Pollution effects on cloud condensation nucleus (CCN) concentration vary widely, sometimes even decreasing the CCN. The interpretation of these pollution effects may be invalid if the nuclei are measured by expansion-type Aitken nucleus counters operated at low expansion ratios to provide a readout (with natural nuclei) equal to that of a thermal-gradient diffusion cloud chamber (TGDCC) instrument operated at 1% supersaturation. Calculations indicate that the expansion chamber operated at a ΔP of 1.4 inch Hg produces a supersaturation of 26%, allowing for vapor depletion effects during the first few hundredths of a second after expansion. The maximum supersaturation in clouds is typically 0.1–1%; therefore, 26% causes a, large positive error. Further, vapor depletion in the instrument during droplet growth limits the growth rate and size of droplets at supersaturations which are low compared to those for which the instrument is designed. Therefore, use of the standard calibration curve contributes a large negative error in the readout. The combination of a large positive error from excessive supersaturation and a large negative error in calibration appears to explain the systematic errors found at the second International Workshop on Condensation and Ice Nuclei (IWCIN). With NaCl nuclei the expansion instrument undercounted 90% of the time, averaging 0.6 of the average of five TGDCC instruments. On the other hand, with the fairly nonwettable nuclei of Teflon it read 25 times the TGDCC average.

An experimental development model of a continuous mixing-jet type instrument for CCN exhibited the same tendency, reading 0.04–0.33 of the TGDCC average with NaCl and 8 times too high with Teflon. If further development can correct the uncertainties in this NCAR instrument, it promises to be convenient and continuous.

Unless proven differently by future comparison tests which are more definitive than those of the IWCIN, interpretations of data on CCN in pollution should be evaluated in terms of the supersaturation at which the data are taken as compared to that in clouds. Future decisions on instrumentation for measurements of CCN should take into account the results of the IWCIN, particularly for measurements in polluted air where most Aitken nuclei do not act as CCN.

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