Water droplet calibration of the Cloud Droplet Probe (CDP) and in-flight performance in liquid, ice and mixed-phase clouds during ARCPAC

Abstract

Abstract. Laboratory calibrations of the Cloud DropletProbe (CDP) sample area and droplet sizing are performed using water droplets of known size, generated at a known rate. Although calibrations with PSL and glass beads were consistent with theoretical instrument response, liquid water droplet calibrations were not, and necessitated a 2 µm shift in the manufacturer's calibration. We show that much of this response shift may be attributable to a misalignment of the optics relative to the axis of the laser beam. Comparison with an independent measure of liquid water content (LWC) during in-flight operation suggests much greater biases in the droplet size and/or droplet concentration measured by the CDP than would be expected based on the laboratory calibrations. Since the bias in CDP-LWC is strongly concentration dependent, we hypothesize that this discrepancy is a result of coincidence, when two or more droplets pass through the CDP laser beam within a very short time. The coincidence error, most frequently resulting from the passage of one droplet outside and one inside the instrument sample area at the same time, is evaluated in terms of an "extended sample area" (SA E ), the area in which individual droplets can affect the sizing detector without necessarily registering on the qualifier. SA E is calibrated with standardized water droplets, and used in a Monte-Carlo simulation to estimate the effect of coincidence on the measured droplet size distributions. The simulations show that extended coincidence errors are important for the CDP at droplet concentrations even as low as 200 cm −3 , and these errors are necessary to explain the trend between calculated and measured LWC observed in liquid and mixed-phase clouds during the Aerosol, RadiationCorrespondence to: S. Lance (sara.m.lance@noaa.gov) and Cloud Processes Affecting Arctic Climate (ARCPAC) study. We estimate from the simulations that 60% oversizing error and 50% undercounting error can occur at droplet concentrations exceeding 400 cm −3 . Modification of the optical design of the CDP is currently being explored in an effort to reduce this coincidence bias.

Particle sizing calibration with refractive index correction for light scattering optical particle counters and impacts upon PCASP and CDP data collected during the Fennec campaign

“…The thin red and black lines show the impact of moving the sample/laser intersect point or sample volume along the laser axis or laterally in a direction perpendicular to the laser axis. The 1-sigma offsets are estimates of the variation from nominal for a typical instrument and are based observed offsets of FAAM's PCASP after realignments, measurements of a number of CDPs by the manufacturer (Baumgardner, 2012) and measurements by Lance et al (2010).…”

“…Multiplying the sample area by the speed of the airflow through the laser and the measurement time interval provides a sample volume allowing a concentration to be measured. Lance et al (2010) showed that the CDP sample area can be measured using a droplet gun on a micro-positioning system. It should be noted, however, that due to the method used to define the sample area, described in Sect.…”

“…The flow rate from the compressed gas supply is manually regulated in to provide a few hundred particles per second as measured by the CDP. This concentration is used to avoid coincidence which can cause problems above ∼500 s −1 (Lance et al, 2010) and yet provide a useful particle distribution in a short time period. The concentration of smaller bead sizes in the sample tends to be more difficult to control which can result in high concentrations and coincidence of particles in the laser beam.…”

“…Hand and Kreidenweis (2002) calibrated with aerosol of three different real refractive indices and used a polynomial interpolation for intermediate values. Covert et al (1990) and Lance et al (2010) calibrated using particles of the same composition as the particles measured in the real world. Others have smoothed theoretical curves or widened the bins of the OPC to generate a monotonically increasing function or reduce the effect of this ambiguity upon each bin (Johnson and Osborne, 2011;Liu et al, 1974).…”

“…The unscattered radiation and a subset of scattered radiation pass through a second sapphire window into the detector arm and the intensity of the unscattered beam is measured by a dump spot monitor. Light scattered within the range 4-12 • is passed onto an optical beam splitter where 33 % and 67 % of the light is directed to two detectors known as the Sizer and Qualifier respectively (Lance et al, 2010).…”