over the study sites. When comparing the sites monitored in the morning, Secunda was found to have a higher morning NO average concentration of 0.75 ppb than that over Rustenburg of 0.22 ppb. The distribution of NO concentration in space was less variable over Secunda than over 98 Rustenburg. Secunda had NO2 concentration relative standard deviation of 68.53% and Rustenburg of 135.82%. Witbank was found to have a higher afternoon NO average concentration of 0.32 ppb than that over the Vaal Triangle of 0.012 ppb. The higher afternoon NO concentration over Witbank could be ascribed to the subsidence inversion that occurred at 1464 magl on 23/09/2005 (Figure 3.9(b)). The NO concentration relative standard deviation of 137.22%% over Witbank and of 132.29% over the Vaal Triangle suggest that the spatial variations of NO concentrations over the two sites were comparable and high. The morning Secunda NO average concentration and NO concentrations distribution in Table 3.15 are generally higher than both the ones over Witbank and the Vaal Triangle monitored in the afternoon. On the other hand Rustenburg morning NO average concentration is higher than that over the Vaal Triangle but lower than that over Witbank. Normally it would be expected that the morning Rustenburg NO average concentration be higher than both the afternoon Vaal Triangle and Witbank NO average concentrations, because of dilution and photochemical consumption in the afternoon. The relatively low morning NO levels over Rustenburg could be the result of less emissions of NO over the site. Table 3.15: Spring campaign Highveld hotspots comparison: NO concentration distribution at approximately 167 m above ground level. Site Min (ppb) 25% (ppb) Median (ppb) 75% (ppb) Max (ppb) Average (ppb) StdDev % Number Secunda 0 0.37 0.67 0.92 3.2 0.75 68.53 2516 Vaal Triangle 0 0.003 0.006 0.018 0.03 0.012 132.29 1052 Rustenburg 0 0.074 0.14 0.22 1.61 0.22 135.82 3742 Witbank 0 0.065 0.13 0.35 1.89 0.32 137.22 1899 99 Nitrogen dioxide Table 3.16 shows NO2 concentration distribution data over the study sites. Secunda was found to have a higher morning NO2 average concentration of 7.06 ppb than that over Rustenburg of 4.69 ppb. The NO2 concentration relative standard deviations of 29.83% over Secunda and of 19.60% over Rustenburg suggest that the NO2 concentration was more spatially variable over Secunda than over Rustenburg. Witbank was found to have a higher afternoon NO2 average concentration of 9.07 ppb than that over the Vaal Triangle of 7.45 ppb. The inversion at 1464 magl (Figure 3.9(b)) on 23/09/2005 could have contributed to the relative high NO2 concentration at Witbank. The NO2 concentration relative standard deviation of 16.32% over Witbank and 10.19% over the Vaal Triangle indicate that the NO2 concentration was more variable in space over Witbank than over the Vaal Triangle. The morning Secunda NO2 average concentration is comparable to the Vaal Triangle afternoon NO2 average concentration, but lower than the Witbank afternoon NO2 average concentration. On the other hand, Rustenburg morning NO2 average concentration is lower than both the Vaal Triangle and Witbank afternoon NO2 average concentrations. The high afternoon NO2 average concentrations over the Vaal Triangle and Witbank could be due to continuous sources emitting throughout the day, which are likely to be industrial sources. The subsidence inversion at 1464 magl on 23/09/2005 in Figure 3.9(b) could have also contributed to the relatively high afternoon NO2 levels at Witbank. Table 3.16: Spring campaign Highveld hotspots comparison: NO2 concentration distribution at approximately 167 m above ground level. Site Min (ppb) 25% (ppb) Median (ppb) 75% (ppb) Max (ppb) Average (ppb) StdDev % Number Secunda 3.15 5.26 7.11 8.09 12.71 7.06 29.83 2516 Vaal Triangle 6.27 6.85 7.15 7.86 9.38 7.45 10.19 1052 Rustenburg 3.30 4.05 4.44 5.08 9.92 4.69 19.60 3742 Witbank 3.38 8.46 8.81 9.28 17.01 9.07 16.32 1899 100 Sulphur dioxide Table 3.17 shows the SO2 concentration distribution data over the study sites. Secunda was found to have a higher morning SO2 average concentration of 14.50 ppb as compared to Rustenburg with an average concentration of 3.96 ppb. The relative standard deviation of SO2 concentration of 34.06% over Secunda and 55.00% over Rustenburg show that SO2 concentration was more variable in space over Rustenburg than over Secunda. The Vaal Triangle was found to have a slightly higher afternoon SO2 average concentration of 8.41 ppb than that over Witbank of 7.39 ppb. The SO2 concentration was more variable in space over Witbank than over the Vaal Triangle. It had a relative standard deviation of 52.45% over Witbank and 12.82% over the Vaal Triangle. The morning Secunda SO2 average concentration and SO2 concentration distribution in Table 3.17 are generally higher than the ones over the Vaal Triangle and Witbank monitored in the afternoon. On the other hand, morning Rustenburg SO2 average concentration and SO2 concentration distribution is generally lower than the ones over the Vaal Triangle and Witbank monitored in the afternoon. The high afternoon SO2 concentrations over the Vaal Triangle and Witbank as compared to Rustenburg in the morning could be due to more and stronger emission sources with small emitting diurnal cycle (emitting throughout the day), which are likely to be industrial sources. The subsidence inversion at 1464 magl on 23/09/2005 in Figure 3.9(b) could have also contributed to the relatively high afternoon SO2 levels at Witbank. Table 3.17: Spring campaign Highveld hotspots comparison: SO2 concentration distribution at approximately 167 m above ground level. Site Min (ppb) 25% (ppb) Median (ppb) 75% (ppb) Max (ppb) Average (ppb) StdDev % Number Secunda 7.26 10.74 13.31 17.21 29.66 14.50 34.06 2516 Vaal Triangle 7.03 7.73 7.87 8.85 11.29 8.41 12.82 1052 Rustenburg 2.40 2.66 3.13 3.99 15.53 3.96 55.00 3742 Witbank 4.25 5.36 5.46 7.27 23.95 7.39 52.45 1899 101 PM2.5 aerosols Table 3.18 shows the PM2.5 aerosols concentration distribution data over the study sites. Rustenburg was found to have a higher morning aerosols average concentration of 1021.56 cm-3 as compared to Secunda with an average of 822.57 cm-3. The aerosols concentration was more variable in space over Rustenburg than over Secunda, with relative standard deviations of 24.52% and 17.40% respectively. Witbank was found to have a higher afternoon aerosols average concentration of 785.56 cm-3 as compared to the Vaal Triangle with an average concentration of 609.08 cm-3. The inversion at 1464 magl (Figure 3.9(b)) on 23/09/2005 could have contributed to the relative high aerosols concentration at Witbank. The relative standard deviations of aerosols concentrations of 20.50% over Witbank and 19.30% over the Vaal Triangle, show that the aerosols concentration variability in space over the two sites was comparable. The morning Secunda and Rustenburg aerosols average concentrations and concentrations distribution were generally higher than those over Witbank and the Vaal Triangle monitored in the afternoon. Though the morning aerosols concentrations were higher in comparison with afternoon concentrations, the difference in aerosols average concentrations between Witbank and the Vaal Triangle with Secunda is not marked. This suggests that there is some contribution to aerosols loading over the Vaal Triangle and Witbank from relatively strong and continuous sources, which are likely to be industrial sources. 102 Table 3.18: Spring campaign Highveld hotspots comparison: PM2.5 aerosols concentration distribution at approximately 167 m above ground level. Site Min (#/cm 3 ) 25% (#/cm 3 ) Median (#/cm 3 ) 75% (#/cm 3 ) Max (#/cm 3 ) Average (#/cm 3 ) StdDev % Number Secunda 326.44 726.24 808.22 909.50 1326.15 822.57 17.40 2516 Vaal Triangle 218.60 541.80 600.62 669.78 1061.60 609.08 19.30 1052 Rustenburg 176.39 854.87 1017.99 1190.99 1933.86 1021.56 24.52 3742 Witbank 160.35 691.18 788.24 885.72 1394.54 785.56 20.50 1899 Comparison of seasonal variation of air pollutants levels over the Highveld air pollution hotspots The Tables 3.19 to 3.23 show seasonal spatial average concentrations of O3, NO, NO2, SO2 and PM2.5 aerosols respectively over the four Highveld air pollution hotspots. The data is extracted from the considered seasonal case studies in Tables 3.2 to 3.6, 3.8 to 3.12, and 3.14 to 3.18. Average concentrations were determined from data collected at 167 magl over all the study sites and on the three studied seasons. The seasons that are compared are autumn, winter and spring. Ozone Surface O3 loading varies seasonally. It has a broad peak in the dry season that is due to a large photochemical generation occurring in this season (Jacobs et al., 1995; Betts et al., 2002). In southern Africa it has peak concentrations in spring months from August to November (Zunckel et al., 2004). Emissions from wide spread biomass burning (Figure 3.10), both regional and long-range transported from countries lying north of South Africa, lightning during the inter-dry and wet season period and biogenic emissions contribute to the peak O3 concentrations throughout the lower troposphere from August to November (Betts et al., 2002; Diab et al., 2004). The warm temperature around this period supports the emission of biogenic hydrocarbons and anthropogenic volatile organic compounds, both precursors of O3 (Sillman and Samson, 1995). From the Southern Hemisphere Additional Ozonesondes (SHADOZ) project, Diab et al., (2004) 103 established using data from 1998-2002 that the Total Tropospheric Ozone over the industrial Highveld (Irene) has changed from having a broad peak from September to November to a sharper October maximum in the recent period. Figure 3.10: Monthly fires and their location detected by satellite over the region shown by red spots, the blue spots represents towns. Figure 3.10(a) shows the fires detected during the month of August 2005. Figure 3.10(b) shows the fires detected during the month of September 2005. (AFIS). From the monitoring results of the three season campaigns shown in Table 3.19, all the study sites generally show a seasonal variation in O3 average concentrations. The low values were observed during the autumn campaign. During the winter campaign higher values were observed and the highest values were observed during the spring campaign. Secunda and the Vaal Triangle were monitored consistently at the same time of the day, Secunda in the morning and the Vaal Triangle in the afternoon. Both these sites had seasonal change increments of +/- 10 ppb in spatial O3 average concentrations. The Witbank spatial O3 average concentration difference between winter and autumn was 21.62 ppb, in both cases O3 was monitored in the morning. The Rustenburg spatial O3 average concentration difference between spring and autumn was 30.32 ppb, in both cases O3 was also monitored in the morning. a b 104 Table 3.19: Highveld hotspots seasonal spatial O3 average concentrations comparison Seasonal O3 average concentrations (ppb) Site Autumn Winter Spring Secunda 30.00 46.55 54.20 Witbank 24.41 46.03 71.26* Rustenburg 28.48 53.15* 58.80 Vaal Triangle 42.51* 53.52* 63.46* * O3 monitored in the afternoon Nitrogen oxides Surface NOX have seasonal peak concentrations in winter (Parish et al., 1990; Doddridge et al., 1992). The peak NOX concentrations in winter over the study sites are due to stronger emissions in winter from wide spread biomass burning (GDACE, 2004), domestic fossil fuel burning and power generating plants for space heating (Annegarn et al., 1996b; Diab et al., 2004), and slower removal by less efficient photochemical processes (Parish et al., 1990; Doddridge et al., 1992). From the monitoring results of the three season campaigns in Table 3.20, all the study sites show winter peak spatial NO average concentrations except Secunda. The Witbank winter peak would probably have been higher than the one reported, and the Secunda NO concentration winter peak would probably have been observed, if the sites did not experience relatively strong winds which were prevailing during the monitoring of these two sites. Rustenburg and the Vaal Triangle winter NO concentrations were monitored in the afternoon, which implies the concentrations could have been at their diurnal minimum (Annegarn et al., 1996a). The Rustenburg spatial NO average concentration derived from afternoon monitoring was higher than both autumn and spring spatial NO average concentrations derived from morning monitoring. The Vaal Triangle winter peak spatial NO average concentration was the highest. 105 Table 3.20: Highveld hotspots seasonal spatial NO average concentrations comparison Seasonal NO average concentrations (ppb) Site Autumn Winter Spring Secunda 1.20 0.74 0.75 Witbank 0.65 0.93 0.32* Rustenburg 0.26 0.71* 0.22 Vaal Triangle 0* 3.14* 0.012* * NO monitored in the afternoon From the monitoring results of the three season campaigns in Table 3.21, only Rustenburg and the Vaal Triangle show peak winter spatial NO2 average concentration. Both these sites were monitored in the afternoon, which implies the concentrations must have been at their diurnal minimum (Annegarn et al., 1996a). The Rustenburg spatial NO2 average concentration derived from afternoon monitoring was higher than both the autumn and spring spatial NO2 average concentrations derived from morning monitoring. The Vaal Triangle winter spatial NO2 average concentration was higher than the one over Rustenburg. The winter peak NO2 average concentrations were not observed for Secunda and Witbank, as a result of relatively strong winds which were prevailing during the monitoring of these two sites as mentioned in the previous passages. Table 3.21: Highveld hotspots seasonal spatial NO2 average concentrations comparison Seasonal NO2 average concentrations (ppb) Site Autumn Winter Spring Secunda 1.48 1.65 7.06 Witbank 0.65 5.08 9.07* Rustenburg 0.17 6.33* 4.69 Vaal Triangle 0* 19.51* 7.45* * NO2 monitored in the afternoon 106 Sulphur dioxide Surface SO2 loading also varies seasonally with peak concentrations in winter. The peak SO2 concentrations in winter over the study sites are due to stronger emissions from domestic fossil fuel combustion and power generating plants for space heating (Annegarn et al., 1996b; van Horen et al., 1996; Diab et al., 2004). The high pressure systems which are more frequent in winter worsen the high winter SO2 levels by causing subsidence, limiting vertical dispersion of air pollutants (Scheifinger, 1992; Tyson et al., 1996; GDACE, 2004). From the monitoring results of the three season campaigns in Table 3.22, only the Vaal Triangle show peak winter spatial SO2 average concentration. The Vaal Triangle was monitored in the afternoon in all the monitoring campaigns, this imply the SO2 concentrations could have been at their background concentrations (Annegarn et al., 1996a). The Rustenburg winter spatial SO2 average concentration monitored in the afternoon was only higher than the spring, but lower than the autumn spatial SO2 average concentration, the autumn and spring SO2 levels were both monitored in the morning. The winter peak SO2 concentrations were not observed in Table 3.22 for Secunda and Witbank, for the same reason the winter peak of NO and NO2 concentrations were not observed. Table 3.22: Highveld hotspots seasonal spatial SO2 average concentrations comparison Seasonal SO2 average concentrations (ppb) Site Autumn Winter Spring Secunda 17.02 6.65 14.50 Witbank 13.07 7.45 7.39* Rustenburg 12.17 4.62* 3.96 Vaal Triangle 6.97* 11.05* 8.41* * SO2 monitored in the afternoon 107 PM2.5 aerosols Surface atmospheric aerosols loading varies seasonally, with peak concentrations in winter. The high winter aerosols concentrations over the study sites are associated with stronger emissions from wide spread biomass burning (Butler et al., 2003; Eck et al., 2003; Hobbs et a., 2003), domestic fuel burning and power generating plants for space heating (van Horen et al., 1996; Annegarn et al., 1996b; Diab et al., 2004). The semi-permanent, subtropical, continental anticyclones which frequently occur in winter aggravates the high winter aerosol loading by producing subsidence temperature inversions (Scheifinger, 1992; Tyson et al., 1996; Hobbs et al., 2003; GDACE, 2004). From the monitoring results of the three season campaigns in Table 3.23, only the Vaal Triangle show peak winter spatial aerosols average concentration. The Vaal Triangle was monitored in the afternoon in all the monitoring campaigns, this imply the aerosols concentrations might have been at their minimum diurnal concentrations (Annegarn et al., 1996b). The Rustenburg winter spatial aerosols average concentration monitored in the afternoon was lower than both the autumn and spring spatial aerosols average concentration, which were both monitored in the morning. The winter peak PM2.5 aerosols concentrations were not observed in table 3.23 for Secunda and Witbank, for the same reason the winter peak for NO, NO2 and SO2 concentrations were not observed. Table 3.23: Highveld hotspots seasonal spatial PM2.5 aerosols average concentrations comparison Seasonal PM2.5 aerosols average concentrations (#/cm 3 ) Site Autumn Winter Spring Secunda 1052.33 393.42 822.57 Witbank 1166.23 709.61 785.56* Rustenburg 872.52 482.02* 1021.56 Vaal Triangle 580.89* 1010.77* 609.08* * PM2.5 aerosols monitored in the afternoon 108 ************************************ The concentration distribution of O3, NOX, SO2 and PM2.5 aerosols over the Highveld air pollution hotspots were compared in all monitored seasons. Seasonal change in atmospheric loading of these air pollutants over the Highveld air pollution hotspots was also assessed. In the comparisons of the Highveld air pollution hotspots, it was taken into account the prevailing meteorological conditions, and the temporal cycle of the photochemical processes, which both affect the atmospheric loading of air pollutants.