CN counter

Langley Aerosol Research Group Experiment (LARGE).  The "classic" suite of instrumenation measures in-situ aerosol micrphysical and optical properties. The package can be tailored for specific science objectives and to operate on a variety of aircraft. Depending on the aircraft, measurments are made from either a shrouded single-diffuser "Clarke" inlet, from a BMI (Brechtel Manufacturing Inc.) isokinetic inlet, or from a HIML inlet. Primary measurements include:

1.) total and non-volatile particle concentrations (3nm and 10nm nominal size cuts),
2.) dry size distributions from 3nm to 5µm diameter using a combination of mobilty-optical-aerodynamic sizing techniques,
3.) dry and humidified scattering coefficients (at 450, 550, and 700nm wavelength), and
4.) dry absorption coefficients (470, 532, and 670nm wavelength).

LARGE derived products include particle size statistics (integrated number, surface area, and volume concentrations for ultrafine, accumulation, and coarse modes), dry and ambient aerosol extinction coefficients, single scattering albedo, angstrom exponent coefficients, and scattering hygroscopicity parameter f(RH).

The nucleation-mode aerosol size spectrometer (NMASS) measures the concentration of particles as a function of diameter from approximately 4 to 60 nm. A sample flow is continuously extracted from the free stream using a decelerating inlet and is transported to the NMASS. Within the instrument, the sample flow is carried to 5 parallel condensation nucleus counters (CNCs) as shown in Fig. 1. Each CNC is tuned to measure the cumulative concentration of particles larger than certain diameter. The minimum detectable diameters for the 5 CNCs are 4.0, 7.5, 15, 30 and 55 nm, respectively. An inversion algorithm is applied to recover a continuous size distribution in the 4 to 60 nm diameter range.

The NMASS has been proven particularly useful in measurements of nucleation-mode size distribution in environments where concentrations are relatively high and fast instrumental response is required. The instrument has made valuable measurements vicinity of cirrus clouds in the upper troposphere and lower stratosphere (WAM), in the near-field exhaust of flying aircraft (SULFUR 6), in newly created rocket plumes (ACCENT), and in the plumes of coal-fired power plants (SOS ’99). The instrument has flown on 3 different aircraft and operated effectively at altitudes from 50 m to 19 km and ambient temperatures from 35 to -80ºC.

Accuracy. The instrument is calibrated using condensationally generated particles that are singly charged and classified by differential electrical mobility. Absolute counting efficiencies are determined by comparison with an electrometer. Monte carlo simulations of the propagation of uncertainties through the numerical inversion algorithm and comparison with established laboratory techniques are used to establish accuracies for particular size distributions, and may vary for different particle size distributions. A study of uncertainties in aircraft plume measurements demonstrated a combined uncertainty (accuracy and precision) of 38%, 36% and 38% for number, surface and volume, respectively.

Precision. The precision is controlled by particle counting statistics for each channel. If better precision is desired, it is necessary only to accumulate over longer time intervals.

Response Time: Data are recorded with 10 Hz resolution, and the instrument has demonstrated response times of this speed in airborne sampling. However the effective response time depends upon the precision required to detect the change in question. Small changes may require longer times to detect. Plume measurements with high concentrations of nucleation-mode particles may be processed at 10 Hz.

Specifications: Weight is approximately 96 lbs, including an external pump. External dimensions are approximately 15”x16”x32”. Power consumption is 350 W at 28 VDC, including the pump.

1) Time of Flight Aerosol Mass Spectrometer (ToF-AMS)

Total and single particle characterization of volatile aerosol ionic and organic components (50-700nm). Uncertainty depends on species and concentration.

2) Single Particle Soot Photometer (SP2)

Single particle measure of BC (soot) mass in particles and determination of mixed particle size and non-BC coating using laser scattering and incandescence. 70-700nm. Single particle counting up to 10,000 per sec.

3) A size-resolved thermo-optic aerosol discriminator (30 s avg.):

Aerosol size distribution from 0.12 up to 7.0 μm, often where most aerosol mass, surface area and optical effects are dominant. Uses a modified Laser Optical Particle Counter (OPC) and computer controlled thermal conditioning system is used upstream (airstream dilution dried). Characterizes aerosol components volatile at 150, 300 and 400C and refractory aerosol at 400C (sea salt, dust and soot/flyash). (Clarke, 1991, Clarke et al., 2004). Uncertianty about 15%

4) Condensation Nuclei - heated and unheated (available at 1Hz)

Two butanol based condensation nuclei (CN) counter (TSI 3010) count all particles between 0.01-3.0 um. Total CN, refractory CN (those remaining at 300C after sulfate is removed) and volatile CN (by difference) are obtained as a continuous readout as a fundamental air mass indicator (Clarke et al. 1996). Uncertainty ~ 5%.

5) Aerodynamic Particle Sizer – (APS-TSI3320) – (<5min/scan)

To further characterize larger “dry” particles, including dust, an APS is operated which sizes particles aerodynamically from 0.8 to 20 μm into 50 channels. Uncertainty~10%.

6) Differential Mobility Analyzer with thermal conditioning – (<3 min/scan)

Volatility tandem thermal differential mobility analyzer (VTTDMA) with thermal analysis that provides size information (mass, surface area, number distributions) and their state of mixing over the 0.01 to 0.3μm size range (Clarke et al., 1998, 2007) for sampling times of about 1-3 minutes. Uncertainty ~10%

7) Nephelometer (10-7 m-1 detection for 60s avg., recorded every 1 sec.)

A 3 wavelength nephelometer (450, 550, 700nm) is used for total scattering and submicrometer scattering values using a Radiance Research single wavelength nephelometer (and thereby coarse dust scattering by difference).

8) Two Particle Soot Absorption Photometers (PSAP-Radiance Research; detection <0.1μg m-3 for 5 min. avg. )

The PSAP is used to quantify the spectral light absorption coefficient of the total and submicron aerosol (eg. soot, BC) at three wavelengths (450, 550, 660nm).

9) Humidity Dependent Light-Scattering (10-6 m-1 detection for 60s avg.; recorded every 1 s)

Two additional Radiance Research single-wavelength nephelometers are operated at two humidities (high/low) to establish the humidity dependence of light scattering, f(RH).

The CNC counts particles in the approximate diameter range from 0.006 m to 2 m. The instrument operates by exposing the articles to saturated Flourinert vapor at 28 C and then cooling the sample in a condenser at 5 C. The supersaturation of the vapor increases as it is cooled and the vapor condenses on the particles causing them to grow to sizes which are easily detected. The resulting droplets are passed through a laser beam and the scattered light is detected. Individual particles are counted and are referred to as condensation nuclei (CN). Two CN Counters are provided in the instrument. One counts the particles after sampling from the atmosphere and the second counts particles that have survived heating to 192C. Lab experiments show that pure sulfuric acid particles smaller than 0.05 mm are volatilized in the heater. The heated channel detects when small particles are volatile and permits speculation about the composition. The CNC II contains an impactor collector which permits the collection of particles on electron microscope grids for later analysis. The collector consists of a two stages. In the first stage the pressure of the sample is reduced by a factor of two without loosing particles by impaction on walls. The second stage consists of a thin plate impactor which collect efficiently even at small Reynolds numbers. The system collects particles as small as 0.02 m at WB-57 cruise altitudes. As many as 25 samples can be collected in a flight.

Developed by Droplet Measurement Technologies, the CFSTGC is based on a concept by Roberts and Nenes [2005]. The instrument counts the fraction of aerosol particles that become droplets when exposed to a given water vapor supersaturation (RH > 100%).

As with all CCN counters, a temperature gradient is applied to produce a supersaturation of water vapor. However, the mechanism for generating supersaturation is not the same for all CCN counters. For example, for continuous flow parallel plate diffusion chambers, the temperature gradient is perpendicular to the flow, and supersaturation is a result of the nonlinear dependence of vapor pressure upon temperature. The same mechanism applies for static diffusion cloud chambers, where there is no flow at all.

However, as the name implies, for the Continuous Flow Streamwise Thermal Gradient CCN Counter, the temperature gradient is in the streamwise direction (maintained by thermoelectric coolers). In this case, supersaturation results as a consequence of the greater rate of mass transfer over heat transfer.

With laminar flow, heat and water vapor are transferred to the centerline of the column from the walls only by diffusion.

Since molecular diffusivity is greater than thermal diffusivity, the distance downstream that a water molecule travels before reaching the centerline is less than the distance the heat travels downstream before reaching the centerline. If you pick a point at the centerline, the heat originated from a greater distance upstream than the water vapor.

There are four facts that are necessary to explain how supersaturation is generated within the CFSTGC:

1) Assuming that the inner surface of the column is saturated with water vapor at all points, since the temperature is greater at point B than at point A, the water vapor partial pressure is also greater at point B than at point A.

2) The actual partial pressure of water vapor at point C is equal to the partial pressure of water vapor at point B.

3) However, since the temperature at point C is the same as at point A, the equilibrium water vapor pressure at point C is equal to the water vapor partial pressure at point A.

4) The saturation ratio is the ratio between the actual partial pressure of water vapor and the equilibrium vapor pressure. This is equivalent to the partial pressure at point B divided by the partial pressure at point A, which is always greater than one. Thus supersaturation is generated through a dynamic equilibrium.