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© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  21 Jul 2020

21 Jul 2020

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This preprint is currently under review for the journal ACP.

The potential role of organics in new particle formation and initial growth in the remote tropical upper troposphere

Agnieszka Kupc1,2, Christina J. Williamson1,3, Anna L. Hodshire4, Jan Kazil1,3, Eric Ray1,3, T. Paul Bui5, Maximilian Dollner2, Karl D. Froyd1,3, Kathryn McKain3,6, Andrew Rollins1, Gregory P. Schill1,3, Alexander Thames7, Bernadett B. Weinzierl2, Jeffrey R. Pierce4, and Charles A. Brock1 Agnieszka Kupc et al.
  • 1Chemical Sciences Laboratory, National Oceanic and Atmospheric Administration, Boulder, CO 80305, USA
  • 2Faculty of Physics, Aerosol Physics and Environmental Physics, University of Vienna, 1090 Vienna, Austria
  • 3Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309, USA
  • 4Department of Atmospheric Science, Colorado State University, Fort Collins, CO 80523, USA
  • 5Earth Science Division, NASA Ames Research Center, Moffett Field, California, USA
  • 6Global Monitoring Laboratory, National Oceanic and Atmospheric Administration, Boulder, CO, 80305, USA
  • 7Department of Meteorology and Atmospheric Science, Pennsylvania State University, University Park, PA, USA

Abstract. Global observations and model studies indicate that new particle formation (NPF) in the upper troposphere (UT) and subsequent particles supply 40–60 % of cloud condensation nuclei (CCN) in the lower troposphere, thus affecting the Earth's radiative budget. There are several plausible nucleation mechanisms and precursor species in this atmospheric region, which, in the absence of observational constraints, lead to uncertainties in modeled aerosols. In particular, the type of nucleation mechanism and concentrations of nucleation precursors, in part, determine the spatial distribution of new particles and resulting spatial distribution of CCN from this source. Although substantial advances in understanding NPF have been made in recent years, NPF processes in the UT in pristine marine regions are still poorly understood and are inadequately represented in global models.

Here, we evaluate commonly used and state-of-the-art NPF schemes in a Lagrangian box model to assess which schemes and precursor concentrations best reproduce detailed in situ observations. Using measurements of aerosol size distributions (0.003 < Dp < 4.8 μm) in the remote marine troposphere between ~ 0.18 and 13 km altitude obtained during the NASA Atmospheric Tomography (ATom) mission, we show that high concentrations of newly formed particles in the tropical UT over both the Atlantic and Pacific oceans are associated with outflow regions of deep convective clouds. We focus analysis on observations over the remote Pacific Ocean, which is a region less perturbed by continental emissions than the Atlantic. Comparing aerosol size distribution measurements over the remote Pacific with box-model simulations for 32 cases shows that none of the NPF schemes most commonly used in global models, including binary nucleation of sulfuric acid and water (neutral and ion-assisted) and ternary involving sulfuric acid, water, and ammonia, are consistent with observations, regardless of precursor concentrations. Through sensitivity studies, we find that the nucleation scheme among those tested that is able to explain most consistently (22 of 32 cases) the observed size distributions is that of Riccobono et al. (2014), which involves both organic species and sulfuric acid. The method of Dunne et al. (2016), involving charged sulfuric acid-water-ammonia nucleation, when coupled with organic growth of the nucleated particles, was most consistent with the observations for 5 of 32 cases. Similarly, the neutral sulfuric acid-water-ammonia method of Napari (2002), when scaled with a tuning factor and with organic growth added was most consistent for 6 of 32 cases. We find that to best reproduce both nucleation and growth rates, the mixing ratios of gas-phase organic precursors generally need to be at least twice that of SO2, a proxy for dimethyl sulfide (DMS). Unfortunately, we have no information on the nature of oxidized organic species that participated in NPF in this region. Global models rarely include organic-driven nucleation and growth pathways in UT conditions where globally significant NPF takes place, which may result in poor estimates of NPF and CCN abundance and contribute to uncertainties in aerosol-cloud-radiation effects. Furthermore, our results indicate the organic aerosol precursor vapors may be important in the tropical UT above marine regions, a finding that should guide future observational efforts.

Agnieszka Kupc et al.

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Publications Copernicus
Short summary
Tropical upper troposphere over the Atlantic and Pacific Ocean is a major source region of new particles. These particles are associated with the outflow from deep convection. We investigate the processes that govern the formation of these particles and their initial growth, and show that none of the formation schemes commonly used in global models are consistent with observations. Using newer schemes indicates that organic compounds are likely important as nucleating and initial growth agents.
Tropical upper troposphere over the Atlantic and Pacific Ocean is a major source region of new...