Results and discussion of the old LIDAR observations
5.3 Low Altitude Aerosols
5.3.1 Raw Data
The earliest LIDAR profiles were takenon July 3rd 1997 when interesting be- haviour ofthe low altitude aerosol was seen. Fig.(5.1) shows a time evolution with profiles taken from 18:15 to 19:00 at intervals of 15 mins.
The x-axis represents altitude in km and the y-axis photon counts from the photomultiplier. These profiles are raw as they are recorded by the data capturing system.
Three layers of aerosols are clearly evident from the plot. The first layer is at 1.2 km. The second and third layer are more prominent and are found at 2.4 km and 4 km respectively. In all five profiles the photomultiplier is saturated for the first 400 m.
In order to see whether these results persist for the rest of the month, a set of profiles were taken three weeks later on July 23rd 1997 (fig.(5.2)).
Each profile was taken in intervals of 15 mins as before. The three layers (1.2 km, 2.4 km and 4 km) which appeared on July 3rd 1997 seem to reinforce and appear at greater heights on July 23rd 1997. Theestimated thickness of the each layer is 800 m.
From the profiles of figs.(5.1) - (5.2), the following conclusions can be drawn:
(i) The 1.2 km layer seems to be enhanced from July 3rd to July 23rd.
(ii) The 2.4 km and 4 km layers remain fixed in altitude.
(iii) These two layers seem to behave independently of each other.
8 10
J
July 3rt!1997
18 :30 2JO shot
400~
;; 300 :': 200
C) 100
o
o 2 4 6
Altimdelbn
8
J
10Ju lyJrd 1997 18:15225 shot
Akiude/bn
400 b : 1
;; 300 :': 200
C) 100
o Ic..oo_-","'"
o 2 4 6
July3rrl1997 19:00225sht
JllC o~,
••
J
o 2 4 6 8 10
AtindeJl::m
,.]
8 10
JulyJrd 19 1 :S5 2s1l.jJ:
;; ~~KM
:': 20
C) 1 0
o ....bt-I~i"'-'''''-
0 2 4 6
_ .nm
Figu re 5.1: Low altitude L1DAR profiles taken on July 3 1997 from 18:15 to 19:00.
July23rd IJ97 18:45 200 shot
400
:J
l ~b.
o2 4 6 8 10
A1tiludeA;m o 2
July23<d997 19:00200shots
4 6
AliudeA<m
8 1
July23rd lJ97
19:15200shot July19:) 020 0 shots2Jrd 991
400~
" 300=200
8 100
o
o 2 4 6 8 10
Altitude/km
dl&5::J
o 2 4 6 8 l Cmud.non
Figu re 5.2: Low altitude L1DAR profiles taken on July 23 1997 from 18:45 to 19:30.
Another set of profiles were taken more than four weeks later on August 21st 1997 (fig.(5.3)) in order to see whether the aerosol layers detect ed on July 3rd and July 23rd persist.
We can therefore conclude that the three aerosol layers seem to persist for most of July. The layers disappear towards the end ofthe mont h. This is clearly seen in the profiles of August 21st 1997 (figs.(5.3)) where all the three peaks have disappeared.
Ang21st1997 Mg2lst 1997
18:3050shou 18:45150shot
1 :,:K:.:J I
200:~t\A:<::YJ
0 2 4 6 8 10 0 2 4 6 8 10
illudet1;m Altitude/km
Aug21st199' hJg21st199;
19:00150 sh 19:15150sh1
] ~\::J 1 ~\:::J
0 2 4 6 8 10 0 2 4 6 8 10
AltitudeJkrn All:itudelhn
hJg 21st1997 19:3015Osoo~
] :'~4\::J
0 2 4 6 8 10Altitudeikm
Figure 5.3: Low altitude L1DAR profilestaken on August 211997 from 18:30 to 19:30.
Low altitudeLIDARraw data profiles are plottedfor the datesSeptember
26thto October21st 1997(figs.(5.4) - (5.6)) inorder to see whetherthetrend in August persists.
From the Sept emb er 26th results (fig.(5.4)), it appears there has been some development s at 2.8 km and 3.6 km. The layers seem to be so close that they appear as two broad peaks in the LIDAR profiles. Comparing with the July 3rd results , it seems that the 2.4km layer has moved to higher altitude (2.8 km) and the 4 km layer to lower altitude (3.6 km). The two layers at 2.8 km and 3.6 km seem to be stable for the whole of September and October (figs.(5.5) - (5.6)). The persistence ofthese layers suggest that they are unaffected by circulation changes in the atmosphere .
Three possibleexplanat ions whichcould account forthiserrat ic behaviour of the aerosols are:
(i) As shown in fig.(5.7) ,over most of SouthernAfrica ,layers of absolutely stable air occur preferentially at around 700 hPa (rv 3 km), 500 hPa (rv 5 km) and 300 hPa (rv 9 km) (Cosijn and Tyson 1996). A fourth layer also occurs over coastal areas around 850 hPa (rv 1.5 km). All four layers have the effect of inhibiting vertical transfer of aerosols and trace gases and of trapping them in theinterveninglayers (Cosijn and Tyson 1996). The solid line refers to the height ofthe escarpment above sea level.
(ii) Biomass burningoccur s frequentlyin South Africa, eit her in the form of prescribed burns or wild fires (P illay et al. 1995). The ext ent of these burns on the east coast of Sout h Africa rea ches its maximum in the winter season (P illay et al. 1995). Thusthe loading of the atmosphere will behigher in winter.
Figure 5.4: Low altitude L1DAR profiles taken on September 26 1997from 18:30 to 19:00.
Figure 5.5: Low altitude L1DAR profiles taken on October 20 1997from 18:30 to 19:15.
Figure 5.6: Low altitude L1DAR profiles taken on October 21 1997from 18:30 to 19:00.
(iii) The loading of the atmosphere when sugar cane burning began in June is different in July and October.
The September 26th and October 20th results show a higher concentra- tion of aerosols at 2.8 km and 3.6 km. This observation can be accounted for by the fact that the burning of sugar cane has reached its end in September and also the largest number of active fires detected in South Africa occurred between July and September (Justice et al. 1996). Therefore the aerosol loading of the atmosphere would be higher in September than in June. In- deed this is what can be seen from the LIDAR profiles (fig.(5.4)). The stable discontinuities (point (i) above) which are present during most of the year (Cosijn and Tyson 1996) serve to trap the aerosols produced during the burn- ing of sugar cane and biomass burning. Towards the end of September and
200
JOO
400
500
";i' a..£:;
..
600..
~a: 700
BOO
900
1000
• JOOhPa layer
•500h"a layer
- 700hPalayer
·850 hPa layer
PI PR 3E BL UP SP CT PE DB
Figure 5.7: Mean annual spatial variation of absolutely stable layers at 850,700, 500 and 300 hPa over South Africa. PI denotes Pietersburg, PR Pretoria, BE Bethlehem, BL Bloemfontein, UP Upington, SP Spingbok, eT Cape Town, PE Port Elizabeth and DB Durban (Cosijn and Tyson(1996)).
October, although mixing is taking place, cont inuous loa ding of the atrno- sphere has result ed in a higher concentration of aerosols. Fig.(5.5) shows a strong return bet ween 8km and 10 km on October 20th 1997at 18:30. This may be due to the stable discont inuit y at 300hPa ("" 9 km) (see fig.(5.7)).