Characteristics of Northwest Atlantic Cloud Water: PMF Applied to ACTIVATE’s Cloud Water Data Set

Cloud water serves as a chemical reactor where gases and particles undergo chemical transformations and thus is a key component in aerosol-cloud interactions. A total of 535 cloud water samples collected aboard the NASA ACTIVATE HU-25 Falcon (2020–2022) over the northwest Atlantic (U.S. East Coast to Bermuda) are analyzed, and a set of 31 dissolved species spanning major inorganic ions, selected organic acids, and trace elements are used here for source apportionment analysis. The EPA positive matrix factorization (PMF) 5.0 model yields a six-factor solution that explains considerable variability in dissolved solute mass (r = 0.77), with the factors being sea salt (81.3% of reconstructed mass), secondary aerosol (12.2%), traffic/combustion (5.2%), metal-enriched dust (0.9%), aged dust (0.3%), and an industrial/metallurgical factor (0.07%). Seasonal and spatial patterns showed strong sea salt mass contributions in all months, but especially in winter enhanced aged dust influence near Bermuda in June, and larger relative contributions of metal and traffic/combustion-related factors near the coast. Concentration-weighted trajectory maps link the dust factor to trans-Atlantic transport from North Africa and most other factors to export from the eastern United States. Overall, the results show that mass-based cloud water chemistry is strongly weighted toward larger, highly hygroscopic particles, while still retaining clear signatures of continental pollution and long-range dust transport.