The relationship between cloud droplet number concentrations and anthropogenic pollution: Observations and climatic implications

Measurements of the concentrations of sulfate and nitrate in approximately 400 cloud water samples collected during four field studies carried out since 1982 are used with coincident measurements of cloud droplet number concentrations (CDNC) and liquid water content (LWC) to examine the relationship between CDNC and anthropogenic pollution, where sulfate concentration is used as the measure of the latter. The number of samples is compressed to 92 by averaging duplicates and multiple samples at similar altitudes during any particular flight, with 85 including CDNC measurements. Positive linear regressions between log (CDNC) and log (cloud water sulfate concentration) are determined for both stratiform and cumuliform cloud. Because of the number of factors affecting the CDNC, the coefficients of determination are only 0.30 and 0.49 for the respective cloud types. The LWC is relatively invariant with the cloud water sulfate concentration. The observed range of CDNC for the study region is 20–600 cm −3 (median of 59 observations is 210 cm −3 ) for stratiform clouds and 170–1100 cm −3 (median of 26 observations is 400 cm −3 ) for cumuliform clouds. The median CDNC for all sampled clouds is 250 cm −3 . CDNC are also determined for “clean‐air” conditions. The latter is defined as cases for which the concentrations of both cloud water sulfate and cloud water nitrate are comparable to aerosol sulfate concentrations and aerosol nitrate plus HNO 3 concentrations, respectively, as reported for remote regions of the globe. For the clean‐air clouds the observed range of CDNC for the study region is 20–250 cm −3 (median of 12 observations is 120 cm −3 ) for stratiform clouds and 170–370 cm −3 (median of four observations is 240 cm −3 ) for cumuliform clouds. The median CDNC for all clean‐air clouds is 160 cm −3 . The median CDNC for the complete population is 56% greater than the clean‐air CDNC; the hypothesis that the clean‐air CDNC is not different from the median CDNC is rejected at a confidence level of >99.5%. The present‐day climatic forcing due to cloud albedo change arising from increased CDNC is estimated from a rudimentary model at between −2 W m −2 and −3 W m −2 for eastern North America.