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This work is licensed under a Creative Commons Attribution 4.0 International License.


Benjamin A. Nault, Aerodyne Research, Inc.
Pedro Campuzano-Jost, University of Colorado
Douglas A. Day, University of Colorado
Duseong S. Jo, University of Colorado
Jason C. Schroder, University of Colorado
Hannah M. Allen, California Institute of Technology
Roya Bahreini, University of California
Huisheng Bian, Joint Center for Environmental Technology
Donald R. Blake, University of California
Mian Chin, NASA Goddard Space Flight Center
Simon L. Clegg, University of East Anglia
Peter R. Colarco, NASA Goddard Space Flight Center
John D. Crounse, California Institute of Technology
Michael J. Cubison, TOFWERK USA
Peter F. DeCarlo, Johns Hopkins University
Jack E. Dibb, University of New HampshireFollow
Glenn S. Diskin, NASA Langley Research Center
Alma Hodzic, National Center for Atmospheric Research
Weiwei Hu, Chinese Academy of Sciences
Joseph M. Katich, University of Colorado
Michelle J. Kim, California Institute of Technology
John K. Kodros, Colorado State University
Agnieszka Kupc, NOAA Chemical Sciences Laboratory
Felipe D. Lopez-Hilfiker, TOFWERK AG
Eloise A. Marais, University College London
Ann M. Middlebrook, NOAA Chemical Sciences Laboratory
J. Andrew Neuman, University of Colorado
John B. Nowak, NASA Langley Research Center
Brett B. Palm, University of Washington
Fabien Paulot, NOAA Geophysical Fluid Dynamics Laboratory
Jeffrey R. Pierce, Colorado State University
Gregory P. Schill, University of Colorado
Eric Scheuer, University of New Hampshire, Durham
Joel A. Thornton, University of Washington
Kostas Tsigaridis, Columbia University
Paul O. Wennberg, California Institute of Technology
Christina J. Williamson, University of Colorado
Jose L. Jimenez, University of Colorado


The inorganic fraction of fine particles affects numerous physicochemical processes in the atmosphere. However, there is large uncertainty in its burden and composition due to limited global measurements. Here, we present observations from eleven different aircraft campaigns from around the globe and investigate how aerosol pH and ammonium balance change from polluted to remote regions, such as over the oceans. Both parameters show increasing acidity with remoteness, at all altitudes, with pH decreasing from about 3 to about −1 and ammonium balance decreasing from almost 1 to nearly 0. We compare these observations against nine widely used chemical transport models and find that the simulations show more scatter (generally R2 < 0.50) and typically predict less acidic aerosol in the most remote regions. These differences in observations and predictions are likely to result in underestimating the model-predicted direct radiative cooling effect for sulfate, nitrate, and ammonium aerosol by 15–39%.


Earth Systems Research Center

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Communications Earth & Environment



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This is an open access article published by Nature in 2021 in Communications Earth & Environment, available online: