Nucleation-Mode Aerosol Size Spectrometer (N-MASS)

Measurement Description:

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-NASS. Within the instrument, the sample flow is carried to 5 parallel condensation nucleus counters (CNCs) as shown in Figure 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 Figure 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.

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 uncertainies 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 (Figure 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.
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.

Figure 1. Schematic diagram of the internal flow system of the N-MASS.

Figure 2. Particle size distributions measured by the N-MASS in the near-field exhaust of a Boeing 737 in flight at 8 km altitude. The three curves in each panel correspond to measurements taken 0.2, 0.4, and 0.6 s after emission and are presented in units of particle number, surface and volume produced per kg of fuel burned. The growth of the aerosol with increasing plume age can be clearly seen.