Ice Nuclei Counter (CFDC) Data
Paul DeMott and Gavin McMeeking Contact: gavin AT atmos DOT colostate DOT edu; pdemott AT lamar DOT colostate DOT edu; 970 491 8667 Dept. of Atmospheric Science Colorado State University Fort Collins, CO 80524-1371
1.0 DATA SET OVERVIEW:
Please note this is QUALITY CONTROLLED, THOUGH STILL preliminary data THAT MAY BE SUBJECT TO FUTURE REVISION. This dataset contains ASCII text files listing ice nuclei concentrations averaged over investigator-defined sampling periods (typically the interval between background filter checks or single cloud penetrations when sampling from the CVI). It covers the entire ICE-T campaign period (1-31 July 2011) and the region covered by the C-130 aircraft: latitude: -10.00 to 40.00 longitude: -90.00 to -40.00
2.0 INSTRUMENT DESCRIPTION:
The continuous flow diffusion chamber samples aerosol particles through a region of controlled temperature and super-saturation (Rogers et al., 2001; Eidhammer et al. 2010). Aerosol that serve as ice nuclei activate into ice crystals that are detected optically. The technique is sensitive to deposition and condensation/immersion freezing. Sample conditions during ICE-T range between 80-105% RH w/ respect to water and chamber temperatures between -30 and -15 C. Sample passes a 2.4 micron impactor to remove large particles to allow optical ice crystal detection. The instrument periodically samples filtered air to determine the background ice crystal count rate that arises from the formation of frost on various surfaces within the chamber, which can be released and detected as IN. The instrument provides 1 Hz data, but due to extremely low concentrations of IN the results are typically averaged over user-defined sampling intervals to produce reasonable statistics. The detection limit of the instrument is a function of both the sampling interval time/volume and the background conditions, which depend on a wide range of variables including stability of pressure, relative humidity of the sampled air, time since walls were first coated with ice, quality of initial ice coating, and a host of other factors at times such as aircraft vibration and sharp changes in angle of attack at the inlet. Reported uncertainties are standard deviations of average concentrations assuming IN counts are described by a Poisson distribution and propagated over the background correction calculations.
3.0 DATA COLLECTION AND PROCESSING:
Data are recorded as a distribution of peak heights by a multi-channel analyzer connected to the optical particle counter at the base of the chamber. Particles with peak heights above a critical size equating to approximately 3 microns are considered to be IN, while those below the critical size are treated as aerosol. Aerosol concentrations represent the approximate size range 0.4-2.5 um, but the exact size range is not well defined. Counts are corrected for the MCA live time and converted to concentrations at STP using measured total and sheath flow rates at STP. Aerosol location temperature is calculated from recorded inner and outer wall temperatures and measured air flow rate through the chamber. Aerosol location super-saturation is also calculated from recorded inner and outer wall temperatures assuming they are coated completely with a thin layer of ice at the same temperature as the walls, and following equations in Rogers (1988). Data are corrected for background concentrations using IN concentrations measured during filter periods. Data are screened for periods with flows outside the nominal range (occurs at low ambient pressures), if wall temperatures are not stable, or if electrical noise is present in the MCA channel. Additional manual screening is performed to remove periods when instrument tests were occuring or other routine procedures were being performed to reduce background (e.g., repeated evacuations to low chamber pressure).
4.0 DATA FORMAT:
Data are reported in ASCII format as rows containing average values and accompanying standard deviations for the period specified by the start and end times (in UTC), position data, and a range of variables containing CFDC IN data. All concentrations are reported at standard temperature and pressure (defined as 273.15 K and 1013.25 hPa). Times are reported as YYYYMMDDHHmmss. Latitude and longitude are reported in degrees. Altitude is reported in m. The CVI flag indicates when the CFDC was sampling from the CVI inlet (CVI = 1) and when it sampled from the Wyoming heated inlet (CVI = 0). The CVI enhancement or correction factor is reported for all periods, including those when the CFDC was NOT sampling from the CVI. Reported CFDC data are NOT corrected for the CVI enhancement factor. Caution should be applied when using CFDC data from the CVI inlet due to a wide range of factors specific to the CVI itself. Note that CVI IN concentrations may also underestimate peak values in cloudy segements depending on cloud properties and their consistency over the averaging period. Users are required to contact CSU for Average concentrations of aerosol (detected by the CFDC and only for particles with optical diameters > 0.2 um) are reported as number per cubic centimeter. Ice nuclei concentrations are reported in number per L both as raw values and those corrected for background (IN_COR). The IN significance flag is set to 1 for samples that are significantly different from 0 at the 95% confidence level. Average instrument conditions for each sample are also listed (temperature, supersturation w/ respect to water, and chamber pressure).
5.0 DATA REMARKS:
This is preliminary data. Please contact authors with any questions or for higher resolution data. The CFDC instrument suffered from high background levels during many ICE-T flights due to a number of environmental factors affecting physical and electronic systems. As a result only a subset of reported values were significantly different from zero. We stress that high backgrounds do not result in questionable data, only in a higher detection limit that may mask periods of lower IN concentrations and produce large data gaps. A data recording problem limited information available for RF05. IN error estimates could not be obtained for that flight and are not reported. Significance is estimated from visual inspection and users are encouraged to contact CSU if they plan to use RF05 data in their analysis.
List of documents cited in this data set description
Eidhammer, T., P. J. DeMott, A. J. Prenni, M. D. Petters, C. H. Twohy, D. C. Rogers, J. Stith, A. Heymsfield, Z. Wang, S. Haimov, J. French, K. Pratt, K. Prather, S. Murphy, J. Seinfeld, R. Subramanian, and S. M. Kreidenweis, 2010: Ice initiation by aerosol particles: Measured and predicted ice nuclei concentrations versus measured ice crystal concentrations in an orographic wave cloud. J. Atmos. Sci., 67, 2417–2436.
Rogers, D. C., 1988, Development of a continuous flow thermal gradient diffusion chamber for ice nucleation studies, Atmos. Res., 22, 149– 181.
Rogers, D. C., P. J. DeMott, S. M. Kreidenweis and Y. Chen, 2001: A continuous flow diffusion chamber for airborne measurements of ice nuclei, J. Atmos. Oceanic Technol., 18, 725-741.