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Published July 2, 2020 | Published + Supplemental Material
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On the relationship between cloud water composition and cloud droplet number concentration


Aerosol–cloud interactions are the largest source of uncertainty in quantifying anthropogenic radiative forcing. The large uncertainty is, in part, due to the difficulty of predicting cloud microphysical parameters, such as the cloud droplet number concentration (N_d). Even though rigorous first-principle approaches exist to calculate Nd, the cloud and aerosol research community also relies on empirical approaches such as relating N_d to aerosol mass concentration. Here we analyze relationships between Nd and cloud water chemical composition, in addition to the effect of environmental factors on the degree of the relationships. Warm, marine, stratocumulus clouds off the California coast were sampled throughout four summer campaigns between 2011 and 2016. A total of 385 cloud water samples were collected and analyzed for 80 chemical species. Single- and multispecies log–log linear regressions were performed to predict N_d using chemical composition. Single-species regressions reveal that the species that best predicts N_d is total sulfate (R²_(adj) = 0.40). Multispecies regressions reveal that adding more species does not necessarily produce a better model, as six or more species yield regressions that are statistically insignificant. A commonality among the multispecies regressions that produce the highest correlation with N_d was that most included sulfate (either total or non-sea-salt), an ocean emissions tracer (such as sodium), and an organic tracer (such as oxalate). Binning the data according to turbulence, smoke influence, and in-cloud height allowed for examination of the effect of these environmental factors on the composition–Nd correlation. Accounting for turbulence, quantified as the standard deviation of vertical wind speed, showed that the correlation between N_d with both total sulfate and sodium increased at higher turbulence conditions, consistent with turbulence promoting the mixing between ocean surface and cloud base. Considering the influence of smoke significantly improved the correlation with N_d for two biomass burning tracer species in the study region, specifically oxalate and iron. When binning by in-cloud height, non-sea-salt sulfate and sodium correlated best with Nd at cloud top, whereas iron and oxalate correlated best with N_d at cloud base.

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© Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 License. Received: 13 Mar 2020 – Discussion started: 19 Mar 2020 – Revised: 28 May 2020 – Accepted: 07 Jun 2020 – Published: 02 Jul 2020. Alexander B. MacDonald acknowledges support from the Mexican National Council for Science and Technology (CONACYT). We acknowledge Agilent Technologies for their support and Shane Snyder's laboratories for ICP-QQQ data. This research has been supported by the Office of Naval Research (grant nos. N00014-10-1-0811, N00014-11-1- 0783, N00014-10-1-0200, N00014-04-1-0118, and N00014-16-1-35 2567), and the National Aeronautics and Space Administration (grant no. 80NSSC19K0442), in support of the ACTIVATE Earth Venture Suborbital-3 (EVS-3) investigation, which is funded by NASA's Earth Science Division and managed through the Earth System Science Pathfinder Program Office. Author contributions. All coauthors contributed to some aspect of the data collection. ABM and AS conducted the data analysis and interpretation. ABM and AS prepared the article with contributions from all coauthors. Data availability. All data used in this work can be found on the Figshare database (Sorooshian et al., 2017; https://figshare.com/articles/A_Multi-Year_Data_Set_on_Aerosol-Cloud-Precipitation-Meteorology_Interactions_for_Marine_Stratocumulus_Clouds/5099983, last access: 27 June 2020). The supplement related to this article is available online at: https://doi.org/10.5194/acp-20-7645-2020-supplement. The authors declare that they have no conflict of interest. Review statement. This paper was edited by Lynn M. Russell and reviewed by two anonymous referees.

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