SOP - Mass Concentration - Beta Attenuation - Met One Instruments BAM-1020

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Measurement of
Particulate Mass Concentration
with the
Met One Instruments BAM-1020
Beta Attenuation Monitor
 

Return to Standard Operating Procedures

Contents

Aim

This manual outlines Standard Operating Procedures (SOP) for measurement of aerosol mass concentration using a Met-One BAM-1020 beta attenuation mass monitor.  It is intended for use by staff carrying out aerosol mass measurements at WMO GAW stations.

 

Guidance on setting up the sampling system and measurement and reporting of aerosol mass concentrations can be obtained in GAW 153 (2003).  Sample conditioning to standardized relative humidity is particularly important for aerosol mass determination and care is required to achieve this without significant mass loss.  Attention to sample conditioning is also required in humid environments where problems with condensation are commonly encountered.

 

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

Background

Continuous aerosol mass measurements are part of the suite of aerosol measurements recommended by the WMO Aerosol SAG to better understand 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
  • BAM=Beta Attenuation Monitor
  • NARSTO,DES=North American Research Strategy for Tropospheric Ozone, Data Exchange Standard

Safety and precautions

BAM instruments utilize a low activity beta-emitting radioactive source.  Operators should not attempt to access the source.  Disposal of an unwanted source should conform with relevant national regulations or legislation.

Statement of traceability

All mass measurements are referenced using the internal calibration membrane mass, which is factory calibrated to traceable standards.  Flow measurements need to calibrated using a suitable flow measuring device that can traced to a recognized national or international standard, also the reference thermometer and barometer are required to have traceable calibration.

Equipment

  • Beta attenuation monitor
  • Calibration equipment includes:
  • Inlet adapter
  • Connecting tubing
  • Reference volumetric flow meter
  • Reference thermometer
  • Reference barometer

Operating Principles

Beta Attenuation Mass Monitors (BAM) measure the mass concentration of particulate matter in the atmosphere by attenuation of beta radiation. A 14C source emits beta rays, which are attenuated as they collide with particulate matter (PM) collected on a filter tape. The degree of attenuation of the beta signal is directly proportional to the mass loading of PM of the filter tape. Although highly different attenuating materials have somewhat different mass proportionality constants, in practice a single mass attenuation coefficient is used, and during operation of the instrument this is automatically referenced using an inserted calibrating membrane.

The BAM-1020 has three component sections: the sample inlet system, the controller and sensor assembly, and the sample pump.

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 Mass Monitor

A number of options are available for setting up the BAM-1020 flow values used to calculate mass loading; in part this depends on the hardware options for the particular instrument.  Flows available are named “metered” (for instruments with manual flow control), “actual” (uses ambient flow) or “STD” (uses flow at 25 °C and 1 atmosphere).  In setting up the instrument for GAW operation the STD option should be selected although this requires an additional density correction (x 1.0915, for 25°C to 0°C) to the mass loadings reported by the instrument before reporting these data to the GAW data centre.  

Daily check

Procedure

1.  CLOCK CHECK: check BAM clock is correct for Time and Date against a known accurate time, preferably UTC, (or UTC ±  fixed time zone adjustment). Adjust if necessary (if greater than 30 seconds).

2.  ERROR CHECK. Note any recent error messages recorded by the BAM. Press F3 from the Top Menu to display the last 10 error messages. Review sections 4.16, 5.2, and 9.2 of the Operation Manual for corrective actions.

3.  TAPE CHECK. Inspect filter tape for damage and the amount remaining. One filter tape should last for 2 months.

4.  REFERENCE CHECK: record and track the density of the Reference Membrane by observing the “LAST m:” value from the main screen.Compare the displayed value with the value given in Appendix B of the Operation Manual.A difference of greater than ± 5% indicates instrument drift and corrective action needs to be taken.

5.  Record all results in daily check sheet.

Monthly check

Procedure

1.  Perform the daily checks as per Section 10.1. Daily Checks of this Work Instruction.

2.  Perform a LEAK CHECK as per Section 4.8 of Operation Manual. Record the leak rate displayed on the screen and track for deterioration.

3.  Perform the FLOW CHECK as per section 6.3 of Operation Manual. Typical BAM-1020 configurations have an ambient temperature and barometric pressure sensor to convert Mass flow to Volumetric flows. Perform the Flow Check while in “Normal” Operating Mode, ensuring that the pump is running. Flows should be 16.7 L/min ± 2%. If outside this tolerance then a Flow Calibration should be performed (note, this is the inlet flow at ambient density conditions, this value matches normal size selective inlet device flow requirements and is not the value used for mass concentration reporting.  See  section 10.). If using a Mass Flow device to check the flows, they will need to be corrected to local temperature and pressure conditions for Volumetric flow.

4.   Record all results in Monthly check sheet.

2-Monthly Check and Maintenance.

Procedure

1.  Perform the daily checks as per Section 10.1. Daily Checks of this Work Instruction.

2.  Perform the Monthly checks as per Section 10.2. Monthly Checks of this Work Instruction.

3.  Replace FILTER TAPE as per section 4.7 of the Operation Manual. Note that typically the tapes only last for two months. Wrap the end of the new tape about 1½ turns around the take-up spool. Advance the tape about 10 spaces, then reverse about 5 spaces. This helps to position the tape on the rollers and indicates if there are any problems with the tape loading before sampling commences. Check remaining stock of filter tape. Order new tape if needed.

4.  Perform FLOW CALIBRATION as per section 6.3 of Operation Manual. Ensure instrument is in “flow calibration” mode. Flows should be 16.7 L/min ± 2%. If outside this tolerance then an adjustment should be performed. If using a Mass Flow device to check the flows, they will need to be corrected to local temperature and pressure conditions for Volumetric flow.

5.  Perform a NOZZLE CLEAN, which cleans the inlet nozzle and nozzle area, as per section 4.8 of the Operation Manual. Particulate build up on the nozzle may damage the filter tapes. The nozzle should be cleaned each time the filter tape is replaced. Isopropanol (Isopropyl alcohol) may be used as the cleaning solution.

6.  Perform an INLET HEAD CLEAN as per section 10.1.3 of the Operation Manual. Follow the instructions according to the type of Inlet Head in operation. Clean surfaces with a mild soapy solution and rinse with clean water. Pay particular attention that nozzles and impaction surfaces of inlet head are clean from particulate build up. Inspect and replace all O-rings as required. Ensure all Inlet Head components are dry before re-installing head on the sampler.

7.  Check instrument zero by attaching a HEPA filter to the inlet (via the inlet flow adapter) and running for a sufficient period to include a full measurement cycle (minimum of one hour).

8.  Record all results in 2-Monthly check sheet.

12-Monthly Check & Maintenance.

Procedure

1.  Perform the daily checks as per Section 10.1. Daily Checks of this Work Instruction.

2.  Perform the Monthly checks as per Section 10.2. Monthly Checks of this Work Instruction.

3.  Perform the 2-Monthly checks as per Section 10.3 2-Monthly Checks of this Work Instruction.

4.  Perform the complete clean of the Inlet System (including Inlet tube) as per section 10.1.5 of the Operation Manual.

5.  Perform a TEMPERATURE SENSOR CALIBRATION as per section 6 of Operation Manual. Reference Thermometer should have up-to-date certification. Position Reference Thermometer within close proximity of BAM-1020 Outside Temperature sensor for best results.

6.  Perform a PRESSURE SENSOR CALIBRATION as per section 6 of Operation Manual. Reference Barometer should have up-to-date certification.

7.  Record all results in 12-Monthly check sheet.

Proformas & Paperwork

When completed all check sheets should be collated and kept with the instrument for future analysis. Electronic checklists are recommended instead of paper lists. Backup copies of the electronic files must be kept in a safe place. Electronic checklists should be printed out once every month.

BAM-1020 Daily Checklist

Date /

Operator:

                                                                                                                                           
 
ERROR LOG: any errors recorded?                                                                                                                                            
TAPE CHECK:

Tape OK?

                                                                                                                                           
Reference Check:                                                                                                                              
“Last m:” value                                                                                                                                            
Pass / Fail?                                                                                                                                            
                                                                                                                                                         
Comments:

             

             

             

             

             

                                                                                                                                           

BAM-1020 Daily Checklist

BAM-1020 Monthly Checklist

Date /

Operator:

                                                                                                                                           
Leak Check
Leak rate LPM                                                                                                                                            
PASS / FAIL ?                                                                                                                                            
Flow Check:                                                                                                  
Reference Flowmeter make/model                                                                                                                                            
Reference Flowmeter s/n                                                                                                                                            
Measured Volumetric flow                                                                                                                                            
Displayed Volumetric flow                                                                                                                                            
 % difference
 
 
 
 
 
 
 
 
 
 
PASS / FAIL?
 
 
 
 
 
 
 
 
 
 
                                                                                                                                                         
Comments:

             

             

             

             

                                                                                                                                           

BAM-1020 Monthly Checklist

BAM-1020 2-Monthly Checklist

Date /

Operator:

                                                                                                                                           
Filter Tape                                                                                                                                            
Replaced Filter Tape?                                                                                                                                            
Flow Calibration                                                                                                                                            
Reference Flowmeter make/model                                                                                                                                            
Reference Flowmeter s/n                                                                                                                                            
Measured Volumetric flow                                                                                                                                            
Displayed Volumetric flow                                                                                                                                            
 % difference
 
 
 
 
 
 
 
 
 
 
PASS / FAIL?
 
 
 
 
 
 
 
 
 
 
Adjusted?
 
 
 
 
 
 
 
 
 
 
Nozzle clean                                                                                                                                            
Nozzle Cleaned OK?                                                                                                                                            
Inlet Head Clean                                                                                                                                            
Cleaned Inlet OK?                                                                                                                                            
Aerosol zero with HEPA filter                                                                                                                                            
Comments:

             

             

             

                                                                                                                                           

BAM-1020 2-Monthly Checklist

BAM-1020 12-Monthly Checklist

Date /

Operator:

                                                                                                                                           
Complete Inlet clean                                                                                                                                            
Cleaned whole of inlet system?                                                                                                                                            
Temperature Sensor Cal                                                                                                                                            
Reference Thermometer make/model                                                                                                                                            
Reference Thermometer s/n                                                                                                                                            
Measured (REF) temperature                                                                                                                                            
Displayed (BAM) temperature                                                                                                                                            
 % difference
 
 
 
 
 
 
 
 
 
 
PASS / FAIL?
 
 
 
 
 
 
 
 
 
 
Adjusted?
 
 
 
 
 
 
 
 
 
 
Pressure Sensor Cal                                                                                                                                            
Reference Barometer make/model                                                                                                                                            
Reference Barometer s/n                                                                                                                                            
Measured (REF) Baro.Press                                                                                                                                            
Displayed (BAM) Baro.Press                                                                                                                                            
 % difference
 
 
 
 
 
 
 
 
 
 
PASS / FAIL?
 
 
 
 
 
 
 
 
 
 
Adjusted?
 
 
 
 
 
 
 
 
 
 
Comments:

             

             

                                                                                                                                           

BAM-1020 12-Monthly Checklist

Data handling/processing

Data can be transferred from the BAM1020 to either a stand-alone PC or central data facility via RS232 communication protocols.  The normal mode of operation is to draw sample air through the filter tape for 50 minutes with the remaining time being taken for other instrument functions, giving an overall sample rate of one mass sample per hour.  This determines that the only mass data hourly statistic is the mass concentration (together with any flags for indicating corrupt or contaminated data).

 

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. 

 

Mass loading data reported to the GAW data centre should be calculated using flow normalized to STP (0 °C, 1 atmosphere).  For the BAM-1020 utilizing the STD option, a density correction of x 1.0915, is required to convert mass concentrations reported by the instrument (using flow referenced to 25°C) to those recommended by WMO (using a flow referenced to 0°C).  

 

Minimum requirements for data examination include plotting the time series of mass concentration data and examination of the station’s mass monitor log to identify possible sample contamination or instrumental problems, such as flow problems.  Any such episodes should be flagged in the subsequent edited data file and excluded from processing.  Any additional corrections, calibrations, or other information, such as time zone, that are required to interpret the data (i.e. meta data) should be recorded in the meta data file for this period and form part of the data 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 any selection procedures should also form part of the submitted meta data 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). 

 

Edited hourly statistics files, including all relevant meta data 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.  Details on the archiving procedures and contact details can be obtained from GAW 153 pp 44-47).

References

Met One Inc. BAM-1020 Operation Manual.

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

sag-aero@tropos.de

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