CVI (Counterflow Virtual Impactor) AUTHOR: Cynthia Twohy Ocean Admin 104 Oregon State University Corvallis, OR 97331-5503 Phone: (541) 737-5690 Fax: (541) 737-2540 twohy@coas.oregonstate.edu ; http://www.oce.orst.edu/ats/twohy/ OVERVIEW: The counterflow virtual impactor (CVI) separates droplets or ice crystals from the ambient aerosol and evaporates the condensed water, producing cloud droplet residual particles and water vapor. During DYCOMS, the NCAR CVI was used to measure cloud droplet number (as number of residual particles after evaporation) and condensed water content. The CVI inlet was mounted on the right side, forward, on the C-130 aircraft. Particle number was measured with a TSI 3760 condensation nucleus counter and condensed water content was measured with a modified Lyman-alpha hygrometer. Other measurements made behind the CVI included residual particle size using a LAS-AIR particle counter, filter samples for organic analysis by Lynn Russell (Princeton), and residual particle composition by electron microscopy (Jim Anderson of Arizona State University). Additional information on the CVI can be found on Twohy's website (listed above) under "Current Research." DATA FILES: The following 1 Hz variables in the space-delimited ascii data files relate to the CVI: TIME: Time in UTC seconds after midnight. For example, 10:30 UTC would equal 37800 sec (10*3600+30*60) CVINFLAG: inlet flag for current sample: 0=CVI, 1= total (whole air sample through CVI inlet). CVCWC: CVI condensed water content in g/m3, corrected back to ambient concentrations and conditions. CVN: CVI cloud particle number concentration in cm-3 (see notes, below), corrected back to ambient conditions and conditions. CFACT: concentration factor inside the CVI sample line, relative to ambient concentrations. CVF2C: corrected volumetric flow rate (l/min) for Lynn Russell's filter samples. CVFCN2C: corrected volumetric flow rate (l/min) for Prof. Jim Anderson's SEM filter sampler behind the CVI. CVFX1C: corrected volumetric flow rate (l/min) for Prof. Jim Anderson's TEM sampler behind the CVI. CVRH: Relative humidity (%) inside the CVI sample line. CVPCN: Pressure (mb) inside the CVI sample line. CNT1: Temperature (K) inside CVI sample line. DATA COLLECTION AND ANALYSIS: The CVI data were collected on a Dell Inspiron laptop PC with National Instruments (SCXI) acquisition hardware. Data were interpolated during the 4-5 s gap between data files which terminated at the end of each hour of data collection. Data from takeoff and landing were often contaminated with exhaust particles and gases and were deleted. To correct for the lag time in the CVI plumbing and differences in timing between the aircraft and the CVI data system, the CVI data were adjusted in time by comparison to the FSSP data. DATA INTERPRETATION: The range of aerodynamic cloud particle sizes which the CVI measures is between the lower cut size of about 4 microns radius and the upper cut size of about 25 microns radius. Larger droplets or ice crystals will impact on the bend downstream of the CVI inlet tip. The evaporated residue from these large particles will not be transmitted to the sensors measuring aerosol particles, although the condensed water content for the larger particles will be detected by the Lyman-alpha (Twohy et al., 1997). In polluted stratus clouds, the mean droplet diameter may be at or below the lower cut size, resulting in CVI number concentrations and water contents substantially lower than corresponding parameters from the FSSP. Since droplet number and water contents are enhanced during the impaction process inside the CVI, measured values inside the instruments are divided by the concentration factor, CFACT, back to ambient (in cloud) concentrations at ambient temperature and pressure. Number concentration, CVN, also may be artificially enhanced on any flight due to breakup of large (>100 micron) droplets in the CVI inlet. Use the 260-X, 2D-C, or 2D- P data to determine when these are present, or compare CVN to number concentration from the FSSP (CONCF). If CVN>>CONCF, breakup is likely to be occurring and the data are questionable. Since the Lyman-alpha hygrometer drifts, baseline values were determined by comparison of water vapor values with CVI number concentration values, which were essentially zero when outside of cloud. These water vapor baseline values were combined to produce a smooth baseline curve for an entire flight that was subtracted from condensed water values throughout the flight; this produces approximately zero values outside of cloud. The CVI was periodically used as a total aerosol inlet to collect aerosol and water vapor data outside of cloud. Please note thses times when INFLAG = 1, as data is distinctly different during these times. When INFLAG = 1, CVN and CVCWC represent the actual particle number and water vapor content inside the CVI, which are close to ambient values. Actual ambient values can be determined by correcting for the density difference between the CVI and ambient air using the ideal gas law. Variables to use include the CVI pressure and temperature (CVPCN and CNT1) from this file, and the ambient pressure and temperature from the C-130 RAF aircraft data set. Spikes in CVCWC may occur on either side of the ambient sample if the flag was set slightly after the sample begun. DATA PROBLEMS: Flights with missing or problem data are given below: R1 and R2 (10 July and 11 July 2002): CVI inoperative due to computer hard drive problems. R3 (13 July 2002): Files are split into "a" and "b" due to data gap between 36747- 37394 s (computer problem). Also, CVCWC signal is artificially high due to contamination of system by ambient water vapor (leak in EM samplers). R4 (17 July 2002): Enhancement in CVCWC during following times due to continued leak in EM samplers: 27400 to 31176 s, 40590 to 47060 s. R7 (24 July 2002): Bad CVN and CVCWC data between 47030 to 48460 s due to flow testing. R8 (25 July 2002): Bad CVN and CVCWC data between 86500 to 87800 s due to leak. R10 (28 July 2002): Butanol was low on CN counter. CVN was compared to aircraft CONCN during ambient samples to a develop power curve correction to the CVN data as a function of time. References: Twohy, C. H., A.J. Schanot and W. A. Cooper, 1997: Measurement of condensed water content in liquid and ice clouds using an airborne counterflow virtual impactor. J. Atmos. Oceanic Tech., 14, 197-202.