The CCN counter operates on the principle that diffusion of heat in air is slower than diffusion of water vapor (Roberts and Nenes, 2005). Inside the column, a thermodynamically unstable, supersaturated water vapor condition is created as follows. Water vapor diffuses from the warm, wet column walls toward the centerline at a faster rate than the heat. Figure (left) shows point C along the centerline where the diffusing heat originated higher on the column (red-line, point A) than the diffusing mass (blue line, point B). Assuming the water vapor is saturated at the column wall at all points and 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. The actual partial pressure of water vapor at point C is equal to the partial pressure of water vapor at point B. The temperature at point C is lower than at point B, however, which means that there is more water vapor (corresponding to the saturation vapor pressure at point B) than thermodynamically allowed.

The CCN measures aerosol particles called cloud condensation nuclei that can form into cloud droplets. The instrument operates by supersaturating sample air to the point where the CCN become detectable particles, which are then sized using an optical particle counter and distributed into 20 bins.

The Scanning Electrical Mobility system (SEMS)’s Differential Mobility Analyzer (DMA) is optimized to size particles over the 10 to 1000 nm or 20 to 2000 nm diameter range for DMA sheath air flow rates between 2.5 and 10 lpm. The DMA selects particles based on the principle of electrical mobility. The size selected monodisperse particles than detected by a Mixing-based Condensation Particle Counter (MCPC) based on the principle of scattering.

SEMS is a scanning mobility spectrometer providing rapid aerosol size distribution measurements from 10 nm to 1.4 µm.

Differential Mobility Analyzer (DMA) is optimized to size particles over the 10 to 1000 nm or 20 to 2000 nm diameter range for DMA sheath air flow rates between 2.5 and 10 lpm. The DMA selects particles based on the principle of electrical mobility. 

The DMA consists of two electrodes and a flow path in which particles move. When a voltage is applied charged particles are diverted from the straight path in the resulting electrical field and classified based on their electrical mobility. When the voltage is constant the DMA generates monodisperse aerosols from a polydisperse particle source. DMA will alternatively be associated with CCN counter to quantify the hygroscopic characteristics/cloud condensation nuclei activations of size classified aerosols as a function of their supersaturations.

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SP2-XR uses the laser-induced incandescence principle to accurately quantify the soot fractions in ambient aerosol for long-term monitoring applications. It uses one scattering and one incandescence detector to count the total and soot particles. It utilizes the high optical power intracavity Nd:YAG laser. Light-absorbing particles, mainly black or elemental carbon, absorb energy and are heated to the point of incandescence. The energy emitted in this incandescence is measured, and a quantitative determination of the black carbon mass of the particle is made. This mass measurement is independent of the particle mixing state, and hence the SP2-XR is a reliable measure of the black carbon mass concentration. Since the SP2-XR detects single particles, it also measures the black carbon particle concentration.

Single Particle Soot Photometer-Extended Range (SP2-XR) directly measures refractory black carbon (rBC) in individual particles.

The Wideband Integrated Bioaerosol Sensor-New Electronics Option (WIBS-NEO) is designed to provide highly sensitive measurements of bioaerosols. The instrument uses two UV filtered flashlamp sources to excite fluorescence in individual particles. Detection wavebands have been selected to optimize detection of common bioaerosol components such as tryptophan and nicotinamide adenine di-nucleotide (NADH).

The single-particle fluorescence sensor, WIBS-NEO, employs a central optical chamber around which are arranged the following components:

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The Continuous Light Absorption Photometer (CLAP) measures the aerosol absorption of radiation at three visible wavelengths; 461, 522 and 653 nanometers (nm) based on the continuous measurements of light transmitted through a filter while the aerosols are being deposited on the filter. Data from this measurement is used in radiative forcing calculations, atmospheric heating rates, and as a prediction of the amount of equivalent black carbon in atmospheric aerosol and in models of aerosol semi-direct forcing. Aerosol absorption measurements are essential to modeling the energy balance of the atmosphere.

Continuous Light Absorption Photometer (CLAP) operates at three visible wavelengths (blue, green red), to allow calculation of aerosol absorption coefficients.

The CLAP provides measurement of the light absorption of particles deposited on a filter. It utilizes solenoid valves to cycle through 8 sample filter spots and 2 reference filter spots, enabling the instrument to run at ideal conditions (filter transmittance greater than 0.7). The CLAP uses 47-mm diameter, glass-fiber filters (Pallflex type E70-2075W). These filters are made of two fibrous layers, borosilicate glass fibers overlaying a cellulose fiber backing material (for strength and stability). The cellulose fiber layer is thought to take up water under conditions of high humidity, which is one reason that CLAP has an internal heater to lower the sample relative humidity inside the instrument. The CLAP is typically installed so that it draws its sample air through a modified of ACAS setup. The CLAP vacuum line is connected to a vacuum diaphragm pump.