Clean air slots amid dense atmospheric pollution in southern Africa

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. D13, 8490, doi: /2002jd002156, 2003 Clean air slots amid dense atmospheric pollution in southern Africa Peter V. Hobbs Department of Atmospheric Sciences, University of Washington, Seattle, Washington, USA Received 31 January 2002; revised 9 May 2002; accepted 28 May 2002; published 28 March [1] During the flights of the University of Washington s Convair-580 in the Southern African Regional Science Initiative (SAFARI 2000) in southern Africa, a phenomenon was observed that has not been reported previously. This was the occurrence of thin layers of remarkably clean air, sandwiched between heavily polluted air, which persisted for many hours during the day. Photographs are shown of these clean air slots (CAS), and particle concentrations and light scattering coefficients in and around such slot are presented. An explanation is proposed for the propensity of CAS to form in southern Africa during the dry season. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0345 Atmospheric Composition and Structure: Pollution urban and regional (0305); KEYWORDS: clean air slots, clear air slots, air pollution, stable atmospheric layers, atmospheric layers Citation: Hobbs, P. V., Clean air slots amid dense atmospheric pollution in southern Africa, J. Geophys. Res., 108(D13), 8490, doi: /2002jd002156, Introduction [2] The Earth s atmosphere is often highly layered. Layering is most common when the atmosphere is stable to vertical motions, particularly when the temperature increases with height over a certain altitude range. This phenomenon has received considerable attention in connection with air pollution, since pollutants tend to accumulate beneath stable layers [e.g., Tyson and Preston-Whyte, 2000; Wu et al., 1997; Swap and Tyson, 1999; Newell, 1999; Garstang et al., 1996]. Consequently, increases in visibility above stable layers are a common phenomenon. This paper describes another consequence of stable layers, which has not been reported previously, which is to produce thin, horizontal layers of remarkably clean air, which we will call clean air slots (CAS), that separate polluted air below and above. [3] Although we have observed CAS in various locations around the world, we found them to be particularly well defined and prevalent in southern Africa during the dry season (August and September). In the dry season, southern Africa is dominated by a semipermanent, subtropical, continental anticyclone, which produces spatially ubiquitous and temporarily persistent subsidence temperature inversions [Tyson and Preston-Whyte, 2000; Garstang et al., 1996]. Also, in the dry season in southern Africa there is widespread biomass burning, which produces extensive pollution. Consequently, CAS in southern Africa occurs against a background of heavy pollution and poor visibility, which makes them particularly striking when viewed from an aircraft. Copyright 2003 by the American Geophysical Union /03/2002JD Instrumentation [4] The measurements and observations reported here were obtained aboard the University of Washington s Convair-580 research aircraft. Information on all of the instruments aboard this aircraft in the Southern African Regional Science Initiative (SAFARI 2000) is given in Appendix A by P. V. Hobbs in the work of Sinha et al. [2003]. Here we describe briefly those instruments used to obtain the measurements presented in this paper. [5] Static air temperature (T) was measured with an inhouse built temperature sensor with a reverse-flow head, which essentially eliminates dynamic heating. Relative humidity (RH) was derived from T and the dewpoint temperature measured with a Cambridge Systems cooledmirror devise (model TH73-244). [6] Aitken or condensation nucleus (CN) concentrations were obtained with a TSI model 3025A condensation particle counter, which measures the total concentration of particles with diameters >0.003 mm. The light scattering coefficient for particles, s sp (i.e., with molecular scattering subtracted from the measured total scattering), at a wavelength of 550 nm was provided by an MS Electron model 3W-02 integrating nephelometer. The airflow to the nephelometer was warmed to dry the aerosol prior to the measurements. The measured values of s sp were corrected following the procedures described by Anderson and Ogren [1998]. [7] Particle size distributions over the diameter range mm were measured with a Particle Measuring Systems Passive Cavity Aerosol Spectrometer Probe (PCASP-100X) light scattering instrument. The total concentration of particles measured with this instrument is referred to as the PCASP concentration. The PCASP was located under the wing of the aircraft, and a preheater to the instrument was switched on to dry the particles. 3. Observations and Measurements [8] CAS in southern Africa could be observed visually from the aircraft by looking out toward the horizon as the SAF 26-1

2 SAF 26-2 HOBBS: CLEAN AIR SLOTS IN SOUTHERN AFRICA measurements of various particle parameters (e.g., CN and s sp ) decreased sharply. [9] Over a period of 37 days in southern Africa (10 August to 16 September 2000), with flights on 22 of these days, CAS were observed, and measurements obtained below, within and above them, on 11 occasions covering 6 days (Table 1). On 6 of the 11 occasions, CAS were overland (near Pietersburg, South Africa; over the Namib Desert, Namibia; and over the Etosha National Park, Namibia), and on the other five occasions the CAS were just off the east and west coasts of southern Africa (over Inhaca Island, Mozambique, and off the southwest African coast). The earliest time that a CAS was observed was 0813 UTC (1013 local time) over Inhaca Island, and the latest time was 1531 UTC (1731 local time) over Pietersburg. However, flights took place only between 0530 and 1650 UTC. [10] Listed in Table 1 are the heights of the base and top and the thickness of each of the 11 CAS, and the average light scattering coefficient, CN concentration and PCASP concentration in, below and above each CAS. s of these parameters for the 11 cases of CAS are also given in Table 1. For example, the average thickness of the CAS was 763 ± 445 m. The last column in Table 1 lists the static stability (S z ) of the air in which the CAS was located, which is defined here as S Z ¼ 1 ð T d Þ Figure 1. CAS over southern Africa. (a) Near Maputo, Mozambique at 0956 UTC on 24 August (b) Over the Namib Desert, Namibia at 1130 UTC on 11 September (c) Off the Namibian coast at 1333 UTC on 13 September (Photos: Peter V. Hobbs.) aircraft changed altitude. Because of the great visual range in a CAS compared to that in the polluted air above and below, CAS appeared as thin, white, horizontal lines (Figure 1). In the polluted air above and below a CAS, visibility was very limited. However, on penetrating into a CAS, the visibility increased dramatically, often to in excess of 250 km, and where T is the temperature (in K), d the dry adiabatic lapse rate (9.8 K km 1 ), and the temperature lapse rate in the region of the CAS. The larger the positive value of S z,the greater is the stability to vertical displacements of the air in the CAS. As can be seen in Table 1, all of the CAS was located in regions of positive static stability. [11] Shown in Figure 2a are measurements of CN and s sp that are characteristic of CAS and the air above and below them in southern Africa. For the case shown in Figure 2a, the average CN concentration within the CAS was about 50% of that below and above the CAS, and the average light scattering coefficient in the CAS was about 40% of that below and above the CAS. Comparison of Figures 2a and 2b shows that the lower part of the CAS was located in a stable region where the temperature was essentially constant with height, and the center of the CAS was coincident with an even more stable temperature inversion. Although the temperature increased with height in the upper portion of the CAS, the air was still stable in this region. [12] Figure 2a can be used to illustrate what is meant in Table 1 by in the CAS, below the CAS, and above the CAS. For the CN curve shown in Figure 2a, below the CAS is the region marked by A, in the CAS the region between B and C, and above the CAS the region D. [13] Figure 3 shows the vertical profile of s sp through the CAS on 24 August (see Table 1). Since the lowest value of the light scattering coefficient that can be measured by the nephelometer is m 1, values below this are shown by dots in Figure 3. In determining the average values of s sp in the CAS (Table 1) only values of s sp > m 1 were used. The average value of s sp in the CAS observed on 24 August was m 1. By comparison, the light scattering coefficient for molecular scattering

3 HOBBS: CLEAN AIR SLOTS IN SOUTHERN AFRICA SAF 26-3 Table 1. Summary of Airborne Measurements in, Below, and Above CAS in Southern Africa UW Flight Number Date (2000) Location Period in CAS (hhmm UTC) a Height of Base of CAS (m) Height of Top of CAS (m) Thickness of CAS (m) CN in CAS CN Below CAS CN Above CAS Light Scattering Coefficient, s sp, in CAS (in units of 10 6 m 1 ) Light Scattering Coefficient, s sp, Below CAS (in units of 10 6 m 1 ) Light Scattering Coefficient, s sp, Above CAS (in units of 10 6 m 1 ) PCASP in CAS PCASP Below CAS PCASP Above CAS Static stability (Sz) (in units of 10 5 m 1 ) Aug. Near Pietersburg, South Africa Aug. Near Pietersburg, South Africa Aug. Over Inhaca Island, Mozambique Sept. South Atlantic Ocean, 350 km NW of Sept. South Atlantic Ocean, 350 km W-NW of Sept. Namib Desert, 80 km SW of Walvis Bay, Namibia Sept. South Atlantic Ocean, 300 km NW of Sept. Namib Desert, 50 km south of Sept. Atlantic Ocean, 400 km SW of Sept. Etosha National Park, Namibia Sept. Etosha National Park, Namibia a Local time = UTC + 2 hours Value Standard Deviation of Value

4 SAF 26-4 HOBBS: CLEAN AIR SLOTS IN SOUTHERN AFRICA Light Scattering Coefficient, σ sp (m -1 ) Relative Humidity (%) D 3000 C Altitude, MSL (m) CN B σ sp A Center of Clean Air Slot Altitude, msl (m) T Center of Clean Air Slot RH Ground Level Total Particle, CN (cm -3 ) (a) Temperature ( o C) (b) Figure 2. Vertical profiles through a CAS over Pietersburg, South Africa on 20 August This is the second CAS on 20 August listed in Table 1. (a) Light scattering coefficient (s sp ) at a wavelength of 550 nm and total particle concentrations (CN). (b) Temperature (T) and relative humidity (RH) Altitude (m) 3000 Clean Air Slot Light Scattering Coefficient, σ sp (m-1) Figure 3. Vertical profile of the light scattering coefficient (s sp ) through a CAS over Inhaca Island, Mozambique on 24 August The lowest reliable detection limit of the nephelometer is m 1. At 3000 m, the light scattering coefficient due to molecular scattering is m 1.

5 dn/d(log D) (cm -3 µm -1 ) IN CAS ABOVE CAS BELOW CAS Particle Diameter (µm) Figure 4. Particle size distributions measured by the PCASP-100X below (triangles), within (squares), and above (circles) the CAS through which a climb was made from UTC over Pietersburg, South Africa on 20 August The spectra below and above the CAS are each averages obtained over a time period of 1 min, and the spectra in the CAS is an average over a period of 0.5 min Height (m) HOBBS: CLEAN AIR SLOTS IN SOUTHERN AFRICA SAF 26-5 ULTRA CLEAN FREE TROPOSPHERIC AIR at an altitude of 3 km is m 1. Hence, on this occasion (and others) scattering by particles in the CAS was less than molecular scattering. [14] Particle size spectra measured with the PCASP in, below and above the CAS on 20 August (the same case illustrated in Figure 2) are shown in Figure 4. Over the size range depicted, the concentrations of particles are lower in the CAS than either above or below it. 4. Scenario for Formation of CAS [15] Shown in Figure 5 is a schematic of the situation that we suggest gives rise to CAS in southern Africa during the dry (biomass burning) season. [16] It is clear from the facts presented in section 3 that CAS reside in relatively shallow regions of stable air. This region is depicted as a temperature inversion in Figure 5, although the temperature in a stable region does not necessarily have to increase with height. [17] What is the origin of the clean air in a CAS? Clearly it cannot come from below, because the air is generally polluted all the way to the ground; neither can the clean air come from immediately above the CAS, since the air in that region is also polluted (e.g., Figure 2). We postulate that the clean air in a CAS comes from the free troposphere, which in southern Africa in winter is generally located above about 4 km msl (620 hpa). This supposition is supported by the following observations. 1. On 14 and 16 September, measurements were obtained in both CAS and in the free troposphere. The average value of s sp measured in the free troposphere on General subsidence over region Temperature Profile 3000 ELEVATED POLLUTED LAYER Aged pollutants transported aloft B A C D HIGH VISIBILITY CLEAN AIR SLOT (CAS) 2000 CONVECTIVE BOUNDARY LAYER Local sources of pollution Townships Biomass burning Industrial sources Figure 5. Schematic representation of a CAS and its environment in southern Africa during the dry, biomass burning season.

6 SAF 26-6 HOBBS: CLEAN AIR SLOTS IN SOUTHERN AFRICA 100 dn/d(log D) (cm -3 µm -1 ) IN CAS 0.01 IN FREE TROPOSPHERE Particle Diameter (µm) Figure 6. size spectra of particles measured in four CAS and in the free troposphere on 14 and 16 September The spectrum for the CAS is an average over 34 min, and the spectrum in the free troposphere is an average over 83 min. these 2 days was m 1, and the average of the measurable minimum values of s sp in the CAS was m 1. This suggests that the air in the CAS originated in the free troposphere, but underwent some degradation as it was transported downward. 2. As shown in Figure 6, the average particle size spectra measured in the free troposphere on 14 and 16 September was similar in both concentrations and shape to the average spectra measured in the CAS. 3. Using the NOAA Air Resources Laboratory software ( back trajectories were run for air parcels in the CAS for all 11 cases listed in Table 1. In each case, the air in the CAS was advected from above. Figure 7 illustrates this for the CAS observed over Inhaca Island on 24 August Twenty-six hours earlier, the air in this CAS was over the central part of South Africa at a pressure level of 500 hpa, that is, well in the free troposphere. [18] How does a slot of clean air from the free troposphere descend to form a CAS? Due to the anticyclonic conditions that prevail in southern Africa in the dry season, there is widespread subsidence of air. As a layer of air from the free troposphere subsides, its upper portion will descend faster than its lower portion due to the restriction on descending motion imposed by the Earth s surface. Therefore, the upper portion of the descending layer will warm relative to the lower portion due to the faster rate of compression and adiabatic heating. Consequently, the layer as a whole will become progressively more stable as it descends, and may eventually produce a temperature inversion. As we have seen, CAS are situated in stable layers (Table 1). [19] How does the air in a CAS remain so clean for many hours? First of all, since the air in a CAS originates in the free troposphere it starts off very clean. Also, the stability of a CAS to vertical motions will inhibit turbulent eddies (carrying pollutants) from penetrating into the CAS from either above it or below it. This can be seen from the idealized temperature profile through a CAS shown in Figure 5. If a parcel of polluted air at A moves upward into the CAS, its temperature will decrease along AB due to expansion. Therefore, the temperature of the rising parcel will fall progressively below the ambient air temperatures, represented by AC. Consequently, the parcel will become denser than the ambient air in the CAS, and it will sink back to A. If a parcel of polluted air at C attempts to move downward into the CAS, its temperature will increase along CD due to compression. Therefore, the temperature of the descending parcel will increase progressively relative to the ambient temperatures in the CAS. Consequently, the parcel will become less dense than the ambient air in the CAS, and it will tend to rise back to C. Molecular diffusion may transport polluted air into the CAS from both above and below, but this takes place only over very short distances. Particles may sediment into a CAS from above. However, even if we consider a relatively large particle, say 0.5 mm in diameter (Figure 4), which has a sedimentation velocity of ms 1, it would take 10,000 s or 28 hours to fall through 1 m. Also, any appreciably large particles very low down in a CAS will sediment out of the CAS. Visibility in a CAS may also be enhanced by the low relative humidity (Figure 2), which will cause evaporation of any water condensed on the particles [Magi and Hobbs, 2003]. [20] As depicted in Figure 5, the heavily polluted air below CAS clearly derives from biomass burning and industrial pollution. We postulate the polluted air above CAS has similar origins but, on average, is more aged than the pollutants below CAS. The reasoning behind this postulate is as follows. As convection builds with warming of the Earth s surface during the day, stable layers and CAS will tend to dissipate (note that the latest time at which a CAS was observed was 1531 UTC, and most of the CAS were observed well before this time) (see Table 1). Once the

7 HOBBS: CLEAN AIR SLOTS IN SOUTHERN AFRICA SAF E MOZAMBIQUE 20 S 25 E 30 E 35 E 40 E SOUTH AFRICA Inhaca Is. 08 UTC 08/24/00 25 S 00 UTC 08/23/00 30 S 35 S Pressure (hpa) Inhaca Is /24/ /23/00 UTC Figure 7. Trajectory for an air parcel arriving at the CAS observed over Inhaca Island, Mozambique at 3000 m msl (712 hpa) at 0800 UTC on 24 August The upper diagram shows a plain view of the trajectory and the lower diagram a vertical view (note that earlier times are to the right). stable layers have dissipated, pollutants from the surface can be transported to higher altitudes, most likely in late afternoon and evening. During the night, stable layers and CAS can be reestablished, leaving polluted air above the CAS. Support for this scenario is provided by the similarity of the particles spectra above and below CAS (Figure 4). Note that for the smaller particles in particular, the concentrations are somewhat greater above the CAS than they are below it (Figure 4). This could be due to greater gas-to-particle conversion, and coagulation of particles <0.1 mm into particle >0.1 mm, in the more aged polluted air above the CAS. [21] On one occasion (16 September) we obtained hydrocarbon and CO measurements in the polluted air above and below a CAS. An indicator of the relative ages of the air in these two regions is provided by the ratio of the concentrations of ethylene (C 2 H 4 ) to carbon monoxide (CO), since the primary source of both of these species in southern Africa during the dry season is biomass burning but ethylene has a lifetime of 0.5 day due to its reaction with the hydroxyl radical (OH) and CO a lifetime of months [Mauzerall et al., 1998]. The measured value of the ratio C 2 H 4 /CO for the air below the CAS on 16 September was about 3 times larger than in the air above the CAS. This indicates that the air above the CAS was at least 0.5 day older than that below the CAS. However, since the OH concentration may have been reduced in the time taken for the air to be transported above the CAS, and since OH is not present at all after sunset, the air above the CAS may well have been more than 0.5 day older than that below the CAS. [22] We envisage that CAS are transported downward intermittently as high pressure ridges move across southern Africa. As a CAS moves downward, it is likely to be progressively eroded from both above and below. Thus, the CAS documented here, which were observed well down in the lower troposphere, may have been the remnants of much thicker CAS aloft. 5. Summary [23] In this paper we described a previously unreported phenomenon, namely, thin regions of very clean air separating polluted air above and below. Although we have observed these CAS in other parts of the world, they are

8 SAF 26-8 HOBBS: CLEAN AIR SLOTS IN SOUTHERN AFRICA particularly prevalent in southern Africa during the dry (biomass) burning season. [24] The properties of 11 CAS observed over South Africa have been listed (see Table 1), and several of the CAS have been described in more detail (see Figures 1 7). [25] We have proposed that during the dry season in southern Africa CAS originate in the free troposphere, and that they are transported downward by the widespread subsidence associated with the continental anticyclone that dominates the region. The polluted air below the CAS derives from biomass burning and industrial emissions. It is postulated that the polluted air above the CAS derives from these same sources, but is more aged than that below the CAS. Evidence has been presented to support this scenario. [26] Acknowledgments. Thanks are due to Arthur Rangno for his valuable help in this study. This research was supported by NASA grants NAG and NAG and NSF grant ATM as part of the SAFARI 2000 Southern African Regional Science Initiative. References Anderson, T. L., and J. A. Ogren, Determining aerosol radiative properties using the TSI 3563 integrating nephelometer, Aerosol Sci. Technol., 29, 57 69, Garstang, M., P. D. Tyson, R. Swap, M. Edwards, P. Kållberg, and J. A. Lindesay, Horizontal and vertical transport of air over southern Africa, J. Geophys. Res., 101, 23,721 23,736, Magi, B. M., and P. V. Hobbs, The effects of humidity on aerosols in southern Africa during the biomass burning season, J. Geophys. Res., 108, doi: /2002jd002144, in press, Mauzerall, D. L., et al., Photochemistry in biomass burning plumes and implications for tropospheric ozone over the tropical South Atlantic, J. Geophys. Res., 103, , Newell, R. E., Ubiquity of quasi-horizontal layers in the troposphere, Nature, 398, , Sinha,P.,P.V.Hobbs,R.J.Yokelson,I.Bertschi,D.R.Blake,I.J. Simpson, S. Gao, T. L. Kirchstetter, and T. Novakov, Emissions of trace gases and particles from savanna fires in Southern Africa, J. Geophys. Res., 108, doi: /2002jd002325, in press, Swap, R. J., and P. D. Tyson, Stable discontinuities as determinants of the vertical distribution of aerosols and trace gases in the atmosphere, S. Afr. J. Sci., 95, 63 71, Tyson, P. D., and R. A. Preston-Whyte, The Weather and Climate of Southern Africa, Oxford Univ. Press, New York, Wu,Z.,R.E.Newell,Y.Zhu,B.E.Anderson,E.V.Browell,G.L. Gregory, G. W. Sachse, and J. E. Collins Jr., Atmospheric layers measured from the NASA DC-8 during PEM-West B and comparisons with PEM-West A, J. Geophys. Res., 102, 28,353 28,365, P. V. Hobbs, Department of Atmospheric Sciences, University of Washington, Box , Seattle, WA , USA. (phobbs@ atmos.washington.edu)