Cloud condensation nuclei: Measurement, prediction and effects on remote marine stratocumulus clouds.

摘要

Laboratory studies that constrain uncertainties in the calibration of two static thermal gradient diffusion cloud concentration nuclei (CCN) instruments are presented. The relative uncertainty in CCN concentration is determined to be between 10 to 30%, depending on the applied chamber supersaturation. The latter was inferred by challenging the instruments with particles of known composition made monodisperse in a differential mobility analyzer (DMA) at two relative humidities (73% and 10%). Results show that the effect of test particle asphericity or degree of compactness are not responsible for a significant bias in the experimental assessment of applied chamber supersaturation.; CCN concentrations observed during the second Dynamics and Chemistry of Marine Stratocumulus experiment (DYCOMS-II) are also presented. A formalism that combines atmospheric variability and measurement uncertainties associated with the fitted CCN activation spectra is developed. Observed CCN data are compared with predictions based on aerosol size spectra and chemical composition measurements. Predictions and observations are in agreement within the uncertainties imposed by both measurement error and computational assumptions, with the exception of measurements made at the effective supersaturation s = 0.1%. Sensitivity studies indicate that the discrepancy at s = 0.1% could be explained by assuming that the particles were internally mixed with a soluble mass fraction of 0.25, or that the passive cavity aerosol size spectrometer (PCASP) instrument was not completely removing chemically bonded water.; A case study analyzing Pockets of Open Cells (POCs) during the second DYCOMS-II research flight is also presented. POCs are elucidated based on an aerosol/cloud microphysical hypothesis. If this hypothesis is correct, pristine marine boundary layer clouds may be more susceptible to aerosol indirect effects than thought previously. Also analyzed are Aitken mode particles that are likely to be the result of a recent homogeneous particle nucleation event that occurred within the marine boundary layer and appeared to be associated with POCs. These particles were not effective cloud nuclei and are thought to be non-hygroscopic.