From TROPOS Wiki

Current revision as of 17:35, 9 July 2009

Return to Standard Operating Procedures

Aim

This manual outlines Standard Operating Procedures (SOP) for measurement of aerosol light scattering coefficient using an Ecotech M9003 integrating nephelometer.  It is intended for use by staff carrying out aerosol light scattering measurements, using such instruments at WMO GAW stations.

Guidance on setting up the sampling system and measurement and reporting of aerosol properties can be obtained in GAW 153 (2003).  Sample conditioning to standardized relative humidity is particularly important for aerosol light scattering coefficient determination and particular care is required to achieve this without significant mass loss, or in humid environments problems with condensation.

Additional information can be obtained from sag-aero@tropos.de

Background

Continuous measurements of aerosol light scattering coefficient is a high priority component of the suite of aerosol measurements recommended by the WMO Aerosol SAG to better understand the global climate and air quality impacts of aerosol particles.

Terms and definitions

WMO GAW=World Meteorological Organization, Global Atmosphere Watch

Aerosol SAG=GAW Scientific Advisory Group for aerosol

NARSTO DES=North American Research Strategy for Tropospheric Ozone, Data Exchange Standard

Nephelometer.=The integrating Nephelometer used to determine aerosol scattering coefficient.

σ_sp =The coefficient of aerosol scattering as displayed by the M9003.

Mm^-1=Unit of measure for σ_sp , inverse megameters (1 Mm^-1 = 10^-3 km^-1 = 10^-6 m^-1).

Reference Thermometer. =A traceable reference thermometer must be used for the calibration of the temperature compensation sensor and must have an accuracy of  ± 0.1 degrees Celsius (°C).

Reference Barometer.=A traceable reference barometer must be used for the calibration of the pressure compensation sensor and must have an accuracy of ± 1 kiloPascals (kPa).

CO₂=Typical gas used for span calibration.

FM200=(1,1,1,2,3,3,3) heptafluoropropane,  alternative gas used for span calibration. Gives a signal response closer to the full-scale output than CO₂. 

DFU=Disposable Filter Unit

HEPA=High efficiency particulate aerosol (filter)

SSI=Size selective Inlet

Safety and precautions

Calibration of nephelometers usually involves the use of inert pressurized gases; appropriate safety precautions for safe handling of gas cylinders including bottle restraint, use of regulators in good working condition need to be observed.  Venting of calibration gases into poorly ventilated or small enclosed spaces must be avoided to prevent asphyxiation hazards.  Venting should also be carried out in a manner that minimizes impact on other measurements.  Ozone depleting and high greenhouse potential calibration gases (e.g chlorofluorocarbons) should be avoided.  

Statement of traceability

Light scattering measurements are referenced to pure calibration gases for which accepted light scattering coefficient values have been published in the scientific literature.  Temperature and pressure calibrations need to be traceable to an appropriate national or international standard.

Equipment

• Integrating nephelometer
• Calibration equipment includes:
• High efficiency filters (e.g. HEPA cartridge, high grade DFU - e.g. Balston AQ)
• Connecting tubing
• Flow indicator (for calibration gases)
• Reference gases
• Pressure regulator or other devices for reducing the calibration gas flow to a steady low flow rate
• Reference thermometer
• Reference barometer

Operating Principles of the M9003 Integrating Nephelometer

In its most common configuration, an integrating nephelometer measures the amount of light scattered by a sample of air at one or more wavelengths, using a small near-Lambertian, diffuse, light source.  The sample air is drawn into a blackened sample chamber where it is illuminated by a very bright LED array (light source).  The light scattered by the sample of air is detected by a photomultiplier operating in photon-counting mode.  The fraction of the scattering due to particles is separated from that due to air.  It is scaled in the instrument’s controller using stored calibration factors, and then logged in the data memory.  The instrument is calibrated by introducing filtered air, to give an aerosol zero, and a gas with a known scattering coefficient, typically CO₂ to give the span.

The Ecotech M9003 integrating nephelometer displays the aerosol scattering coefficient as σ in Mm^-1 (inverse Mega meters). It can be operated stand-alone with its own internal data logger or connected to an external data logger.  It can condition incoming sample air to provide a fixed upper relative humidity level.  Calibration sequences can be initiated manually or automatically.

Responsibilities

Each GAW station that utilizes this type of equipment has the responsibility to ensure that the outlined operating procedures are followed. The WMO GAW Aerosol SAG has the responsibility for updating this manual. 

Operation and maintenance of the Nephelometer

Procedures covered

This document describes the procedures used to perform routine maintenance, audits and calibrations on the Nephelometer at weekly, 3-monthly, and 6-monthly intervals. Because many of the maintenance activities are performed jointly with the calibrations and audits all are contained within this document.

Setting up the instrument for Operation.

The M9003 nephelometer may be mounted on a table top, bench or wall mounted.  It should be located at a level that is convenient for calibration and arranged so that the inlet is as direct as possible, preferably with no bends or changes in tubing diameter.  The inlet should have a rain hood or shroud to prevent rain entering the sample line.  The M9003 should preferably use a dedicated inlet to prevent back flow of calibration gas during (automatic) calibration interfering with other measurements.  Further details on inlet characteristics are given in the WMO report GAW 153 (p4).  If the laboratory operates at a temperature lower than ambient and the humidity is high, an inlet heating system (and/or other drying device) will be necessary, also the inlet line inside the laboratory will need thermal insulation (using regular air conditioner foam insulation) and possibly additional heating.  The nephelometer inlet can operate to maintain the sample RH at less than 40%, which is the recommended way to operate. For further details see the Ecotech M9003 manual.

• Size selective inlet – the GAW aerosol program recommends use of a size selective inlet, although it is also common practice to operate nephelometers looking at the total scattering coefficient (with no size selection, or Total Suspended Particulate, TSP).  For GAW a size cut of 1 mm diameter at low RH for sampling the submicrometer diameter aerosol fraction is recommended (see GAW153 , p26); the GAW program also suggests use of a 10 mm diameter cut or switched inlets.                                                                  In its standard configuration using an internal pump the M9003 cannot be operated with a SSI (e.g cyclone), use of a SSI requires modification of the flow system.  Information on the type of inlet used should be included as part of the meta-data provided together with the observations to the data repository.  

Set-up for Calibration.

See Figure 1 below (and Ecotech M9003 Users Manual).

• Calibration gas - pure gas (~100% v/v) and regulator or an adjustable valve for lower pressure gases. Refrigerant gas, fire retardant gas (eg FM200) or CO2 may be used (‘food grade’ is acceptable) – additional filters for zero air and cal gas are recommended. Configure Settings as per Section 3.6 of this work instruction.
• Length of copper tubing - around 5 to 6 m long is needed for the calibration gas to warm up to room temperature after it leaves the gas bottle.
• Flow indicator - ball flowmeter etc (around 0-5 l/min).
• Zero air line – install minimum 1 meter of ¼” tubing into ‘Zero Air’ port and place additional filter on end of tubing. Keep tubing away from calibration gas cylinder.

Note: Span Calibrations are typically initiated manually at 1 to 3 month intervals.  If calibrations are to be initiated automatically then the gas lines must be pressurized and flows checked first. The line will stay pressurized when the cal cycle is off.

Default Configuration.

All instrument operating parameters can be accessed through the keypad and a simple menu that is displayed on a LCD display.  Default parameters are included for a typical operating situation. 

Settings on the menu screen – these are the recommended normal settings.

Report preferences Screen:

Date:                            D/M/Y

            Temp:                           ºC

            Press:                           mB (milliBar =hPa)

            Normalise to:                0 ºC (the Standard Temperature to which to normalize)

Calibration Screen:

            Zero check:                  24 hrs

            Wavelength:                  520 nm.

            Span Gas                     CO₂ (or whichever gas is used)

            Min cal time:                 10 min

            Max cal time:                20 min

            % stability:                    97%

Control Screen:

            Cell Heater:                  yes

            Inlet heater:                   yes

            Desired RH:                 40%

            RH buffer:                    2%

Analog out Screen:                 Not used here

Serial I/O Screen:

            Baud:                           9600 (but depends on Logging system)

            Parity:                           none

Adjust Clock Screen:              Adjust clock as necessary (see manual) normally operate UTC, with the time zone used reported as part of metadata file

Data Logging Screen:

            Log period:                   5 min

            Clear Datalg:                (only when you need to)

            Log data now:              (only when extra readings are wanted at a particular time)

Remote menu:                          No

Summary of Work Instructions

Daily Check

Purpose

A daily manual check of the instrument, comprising examination of data on the instrument display will ensure that instrument malfunction is discovered quickly and that corrective action can be taken without delay.

Weekly Check

Purpose

This procedure describes how to check that the regular (every 24 hours) zero measurement has been made and that operating parameters are within acceptable tolerances.

Frequency

This should be performed at weekly intervals.

Equipment

M9003 Weekly check sheet.

Procedure

1.  Confirm Instrument is powered up and operational. Confirm Sample Fan is operational by placing hand over exhaust grill.

2.  Enter READINGS screen from MAIN MENU and record following parameters into C

3.  heck sheet:

Readings Screen:

                        Scattering Coefficient                xxx.

                        Atm Pressure                            xxxx

                        Air Temp                                  xx

                        Cell Temp                                xx

                        Rel Humidity                             xx

4. Enter CALIBRATION Screen from the MAIN MENU and record the following parameters in the Check sheet:

Calibration Screen:

                        Wavelength:                              520 nm.

                        Last Zero check:                       x.xxx

                        Zero Check stability:                 xx%

                        Wall signal                                xx

5. Enter the CONTROL screen from the MAIN MENU and record the following parameters in the Check sheet:

Control Screen:

            Cell Heater:                              yes

            Inlet heater:                               yes

            Desired RH:                             40%

            RH buffer:                                2%

6. Enter the HIDDEN MENU by pressing on the FIRST “C” in the ECOTECH label below the keypad, and record the following parameters in the check sheet”

Hidden Menu ® Counts Screen:

            Dark count:                              xxxxx (within 0-10, >20 is BAD!)

            LED Set point:              xx (within 0-40, typically 5-25)

The DARK count gives the PMT signal with the light source (LEDs) OFF. A high value indicates a light leak into the cell.

The SHUTTER count gives the PMT signal during the Reference cycle (every 15 seconds). It should approximate 1,200,000 counts.

7.  Confirm Nephelometer clock time against UTC time and adjust if necessary. If adjustment is necessary, report both the old and the new setting of the clock.

8.  Enter any additional comments into Comments Box of Check Sheet.

3-Monthly check.

Purpose

This procedure describes how to perform the 3-monthly checks and calibrations

Frequency

This should be performed every 3 months.

Equipment

• Monthly check sheet.
• Calibration gas

Procedure

1.  Perform the Weekly checks (as per this Work Instruction: 9.7).

2.  Perform the Manual Zero/Span Calibration (as per this Work Instruction: 9.11).

3.  Inspect and clean inlet (remove trapped dust, bugs, etc).

6-Monthly Check and Maintenance.

 Purpose

This procedure includes more frequent checks plus checking the DFUs.

Scope

This should be performed at least twice per year.

Equipment

• Monthly check sheet.
• Calibration gas
• New DFU’s

Procedure

1.  Perform the Weekly checks (as per this Work Instruction: 9.7).

2.  Perform the 3-Monthly checks (as per this Work Instruction: 9.8).

3.  Inspect and replace Zero Air Pump DFU by following Maintenance section of Users Manual. Replace only if necessary (noticeable particle loading).

4.  Inspect and replace Zero/Span Air DFU by following Maintenance section of Users Manual. Replace only if necessary (noticeable particle loading).

5.  Add entry into Comments section of Weekly check sheet if replaced DFU’s.

12-Monthly Check & Maintenance.

Purpose

This procedure includes more frequent checks plus checking the nephelometer temperature and pressure calibration

Scope

Temperature and pressure checks should be carried out at least every 12 months.

Equipment

• Reference thermometer
• Reference barometer
• Audit sheet No. 4

Procedure

1.  Perform the Weekly checks (as per this Work Instruction: 9.7).

2.  Perform the 3-Monthly checks (as per this Work Instruction: 9.8).

3.  Perform the 6-Monthly checks (as per this Work Instruction: 9.9).

4.  Replace the two AA back-up batteries by following Maintenance section and page 7 (diagram) of Users Manual.

5.  Add entry into Comments section of Weekly Check sheet if replaced batteries.

6.  Carry out temperature and pressure checks (as per this Work instruction: 9.12)

Manual Zero/Span calibration.

Purpose

This work instruction describes how to perfom manually initiated zero & span calibrations.

Equipment

• Calibration Gas

Procedure

1.  Use the M9003 calibration proforma located at the end of this procedure to record calibration information.

2.  Ensure the Span gas type is set in the CALIBRATION Menu (e.g to FM 200)

3.  Follow instructions in section 3.5 of this Work Instruction for connecting the calibration gas to the span port of the M9003.

4.  From the CALIBRATION menu select the DO SPAN CHECK and press <enter>. The instrument span port will open with an audible “click”. When the span gas solenoid is heard to click open, open the calibration gas cylinder valve, set the regulator to 50 kPa and the gas flow to 1.8 – 2.0 L/min on the rotameter. After the 20-minute cycle is complete enter the CALIBRATION menu and record the instrument response to the cal gas under LAST SPAN CHECK. Record this value into the proforma.

5.  From the CALIBRATION Menu select the DO FULL CAL command and press <enter>. The message “ Full calibration will commence within 30 seconds” will appear on the display, and the instrument span port will open with an audible “click”.

6.  The DO FULL CAL command is a fully automatic process and the operator need not do anything during the calibration.  The span portion of the calibration will run for 20 minutes.

7.  When the span calibration is completed the instrument will automatically close the span port, open the zero port for zero calibration, and update the last span check and span check stability fields in the calibration menu.

8.  The zero portion of the calibration will run for 20 minutes.  After the zero calibration is finished the instrument will update last zero check and zero check stability fields in the calibration menu.

9.  Enter the analyser zero, zero stability, span check and span check stability information to the M9003 calibration proforma.

Temperature, Pressure, and RH Calibration.

Purpose

This work instruction describes how to check the temperature and pressure measured by the nephelometer

Scope

Temperature and pressure checks should be carried out at least every 12 months.

Equipment

• Reference thermometer
• Reference barometer
• Audit sheet No. 4

Procedure

Remove the Casing of the Integrating Nephelometer M9003

(i)  Ensure M9003 integrating Nephelometer is in a stable environment and has been operating for a period of > 1 hour.

(ii)  Remove 4 black screws on the front of Nephelometer and remove case, taking care to remove the earth trap as well (refer to Figure 2 for locations).  Your view should be as shown below in Figure 2.

Figure 2 Photo of the inside of the Integrating Nephelometer M9003 showing positioning of the Air Temperature/Humidity Sensor, Cell Pressure Sensor, Cell Temperature Thermistor and Sample Pump. The 4 screw positions are denoted by *. The inset shows the display and keypad in Environmental Cal Mode on the Integrating Nephelometer M9003. The location of the hidden MET Calib menu is also shown.

Place the M9003 Nephelometer in Environmental Calibration Mode

The calibration procedure for all the internal sensors requires the M9003 Integrating Nephelometer to be in Environmental Calibration Mode.  Before doing this, ensure that the M9003 Integrating Nephelometer is in a stable environment and has been operating for a period of >1 hour.

To place the M9003 in Environmental Calibration Mode:

(i)  Access the MET Cal hidden menu by pressing the 2nd C in ECOTECH on the keypad. (Refer to Figure 2).

(ii)  Then select MET Calib from the menu (1st item on the menu) and press enter.

(iii)  On opening this menu, the display should look similar to that shown in Figure 1.2.

Scroll down the menu to Enter Env Cal, click enter.  The M9003 is now in Environmental Calibration Mode.

(iv)  Press exit on the keypad.  All the M9003 internal sensors (RH, CT, P, and Air) should now be displayed on the screen (this may take 30 seconds).  Measurements displayed are updated continuously as they are read.

Figure 3: Screen display on activation of the MET Calib menu. Refer to manual for further instructions.

Calibration of the Barometric Pressure Sensor

(i)  Place the M9003 Integrating nephelometer in Environmental Calibration Mode (refer to Section 9.12.2)

(ii)  Disconnect the sample pump and allow the pressure reading on the M9003 display to stabilize.

(iii)  Determine the current barometric pressure using the Calibrated Barometric Pressure Sensor.

(iv)  Read off the pressure displayed on the M9003 screen and calculate the difference between this pressure and that measured using the Calibrated Barometric Pressure Sensor.

(v)  Access the MET Calib hidden menu (refer to Section 9.12.2 and Figure 2).

(vi)  Scroll down the menu (refer to Figure 3) and verify that the parameter Cal Press Y=26768

(vii)  If the difference between the pressure measured using the calibrated standard and the M9003 is >1hPa, adjust the Cal Press X value by the difference calculated in step (v) in this section (e.g. If Pressure (Calibrated Sensor) - Pressure (M9003) = +2.0 hPa, then increase the Cal Pres X value by 2.0 hPa (e.g. from 1013.2 to 1015.2)).

(viii)  Return to the main menu by pressing Exit on the keypad and check the new pressure reading.

(ix)  If M9003 does not read within 1 hPa of the value measured using the Calibrated Barometric Sensor, repeat steps (v) to (viii) of this section.

Calibration of the Cell Temperature (Thermistor)

(i)  Place the M9003 Integrating Nephelometer in Environmental Calibration Mode (refer to Section 9.12.2)

(ii)  Place the Calibrated Temperature Probe on the cell housing near the Cell Temperature Thermistor, and temporarily attach in order to ensure good thermal contact (refer to Figure 2 for location)

(iii)  Measure the temperature at this position (allowing sufficient time for the reference thermometer to stabilize)

(iv)  Calculate the difference in this temperature and the one shown on the M9003 display for the Cell Temperature (i.e. CT).

(v)  If the difference is >0.1°C, Access the MET Calib hidden menu (refer to steps in users manual and Figure 2) and scroll down to Cal Therm.

(vi)  If Cell Temperature (Calibrated Sensor) – Cell Temperature (M9003) = +0.2°C, then increase the Cal Therm value by 100 units (e.g. from 18089 to 18189).  If Cell Temperature (Calibrated Sensor) – Cell Temperature (M9003) = - 0.2°C, then decrease the Cal Therm value by 100 units (e.g. from 18089 to 17989).  NOTE: This works out to be a change of 50 units per 0.1°C.

(vii)  Return to the main menu by pressing Exit on the keypad and check the new cell temperature reading.

(viii)  If M9003 value does not read within 0.1°C of the value measured using the Calibrated Temperature Probe, repeat steps (ii) to (vii) of this section until this is the case.

Calibration of the Ambient Temperature Sensor (Vaisala)

(i)  Place the M9003 Integrating Nephelometer in Environmental Calibration Mode (refer to Section 9.12.2)

(ii)  Remove Vaisala Temperature/Humidity Sensor by unscrewing the bolt at brass T section (refer to Figure 2 for location).

(iii)  Place the Calibrated Temperature Probe close to the sensor (such that good thermal coupling is obtained)

(iv)  and allow the sensors to stabilise in the ambient environment (at least 1 minute).

(v)  Record the ambient temperature from the Calibrated probe and the M9003.

(vi)  Calculate the difference between these 2 values.

(vii)  If the difference is > 0.1°C, access the MET Calib hidden menu (refer to steps in users manual and Figure 2), scroll down to Cal VaisTO and adjust this value accordingly.  e.g. If Ambient Air Temperature (Calibrated Sensor) – Ambient Air Temperature (M9003) = +2°C, then increase the Cal VaisTO value by 2 units (e.g. from 234.25 to 236.25).  If Ambient Air Temperature (Calibrated Sensor) – Ambient Air Temperature (M9003) = -2°C, then decrease the Cal VaisTO value by 2 units (e.g. from 234.25 to 232.25).

(viii)  Return to the main menu by pressing Exit on the keypad and check the new Ambient Air Temperature reading.

(ix)  If M9003 value does not read within 0.1°C of the value measured using the Calibrated Temperature Probe, repeat steps (iii) to (vii) of this section until this is the case.

Calibration of the Relative Humidity Sensor

(i)  Place the M9003 Integrating Nephelometer in Environmental Calibration Mode (refer to Section 9.12.2)

(ii)  Remove Vaisala Temperature/Humidity Sensor by unscrewing the bolt at brass T section (refer to Figure 2 for location).

(iii)  Allow Vaisala Temperature/Relative Humidity Sensor to stabilise in the ambient environment.

(iv)  Access the MET Calib hidden menu (refer to steps (i) & (ii) in Section 9.12.2 and Figure 2) and scroll down the menu to Cal RH Gra

(v)  Check Cal RH Gra=1.0000

(vi)  Press Exit to return to the main menu.

(vii)  Measure the Relative Humidity of the ambient air using the Calibrated Sensor and record value (if using a psychrometer to do this, refer to users manual for instructions).

(viii)  Record the RH measured by the M9003 as shown on the display.

(ix)  Calculate the difference between these 2 values.  If the difference is >1% select MET CALIB and adjust Cal RH Off accordingly.  (e.g. If the difference between the calibrated sensor and the RH displayed by the M9003 is +3%, increase Cal RH Off from 0.0000 to 3.0000.

(x)  Press Exit to return to the main menu and check the RH reading on the M9003 display.

Repeat steps (iii) to (x) until the reading is within 1% of the calibrated sensor value.

(xi)  When this is the case, replace Visala Temperature/Relative Humidity sensor back into the T-piece fitting on the inlet.

Proformas & Paperwork

When completed, all log sheets and audit sheets are to be filed in a dedicated folder at the station or central office facility.

Weekly Check sheets may be kept in a folder with instrument.

Use of electronic checklists instead of  paper-based lists is recommended. Backups of electronic checklists are to be kept with backups of the data at a different location from the primary storage place. Checklists should be printed out once per month and kept in a safe place. 

Ecotech M9003 Weekly Checklist

Ecotech M9003 Weekly Checklist

Nephelometer Audit Sheet No. 1

Calibration

Date………………………. Operator…………………………

M9003 s/n:……………………………………………………..

Nephelometer Audit Sheet No. 1

Nephelometer Audit Sheet No. 2

Date………………………. Operator…………………………

M9003 s/n:……………………………………………………..

Reference Thermometer.

            Make:

            Model:

            s/n:

            last calibrated:

Reference Barometer.

            Make:

            Model:

            s/n:

            last calibrated:

Reference Psychrometer:

            Make:

            Model:

            s/n:

            last calibrated:

PRE CAL Check

PRE CAL Check

POST CAL Check

POST CAL Check

Data handling/processing

Light scattering, instrument and environmental parameter data from the Ecotech nephelometer may be retrieved continuously, via the communication port, using an external data system (e.g. a station’s central data system) or logged internally by the nephelometer’s controlling system and then periodically downloaded to a central or stand-alone system, e.g. a PC.  In the latter case Ecotech software is available (from www.ecotech.com.au) to facilitate data down-loading; this software will allow the data to be saved in a format that is suitable for use in a spreadsheet.

In either case the raw data files should be retained and archived at the station or central facility.  Data backup and processing at the station or central office should be carried out regularly, with a recommended minimum frequency of weekly. 

Minimum requirements for data examination include plotting the time series of light scattering data and examination of the station’s nephelometer log to identify possible sample contamination or instrumental problems, such as zero drift.  Any such episodes should be flagged in the subsequent edited data file and excluded from processing in the hourly statistics.  Other levels of processing may be applied, for example zero and span drift correction if appropriate.  If these procedures are carried out, this information should be added to the metadata file for this period and form part of the metadata submission to the GAW archive.  At some stations/locations data are screened using selected wind-sectors or species concentration data e.g. radon, to ensure that measurements represent specifically defined air masses.  A description of the selection procedures should also form part of the submitted metadata file. (Some limited information on screening procedures is given in the GAW Aerosol Procedures manual, GAW 153, p 6, 2003, and more detailed information can be obtained from the Aerosol SAG).   Higher level processing, for example to include truncation errors may also be carried out using the relevant instrument transfer function when this is known, any such processing should be indicated in the metadata file.

The GAW aerosol sampling manual recommends minimum sampling rates of 1 to 3 minutes and the minimum statistics for light scattering data are arithmetic mean and standard deviation, median, 5 and 95 percentiles for each hour (GAW 153, p 28).  These hourly data statistics should also include a flag to indicate corrupt or contaminated data.    

Edited hourly statistics files, including all relevant metadata should be prepared into the NARSTO DES format required by the GAW archive and submitted regularly to the GAW aerosol data centre.  These data should also be retained and archived at the station or central facility. 

References

Ecotech M9003 Nephelometer Users Manual,

Australian Standard 3580.12.1:2001 Methods of sampling and analysis of ambient air. Method 12.1: Determination of light scattering-Integrating nephelometer method.

GAW 153 (2003)  http://www.wmo.ch/pages/prog/arep/gaw/documents/gaw153.pdf

Contact for further information: sag-aero@tropos.de