1) Instrument Function.
The nucleation-mode aerosol size spectrometer (N-MASS) 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 N-MASS. 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.
2) Scientific Utility of the Data.
The N-MASS 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--see Fig. 2), 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 -80 oC.
3) Instrument Characteristics.
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 (Fig. 2--a
difficult case in which most particles were near the lower detection
diameter of the instrument) demonstrated
a combined uncertainty (accuracy and precision) of 38%, 36% and 38%
for number, surface and volume, respectively.
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
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.