The potential role of organics in new particle formation and initial growth in...

Kupc, A., C. Williamson, A. Hodshire, J. Kazil, E. Ray, T. P. Bui, M. Dollner, K. Froyd, K. McKain, A. Rollins, G. Schill, A. Thames, B. Weinzierl, J. Pierce, and C. Brock (2020), The potential role of organics in new particle formation and initial growth in the remote tropical upper troposphere, Atmos. Chem. Phys., doi:10.5194/acp-2020-675.

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

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Tropospheric Composition Program (TCP)