Counter-flow Virtual Impactor inlet

CVI General Information

The NCAR counterflow virtual impactor (CVI) (Noone et al., 1988; Twohy et al., 1997) is an airborne instrument that can be used for studies of aerosol/cloud interactions, cloud physics, and climate. At the CVI inlet tip (Figure 1), cloud droplets or ice crystals larger than about 8 µm aerodynamic diameter are separated from the interstitial aerosol and impacted into dry nitrogen gas. This separation is possible via a counterflow stream of nitrogen out the CVI tip, which assures that only larger particles (cloud droplets or ice crystals) are sampled. Cloud particles larger than the "cut-size" are injected into dry carrier gas flow (usually N2) where they evaporate. Because droplets or crystals in a sampling volume of about 200 l/min are impacted into a sample stream of approximately 10 l/min, concentrations within the CVI are significantly enhanced. The water vapor and non-volatile residual nuclei remaining after droplet evaporation are sampled downstream of the inlet with selected instruments. These may include a Lyman-alpha or similar hygrometer, a condensation nucleus counter, an optical particle counter, filters for chemical analyses, or user instruments.

A variety of instruments can be fed from the CVI, and they monitor characteristics of the evaporated cloud residuals. CVI is anti-ice heated and suitable for supercooled liquid water clouds. System uses heated tubing, fittings to divide the sample, and flow control for the cut-size. RAF operates two independent CVI systems that are specially designed for the GV or C-130. These systems have been used on NASA DC-8 and UND Citation-II.

Measurements Provided: Depends on the choice of instruments that are fed by CVI. May include cloud particle concentration, condensate mass, water vapor (for isotopic analysis) and aerosol particle residuals.

Measurement Characteristics: 

• Overall estimate of uncertainty: depends on downstream instrument characteristics
• Response time: ~1 to 3 sec, depending on flow rates
• Precision:
• Other measurement characteristics (comments on signal/noise, bias limits, etc): Susceptible to contamination when cloud particles hit the tip and break into fragments. May exhibit long hysteres is times when passing through high watercontent clouds.

History of Significant Changes: Stainless steel tip and piping were replaced with Titanium to address concerns about contamination when SS was ablated by high speeds and cloud particle impaction.