Task C Determination of the aerosol extinction profiles
In this document, my procedure for the extraction of aerosol extinction profiles is described. Other people might follow this procedure or make their own choices.
The extrated profiles are used as input for the radiative transfer simulations.
C1 Raw data used to extract aerosol extinction profiles
The aerosol profiles are derived from the ceilometer measurements (yielding the profile information) in combination with the sun photometer measurements (yielding the vertically integrated aerosol extinction). In addition, also measurements of the surface aerosol concentration from in-situ measurements (pm2.5 and pm10) in Mainz and Wiesbaden are used. The raw data are provided in the zip file ‘aerosool_raw_data.7z’. It contains the follwing files: a) Ceilometer data: 20130618_MADCAT_CHX070040_Correction2_15minaveraged.txt 20130708_MADCAT_CHX070040_Correction2_15minaveraged.txt These files contain the range-corrected backscatter profiles averaged over 15 minutes. In each row prfiles starting from the surface to an altitude of 15360m are given in height intervals of 15m. Note that the ceilometer is located on the roof of the MPI, which is about 150m above sea level. Thus the first altitude given in the files represent the altitude range from 150 to 165m. The first row contains the profile from 00:00 to 00:15, the second line the profile from 00:15 to 00:30, etc. Note that on 18.06. profiles are available from 00:00 to 22:30; on 08.07. profiles are available from 00:00 to 23:45. b) AERONET data: AERONET_Mainz_18062013_lev20.txt AERONET_Mainz_08072013_lev20.txt These files contain the level 2 AOD data derived from the sun photometer measurements for different wavelengths. For the ‘calibration’ of the ceilometer profiles AOD values for 360 nm are needed, which are calculated as the average of the AOD at 340 nm and 360 nm. c) In situ aerosol concentrations (pm2.5 and pm10) File name content
Aerosols_pm_Mainz_Mombach.txt pm10
Aerosols_pm_Mainz_Parcusstrasse.txt pm2.5 and pm10
Aerosols_pm_Mainz_Zitadelle.txt pm2.5 and pm10
Aerosols_pm_Wiesbaden_Sued.txt pm10
Aerosols_pm_Wiesbaden_Schierstein.txt pm10
Aerosols_pm_Wiesbaden_Ringkirche.txt pm2.5 and pm10 Note that the stations in Mainz provide hourly averages, while the stations in Wiesbaden provide half hour averages.
The aerosol raw data for both selected days are shown in Fig. C1 below. 50 70 blue: 18.06. Zitadelle blue: 18.06. Parcusstrasse red: 08.07. Parcusstrasse 60 red: 08.07. Zitadelle 40 WI Ringkirche Mombach 50 ]
] WI Schierst. ³ ³ m m WI Sued / 30 / g g 40 µ µ [ [
5 0 . 1
2 30 20 m m p p 20 10 10
0 0 3:00 7:00 11:00 15:00 19:00 3:00 7:00 11:00 15:00 19:00 Time Time Fig. C1 Top: Range-corrected backscatter profiles derived from the ceilometer measurements (15 minute averages). Center: AOD derived from the sun photometer. Bottom: Aerosol surface concentrations (pm 2.5 and pm 10) measured at different stations.
C2 Extraction of the aerosol extinction profiles
The vertical structure of the aerosol profile is derived from the ceilometer backscatter data. The time series of the ceilometer profiles are shown in Fig. C1 (top). To determine profiles of aerosol extinction from the ceilometer backscatter data, several modifications and assumptions have to be made. a) The ceilometer data are noisy. Thus the data are averaged over periods of several hours, during which the backscatter profiles are similar. They are also vertically smoothed, and data above 5 to 6 km are set to zero. b) Because of the missing overlap between the outgoing laser beam and the field of view of the telescope, the ceilometer is insensitive to layers below 180m (see Fig. 2). Thus the aerosol extinction profiles below 180m have to be extrapolated. c) The (relative) backscatter profiles are scaled AOD derived from the sun photometer data (at 1020nm) to determine extinction profiles at 1020 (close to the wavelength of 1064 nm ued by the ceilometer). Here the assumption of a constant extinction to backscatter (LIDAR-) ratio is made. This assumption introduces uncertainties in the relative height profile, which are different to quantify. Also the variation of the LIDAR ratio with altitude might be different for the two wavelengths (1020 and 360 nm). The determination of the extinction profile at 1020 nm is an intermediate step to allow the correction of the aeosol self-extinction (see next step). d) The aerosol self-extinction has to be corrected, because the backscatter signal is attenuated by the aerosol extinction between the surface and the altitude at which the scattering occurs. Note that the molecular scattering can be ignored, because of the long wavelength (1064 nm) at which the ceilometer is operated. e) After the aerosol self-extinction correction was applied, a second scaling by the AERONET AOD at 360 nm is applied to derive the aerosol extinction profiles at the wavelength needed for the radiative transfer simulations.
The different steps are described in the following sub-sections.
Fig. C2 Overlap function of the CHM15k and the low-access version CHM15k-X. Figure taken from Frey S., Poenitz K., Teschke G., Wille H, Detection of aerosol layers with ceilometers and the recognition of the mixed layer depth, International Symposium for the Advancement of Boundary Layer Remote Sensing (ISARS), 2010.
C2.1 Averaging and smoothing of the ceilometer backscatter data
As a first step, from the raw backscatter profiles one-hour averages are formed. These profiles are shown for three periods on both days (Fig. C3). These periods were chosen, because the variation of the individual ceilometer profiles during these periods was rather low. 18.06., 08:00 to 11:00 18.06., 11:00 – 14:00 18.06., 11:00 – 19:00
3.00E+09 3.00E+09 3.00E+09 14:30 15:30 16:30 11:30 8:30 12:30 l 17:30 18:30 14_19 l a l a
9:30 n a n
2.00E+09 13:30 g 2.00E+09
n 2.00E+09 i g i s 10:30 g i
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8_11h r t e t t e t t a t a c a c s c s k k s c c c 1.00E+09 1.00E+09 1.00E+09 a a a b b b
d d d e e e t t t a a a u u u n n n e e e t t t t t
0.00E+00 t 0.00E+00
a 0.00E+00 a a
-1.00E+09 -1.00E+09 -1.00E+09 0 1000 2000 3000 4000 5000 6000 0 1000 2000 3000 4000 5000 6000 0 1000 2000 3000 4000 5000 6000 Altitude [m] altitude [m] altitude [m] 08.07., 04:00 to 07:00 08.07., 07:00 – 11:00 08.07., 11:00 – 19:00
3.00E+09 3.00E+09 3.00E+09 11:30 12:30 13:30 7:30 14:30 15:30 16:30 4:30 8:30
l 17:30 18:30 11_19h l
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4_7h 7_11h r r e t e t e t t t a t a c a c s c s k s k c
1.00E+09 c c
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d d d e e t e t t a a a u u u n n n e t e e t 0.00E+00 t t t a 0.00E+00 t 0.00E+00 a a
-1.00E+09 -1.00E+09 -1.00E+09 0 1000 2000 3000 4000 5000 6000 0 1000 2000 3000 4000 5000 6000 0 1000 2000 3000 4000 5000 6000 Altitude [m] altitude [m] altitude [m] Fig. C3 Range-corrected backscatter profiles (hourly averages) for three selected periods on both days.
Above 3000m the noise of the backscatter profile increases strongly. Thus above 2000m the profiles are smoothed (by forming the average over 150m), and above 4900m to 5150m (for 18.06.) or 6000m (for 08.07.), the profile is set to zero, because above these altitudes the values become negative over extended altitude ranges.
C2.2 Extrapolation for altitudes below 180m
Below 180m the ceilometer becomes ‘blind’ for the aerosol extinction because of the insufficient overlap between the outgoing laser beam and the field of view of the telescope (see Fig. C2). Thus the profiles have to be extrapolated down to the surface. Here four approaches are applied: i) The value at 180m is used for all altitudes below ii) The values below 180m are linearly extrapolated assuming the same slope as between 180m and 240m iii) The values below 180m are linearly extrapolated using the double slope as between 180m and 240m iv) For the first period on 18.06.2013 (8:00 to 11:00) the values below 180m are set to three times of the lineraly extrapolated values acccording to the in situ measurements on that day, see section C2.6. Here it should be noted that additional and even more strict scenarios might be used, e.g. assuming a decreasing aerosol extinction below 180m.
C2.3 Scalling by the AOD measured by the sun photometer at 1020nm
This (first) scaling is an intermediate step, wich is necessary for the correction of the aerosol self extinction. For this scaling the AOD measured by the AERONET sun photometer is averaged at 1020nm (close to the wavelength of the ceilometer) is used. The average AOD at 1020 nm for the different selected time periods on both days is shown in Table C1. In that table also the averag values at 380 nm re shown, which are needed for the second scaling (section C2.5).
Table C1 Average AOD at 1020 and 360 nm derived from the sun photometer. Time AOD 1020 nm AOD 360 nm 18.06.2013, 08:00 to 11:00 0.124 0.379 18.06.2013, 11:00 to 14:00 0.122 0.367 18.06.2013, 14:00 to 19:00 0.118 0.296
08.07.2013, 04:00 to 07:00 0.045 0.295 08.07.2013, 07:00 to 14:00 0.053 0.333 08.07.2013, 11:00 to 19:00 0.055 0.348
The backscatter profiles are vertically integrated (yielding Bint) and then the whole profiles are scaled by a factor AOD1020nm / Bint. The derived profiles represent the aerosol extinction at 1020nm.
C2.4 Aerosol self extinction correction
The Lidar equation can be expressed as:
1 r I r C I r 2exp r' dr' 0 2 (C1) r 0
Here I indicates the intensity received from a specific altitude r. I0 is the emitted intensity, and (r) is the backscatter coefficient at altitude r. (r) is the extinction coefficient at altitude r. The last term describes the extinction of the light between the surface and the altitude r (the factor 2 before the integral indicates that the attenuation occurs on both ways upward and downward). Since the ceilomteter operates at a long wavelength the contributions of air molecules to (r) and (r) can be neglected. C is the instrument constant of the ceilomter. Equation C1 can be converted into the following equation:
r Br C' r2exp r' dr' (C2) 0
Here B(r) represents the so called range corrected backscatter signal, the detector signal multiplied by the square of the distance r². The range corrected backscatter signal is provided in the ceilometer raw data files. Note that the new calibration constant C’ differs from C. Re- arranging of eq. C2 yields: Br 1 r r C' (C3) 2exp r' dr' 0
The Lidar ratio LR describes the ratio between the aerosol backscatter coefficient and the aerosol extinction coefficient:
LR (C4)
As stated above LR is assumed to be constant here. Thus equation C3 can be converted into:
Br 1 Br 1 r LR r r C' C'' (C5) 2exp r'dr' 2exp r' dr' 0 0
The new calibration C´´ constant was determined in the last sub-section using the AOD from simultaneous sun photometer observations. This determination was based on the assumption that r in equation C5 the aerosol extinction term 2exp r' dr' can be neglected (as a strating 0 point). This assumption is a good approximation because at 1020 nm the aerosol extinction is typically very small. By using the new calibration constant C´´ the aerosol extinction profile was determined in the last sub-section.
Br r (C6) first _ guess C''
Because the aerosol extinction was not considered, we refer to it as the ‘first guess’ aerosol extinction profile (first_guess(r)) in the following. Based on this initial aerosol extinction profile, a more accurate aerosol extinction profile corrected(r) can be derived by considering the aerosol extinction term.
1 r r corrected first _ guess r 2exp r' dr' (C7) corrected 0
This correction is implemented by transforming the integral into a sum over discrete altitude intervals (15 m): i, first _ guess i,corrected zi1 (C8) exp 2 z z j,corrected j j1 z0
Equation C8 is applied iteratively starting from the surface. Here it assumed that for the lowest layer the aerosol extinction correction can be set to 1. This assumption is well justified because of the fine vertical grid used here. In Fig. C4 the effect of the aerosol extinction correction is shown. The effect of this correction is small. It slightly increases the values at higher altitudes and decreases the values close to the surface. Here a small detail should be noted: after the extinction correction the aerosol extinction below 180m for the profiles with originally assumed constant extinction (see section C.2.2) is not constant anymore. Thus the values below 180m are again set to the value at 180m.
18.06., 08:00 to 11:00 18.06., 11:00 – 14:00 18.06., 14:00 – 19:00
0.3 0.3 0.3
slope_180_240moriginal slope_180_240moriginal slope_180_240moriginal slope_180_240mwith extinction correction slope_180_240mwith extinction correction slope_180_240mwith extinction correction
] 0.2 ] 0.2 ] 0.2 m m m k k k / / / 1 1 1 [ [ [
n n n o o o i i i t t t c c c n n n i i i t t t x x x
E 0.1 E 0.1 E 0.1
0 0 0 0 1000 2000 3000 4000 5000 6000 0 1000 2000 3000 4000 5000 6000 0 1000 2000 3000 4000 5000 6000 altitude [m] altitude [m] altitude [m] 08.07., 04:00 to 07:00 08.07., 07:00 – 11:00 08.07., 11:00 – 19:00
0.3 0.3 0.3
slope_180_240moriginal slope_180_240moriginal slope_180_240moriginal slope_180_240mwith extinction correction slope_180_240mwith extinction correction slope_180_240mwith extinction correction ] ] 0.2 ] 0.2 0.2 m m m k k k / / / 1 1 1 [ [ [
n n n o o o i i i t t t c c c n n n i i i t t t x x x E E 0.1 E 0.1 0.1
0 0 0 0 1000 2000 3000 4000 5000 6000 0 1000 2000 3000 4000 5000 6000 0 1000 2000 3000 4000 5000 6000 altitude [m] altitude [m] altitude [m] Fig. C4 Comparison of profiles without (blue) and with (magenta) extinction correction. Both profiles are scaled to the same total AOD (at 360 nm, see section C2.5) determined from the sun photometer.
C2.5 Second scaling by the Aeronet AOD at 360nm
The determine the aerosol extinction profiles at 360 nm, the extinction-corrected profiles (at 1020 nm) are scaled by the AOD aat 360 nm (see table C1). In Fig. C5 the final profiles with different extrapolations below 180m for both days are shown. 18.06., 08:00 to 11:00 18.06., 11:00 – 14:00 18.06., 11:00 – 19:00
0.4 0.4 0.4
slope_180_240m slope_180_240m slope_180_240m double slope_180_240m double slope_180_240m double slope_180_240m const const const 0.3 0.3 0.3 ] ] ] m m m k k k / / / 1 1 1 [ [ [
n n n
o 0.2 o 0.2 o 0.2 i i i t t t c c c n n n i i i t t t x x x E E E
0.1 0.1 0.1
0 0 0 0 1000 2000 3000 4000 5000 6000 0 1000 2000 3000 4000 5000 6000 0 1000 2000 3000 4000 5000 6000 altitude [m] altitude [m] altitude [m] 08.07., 04:00 to 07:00 08.07., 07:00 – 11:00 08.07., 11:00 – 19:00
0.4 0.4 0.4
slope_180_240m slope_180_240m slope_180_240m double slope_180_240m double slope_180_240m double slope_180_240m const const const 0.3 0.3 0.3 ] ] ] m m m k k k / / / 1 1 1 [ [ [
n n n
o 0.2 o 0.2 o 0.2 i i i t t t c c c n n n i i i t t t x x x E E E
0.1 0.1 0.1
0 0 0 0 1000 2000 3000 4000 5000 6000 0 1000 2000 3000 4000 5000 6000 0 1000 2000 3000 4000 5000 6000 altitude [m] altitude [m] altitude [m] Fig. C5 Final aerosol extinction profiles at 360 nm after all corrections are applied for the different periods on both selected days.
C2.6 High surface aerosol extinction in the morning of 18 June 2013
On 18.06.2013, 08:00 to 11:00, largely enhanced surface concentrations of pm2.5 and pm10 were measured (see Fig. C1 bottom). Thus for that period an additional aerosol extinction profile with three times the values of the profile with linear extrapolation below 180m is calculated (see Fig. C6). Also for the scaling of the profile, a slightly higher AOD of 0.41 at 360 nm was assumed, which represents the maximum AOD during that period (the average is 0.38). Thus this aerosol extinction profile can be seen as an extreme profile during that period on 18.06.2013.
0.7 slope_180_240m
0.6 double slope_180_240m const three times extinction_AOD041 0.5 ] m k / 0.4 1 [
n o i t c n
i 0.3 t x E
0.2
0.1
0 0 1000 2000 3000 4000 5000 6000 altitude [m] Fig. C6 Comparison of different aerosol extinction profiles (at 360 nm) for 18.06.2013, 08:00 to 11:00. The light blue profile represents three times the values below 180m compared to the linear extrapolation. It is also scaled to an AOD or 0.41, which was the maximum AOD during the respective period (compared to the average AOD of 0.38 used for the other profiles). C3 Derived aerosol extinction profiles
The finally obtained aerosol extinction profiles are provided on a vertical grid with high (15m) resolution up to an altitude of 6km. To minimise the read-in time of the radiative transfer model, the vertical resolution was reduced at altitudes between 500m and 6000m to 105m.
The derived aerosol profiles are provided in the zip file ‘Extracted_aerosol_profiles.7z’. It contains the individual profiles as described in the table below.
Table C2 Overview on the file names of the different aerosol extinction profiles File name description ext_1806_8_11h.txt profiles with linear interpolation below 180m ext_1806_11_14h.txt ext_1806_14_19h.txt ext_0807_4_7h.txt ext_0807_7_11h.txt ext_0807_11_19h.txt ext_1806_8_11h_2.txt profiles with double increase (slope) below 180m ext_1806_11_14h_2.txt ext_1806_14_19h_2.txt ext_0807_4_7h_2.txt ext_0807_7_11h_2.txt ext_0807_11_19h_2.txt ext_1806_8_11h_c.txt profiles with constant extinction below 180m ext_1806_11_14h_c.txt ext_1806_14_19h_c.txt ext_0807_4_7h_c.txt ext_0807_7_11h_c.txt ext_0807_11_19h_c.txt ext_1806_8_11h_3B_AOD041.txt the extinction below 180m is set to the three times the values of the profile with linear extrapolation below 180m. For he scaling a total AOD of 0.41 instead of 0.38 is used. Note that the height grid of these files starts at zero (which represents 150 m altitude in reality), because in our radiative transfer simulations an altitude of 0km represents the location of the MAX-DOAS instrument.