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Published July 2020 | Supplemental Material + Published
Journal Article Open

Characterization of aerosol hygroscopicity over the Northeast Pacific Ocean: Impacts on prediction of CCN and stratocumulus cloud droplet number concentrations


During the Marine Aerosol Cloud and Wildfire Study (MACAWS) in June and July of 2018, aerosol composition and cloud condensation nuclei (CCN) properties were measured over the N.E. Pacific to characterize the influence of aerosol hygroscopicity on predictions of ambient CCN and stratocumulus cloud droplet number concentrations (CDNC). Three vertical regions were characterized, corresponding to the marine boundary layer (MBL), an above‐cloud organic aerosol layer (AC‐OAL), and the free troposphere (FT) above the AC‐OAL. The aerosol hygroscopicity parameter (κ) was calculated from CCN measurements (κ_(CCN)) and bulk aerosol mass spectrometer (AMS) measurements (κ_(AMS)). Within the MBL, measured hygroscopicities varied between values typical of both continental environments (~0.2) and remote marine locations (~0.7). For most flights, CCN closure was achieved within 20% in the MBL. For five of the seven flights, assuming a constant aerosol size distribution produced similar or better CCN closure than assuming a constant "marine" hygroscopicity (κ = 0.72). An aerosol‐cloud parcel model was used to characterize the sensitivity of predicted stratocumulus CDNC to aerosol hygroscopicity, size distribution properties, and updraft velocity. Average CDNC sensitivity to accumulation mode aerosol hygroscopicity is 39% as large as the sensitivity to the geometric median diameter in this environment. Simulations suggest CDNC sensitivity to hygroscopicity is largest in marine stratocumulus with low updraft velocities (<0.2 m s⁻¹), where accumulation mode particles are most relevant to CDNC, and in marine stratocumulus or cumulus with large updraft velocities (>0.6 m s⁻¹), where hygroscopic properties of the Aitken mode dominate hygroscopicity sensitivity.

Additional Information

© 2020 American Geophysical Union. This is an open access article under the terms of the Creative Commons Attribution‐NonCommercial‐NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non‐commercial and no modifications or adaptations are made. Received 10 FEB 2020; Accepted 23 MAY 2020; Accepted article online 3 JUN 2020. This work was supported by Office of Naval Research Grants N00014‐17‐1‐2719 and N00014‐16‐1‐2567. AS was partially supported by NASA Grant 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. We would like to thank the crew of the CIRPAS Twin Otter for their assistance during the campaign. Data Availability Statement: Airborne field data used in this work can be accessed on the Figshare database (Sorooshian et al., 2017: https://doi.org/10.6084/m9.figshare.5099983.v10).

Attached Files

Published - 2020EA001098.pdf

Supplemental Material - ess2590-sup-0001-2020ea001098-t-ds01.docx

Supplemental Material - ess2590-sup-0002-2020ea001098-t-ds02.docx


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August 22, 2023
October 23, 2023