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Title: Low-CCN concentration air masses over the eastern North Atlantic: Seasonality, meteorology, and drivers

In this paper, a 20 month cloud condensation nucleus concentration (N CCN) data set from Graciosa Island (39°N, 28°W) in the remote North Atlantic is used to characterize air masses with low cloud condensation nuclei (CCN) concentrations. Low-CCN events are defined as 6 h periods with mean N CCN<20 cm -3 (0.1% supersaturation). A total of 47 low-CCN events are identified. Surface, satellite, and reanalysis data are used to explore the meteorological and cloud context for low-CCN air masses. Low-CCN events occur in all seasons, but their frequency was 3 times higher in December–May than during June–November. Composites show that many of the low-CCN events had a common meteorological basis that involves southerly low-level flow and rather low wind speeds at Graciosa. Anomalously low pressure is situated to the west of Graciosa during these events, but back trajectories and lagged SLP composites indicate that low-CCN air masses often originate as cold air outbreaks to the north and west of Graciosa. Finally, low-CCN events were associated with low cloud droplet concentrations (N d) at Graciosa, but liquid water path (LWP) during low-CCN events was not systematically different from that at other times. Satellite N d and LWP estimates from MODIS collocatedmore » with Lagrangian back trajectories show systematically lower N d and higher LWP several days prior to arrival at Graciosa, consistent with the hypothesis that observed low-CCN air masses are often formed by coalescence scavenging in thick warm clouds, often in cold air outbreaks.« less
ORCiD logo [1] ; ORCiD logo [1] ; ORCiD logo [2] ;  [3]
  1. Univ. of Washington, Seattle, WA (United States). Dept. of Atmospheric Science
  2. Stony Brook Univ., NY (United States). School of Marine and Atmospheric Sciences
  3. Cooperative Inst. for Research in Environmental Sciences (CIRES), Boulder, CO (United States)
Publication Date:
Grant/Contract Number:
SC0013489; SC0006865
Accepted Manuscript
Journal Name:
Journal of Geophysical Research: Atmospheres
Additional Journal Information:
Journal Volume: 122; Journal Issue: 2; Journal ID: ISSN 2169-897X
American Geophysical Union
Research Org:
Univ. of Washington, Seattle, WA (United States)
Sponsoring Org:
USDOE Office of Science (SC), Biological and Environmental Research (BER) (SC-23)
Country of Publication:
United States
54 ENVIRONMENTAL SCIENCES; cloud condensation nuclei; clouds; aerosols; Azores
OSTI Identifier:
Alternate Identifier(s):
OSTI ID: 1376749