FAAM CORE Passive Cavity Aerosol Spectrometer Probe

 

PCASP during a desert dust campaign.

The PCASP measures both aerosol number and size in the nominal size range 0.1 to 3 micrometers. It is an optical particle counter (OPC) and uses a laser to illuminate the aerosol-laden ambient air stream from the instrument inlet. Pulses of light scattered from each particle are counted with the intensity of the light scattered into a fixed solid angle giving a measure of particle size. The instrument is fitted to the aircraft in one of the PMS canister under the aircraft wings.
 
 

Table of Contents

Specifications
System Description
Calibration
Data
History
References

 

Specifications

Nominal Measured Aerosol Range: 0.1-3.0 µm
Sample Flow Rate: 1 cc/s
Sheath Flow Rate: 15 cc/s
Number of Size Bins: 30 bins
Sampling Frequency: 0.1 to 10 Hz
Laser: HeNe, wavelength 0.6328 μm
Light Collection Angle: Nominally 35°-120° and 60°-145°
Auxiliary Parameters: Ambient temperature, dynamic pressure, standard and volumetric flows for sample and sheath air
 

System Description

Air enters the instrument through a nose cone which, with an expanding internal cross-section, decelerates the flow by approximately 10 times. An inlet needle samples the air and directs it straight into the sample cavity. A sheath flow of filtered air envelops and focuses the sample flow while isolating the aerosol-laden air flow from the optics. A HeNe laser illuminates the sample flow twice, once before and once after being retro-reflected from the mirror-coated surface of crystal oscillator. The oscillator prevents interference of the beams. Light scattered by particles in the flow is collected and collimated by a parabolic mirror which is then focused onto an avalanche photodiode.

The intensity of the scattered light collected changes by more than six orders of magnitude over this particle size range To cover this dynamic range, a cascade of three amplifier stages are used. The high-, mid-, and low-gain stages cover approximate size ranges 0.1-0.14, 0.14-0.3, and 0.3-3 µm. It should be noted that the use of these three gain stages does lead to complications during data analysis at these junctions, Rosenberg et al discuss this in detail. Pulses of scattered light are sampled by a 12-bit analogue-to-digital converter at 12 MHz and the pulse length and pulse height are recorded. Comparing the peak digital level of the pulse to a threshold table, each pulse is placed in one of 30 size bins. Note that for data logged on the SPP200 electronics, bin 1 is different to all other bins. The lower edge of bin 1 is defined by the peak width threshold not a peak height threshold. This means that bin 1 must always be discounted from any analysis.

 

Calibration

The threshold table specifies the upper and lower bounds of each bin in terms of digital voltage level. Assuming an invariant collection solid angle and alignment, each level corresponds to a scattering cross-section. If the properties of the aerosol are known (or are assumed) then the corresponding particle diameter may be calculated. Given the variability and unknowns of atmospheric aerosols under test these assumptions may be considerable.

Linear fit of scattering cross-section and voltage level for each gain stage.The discrete calibration method used for the FAAM PCASPs is detailed in Rosenberg et al and users of PCASP data are encouraged to familiarize themselves with the use and uncertainties of the calibration data. Aerosolized DEHS (di ethyhexyl sebacate) that has been size-filtered through a differential mobility analyser and polystyrene latex (PSL) beads of known size both have well characterized physical and optical properties. These materials are used to calibrate the response of each gain stage individually which results in a linear fit of digitized pulse height versus scattering cross-section as in figure 2.

Schematic of PCASP calibration setup using DEHS.The laboratory setup for PCASP calibration for sizes 0.1 µm to approximately 0.3 µm with DEHS is shown in figure 3. Calibration standard PSL beads are used for sizes 0.3-3 µm, figure 4 is a schematic of the laboratory setup. Calibrations are usually carried out at the beginning and end of a measurement campaign. Under-wing calibrations are sometimes done mid-campaign, however the unavailability of DEHS means the resolution is greatly reduced. Any cleaning and realignment of a probe will significantly affect its response so a calibration is routinely done after any instrument work. A table of bin boundaries in terms of scattering cross-section is provided from each calibration. Bin boundaries in terms of PSL-equivalent diameter is also provided, however this assumes that all aerosols measured in flight are spherical, homogeneous, and of the same refractive index as PSL. This is likely to be a somewhat dubious assumption thus such data needs to be used with great care.

Schematic of PCASP calibration setup using PSL beads.Far better that the data user convert the scattering cross-section boundaries into diameters of the aerosols under test. In order to aid the user with such corrections FAAM can provide two software tools. Mie Scattering Table Generator uses Mie theory to define a table of scattering cross-sections as a function of diameter based on a given instrument optical geometry, laser wavelength and particle refractive index. Cross Section to Diameter Converter reads a table of calibrated bin boundaries in terms of scattering cross-section data and determines the bin mean and bin width in terms of diameter. To do this scattering data created by Mie Scattering Table Generator or other data provided by the user (if for example Mie theory is not applicable). Both programs are provided as Windows executables, however, C++ and FORTRAN source code is included and should compile on other operating systems (but requires the wxWidgets GUI libraries). These software packages are maintained by Phil Rosenberg and are available on SourceForge.

Mie Scattering Table Generator http://sourceforge.net/projects/mieconscat/ ver 1.1.7
Cross Section to Diameter Converter http://sourceforge.net/projects/cstodconverter/ ver 1.2.10

Plots of calibration data for both PCASP1 and PCASP2 are now available. Calibration data is currently emailed to campaign participants and is available upon application.

 

Data

Relevant FAAM core instrument data such as air speed, ambient temperature and pressure, that are not available in-flight are used to process the raw PCASP data. This processed data is stored in netCDF files and is available for each FAAM flight on the British Atmospheric Data Centre (BADC) archive. Currently all processed data has a temporal resolution of 1Hz. The filename structure is core-cloud-phy_faam_YYYYMMDD_v500_ri_bnnn.nc where YYYYMMDD is the date of the flight, bnnn is the 3-digit flight number, and ri is the revision number. The first revision of a file will include r0 in the filename with the number increasing with each new revision. Unless instructed otherwise, always use the highest revision number. Inside the netCDF all PCASP variables begin with PCAS2. Raw instrument data files is also available for those who require it.

The data is referenced against a time variable called PCAS2SPM which provides the time in seconds past midnight on a given date. Another variable called Time is included in the NetCDF which gives integer seconds past midnight. This variable is provided for easy cross referencing to other flight data and also allow plotting with NCAR's Aeros software. The data consists of concentrations for each of the 30 bins (remember that the data from bin 1 should be discarded) under the variables PCAS2_nn with percentage uncertainties for these concentrations under the variables PCAS2_nn_err (replace nn with the channel number). The uncertainties represent possible offsets and are not independent from one data point to the next, i.e. they cannot be reduced by averaging over multiple data points. The data file also contains the sum of the concentrations over channels 2 to 30 in the variable PCAS2CON, the volumetric flow rate within the PCASP PCAS2_FL, and a flag to indicate data quality in the variable PCAS2_FLAG. The flow rate variable can be used to approximately derive counting uncertainties by the equation counting uncertainty=sqrt(PCAS2_nn/PCAS2_FL). Note that this is only approximate and an extra variable will be added in the future giving counting uncertainties. Counting uncertainties can be reduced by averaging over multiple data points using the usual uncertainty propogation formulae.

The variable PCAS2_FLAG may have one of the following values;

  1. Data is fine.
  2. Absolute concentrations may be incorrect but all bins should be incorrect by the same factor so relative concentrations could be used. This could be because no temperature and pressure data was available to convert measured mass flow to volume flow (in which case the last known good value is used instead) or the flow meter is outside its calibrated range.

    The ratio of air speed inside/outside the subsampler is outside the range originally tested by Belyaev and Levin for calculating the inlet efficiency.

  3. No true air speed is available for performing inlet efficiency calculations. The last good true air speed data has been used.

    The reference voltage (used as a measurement of the laser power) is low. This could be caused by low temperatures, poor alignment or dirty optics. The effect will be to allow electrical noise to be falsely generate particle events creating false particles in the lower bins.

  4. The data is unusable or does not exist. In this case the data are replaced with the value -9999. This can occur if data has not been recorded for this time (e.g. during preflight or post-flight), the total flow through the instrument is so high that it is likely to cause turbulence in the optical cavity or the true air speed is low indicating that the aircraft is still on the ground (in this case the inlet efficiency calculations are not valid and actually tend to infinite for still air).

The netCDF also includes the following variables for calibration data, PCAS2_D_L_NOM, PCAS2_D_U_NOM, PCAS2_D_L_CAL, PCAS2_D_U_CAL, PCAS2_D_L_ERR, PCAS2_D_U_ERR, PCAS2_D_L_RI, and PCAS2_D_U_RI. These variables are no longer used as the calibration data is kept in seperate files. The handling of the calibration data is currently being overhauled with a new netCDF file being designed for calibration data.

 

History

FAAM owns two PCASP-100X instruments purchased from Particle Measurement Systems (PMS), these are generally referred to with the unimaginative titles PCASP1 (instrument serial number 17884-0190-04) and PCASP2 (s/n PMI-1022-1202-31). In December 2008 PCASP2 was sent to Droplet Measurement Technologies (DMT) to have its internal electronics brought up to date. This involved removing all data processing electronics in the instrument and replacing them with DMT's SPP200 electronics package. In addition to providing up to date, currently supported and hopefully more reliable electronics this provided the instrument with 30 software programmable size channels - a significant improvement on the previous 16 hardware configured size channels. In April 2009 both probes were flown on the MEVEX detachment, during which PCASP1 developed a serious fault. In Early 2010 PCASP1 was repaired and also provided with DMT's SPP200 electronics package. This means that all data from May 2009 onwards is derived from instruments with the SPP200 electronics.

 

References

S.P. Belyaev and L.M. Levin, Techniques for Collection of Representative Aerosol, Aerosol Science, 5, 325-338, 1974.

P.D. Rosenberg, A.R. Dean, P.I. Williams, J.R. Dorsey, A. Minikin, M.A. Pickering and A. Petzold, Particle sizing calibration with refractive index correction for light scattering optical particle counters and impacts upon PCASP and CDP data collected during the Fennec campaign, Atmos. Meas. Tech., 5, 1147-1163, doi:10.5194/amt-5-1147-2012, 2012.

DMT PCASP Manual, DOC-0228, Rev C. http://www.dropletmeasurement.com/resources/manuals-guides.

 

Contact

Graeme Nott at FAAM.
Last update: 9 Oct 2013.

Additional information