**2. Separation techniques**

The separation of CN from interstitial aerosol technique is based on the particle's inertia. The instrument that employs this technique is called counterflow virtual impactor (CVI). The separation is achieved by stopping and removing the gas phase and small particles but capturing large particles with sufficient inertia to cross gas streamlines. Particles with insufficient inertia to be captured follow the deflected streamlines and are removed from the system. Higher inertia particles are injected into a typically clean, dry, and warm counterflow carrier gas that causes evaporation of condensed phase water. This technique has the advantage that a broad cut size range can be achieved by varying the flow rates associated with the CVI without changing the physical dimensions of the instrument.

Separating Cloud Forming Nuclei from Interstitial Aerosol 409

the distance between the tip of the CVI nozzle till the beginning of the sample flow is larger than the particle stopping distance, then the particle joins the sample flow and is transmitted. Figure 3 shows the relationship between the droplet diameters to the stopping distance for different flows. The flow can be varied either by increasing the input flow, while keeping other

flows constant, or by varying the CVI geometry. The former option is always desired.

**Figure 1.** a: Flow schematics within the pumped counterflow virtual impactor (PCVI). Input flow carries the condensation nuclei and interstitial particles (non-activated aerosol particles). The large particles that have sufficient inertia to cross the streamlines enter the counterflow; these particles that

The CVI used in the laboratory set up is called pumped CVI (PCVI) and the CVIs used for *in-situ* measurements are called airborne CVI (ACVI). The flow schematics of these designs are shown in Figure 1 a) and b), respectively. In PCVI design the aerosol particles are pulled inside the instrument and undergo inertial separation, while in ACVI the aircraft velocity imparts motion for aerosol particles that are again separated based on the inertia. Both designs are widely used and their performance characteristics are documented.
